Blog updates are in ascending order by date entry.
Kit IM-13............................$34.95
Assembled IMW-13.....................$52.95
Obviously this will be my blog for restoring the meter. Quite the find actually and gladly adding it to my growing collection of Heathkit hardware. Updates will be posted at the end of the last update as progress is made.
The IM-13 VTVM was produced from 1964-1968 to the best of my knowledge. According the website link above, all of their VTVM's essentially had the same circuit design with the exception of the tubes used as the technology progressed. I suggest having a read of the webpage. Interesting info.
Another thing I'd like to recommend is Mr. Carlson's Lab on Youtube. This one goes into repair of a Knight KG-625 VTVM and it is built very similar to the Heathkit. It had to do the same job thus similar construction and design. If you are not interested in watching the repair itself, just watch the first two and a half minutes. Check it out here.
I wanted a VTVM to replace one my father had built from a company that had kits like Heathkit. He built a PACO VTVM (model V70) used to align the GR-180 color TV, SB-401, and SB-301 he also built.
The IM-13 arrived in a box which was well padded. Can't say anything about how it was handled during transport as one of the corners was slightly compressed showing it had taken a fall or something had impacted it. Once the box was opened the meter was inside another box. Double boxing. I like that! Getting the inner box open and finding the meter was wrapped in bubble wrap was a relief. Despite its harsh treatment by the shipping company, it appears the meter movement is okay. Analog meters don't do well when tossed about. The shock from impacts can damage the movement.
I purchased this meter from a seller having bought it through an estate sale of a hardware store owner who "dabbled" in electronics. If true, he most likely bought it assembled by Heathkit unless his dabbling was quite experienced in construction techniques. I wasn't given any further information about the unit. Either way I believe I have received a very good conditioned and well maintained meter. After removing the mounting bracket and the cover it appears to be in an original condition that also appears to have been professionally built. The solder joints are perfect and the components are placed just right. The serial number on the unit shows it to be number 1294.
As you can see on the ID sticker, the model, some odd number and the serial number are present. That center number is a bit of a puzzle for me. Is it an 8 then a line then a 1 or an 8-1-1 or and 8 and two I's? I never liked Arial font because an upper case I looks like a lower case L or a 1. But here is an example of the I and a 1 looking identical. So can anyone tell me what that middle number is?
I did get a bit surprised and excited to find this battery in the holder inside the meter!
30 cents for a carbon-zinc dry cell battery eh? Has to be late 1960's at the earliest to maybe early 1970's. An article beginning on page 46 in Popular Science dated January, 1971 shows the battery labeled like the one above with several others and mentions a price of 25 cents but didn't name the manufacture. I have emailed Eveready inquiring when this label was discontinued and changed to a regular font stating "Flashlight Battery" like the one in the blurry picture below. I hope to hear from them. At all.
Note the difference in the labels. Now why would I think this is special or unique? Batteries have been found in old radio's, test equipment and the like before. The why is because the battery is in perfect condition. "Okay. Now what?" You ask. Well, when I tested it for voltage, I got 1.45V on the meter. This is why I emailed Eveready about the production change on the label. It could give me an idea on how old the battery really is. My bet is at least 40 years.
I had to take the battery out just because I couldn't believe my eyes and I got the same result. 1.45 volts. I don't have a photo of that test but it read the same as in the one above.
Now this has to be a carbon-zinc battery since the old mercury batteries would have long ago corroded the terminals from electrolysis action. It's definitely not an alkaline even though they've been around since the late 1950's for the same reason. I'm going to put a Zinc-Chloride version made by Eveready labeled Super Heavy Duty into the meter when I'm finished with the restoration. I hope to find them in stores somewhere but if not, I'll just order online. An application manual, written by Eveready can be found here. In it describes the three types of carbon-zinc batteries based on their chemical make up.
Rayovac still manufactures and sells them as well for 99 cents for a pack of two directly from their website. I will most likely go with the Eveready as I've know that name quite a bit longer. Who knows? I might not have to buy another battery for the meter for the rest of my life. Depending on how much I need to measure Ohms or if I forget to switch the mode back to Volts mode that is. ;-)
Colors are now much more visible and defined and I found that the red-ish color in the photo's is gone. Purple doesn't look like a brown and yellow is easier to pick out. However, nothing will fix the idiocy of manufacturers that still make red colored resistors with red color bands. Really people? Learn from Vishay-Dale and print the value on it for cryin' out loud! (End of soapbox rant)
No scratches were found on the initial inspection of the face of the unit. The meter movement cover does not have any cracks but does have a blemish which was expected. What surprised me the most was that the two clips to wrap the power cord around were still there with the cord wrapped around them. I didn't notice that in the photo's. So now I'm wondering about how well this unit really might have been treated.
Not wanting to wait to find out, I tossed the box aside and carefully unwrapped the cord and found it was still flexible. Stiff but flexible. I didn't over flex it to straighten it out, just set it aside. I removed the one thumb knob (the other was missing as expected) to remove the mounting bracket and carefully removed the case cover screws. Once the cover was off, the interior of the IM-13 was clean with the exception of two cobweb lines. Removal of these from the case proved easy but harder to remove from my fingers.
As I mentioned earlier, my first impression was this had been professionally built or done as well by an experienced builder. It's been taken care of too and in original condition so far as I can tell. Looking through the unit I noticed the 12AU7 had the telltale signs of having been equalized as there was the tell-tale signs that the barium of the Getter absorbed as much contaminant as it could creating the milky white substance inside the top of the glass envelope. The 6AL5 appears to be okay by sight initially. The TC-2 tube checker will give me slightly more information as to the condition of the tube but not as good as a trans-conductance tube tester would.
Nothing appears to be amiss with the precision resistors on the range switch either. However, there are the plastic encapsulated paper capacitors and a paper electrolytic that will be replaced. C1, a .047 1600V capacitor looks to be a polypropylene but I'm not sure. The only copy of the manual I could find starts with page 26. Page 6 contains the main parts list, then I have page 7 and 8 then skips to 26. So if anyone has the rest of the parts list, please let me know.
I have a probe kit on the way so I can start using the meter as soon as it's restoration is complete. I will continue to search for an original probe in good condition or find a way to remake one with a reasonable design to operationally mimic the original. I might spring for the high voltage probe and just switch between them instead of having two probes tied in to one phono plug like the kit uses.
So what did I come away with this find? I felt I got lucky and wound up with a possibly factory assembled (or professionally built) IM-13 (sans probe) in original condition still having a Carbon-Zinc battery that could be over 40 years old.
Prognosis?
We have the technology. We can rebuild it.
73 for now!
