Astron RS-20S

 Astron RS-20S Linear Power Supply

125VAC 5A Input

11-16VDC Output

16A Continuous Output (Internally Settable)

20A Surge

Front Mounted Speaker

Available: 1980-ish

Price: $199-ish

The picture above is of an RS-20A.  I wasn't not able to locate a photo or drawing of mine so I'll have to post a photo of it after I'm done with this project.  The RS-20S looks like it's 'A' cousin with the exception of drilled holes above the label patterned similar in shape to the speaker behind it.  An earlier speaker-equipped supply, the RS-12S, had horizontal slots with rounded ends in a much smaller case.

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May 26th, 2021

My Dad bought one of these for use with his newly acquired Icom IC-745 Transceiver in 1983.  The advantage this power supply had was the built-in speaker was forward facing so the audio could be heard more clearly.  Fast forward to today and I have both of these pieces, albeit, without a working IC-745, yet.  It sits on the shelf awaiting its turn for service.  

A couple of years ago the power supply started having trouble supplying quality constant power.  The voltage would sag and that sometimes caused the radio to blank out during transmit.  Looking into the problem revealed a couple of things that had failed during it's 38 years of service.  The first was the LM723 regulator IC.  It simply got hot and began to wander in it's regulation of the power transistors.  Another part, a TIP29 was affected as well.  It got quite hot and I figured these large temperature swings had pretty much done it in.  Last was the large 33K uF electrolytic filter capacitor.  It wasn't performing at rated values so out it went.

After replacement of the components I added a heatsink to the TIP29 to help it dissipate the heat.  I also socketed the new LM723 to make it easier to replace in the future.  Cleaning the power transistors and their accompanying diodes I installed new mica insulators and put plenty of thermal compound so they would transfer their heat to the big heat sink on the back of the cabinet.  The unit ran much, much more cool now.

Time passed and the need to know what amperage load I'm putting on the supply is a desire.  With 16A continuous supply with a 20A surge might not garner me the amount I may need for my Kenwood.  So I bought a DC Volt/Ammeter from Amazon.  

It can handle up to 50A DC.  It cannot work without the shunt or the readings will be off a bit.

I wanted to install analog meters but there was only room for one and I wasn't interested in installing a switch to go between volts and amps.  I needed to be able to glance at it and know what it's doing without having to manipulate anything.

So far the guts of the power supply have been removed mostly intact.  The power switch, speaker, fuse holder, and the output binding posts needed to be de-soldered to allow me to mark, drill and make the cutout for the meter.  

Which brings us to the end of tonight's (morning?) posting.  More to come as the project progresses.

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May 27, 2021

Today I finished cutting the case to fit a volt/ammeter into the faceplate.  I slipped a couple of times and ruined the faceplate so I will need to find a suitable fix later on.

I'll finish reassembling the unit later on.  Cutting the rivets out that held the transformer require some finishing hardware to keep the bottom of the case low profile in so far as not protruding further than the SCR for the crowbar circuit.

Some trimming of two plastic tabs on the meter the hold it in proved fruitful.  Any further grinding on the case would make the edge of the hole the same size as the outside of the bezel for the meter.  So rather than cutting the tabs off and gluing the meter in it looked like grinding down the tabs some would work.  So far the fit is good even if my grinding isn't.  I nearly dropped the Dremel tool but wound up scoring the front above-right of the meter.  Another mishap occurred when dropping the cutting wheel down too far from the line I thought I was seeing.  The third was with the drum sanding attachment and it grabbed the sharp edge of the hole and I scuffed the front at the lower right corner of the meter.  I'm not fond of the solution to repaint the front at all since it will require removing the grill cloth for the speaker to prevent it from clogging with paint.  Again, I'll fix it later.  I want to get this back in operation for now as it's my only power supply capable of powering the Kenwood TS-450S/AT and LDG RT-100 remote tuner at the moment. 

That's it for now.

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June 6, 2021

Progress has been made and the RS-20S is going back together.  

The Anderson Power Pole panel insert came in earlier than expected so I was able to use this weekend time to get the hole cut out for it.  After a few tweaks with the Dremel and a file it fits really well.  

