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Assistance required with an AA11040 PSU

FelisSapien

Member
Joined
Feb 18, 2021
Messages
18
Location
Birmingham, UK
Hi All,

I've seen a lot of detailed talk problem solving this PSU but I haven't seen chat about quite what I'm experiencing... I have a UK Apple II Europlus. It's complete and cosmetically in pretty good condition. Alas, it doesn't work. I'm starting with the PSU... Firstly I switched out the Rifa caps. After this I reviewed the output and found very low values. After this I replaced the electrolytic capacitors. For a short while this seemed to do the trick. I measured close to the expected +5, +15, -5 and -12. But after a few minutes C20 (660uf 16V) blew. I replaced this capacitor again and the same thing happened again. So I'm guessing too much current is hitting C20? My problem is that although I'm reasonably comfortable replacing like for like components I'm don't have any real experience with power supplies. Is anyone able to help me track back to the cause of the issue?

I have another question around the power supply - as I received it the on/off switch had been bypassed. I removed it and found that on continuity it tests out ok. When in place though it sounds as though it's internals are shorting when switching from on to off and vice-versa. Is this a 'standard' part? I've looked around for a rocker switch around the same size and haven't come across anything quite the same spec. Do I have any hope of pulling the switch I have apart and reassembling?

Thanks in advance,
Carl
 

FelisSapien

Member
Joined
Feb 18, 2021
Messages
18
Location
Birmingham, UK
I might be able to help myself here...

On reviewing my before photos I see that C21 and C22 are not positioned as marked on the board. When I fitted the new caps I for some reason went with the markings not my own notes. When I get a chance I'll put things as-were and retest.
 

daver2

Veteran Member
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Jun 19, 2012
Messages
7,731
Location
UK - Worcester
Just out of interest, is it an AA11040B?

C20 looks like it is the smoothing capacitor for the +12V rail. C21 for -12V and C22 for -5V.

A 16V capacitor is too close to the 12V operating voltage, but it should be OK - especially if you read the correct voltage when it is working.

Although a more likely scenario (as you hint at in post #2) is that the polarised capacitors have been installed ‘back to front’. This will kill the electrolytics over a short period of time as you are observing.

I have seen lots of examples where silk screen markings on the PCBs are incorrect! Deliberate?

If they were tantalum bead capacitors instead of electrolytics - they would have blown up!

Unlikely you will be able to repair the switch, but not out of the question. It is likely that the contacts are arcing. They could be cleaned up by very gently ‘sanding’ them. But beware, the contacts will have a fine coating, and too much sanding will just remove the coating and lead to more problems down the line.

Dave
 

Gary C

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Joined
May 26, 2018
Messages
1,531
Location
Lancashire, UK
The switch will be arcing a bit.

I have overhauled several by popping out the rocker (you can often insert a thin screwdriver down the long side about where the pivot is and twist but it might break the plastic, take care) then once apart, a simple clean is often all it needs.

C20 failing. While its current that causes the blowing, it will be an overvoltage thats caused it (or it was replaced with another faulty cap, which is always a possibility)

Not a PSU expert, but C20 is across the 12V to ground, so for this to exceed by a margin, the 16V rating means T2 is producing voltage from the flyback circuit but its not being regulated.
Regulation is via the 5V line into a ladder of select on test resistors R32 and R25 which is used to set the voltage to IC1 which drives the flyback.

Might be worth sticking in a much higher voltage rated cap so its doesn't blow and see what its regulating at ?

1654255644489.png
 

westveld

Experienced Member
Joined
Dec 31, 2009
Messages
69
Location
Holland, MI
I ran into an early Astec PSU with mis-marked cap orientation on the PCB recently - so that's definitely a thing to look out for.
 

FelisSapien

Member
Joined
Feb 18, 2021
Messages
18
Location
Birmingham, UK
Haven't had the chance to test exhaustively but this seems to be case closed. As the PSU arrived to me C20, C21 and C22 all had polarity the reverse of the PCB markings. Now my new caps are in the same orientation we seem to be stable.

I tried to get into the rocker switch to clean out and resolve the arcing issue... somehow I've now created a situation where the switch is always on. This didn't seem to be obvious when testing with the multi-meter so not sure what I've done there. Will try look again later.

I quickly took the opportunity to plug the board in and the output I get is a screen full of garbage. Based on other systems I've worked on I'd guess this is a RAM issue? As I get more time I'll look get back to this.

Thanks for the advice so far. It's greatly appreciated!
 

daver2

Veteran Member
Joined
Jun 19, 2012
Messages
7,731
Location
UK - Worcester
No problem. At least the switch is permanently ON rather than permanently OFF! The same as the switch on my bench power supply for the last 30 years!

Dave
 
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