__________________________________________
493-14435-ND - 1 CAP ALUM 22UF 20% 160V AXIAL
For tonight, I made up the DC/Ohms probe using the probe kit I purchased on eBay. The kit was meant to have three leads out from a single phono plug. I don't like the idea of a loose lead laying about and less so with a ground that uses a pin-point contact area on the barrel of the plug. I decided to make three leads. I have a plastic housing type phono plug so I won't need to insulate its casing. The ground lead will be secured under the nut that attaches the jack to the faceplate. This way it is always securely grounded. To work with AC voltages, I simply switch probe leads. I will at some point build a switchable probe like the one that was sold with these VTVM's but for now, I'm happy with having separate probes.
Until next time...
73!
_____________________________________
If you have one or are interested in doing a restoration with a capacitor kit, here's the link to my Digikey cart for just the caps.
http://www.digikey.com/short/30f4pr
73!
_____________________________________
Haven't looked for the 12AX7 tube yet. Will need to in order to get it working again to be sure.
73!
_____________________________________
Since I don't want to introduce noise, hum or other interference into the circuit any more than already exists I test capacitors to find out which lead is connected to the foil shell. To do this, I simply powered up my B&K Precision 1570A, bent the leads so that one inserts into the center conductor of the BNC connector and the other touches the chassis ground while holding the capacitor. My fingers act as a signal generator and can use the amplitude of the waveform to determine which lead is the shell. The higher amplitude is the foil as it has a better coupling to my fingers. I do this with all non-polarized capacitors except ceramic disc. The only unshielded portion of the capacitor is then the opposite lead going to the more positive potential of the circuit. A great explanation of this was provided by Mr. Carlson's Lab on YT here.
As the pictures suggest, I have gutted the chassis so I can clean it. It will make replacing the capacitors easier.
I have yet to purchase a 12AX7 tube and may rummage through another box in search of one. If not, I'll go through some other equipment to scavenge parts from I'm not going to restore just yet.
73!
_________________________________________
So I wiped them down, inserted them into their respective sockets in the time-honored tradition of using a piece of cloth to keep from getting fingerprints on the envelopes and pulled up the test and calibration page of the partial manual I found online. It's getting late so I need to go. But before I do, let me leave you with this quick two minute video of the initial warm up and DC Zero Adjust step from the manual. I also performed the switching back and forth from DC+ to DC- and back to finalize the zeroing and to see if the 12AU7A needs to be aged.
73!
_____________________________________
Applying the probes to my new 9 Lives C cell with the DC probe yielded little movement in the needle. About 1/2 of a volt. Adjusting the DC pot gave no change in the reading. So now I'm back to troubleshooting the 12AU7A tube's balance to see if it is indeed a good tube. I'm half-wondering if I should restore the TC-2 to test the tubes from this point forward.
ttfn
______________________________________
It seems the 12AU7A tube is balanced as I can set the meter function to AC and trim the balance to Zero. Doing that and being able to set the needle to Zero in both DC- and DC+ with less than one division difference says that the tube is good. I had to adjust the potentiometers very little to achieve this.
Now then, I decided to de-solder the 10K resistor connecting the plates of the 12AU7A to the rectifier just to get a reading on it even though I can get the needle reading to Zero. It was reading 11.7K which is way outside it's 10% specification. But I can Zero the needle so I'm not worried about it. I checked the AC Balance pot (10K-10 Turn) full resistance since it was now disconnected and it reads 7.80K. Now I've heard than these weren't the most accurate in their day and figured it might be off by a little but a whole 2K? Bad potentiometer? Perhaps. Looks like a few of the other resistors I tested are way out of spec too. I'll now have to systematically go through the whole circuit and check every resistor and potentiometer and replace those parts after I perform a few operating voltage checks. Since I only have the schematic and it doesn't show the voltages at various test points, I'll have to perform these tests myself and document the test point I used and the voltage I read on my DMM. Due to this, the project will take longer than anticipated to complete.
So what have I learned so far? No matter how good a condition the interior and exterior look, the components are old and more than just capacitors will need checking and replacing. So just to be realistic, there is no such thing as a quick fix to vintage equipment and should not take for granted the physical appearance of the unit's condition as an indicator of its operational capability. Check, check and check again.
73!
_______________________________
Now, back to our regularly scheduled blog...
It would appear there are hidden faults within disguised as working parts. I had to have an easier way to get to certain test points without risking a short circuit with the chassis. Granted there's only 120VAC at most but I don't like sparks inside my shack.
After removing the fully-assembled guts from the chassis again and using jumper leads to connect the meter, I plugged in the unit and began some voltage measurements. Since the transformer only steps down in the filament section, 124.5VAC was read on the secondary which matched the mains on the primary. Likewise the filament voltage was spot on 6.3VAC. The transformer tests good. So far, so good.
All voltages are measured with respect to ground as stated in the notes of the schematic. Also note I am using a DMM and a Sears 82373 analog meter to compare with. The schematic notes specifically state that voltages are taken with an 11 megohm (NOT megaohm) VTVM. So my results may vary since my DMM does not have an 11 megohm probe circuit and I don't know how much the circuit tested will be loaded.
The transformer does not center-tap to chassis via the secondary winding and uses a half-wave rectifier circuit. Since many of these have Selenium rectifiers, I was prepared to replace it with a 1N4007 anyway. I found a Heathkit 57-27 silicon rectifier rated 600V @1A in place. It tests good at 723Ω.
Testing the voltage on the lead after the rectifier was within the 50-70 VDC range on the schematic. Testing the lead from the transformer's secondary opposite the rectifier yielded -93.3VDC which is beyond the 60-85VDC negative voltage specified. Again, I'm using a DMM so I'm not sure if that's what I should get or not at this point. From here things seemed to get worse.
In trying to locate why the negative voltage was out seemingly of spec, I found three resistors (carbon composition) that were waaaay out of spec. In the bridge circuit for the 12AU7 there are three of them tied to a 10K potentiometer. A 180K on one side, a 150K on the other side and a 150K attached to the wiper lead. These are part of the balance for the tube's cathodes for the Zero Adjust.
Disconnecting each of them from the potentiometer would allow me to test them without having to calculate their combined parallel/series resistance. I found one 150K that looked cooked and it made the DMM readings jumps all over. I pulled out my old Sears 82373 analog meter and it did the same thing only faster. I tested the other 150K and got 180K. I had to look at the bands again just to be sure and I picked up the right resistor. Yep. Brn/Grn/Yel bands. I checked the 180K resistor (Brn/Gry/Yel) and got a whopping 226K ohms. I removed them from the VTVM and tested them individually without test leads. Plugging them directly into the probe sockets on the DMM I got the same results. So right there I have some serious balance issues with regards to referencing voltages simply because the old carbon composition resistors have either been stressed or absorbed moisture over the decades and ruined them. I cannot trust their stability at this point. I might be able to use them in some simple circuit where they can drift around a lot but never in something that requires a stable circuit. "Hello, Digikey? Yeah. It's me again. I got a BOM list I need you fill. Oh, and while you're at it, double-quadruple everything. I'm in one of those moods."
