Price: $69.95 when introduced.
Tubes used are 12BE6, 12BA6, 12AV6, and a 50C5. It also features superhetrodyne operation at 455kHz IF and a permanent magnet speaker.
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July 26, 2022
I acquired this receiver for the sole purpose of having a SWL rig. It was inexpensive and well worth the small cost for its excellent cosmetic condition alone.
On the back are two screw terminals for an external antenna. The serial number tag shows two numbers. On the left of the tag is a rubber-stamped number: 120004. The right is a much better pronounced stamp: 233060. I'm not sure what the second number really is other than a final inspector's number signifying it is ready for sale or a code for date and place of manufacture. The left is definitely the serial number. There is another six digit number stamped in the center on top of the chassis deck: 199233.
Inside, the components do not show any signs of repair work or modification. It would appear this is an untouched, as-built-from-the-factory example.
Here are some photos of the receiver. Please click the photo for a larger version.
The receiver looks quite clean as if it had been put in a bag, in a box, in a closet and nearly forgotten. I used my isolation transformer and autotransformer to bring up the rig to about 80VAC. The speaker hummed pretty good and knew the filter capacitors needed replacing before anything more would be done. I ordered all the capacitors I would need and a suitable rectifier to replace the Selenium rectifier. I will need a resistor to emulate the voltage drop of the Selenium rectifier's drop to prevent over-voltage downstream since my house voltage sits about 128VAC-130VAC. Almost tempted to build a power supply just for the receiver and bypass the direct power rectification all together.
The four valves were removed and checked on my Heathkit TC-2 Tube Checker my Dad built back in high school. When he gave it to me it had a couple of issues, so I decided to disassemble it, reluctantly, to create what I call a Re-Kit. It's basically breaking down a built kit back into individual components then replacing old, unsafe, or unusable parts with modern equivalent parts. Since most of the TC-2 is comprised of valve sockets and wires I cleaned up the sockets and replaced all of the wire with 600V 18AWG solid core primary wire. It presented a small challenge in some spots using that big a wire. Once completed it passed the tests fine. The transformer put out slightly higher voltages due to the higher line voltage coming in. The plate voltage settings had to be reduced 2V on the dial to be within the chart specification for a valve selected. I don't seem to have an issue with the unit so I left it alone and use it as normal.
The S-120's valves all passed very well. No shorts and no weak valves so I'm happy not to have to locate any. Now my attention can turn to the capacitors and rectifier. Resistors will be desoldered and checked as well with the IM-13 VTVM and the Surpeer DMM for comparison. This work will span the next several weeks as time permits.
All in all, I'm quite happy with this unsullied example of early 1960's electronics manufacturing.
Until next time.
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August 7, 2022
Lots has been happening and I finally got a couple of hours in on the project. The first concern was the high voltage output after the selenium rectifier was replaced with two HER508 rectifiers in parallel. Yes these are overkill but rather than clutter up my storage with so many different values of silicone diodes, I decided on only a few from 50PIV to 1000PIV ranging from 300 mA to the 5A I'm using here. I don't include the Germanium types as they are used as signal diodes only. I could have just as easily used a 2A bridge rectifier for a small complete package but I digress.
The power consumption, according to the specifications is 30W. When I calculated out the current drawn it came to 250mA. Yeah. I didn't think that was right either but I didn't have a 1A rectifier either as my stock is limited due to space. So, I thought about using a wirewound resistor to knock down the 125VAC in the house to a more expected-like 110V so that the output of the power supply was roughly be equal to 130V in the first stage. Not wanting to go after a large wattage resistor I decided to cut the main filter capacitor down from 60uF to 47uF. It's an experiment to get to try out. The results are expected to drop the voltage some to mimic the old selenium rectifier's voltage drop across it. More capacitance would raise the voltage higher and stress the components downstream. I cannot find the formulae for calculating the voltage value. I remember something about a voltage doubler but don't know the math for it. Will check it out later.
