Electronics > Repair

1984 power supply repair

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connorwk:
I'm trying to repair this power supply from an old 1984 computer it seems to not give enough power on its output.
Its a 36v-5v DC-DC power supply branded ASTEC, there was a metal plate screwed to the back of the board to protect the solder joints from the metal case of the computer and on it it says ASTEC AA7281D (think the D is a rev letter as on another one of these power supplies it says AA7281 rev <letter>) 65-00029.
When it is in the computer its 5v output reads only 0.5v. When out of the machine hooked up to 36v and nothing on the 5v output it reads about 5.8v.
Nothing on the board seems obviously bad. The electrolytes seem good and everything else seems ok. Nothing obviously wrong.
Any ideas as to what may be wrong?  :-//

dr.diesel:
Somebody stole R28!  Normal to see unpopulated parts, a bit unusual to see snipped off ones!

Anyway, you can verify the 5v rail isn't being shorted by the computer?  Or perhaps can you load it while out of the PC and see what happens?

JoeO:
Capacitors - If you have an ESR meter, use it on the largest capacitors.  If not, just replace the largest capacitors.

It is 29 years old.  The capacitors are probably "dodgy".

Tita:
Switch mode power supply repair video:

View the entered video if you want to learn something! O0

connorwk:
dr.diesel:
I can verify the machine isn't shorting it. I have two of these computers (1984 Burroughs B25s) and I put the supply from one of them into the one with the bad supply and it worked fine. So obviously it is the supply itself.
JoeO:
I do not have an ESR meter. Guess I should get one if I plan on doing more electronics in the future. Really all I have to test at the moment is a multimeter. I looked up and some people have said you can test the caps by using the resistance mode of a multimeter? Anyway I guess I could replace them although to the fact that they are dodgy, yes they are nearly 30 years old yet I have 3 more of these power supply boards with original caps that say otherwise to being dodgy. (3 more because I have two of the computers and each computer requires two of the power boards)
Tita:
Thanks for the video.
I guess I know what I shall be watching for the next hour!

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