Just for insanity's sake, I removed the 27K from the AC Balance pot and it read
So another lesson learned:
So many do what I just did even though I swore I'd never do it. That is to say, I would never re-use a carbon composition resistor or just assume it's still within tolerance. I made that mistake with this meter because I was foolish in thinking a unit this clean and well cared for would function properly if all I did was replace the old paper-in-oil and electrolytic capacitors. Never again. It's all getting replaced for a total overhaul.
Now I do have to say many will not mess with the "precision" resistors on the range selector switch. I have a deep desire to avoid messing with those as well but since this is out of the chassis, I have the opportunity to test them as well. Looking at the schematic I see on Wafer C-Rear a 9.1Ω, 90Ω, 900Ω, 9K, 90K, 900K and a 9M. Most of these say 1% on them. Now I don't know much about the manufacturing process of these vintage precision resistors but I'm getting concerned when the 1 watt 9.1Ω tested at 9.9Ω. It's carbon composition with the following bands: Wht/Brn/Gold/Gold. This is identified as a 5% tolerance with the double-gold bands. It's out of spec. The others aren't squared off like this one. I don't know how the 'roundies' were made back then. Can't wait to check the 6.838M 1% but I can't as my old DMM can't test above 2M. I need another meter. Have to look at the Sears again but I don't think it will do more than 2M either.
73 for now!
_______________________________
The small clips that make contact with the discs couldn't be scrubbed. I thought about 5000 grit Emery paper but figured that removing any metal here would shorten their life considerably. So I used a generous amount of CRC on these and rotated the discs rapidly to work the mechanism then spray again to wash away any debris broken loose.
After marking the indexing side of the shafts in two places, I disassembled them as well by removing the capture ring and sliding the shafts out of their collets or bushing shells. Cleaning the grease off wasn't too bad on the Range Selector. The Function Selector had hardened and was much more difficult. After cleaning I regreased them with Labelle 106 grease with PTFE I use for my model train gear boxes.
Reassembly is the reverse and after consulting a picture I took before working the problem, got the insulator/spacer tubes in the correct order. Now is the time to be very careful about over tightening the screws that hold the assembly together. Crack the phenolic and you're done. If you're good with superglue, you might have a chance to fix it. Since each wafer is custom made for the application, you cannot use any aftermarket rotary type switch. These wafers are essentially the program of the meter settings. You could sit down and work out a switch replacement but you'll need at least two more switches and a guide to setting them so you don't short out the testing circuits. If I could, I would send these wafers in and have 100 of each made and have a rotary switch rebuild kit to sell. But there are only so many that would buy and I would still have a bunch left over ten years from now so it's not really economical or practical unless I go into business restoring the IM-13.
At this point the rotary switches are cleaned and ready to advance to the next step of restoration. I'll remove the AC Balance and two voltage mode pots and test them for consistency in their sweep from stop to stop like I did with the Zero and Ohms adjustment pots. In the meantime, I still have a BOM to build on my Digikey account.
73!
_______________________________
I created a Bill Of Materials (BOM for short) on Digikey and found everything I needed. While doing that, I decided to check out the IT-28 and get parts for both projects to save a little on shipping. I neglected to include the TC-2's parts so I'll have to eat the shipping for that separately. Bummer.
So testing after the rotary switches were cleaning checked out the switches were making good, clean (relatively) contact. I used my oscilloscope to check the noise on the Pots but didn't check the rotaries as they would be full of spikes during transition. I checked the Pots to see if there were any dead spots or interruptions. So far, so good. With the caps already having been replaced, it was time to get the resistors taken care of and put the old ones to rest.
Since I wasn't planning on putting RF through the meter, (they make probes for that) I decided to use Metal Film resistors from Vishay-Dale. I know what you're thinking. "Why?" Partly because I could get them but also I know it will outlast me in this lifetime so I can count on it anytime I need it. Unless I do something stupid like put 3kV through it before I realize I need the HV probe. Another reason was that I wanted to try my hand at re-engineering the unit with modern equivalent parts. Today's resistors are made on a glass core rod, coated in resistive material and then the resistive material is cut down with a laser in a helical pattern to fine tune the value. This makes them highly inductive and unsuitable for RF use. As the frequency rises above about 11 megacycles the resistor becomes more of a jumper and the resistance is lost. Not good for SWR bridges and other circuits handling RF. But, in low frequency AC and DC applications, they work superbly. Now I could have gotten 1 watt resistors that are the same size as the 1/2 watt carbon composition types but I didn't think there would be a need. Where a 1 watt resistor was needed, I substituted a 2 watt and left the existing 2 watt resistors in the circuit at their respective rating.
Of course I'm taking a chance changing out every single capacitor and resistor (except the precision resistors) to fix this unit. Some might say this is a "shotgun" approach. I disagree. A shotgun solution is that you don't know what's wrong and just replace it all with exact values as before without figuring it out. Okay, I may not have figured it out so the jury is still out on this one.
So the parts have been ordered alongside the parts for the IT-28. There were three capacitors I could not find anywhere that replaces the old Mylar® (Polystyrene) capacitors. Instead of axial leads, I was able to locate PC mount at Mouser in the brick style. It will work but won't look as good. I ordered some Mica caps just in case I change my mind.
73!
____________________________________
On Sunday the 20th I began replacing the resistors. Specifically the 22 megohm 1/2 watt components of R6-R11. I did remove the solder terminal R9 connected between two of them to make it easier to get the new component installed. It also opened up some space to perform a couple of other component swaps. That was all I was able to get it before other things required my attention.
On Wednesday the 23rd, I picked up Sunday's progress and reinstalled the solder terminal and soldered R9 and proceeded to get down to R11's replacement remaining. I replaced the resistor/capacitor pair at R16 and C5 first. The C5 capacitor leads were wrapped around the resistor leads in an attempt to reduce capacitance between the leads going to the same two terminal points.
Getting the cap off the resistor without cutting at least one lead wasn't possible so 'snip' went one lead. The resistor measured over 12 megohms thus putting it outside the acceptable tolerance. The capacitor was a Z5U dielectric ceramic disc and there was no physical damage as could be seen with a magnifying glass. I reused it without re-bending the leads. Once one lead of the resistor was snipped, I could melt the solder and pull the lead out. Pulling the cap off the other lead was easy. Putting it on the new resistor wasn't difficult since the new leads were thinner than the old resistor's. After sliding the cap lead over one lead of the resistor, I managed to semi-wind the other lead through the cap's already formed twisted lead. A slight twist of the pliers to tighten them up and applied solder to complete the connection.