Since I nicked the front with the Dremel when cutting out the hole for the V/A digital meter, I decided it warranted a paint job to cover up most of the nicks and scraped paint.  Instead of just painting the whole thing, with the exception of the silkscreened Astron label on the front, I chose to Semi-Gloss Black the area with the meter, switch, and Anderson panel.  I think the effect is okay.  If I need to take it apart again I'll probably do the whole front this time and mask off the Astron label.  I just didn't relish the idea of having to replace the speaker cloth glued to the backside of the front panel.

I had some debate with myself as to whether or not I wanted to extend the voltage adjustment POT on the circuit board out to the front panel.  I decided against it for now as it wasn't designed as an adjustable model.  The POT is really only used to set the voltage to its advertised value.  To make it an adjustable version I'd need to get the other circuit board that is still available to convert it.  I'd also need to modify the unit to work with four pass transistors IIRC.  I would like to get another one like an RS-35 to do the serious modifications on at a ham fest in the future.  For now, it'll run as an RS-20MSAP (V/A digital meters, speaker, and Anderson Power Pole panel).  Of course they never built such a rig but it's my rig and I'll customize it to fit my needs.  I might even come up with a custom label to put on it when I repaint the whole case.

So here's the progress shots so far:

Anderson Power Pole dual panel
from Powerwerx installed.

Covering up mistakes made.

What the a power supply looks like without
the case.

Meter shunt installed

1N1184A Rectifiers installed.

2N3771 pass transistors installed.

A good PC technician is never without heatsink compound.  Just enough was put where it was needed to make sure the heatsink could do its job.  No more, no less.  If only that stuff didn't stick and get on everything!  Ugh!

You've probably noticed the two nuts for the 1N1184A rectifiers don't have any compound on them.  On the heat-sink's fin-side, there's no need since there's virtually not enough contact area to spread the heat out.  All of of the contact area is on the flange side.

So far I'm happy with the results.  I won't reveal the completed front panel just yet. I want to make sure it doesn't let the smoke out first.  

Sorry buy that's all for this post.  Some things came up and I wasn't able to finish the assembly.

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June 16, 2021

Tonight I finally got a chance to finish the project write-up.  Here a some photos to complete the project.

Looking closer at the binding post I used a 1/4-20x1.5
inch Allen head bolt. With the exception of the two
small ceramic capacitor and an electrolytic, all
connections inside are bound with ring terminals.
The two bolts are stainless steel with flat washers
each, the original binding post insulators, a split
lock washer, and zinc coated nuts. 

The AC side showing the V+/sense lines running
to the meter from the (+) post and shunt

The cover required clearancing for the two nuts
holding the shunt in place.  This was unavoidable
due to the location of the 33KuF capacitor.  I might
have had a little wiggle room with the capacitor
as the circuit board atop it would have a little play
but I chose to work with factory design.
You'll notice the blue plastic cap.  This is a cover
for the stud mounted SCR (+) in the crowbar circuit.
I don't know if this was factory or Dad found
something that fit well.  The factory feet are too
short so I swapped them out with a set I purchased
at a hamfest a couple of years ago that fit Collins
and Heathkit equipment.

Rear overhead view.  Two white wires with the
red heat shrink are (+) to the Anderson Power 
front mounted panel.

Different view.  You can see how the 2200uF cap
tied to the output to smooth any remaining ripple
going to the radio.  The two small capacitors are
to shunt any AC to ground IIRC.

My modified power supply.  Since I damaged the
front panel from the cutouts I needed to do
something to make it look better.  I chose a two-tone
look with semi-gloss black against the factory brown
 to contrast the aesthetics of the colors in the
modifications.  The speaker remains functional and
mostly unobtrusive in its stock form while the
factory silkscreen maintains it origins.

Here you can see the color contrasting against
the black background. The current sensed varies a bit
between .3 and 1.3A with nothing on except the
meter itself.  The Bias-T to power the LDG feedpoint
tuner for my Inverted Vee appears to offer no
appreciable draw.

Powering up the Kenwood TS-450S/AT yields a
current draw as shown on the meter. It's not perfect
but at least I know a little more about what's going on.

Here's the radio at full 100 watts dead key AM 
carrier.  I'll want to do some testing with the Mic
to see how far it may peak during phone mode.
Lower power use evaluation will come with slow
digital/high duty cycle modes like JT65, RTTY,
WSPR, and SSTV.

So that's it for this project.  This was quite a fun little project but now it's time to get back to the SB-401 transmitter and get that back in operation now.  Thanks for following along!

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