With the value decided on I went through the process of figuring out the positioning of the four individual capacitors that will replace the multiple section original filter capacitor. Note here that the original cardboard filter capacitor will remain in place for the "look" but is cut out of the circuit. None of the sections would operate beyond 50V even after an hour of the alleged reforming process some use with a capacitor checker. If it leaks below rated value it is replaced without question. Never have I ever successfully reformed an electrolytic or "waxy", bumble bee, tiny chief, black beauty, etc to properly work at rated voltages. After many experiences I just don't bother. They get replaced. As for authenticity, well, there are some rare examples where an antique capacitor of the aforementioned types have worked but I'm not willing to test my smoke detectors or any of my client's for original equipment or vintage sake. If they want it that way they can take it somewhere else. Again I digress.
So the repair of the power supply is the goal to solve the huge amount of hum from the speaker. With the supply rebuilt I can more reliably find faults during troubleshooting of the other stages. Shotgun replacement of components increases chances the problem isn't found easily by changing too much at once. So, Step 1: Repair power supply. Test.
Tonight, I have almost finished repairing the power supply. All I need is to correct a wiring mistake I discovered while writing this post entry. I also need to install the fuse holder, fuse, and a brown cord with polarized end. Here is a before and after set of the progress thus far.
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| I have wired a .022uF yellow tubular capacitor to the wrong end of the Glow Bar resistor (dog bone) shaped black carbon rod from the long shaft volume potentiometer to the left of center. The capacitor is the one hovering over the big blue .047uF electrolytic. One end (the six o'clock position) is connected correctly but the twelve o'clock end should be connected to the anode end of the rectifiers to the left of the volume potentiometer. Realistically this rectifier is stout enough to handle all of the radio's rectification requirements and could in fact be put in series to double the .5V voltage drop. |
More to come.
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September 2, 2022
Good evening! The S-120 is alive and...well, pretty well. It is an entry level receiver but like many, I don't think it does justice to Hallicrafter's claim it replaces the S-38 series. Even with the Band Spread feature everything is quite packed on the dial. CW reception is possible with the BFO but not very stable so pitch and separation is pretty difficult to maintain. Sideband phone operations on the amateur bands isn't possible but the BFO can be used to clarify AM phone signals. All in all it does pretty good at shortwave AM as well as broadcast AM. Anything else requires a better design.
To solve the high voltage issue I used a 47 ohm 10 watt ceramic power resistor. Yes, it may be overkill but considering the dissipation calculated out to 7.5 watts, a 10 watt was the best choice. I ordered several values from 4 ohms on up at a minimum of 7 watts or higher whichever value was availabe from Mouser. My first choice was the 47 ohm by math and it worked out perfectly.
The Bumble Bee capacitors were replaced as the originals had drifted up by more than 20%. According to the IT-28 capacitor checker, they leaked DC before reaching 50V. The replacements were Illinois Capacitor, owned by Cornell Dublier. I have had very good results with both. All the resistors in the radio were of the Alan Bradley type and within tolerance, if only by a single ohm. Not bad for a 57 year old piece.
The alignment went without a hitch but the receiver didn't seem to be very sensitive so I let it warm up for another half hour and performed the alignment procedure again with better results. Since it isn't the case during this process I should have waited a full half hour instead of the usual 15 minutes. My IM-13 VTVM also had more time to settle in as did the IG-102. The top of the case of the IM-13 reached 88F. It's been a couple of years since I rebuilt it so I should inspect the insides and the battery. I am hoping to locate an original probe instead of the dual probe kit I'm using now.
After the alignment I connected it to my 41 foot ground mounted wire vertical antenna. Pulled in a couple of stations the Icom PCR-100 couldn't hear. So it has it's moments for sure.
All in all it does pretty good at shortwave AM as well as broadcast AM for which it was designed. I'll call this project complete and grab the Heathkit SB-301 as it has gone silent.
Until next time!
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