Bending leads to fit between two solder terminals without unnecessary bends can be a challenge, especially where they are quite close to each other. But without removing a solder terminal lug or a tube socket, it wasn't possible to keep from bending them. So gently bending them and making a 90 degree bend at the point it should hook the terminal worked out pretty well. A straight lead through the other terminal served the purpose well without the need to hook the lead. This was the MO for most of the components. Hooking leads around terminals adds to difficulties later when removal is needed and doesn't really improve the connection since the solder will flow and typically raise or suspend the lead off the surface of the lug unless you crimp the hook down making it even more difficult to remove.
R12 was replaced next. C4 was another Z5U disc cap that didn't require replacing. After remounting the Zero Adjust and Ohm's Adjust pots I installed R31, R32 and R34. I did use heat shrink tubing on the 125VAC exposed leads of these resistors as I didn't like them being exposed even though they were only connected to the secondary of the transformer. Next the rectifier was reinstalled. Yep, I used the original rectifier, D1. It checked out and didn't have any structural issues as far as I could tell. That and it's resistance measured only 20 ohms lower than a 1N4007 I had in stock. So, why not?
C6 was installed as was R35, R36, and R38. C2, a .005µF (5000pF) mica cap was still connected from my earlier replacement of that component. The last component I replaced was R30 after remounting the rotary switches to the chassis. The Vishay-BC replacement was the size of a typical 1/4 watt carbon film but the datasheet confirmed it was a 2 watt value and of metal film construction.
At the last minute I decided to install a fuse cradle type fuse holder. Now, this was for the mains or primary side of the power transformer to protect the transformer from becoming a charcoal briquette. that's the hope anyway. Since the whole device is rated less than 1 amp where .8 is used by the filaments and 10mA used by the rest of the circuit on the secondary, I figured a 1 amp fast-blow fuse might be just enough to handle the short surge by the heaters upon powering up.
With the leads from the rotary switches reattached to their respective terminations, it was time to install the 12AU7A and the 6AL5. Following which side was the fused or HOT side of the power cord, I marked the other side as Neutral so I would know which way to plug it in consistently.
I performed the initial calibration for no power zeroing of the needle. Then turned the IM-13 on and left it for about 30 minutes. After that, I went through the calibration process then left it on to age the 12AU7A tube for a few hours. I turned it on again for about six hours on Friday and another 12 on Saturday. Sunday had about six hours in the DC+ position again and then I performed the calibration tests again. After wiping the meter face with a used dryer sheet to dissipate the static charge it picked up, I was able to complete the calibration process. With that, I did a few measurements with a few batteries after taking readings from a digital meter first for comparison. Next I pulled a small 14 volt AC transformer out and used it to test the AC function. Once done a few resistors were also tested for the Ohm's function. The accuracy of the meter isn't perfect but knowing that I'm still going to use it for what it was meant for. Testing the vacuum tube equipment I have as I restore them without loading the circuits. Can't do that with my DMM. Also, analog meters used for finding a peak or dip is much easier since I'm not waiting for the DMM between one second updates to settle down. I see it as it happens.
Oh, the battery I used for the ohm function was the new Eveready Super Heavy Duty C cell I bought. the cord was wrapped around two tabs the previous owner installed on top of the case. I put them back in as they do provide a pretty good storage solution for the power cord and permanent ground clip lead. I installed a ring terminal on the ground lead so I could capture it with the probe socket's nut.
It was a fun project, albeit a bit longer than I wanted but other things happened preventing me from getting back to it sooner. Now that things have settled down a little bit, I have more time to work the projects.
73!
IM-13 Desktop Vacuum Tube Volt Meter
Heathkit 1968
Catalog 810/68R
PricesKit IM-13............................$34.95
Assembled IMW-13.....................$52.95
Obviously this will be my blog for restoring the meter. Quite the find actually and gladly adding it to my growing collection of Heathkit hardware. Updates will be posted at the end of the last update as progress is made.
 | |
| Photo and text available at: http://www.ohio.edu/people/postr/bapix/vtvms.htm |
The IM-13 VTVM was produced from 1964-1968 to the best of my knowledge. According the website link above, all of their VTVM's essentially had the same circuit design with the exception of the tubes used as the technology progressed. I suggest having a read of the webpage. Interesting info.
Another thing I'd like to recommend is Mr. Carlson's Lab on Youtube. This one goes into repair of a Knight KG-625 VTVM and it is built very similar to the Heathkit. It had to do the same job thus similar construction and design. If you are not interested in watching the repair itself, just watch the first two and a half minutes. Check it out here.
 |
| My own IM-13 |
I wanted a VTVM to replace one my father had built from a company that had kits like Heathkit. He built a PACO VTVM (model V70) used to align the GR-180 color TV, SB-401, and SB-301 he also built.
The IM-13 arrived in a box which was well padded. Can't say anything about how it was handled during transport as one of the corners was slightly compressed showing it had taken a fall or something had impacted it. Once the box was opened the meter was inside another box. Double boxing. I like that! Getting the inner box open and finding the meter was wrapped in bubble wrap was a relief. Despite its harsh treatment by the shipping company, it appears the meter movement is okay. Analog meters don't do well when tossed about. The shock from impacts can damage the movement.
I purchased this meter from a seller having bought it through an estate sale of a hardware store owner who "dabbled" in electronics. If true, he most likely bought it assembled by Heathkit unless his dabbling was quite experienced in construction techniques. I wasn't given any further information about the unit. Either way I believe I have received a very good conditioned and well maintained meter. After removing the mounting bracket and the cover it appears to be in an original condition that also appears to have been professionally built. The solder joints are perfect and the components are placed just right. The serial number on the unit shows it to be number 1294.
I did get a bit surprised and excited to find this battery in the holder inside the meter!
30 cents for a carbon-zinc dry cell battery eh? Has to be late 1960's at the earliest to maybe early 1970's. An article beginning on page 46 in Popular Science dated January, 1971 shows the battery labeled like the one above with several others and mentions a price of 25 cents but didn't name the manufacture. I have emailed Eveready inquiring when this label was discontinued and changed to a regular font stating "Flashlight Battery" like the one in the blurry picture below. I hope to hear from them. At all.
 |
| Tested in the holder as found after removing the cover. |
I had to take the battery out just because I couldn't believe my eyes and I got the same result. 1.45 volts. I don't have a photo of that test but it read the same as in the one above.
Now this has to be a carbon-zinc battery since the old mercury batteries would have long ago corroded the terminals from electrolysis action. It's definitely not an alkaline even though they've been around since the late 1950's for the same reason. I'm going to put a Zinc-Chloride version made by Eveready labeled Super Heavy Duty into the meter when I'm finished with the restoration. I hope to find them in stores somewhere but if not, I'll just order online. An application manual, written by Eveready can be found here. In it describes the three types of carbon-zinc batteries based on their chemical make up.
