Astron RS-20A low voltage problem ?'s

[DELCOLHFC]

Hello all,
I have an Astron RS-20A power supply on my bench to power 10 Pro-2055's that I monitor and tinker with. Recently following a power failure (way too common in this area) I found that the radios running off the power supply were all dead. A quick check with a few wall warts proved all was well with the scanners but the Astron was presumed dead from some unknown event at the time of the power failure. Last week as I was doing some antenna cable work behind the bench I plugged the Astron in and connected a radio to it's power strips, when the power switch was flipped on it worked perfectly. Later in the week (it had been powering 4 scanners for three days) I rearanged some more cables and had to unplug and replug the power supply back in. Following the powering down and back up the Astron is now once again a great paper weight. It shows an output voltage of 1.3 VDC I have tried leaving it unplugged for periods of time and powering it down by the switch and the plug as well as disconnecting all equipment from it. Still dead (well not completely dead but pretty much useless).
Has anyone else seen behavior like this from an Astron (I have used them both at work and at the Fire station without incident for years)? Does anyone have any experience with repair to these units? I am perfectly happy taking things apart and tickling their inards, sometimes they even work again! I am about to do some web searching on this but I figured I would toss this problem out to you folks first to gain any insights from other users here.
Thanks for any help or advice.
Tom

 

[fineshot1]

This is a common occurance with astrons as they are not well protected on the ac input side and the transients propogate to the electronics(usually a regulator). I have seen some fella selling astron repair kits on ebay in the past but not sure if he still does so.

 

[zz0468]

Another common failure mode is the SCR crowbar on the output. I've seen those fail, and put a very low impedance path from the output to ground. Symptoms are very low voltage output, and high current draw, if yours has an ammeter.

 

[daedalus]

I have had a couple of pass transistors fail on Astrons. Easily replaced and cheap fix if it works.

 

[N4DES]

I've also has a wirewound resistors that are on the pass transistors go bad.
I don't recal the value off hand, but they are also something else to look at.

Mark

 

[rescuecomm]

First thing to change is the LM723 voltage regulator chip on the main board. I put a socket in place for mine to make future lightning related repairs easier. If the regulator chip is allowing the voltage to go above the trip point for the crowbar circuit, then the output will be close to zero. Should the overcurrent limit part of the IC still work, it should not get too hot. The only way reset the crowbar circuit is to turn the supply off for about 5 minutes to let the stored voltage go to zero and release the SCR junction.

Bob

 

[DELCOLHFC]

Thanks for all the feedback guys! I just ordered up a handful of the LM723's, it looks like that may be where my problem resides, at least it looks that way now. My Astron already has a socket in place for the voltage regulator IC, maybe the later models all have them....? As I said before, I have used these power supplies for years and always assumed that they were bulletproof, I have never had one fail, but from you folks and looking around the web they do seem to have a couple of issues. Guess I've just been lucky up till now.