Rayovac still manufactures and sells them as well for 99 cents for a pack of two directly from their website. I will most likely go with the Eveready as I've know that name quite a bit longer. Who knows? I might not have to buy another battery for the meter for the rest of my life. Depending on how much I need to measure Ohms or if I forget to switch the mode back to Volts mode that is. ;-)
 | |
| The red-ish hue will be gone from future photo's in future posts. |
Continuing on with my evaluation...
One side note I should mention: After having trouble seeing the color bands on some of the resistors I decided to change out the 2700K soft white CFL in my workbench lamp with a GE 5000K Refresh LED bulb.
Colors are now much more visible and defined and I found that the red-ish color in the photo's is gone. Purple doesn't look like a brown and yellow is easier to pick out. However, nothing will fix the idiocy of manufacturers that still make red colored resistors with red color bands. Really people? Learn from Vishay-Dale and print the value on it for cryin' out loud! (End of soapbox rant)
No scratches were found on the initial inspection of the face of the unit. The meter movement cover does not have any cracks but does have a blemish which was expected. What surprised me the most was that the two clips to wrap the power cord around were still there with the cord wrapped around them. I didn't notice that in the photo's. So now I'm wondering about how well this unit really might have been treated.
Not wanting to wait to find out, I tossed the box aside and carefully unwrapped the cord and found it was still flexible. Stiff but flexible. I didn't over flex it to straighten it out, just set it aside. I removed the one thumb knob (the other was missing as expected) to remove the mounting bracket and carefully removed the case cover screws. Once the cover was off, the interior of the IM-13 was clean with the exception of two cobweb lines. Removal of these from the case proved easy but harder to remove from my fingers.
As I mentioned earlier, my first impression was this had been professionally built or done as well by an experienced builder. It's been taken care of too and in original condition so far as I can tell. Looking through the unit I noticed the 12AU7 had the telltale signs of having been equalized as there was the tell-tale signs that the barium of the Getter absorbed as much contaminant as it could creating the milky white substance inside the top of the glass envelope. The 6AL5 appears to be okay by sight initially. The TC-2 tube checker will give me slightly more information as to the condition of the tube but not as good as a trans-conductance tube tester would.
 |
| The white powdery residue inside the top hat of the 12AU7 Mullard tube. |
Nothing appears to be amiss with the precision resistors on the range switch either. However, there are the plastic encapsulated paper capacitors and a paper electrolytic that will be replaced. C1, a .047 1600V capacitor looks to be a polypropylene but I'm not sure. The only copy of the manual I could find starts with page 26. Page 6 contains the main parts list, then I have page 7 and 8 then skips to 26. So if anyone has the rest of the parts list, please let me know.
I have a probe kit on the way so I can start using the meter as soon as it's restoration is complete. I will continue to search for an original probe in good condition or find a way to remake one with a reasonable design to operationally mimic the original. I might spring for the high voltage probe and just switch between them instead of having two probes tied in to one phono plug like the kit uses.
So what did I come away with this find? I felt I got lucky and wound up with a possibly factory assembled (or professionally built) IM-13 (sans probe) in original condition still having a Carbon-Zinc battery that could be over 40 years old.
Prognosis?
We have the technology. We can rebuild it.
73 for now!
__________________________________________
Parts list for partial restoration - Digikey
493-14435-ND - 1 CAP ALUM 22UF 20% 160V AXIAL
338-3957-ND - 1 CAP FILM 0.047UF 10% 1.6KVDC AXIAL*
P12078-ND - 2 CAP FILM 0.051UF 5% 400VDC RAD**
*Requires 10% tolerance as originally specified in the Heathkit manual.
**Originally was a paper-in-oil .05UF 400VDC Sprague. Paper caps this old have gone acidic and suffers from perforations thus becoming resistors.
NOT replacing these capacitors will burn out plate resistors and transformers. Always test for voltage leakage to determine at what voltage these leak DC voltage before re-using these in any circuit.
June 25, 2017
Just wanted to offer a kind of update. It's been a while since I've dealt with the meter. The SB-630 took priority as I wanted the clock and the SWR meter to use with my Kenwood TS-450S/AT. After completing the rework on the sB-630 a few days ago, I took down the antenna in preparation for some tree branch removal. After taking care of that, I was working with WSPR at about 3 watts (lowest I can get the rig to go) and noticed a hum in the power supply and getting louder. In a panic I thrust my hand toward the power switch knocking everything on the desk off that was in the path to shut it down. I took a whiff over the vents and smelled 'hot'. After letting it cool down, I connected my DMM to it and switched it on for a few seconds. I got only 8 volts out of it. So, hopefully the transformer is OK and just blew a transistor. I'm almost afraid to open it up right now so I'll move on to the tube checker. I do need that for any of the future tube equipment work to provide me with some basic tube check.For tonight, I made up the DC/Ohms probe using the probe kit I purchased on eBay. The kit was meant to have three leads out from a single phono plug. I don't like the idea of a loose lead laying about and less so with a ground that uses a pin-point contact area on the barrel of the plug. I decided to make three leads. I have a plastic housing type phono plug so I won't need to insulate its casing. The ground lead will be secured under the nut that attaches the jack to the faceplate. This way it is always securely grounded. To work with AC voltages, I simply switch probe leads. I will at some point build a switchable probe like the one that was sold with these VTVM's but for now, I'm happy with having separate probes.
Until next time...
73!
_____________________________________
July 2, 2017
Parts have been ordered. All four of the capacitors. One electrolytic and two paper in oil (PIO) capacitors and one wax capacitor will be replaced. A few checks of the resistors check out okay and since the circuits are calibrated with potentiometers I think I can re-use these. Nothing looks burnt or otherwise overheated. The only other things I found wrong with the unit was the reason why the 12AU7 equalized is probably due to it being roughly handled or a harsh impact. The glass envelope has a crack that is visible only when you have the light just right. So, that will be replaced once I get a chance to track one or two down.If you have one or are interested in doing a restoration with a capacitor kit, here's the link to my Digikey cart for just the caps.
http://www.digikey.com/short/30f4pr
73!
_____________________________________
July 10, 2017
I have all the parts to re-cap the IM-13 but there is a question on the .047μF cap as to whether it's an electrolytic or not. The original shows a band on one end and the cap I purchased to replace it was a polypropylene (PP) film cap. I should be able to use it but not sure if the electrolytic was there for anything other than a cheaper alternative to a PP in those days. I'll do a little more looking but the audio crowd seems to prefer the PP over the Electrolytic when doing re-caps.Haven't looked for the 12AX7 tube yet. Will need to in order to get it working again to be sure.
73!
_____________________________________
July 16, 2017
Got a chance to do a little work on the unit tonight. I replaced the .047μF1600V plastic capacitor with the new Cornell Dubilier metallized polypropylene capacitor after using the oscilloscope to determine which lead was attached to the foil shell. Unlike the old days of wax and paper-in-oil capacitors where they marked the "outside foil" of the capacitor, mine weren't marked. Many Orange Drop, Brown Drop and others may have a bar or line which may indicate something. However, since they run down an automated assembly line the capacitor will be printed on whichever side happens to face the machine's imprinter.Since I don't want to introduce noise, hum or other interference into the circuit any more than already exists I test capacitors to find out which lead is connected to the foil shell. To do this, I simply powered up my B&K Precision 1570A, bent the leads so that one inserts into the center conductor of the BNC connector and the other touches the chassis ground while holding the capacitor. My fingers act as a signal generator and can use the amplitude of the waveform to determine which lead is the shell. The higher amplitude is the foil as it has a better coupling to my fingers. I do this with all non-polarized capacitors except ceramic disc. The only unshielded portion of the capacitor is then the opposite lead going to the more positive potential of the circuit. A great explanation of this was provided by Mr. Carlson's Lab on YT here.
As the pictures suggest, I have gutted the chassis so I can clean it. It will make replacing the capacitors easier.
I have yet to purchase a 12AX7 tube and may rummage through another box in search of one. If not, I'll go through some other equipment to scavenge parts from I'm not going to restore just yet.
73!
_________________________________________
July 17, 2017
Tonight I was able to finish installing the last two capacitors, clean the chassis faceplate, reassemble it to test with. The only problem I had was I needed a 12AU7 tube. Looking through some boxes, I found a bag of small tubes. Two of them were GE's and in boxes. The label showed 6AL5 and 12AU7A. Not trusting what was inside I opened them and my jaw dropped. Yep. They were what the box said they were. Now the original IEC Mullard 12AU7 (ECC82) that had equalized was the real prize to have had a replacement that worked. I was expecting to have to find a few online to purchase and hope one of them was actually a good tube. The 6AL5 that was in there originally was made by Unitron. I hadn't known that brand and since I had a boxed GE 6AL5 that I hoped was good, I used it instead. These two tubes were probably the spares my Dad had for his PACO VTVM as the two used the same tubes.So I wiped them down, inserted them into their respective sockets in the time-honored tradition of using a piece of cloth to keep from getting fingerprints on the envelopes and pulled up the test and calibration page of the partial manual I found online. It's getting late so I need to go. But before I do, let me leave you with this quick two minute video of the initial warm up and DC Zero Adjust step from the manual. I also performed the switching back and forth from DC+ to DC- and back to finalize the zeroing and to see if the 12AU7A needs to be aged.
73!
_____________________________________
July 21, 2017
Had a little time to build the probe. I decided to build the aftermarket probe using dual probes as it was intended by the seller since I did not have another shielded phono plug available to make individual probes. After assembling it I tested it with my DMM and found it was sound with no shorts or opens. Next was an initial test plugged into the meter. The first test was DC+ to a "flashlight battery". Back then, the Zinc-Carbon batteries had about 1.5V when new. Today's Zinc-Chloride battery outputs about 1.6V with longer life and greater supply stability. Sold primarily as a "Heavy Duty" battery, the Zinc-Chloride still has it's uses as storage cells for lighting and other low current draw devices. Read more about this type of battery here.Applying the probes to my new 9 Lives C cell with the DC probe yielded little movement in the needle. About 1/2 of a volt. Adjusting the DC pot gave no change in the reading. So now I'm back to troubleshooting the 12AU7A tube's balance to see if it is indeed a good tube. I'm half-wondering if I should restore the TC-2 to test the tubes from this point forward.
ttfn
______________________________________
July 23, 2017
Had a little time to work on meter today to try and figure out why it won't read the Zinc-Chloride (heretofore known as the Heavy Duty battery as it is now called by Eveready) voltage properly. I can zero the meter and set the AC balance but it will not give me the proper voltage reading using the DC probe on any battery. Until I can understand why it won't do that, I won't apply a large AC voltage to it for safety reasons. I may use another power transformer that puts out 14VAC-18VAC from my model train controller for the AC testing.It seems the 12AU7A tube is balanced as I can set the meter function to AC and trim the balance to Zero. Doing that and being able to set the needle to Zero in both DC- and DC+ with less than one division difference says that the tube is good. I had to adjust the potentiometers very little to achieve this.
Now then, I decided to de-solder the 10K resistor connecting the plates of the 12AU7A to the rectifier just to get a reading on it even though I can get the needle reading to Zero. It was reading 11.7K which is way outside it's 10% specification. But I can Zero the needle so I'm not worried about it. I checked the AC Balance pot (10K-10 Turn) full resistance since it was now disconnected and it reads 7.80K. Now I've heard than these weren't the most accurate in their day and figured it might be off by a little but a whole 2K? Bad potentiometer? Perhaps. Looks like a few of the other resistors I tested are way out of spec too. I'll now have to systematically go through the whole circuit and check every resistor and potentiometer and replace those parts after I perform a few operating voltage checks. Since I only have the schematic and it doesn't show the voltages at various test points, I'll have to perform these tests myself and document the test point I used and the voltage I read on my DMM. Due to this, the project will take longer than anticipated to complete.
So what have I learned so far? No matter how good a condition the interior and exterior look, the components are old and more than just capacitors will need checking and replacing. So just to be realistic, there is no such thing as a quick fix to vintage equipment and should not take for granted the physical appearance of the unit's condition as an indicator of its operational capability. Check, check and check again.
73!
_______________________________
July 25, 2017
Just to clarify something that comes up often. One million ohms is written out as megohm and pronounced as megohm.. Not megaohm or mega-ohm or written as meg-ohm (even though it is acceptable in certain types of print). As far as I know from my training and experience, some people hear "meg-a-ohm" because they're used to hearing or saying the 'a' in other contexts of the use of 'meg'. For example: megabyte, megawatt, megavolt or megacycles. It's possible that the word is said so quickly it could be mistaken the 'a' sounds like it is present. It is not. Trust me on this. It's also one of the better ways to irritate your EE instructor. You can also annoy your American geology professor by using the British pronunciation of Aluminium instead of the American version, Aluminum. That's another story for another time.Now, back to our regularly scheduled blog...
It would appear there are hidden faults within disguised as working parts. I had to have an easier way to get to certain test points without risking a short circuit with the chassis. Granted there's only 120VAC at most but I don't like sparks inside my shack.
After removing the fully-assembled guts from the chassis again and using jumper leads to connect the meter, I plugged in the unit and began some voltage measurements. Since the transformer only steps down in the filament section, 124.5VAC was read on the secondary which matched the mains on the primary. Likewise the filament voltage was spot on 6.3VAC. The transformer tests good. So far, so good.
All voltages are measured with respect to ground as stated in the notes of the schematic. Also note I am using a DMM and a Sears 82373 analog meter to compare with. The schematic notes specifically state that voltages are taken with an 11 megohm (NOT megaohm) VTVM. So my results may vary since my DMM does not have an 11 megohm probe circuit and I don't know how much the circuit tested will be loaded.
The transformer does not center-tap to chassis via the secondary winding and uses a half-wave rectifier circuit. Since many of these have Selenium rectifiers, I was prepared to replace it with a 1N4007 anyway. I found a Heathkit 57-27 silicon rectifier rated 600V @1A in place. It tests good at 723Ω.
Testing the voltage on the lead after the rectifier was within the 50-70 VDC range on the schematic. Testing the lead from the transformer's secondary opposite the rectifier yielded -93.3VDC which is beyond the 60-85VDC negative voltage specified. Again, I'm using a DMM so I'm not sure if that's what I should get or not at this point. From here things seemed to get worse.
In trying to locate why the negative voltage was out seemingly of spec, I found three resistors (carbon composition) that were waaaay out of spec. In the bridge circuit for the 12AU7 there are three of them tied to a 10K potentiometer. A 180K on one side, a 150K on the other side and a 150K attached to the wiper lead. These are part of the balance for the tube's cathodes for the Zero Adjust.
Disconnecting each of them from the potentiometer would allow me to test them without having to calculate their combined parallel/series resistance. I found one 150K that looked cooked and it made the DMM readings jumps all over. I pulled out my old Sears 82373 analog meter and it did the same thing only faster. I tested the other 150K and got 180K. I had to look at the bands again just to be sure and I picked up the right resistor. Yep. Brn/Grn/Yel bands. I checked the 180K resistor (Brn/Gry/Yel) and got a whopping 226K ohms. I removed them from the VTVM and tested them individually without test leads. Plugging them directly into the probe sockets on the DMM I got the same results. So right there I have some serious balance issues with regards to referencing voltages simply because the old carbon composition resistors have either been stressed or absorbed moisture over the decades and ruined them. I cannot trust their stability at this point. I might be able to use them in some simple circuit where they can drift around a lot but never in something that requires a stable circuit. "Hello, Digikey? Yeah. It's me again. I got a BOM list I need you fill. Oh, and while you're at it, double-quadruple everything. I'm in one of those moods."
Just for insanity's sake, I removed the 27K from the AC Balance pot and it read
So another lesson learned:
So many do what I just did even though I swore I'd never do it. That is to say, I would never re-use a carbon composition resistor or just assume it's still within tolerance. I made that mistake with this meter because I was foolish in thinking a unit this clean and well cared for would function properly if all I did was replace the old paper-in-oil and electrolytic capacitors. Never again. It's all getting replaced for a total overhaul.
Now I do have to say many will not mess with the "precision" resistors on the range selector switch. I have a deep desire to avoid messing with those as well but since this is out of the chassis, I have the opportunity to test them as well. Looking at the schematic I see on Wafer C-Rear a 9.1Ω, 90Ω, 900Ω, 9K, 90K, 900K and a 9M. Most of these say 1% on them. Now I don't know much about the manufacturing process of these vintage precision resistors but I'm getting concerned when the 1 watt 9.1Ω tested at 9.9Ω. It's carbon composition with the following bands: Wht/Brn/Gold/Gold. This is identified as a 5% tolerance with the double-gold bands. It's out of spec. The others aren't squared off like this one. I don't know how the 'roundies' were made back then. Can't wait to check the 6.838M 1% but I can't as my old DMM can't test above 2M. I need another meter. Have to look at the Sears again but I don't think it will do more than 2M either.
73 for now!
_______________________________
July 26, 2017
I tested the precision resistors on the range selector and don't believe they will be a problem. Since I have no data on the value they actually were when installed originally, the measurements I took will have to serve as the baseline. I'll calculate their percentage of error later. I wanted to finish cleaning the rotary contacts and discs as they looked pretty bad in the good light of my LED desk lamp. I could clearly see the oxidation and crud build-up. I soaked a cotton swab with CRC's Electronic Cleaner (P/N:05103) and scrubbed each disc after disassembling the switch and separating the wafers. Careful manipulation of these allowed me to gain access to the wafers on the Function switch. The Range switch wasn't as easy since all the resistors are soldered between all three wafers. However, there is enough space to utilize a synthetic eraser by Greenbrier International from an art supply store to scrub away the oxidation. A generous spray of CRC 05103 and the copper on the discs now shine.The small clips that make contact with the discs couldn't be scrubbed. I thought about 5000 grit Emery paper but figured that removing any metal here would shorten their life considerably. So I used a generous amount of CRC on these and rotated the discs rapidly to work the mechanism then spray again to wash away any debris broken loose.
After marking the indexing side of the shafts in two places, I disassembled them as well by removing the capture ring and sliding the shafts out of their collets or bushing shells. Cleaning the grease off wasn't too bad on the Range Selector. The Function Selector had hardened and was much more difficult. After cleaning I regreased them with Labelle 106 grease with PTFE I use for my model train gear boxes.
Reassembly is the reverse and after consulting a picture I took before working the problem, got the insulator/spacer tubes in the correct order. Now is the time to be very careful about over tightening the screws that hold the assembly together. Crack the phenolic and you're done. If you're good with superglue, you might have a chance to fix it. Since each wafer is custom made for the application, you cannot use any aftermarket rotary type switch. These wafers are essentially the program of the meter settings. You could sit down and work out a switch replacement but you'll need at least two more switches and a guide to setting them so you don't short out the testing circuits. If I could, I would send these wafers in and have 100 of each made and have a rotary switch rebuild kit to sell. But there are only so many that would buy and I would still have a bunch left over ten years from now so it's not really economical or practical unless I go into business restoring the IM-13.
At this point the rotary switches are cleaned and ready to advance to the next step of restoration. I'll remove the AC Balance and two voltage mode pots and test them for consistency in their sweep from stop to stop like I did with the Zero and Ohms adjustment pots. In the meantime, I still have a BOM to build on my Digikey account.
73!
_______________________________
July 30, 2017
After a bit of messing with the IM-13 and some checks on most of the resistors, it's been decided to do a full electronic restoration as all of the resistors are out of spec by 25% or more with two looking like they got cooked.I created a Bill Of Materials (BOM for short) on Digikey and found everything I needed. While doing that, I decided to check out the IT-28 and get parts for both projects to save a little on shipping. I neglected to include the TC-2's parts so I'll have to eat the shipping for that separately. Bummer.
So testing after the rotary switches were cleaning checked out the switches were making good, clean (relatively) contact. I used my oscilloscope to check the noise on the Pots but didn't check the rotaries as they would be full of spikes during transition. I checked the Pots to see if there were any dead spots or interruptions. So far, so good. With the caps already having been replaced, it was time to get the resistors taken care of and put the old ones to rest.
Since I wasn't planning on putting RF through the meter, (they make probes for that) I decided to use Metal Film resistors from Vishay-Dale. I know what you're thinking. "Why?" Partly because I could get them but also I know it will outlast me in this lifetime so I can count on it anytime I need it. Unless I do something stupid like put 3kV through it before I realize I need the HV probe. Another reason was that I wanted to try my hand at re-engineering the unit with modern equivalent parts. Today's resistors are made on a glass core rod, coated in resistive material and then the resistive material is cut down with a laser in a helical pattern to fine tune the value. This makes them highly inductive and unsuitable for RF use. As the frequency rises above about 11 megacycles the resistor becomes more of a jumper and the resistance is lost. Not good for SWR bridges and other circuits handling RF. But, in low frequency AC and DC applications, they work superbly. Now I could have gotten 1 watt resistors that are the same size as the 1/2 watt carbon composition types but I didn't think there would be a need. Where a 1 watt resistor was needed, I substituted a 2 watt and left the existing 2 watt resistors in the circuit at their respective rating.
Of course I'm taking a chance changing out every single capacitor and resistor (except the precision resistors) to fix this unit. Some might say this is a "shotgun" approach. I disagree. A shotgun solution is that you don't know what's wrong and just replace it all with exact values as before without figuring it out. Okay, I may not have figured it out so the jury is still out on this one.
So the parts have been ordered alongside the parts for the IT-28. There were three capacitors I could not find anywhere that replaces the old Mylar® (Polystyrene) capacitors. Instead of axial leads, I was able to locate PC mount at Mouser in the brick style. It will work but won't look as good. I ordered some Mica caps just in case I change my mind.
73!
____________________________________
August 27, 2017
Now that I've had some time to work on the meter, I can say this was another fun project completed.On Sunday the 20th I began replacing the resistors. Specifically the 22 megohm 1/2 watt components of R6-R11. I did remove the solder terminal R9 connected between two of them to make it easier to get the new component installed. It also opened up some space to perform a couple of other component swaps. That was all I was able to get it before other things required my attention.
On Wednesday the 23rd, I picked up Sunday's progress and reinstalled the solder terminal and soldered R9 and proceeded to get down to R11's replacement remaining. I replaced the resistor/capacitor pair at R16 and C5 first. The C5 capacitor leads were wrapped around the resistor leads in an attempt to reduce capacitance between the leads going to the same two terminal points.
Getting the cap off the resistor without cutting at least one lead wasn't possible so 'snip' went one lead. The resistor measured over 12 megohms thus putting it outside the acceptable tolerance. The capacitor was a Z5U dielectric ceramic disc and there was no physical damage as could be seen with a magnifying glass. I reused it without re-bending the leads. Once one lead of the resistor was snipped, I could melt the solder and pull the lead out. Pulling the cap off the other lead was easy. Putting it on the new resistor wasn't difficult since the new leads were thinner than the old resistor's. After sliding the cap lead over one lead of the resistor, I managed to semi-wind the other lead through the cap's already formed twisted lead. A slight twist of the pliers to tighten them up and applied solder to complete the connection.
Bending leads to fit between two solder terminals without unnecessary bends can be a challenge, especially where they are quite close to each other. But without removing a solder terminal lug or a tube socket, it wasn't possible to keep from bending them. So gently bending them and making a 90 degree bend at the point it should hook the terminal worked out pretty well. A straight lead through the other terminal served the purpose well without the need to hook the lead. This was the MO for most of the components. Hooking leads around terminals adds to difficulties later when removal is needed and doesn't really improve the connection since the solder will flow and typically raise or suspend the lead off the surface of the lug unless you crimp the hook down making it even more difficult to remove.
R12 was replaced next. C4 was another Z5U disc cap that didn't require replacing. After remounting the Zero Adjust and Ohm's Adjust pots I installed R31, R32 and R34. I did use heat shrink tubing on the 125VAC exposed leads of these resistors as I didn't like them being exposed even though they were only connected to the secondary of the transformer. Next the rectifier was reinstalled. Yep, I used the original rectifier, D1. It checked out and didn't have any structural issues as far as I could tell. That and it's resistance measured only 20 ohms lower than a 1N4007 I had in stock. So, why not?
C6 was installed as was R35, R36, and R38. C2, a .005µF (5000pF) mica cap was still connected from my earlier replacement of that component. The last component I replaced was R30 after remounting the rotary switches to the chassis. The Vishay-BC replacement was the size of a typical 1/4 watt carbon film but the datasheet confirmed it was a 2 watt value and of metal film construction.
At the last minute I decided to install a fuse cradle type fuse holder. Now, this was for the mains or primary side of the power transformer to protect the transformer from becoming a charcoal briquette. that's the hope anyway. Since the whole device is rated less than 1 amp where .8 is used by the filaments and 10mA used by the rest of the circuit on the secondary, I figured a 1 amp fast-blow fuse might be just enough to handle the short surge by the heaters upon powering up.
With the leads from the rotary switches reattached to their respective terminations, it was time to install the 12AU7A and the 6AL5. Following which side was the fused or HOT side of the power cord, I marked the other side as Neutral so I would know which way to plug it in consistently.
I performed the initial calibration for no power zeroing of the needle. Then turned the IM-13 on and left it for about 30 minutes. After that, I went through the calibration process then left it on to age the 12AU7A tube for a few hours. I turned it on again for about six hours on Friday and another 12 on Saturday. Sunday had about six hours in the DC+ position again and then I performed the calibration tests again. After wiping the meter face with a used dryer sheet to dissipate the static charge it picked up, I was able to complete the calibration process. With that, I did a few measurements with a few batteries after taking readings from a digital meter first for comparison. Next I pulled a small 14 volt AC transformer out and used it to test the AC function. Once done a few resistors were also tested for the Ohm's function. The accuracy of the meter isn't perfect but knowing that I'm still going to use it for what it was meant for. Testing the vacuum tube equipment I have as I restore them without loading the circuits. Can't do that with my DMM. Also, analog meters used for finding a peak or dip is much easier since I'm not waiting for the DMM between one second updates to settle down. I see it as it happens.
Oh, the battery I used for the ohm function was the new Eveready Super Heavy Duty C cell I bought. the cord was wrapped around two tabs the previous owner installed on top of the case. I put them back in as they do provide a pretty good storage solution for the power cord and permanent ground clip lead. I installed a ring terminal on the ground lead so I could capture it with the probe socket's nut.
It was a fun project, albeit a bit longer than I wanted but other things happened preventing me from getting back to it sooner. Now that things have settled down a little bit, I have more time to work the projects.
 |
| Clean and ready for service. |
Comments
Post a Comment