Issue with asp34-2

[niconoe]

Hello,

I recently took my old STFM from childhood out of storage and trying to fix the PSU (the only issue it had at the time if my memory is correct). I've solved some stuff already, but it's still not working, so I was wondering if someone has sugestions for what to investigate next. Here's the story:

• It's an ASP34 rev 2

• all tests were done with a proper load (resistor)

• Initially, the input side was shorted (burned fuse, and I got light when powering it with a lightbulb in series as a current limiter)

• I realized the main transistor (BU508A) was shorted, so I replaced it by the same reference. The bridge rectifier was working properly

• the other transistor in the switching circuit (C2321) was also giving weird readings with a multimeter (around 0.1v on the junctions), so I also replaced it. Since I was not able to find the same reference, I replaced it with a 2SC1509 which has somewhat similar characteristics to both the C2321 (that was in the machine) and the 2SC1384 (that is on the official schematics). Still I am not 100% sure my substitution is correct

• After replacing the fuse, I can now power it without any short circuit on the input, hooray

• Problem is that there's still 0v on the outputs (5 and 12v)...

• I noticed the main capacitor (after bridge rectifier) properly charges when powered up, and discharge in around a minute after power is cut

• It gives me the impression that the PSU is in fact not switching, but I'm not sure how to test that with just a multimeter

• I tested the TL431 out of the circuit, it is working fine

• The D4 diode on the output side seems fine too

I am a bit unsure of what to test next. Maybe some capacitors, but I have no tools for that. I am tempted to replace some, but most values in this circuits are multiple of 330, and I am not 100% sure that larger capacitors (470) that I have are suitable.

Thanks for all your ideas!

[rubber_jonnie]

Before I go any further I must give the obligatory warning about the danger of working on PSUs. They contain high voltages that can kill. It is your responsibility to be sure you take the necessary precautions to avoid electrocution. If you're not confident working on PSUs then replace rather than repair.

Have you looked across the board with a magnifying glass/Microscope to see is there are any bad joints that need a reflow?

With regards to transistors, whilst I've had some success with replacements, I've found the characteristics need to be very close to be sure that they are suitable for the task in hand, so make sure the replacement is very close to the original specs.

I'd suggest a recap first, exxos has kits in his store, ASP34 Cap kit and of course if you wish, you can source your own. If you don't want to go the whole hog and do all the caps, then try the startup cap, which I think is C8 (Though I'm happy to be corrected)

Then, if the recap doesn't help, get the schematic from here if you don't already have it: Atari schematics

With power disconnected, check the transformer hasn't failed internally between windings with your multimeter. The schematic will help here.

You've already suggested that the rectifier is receiving AC input, and that it is outputting DC as well, but is it going where it should according to the schematic?

If all is good there, and the transistors are known good (As in you've tested them off the board or replaced them) and the caps have been replaced, then you my have an issue with a fuse, and I don't mean the mains fuse. I had an SR98 die because of a fuse in the high voltage side. Looking at an ASP34-2 I have, I don't see a similar fuse on it, but it's worth double checking on the schematic.

I'd also suggest you have a look at the optocoupler, that's the feedback loop for 5v IIRC, if that's failed, then there is no feedback loop.

My other option would be to get a Meanwell RD-50A and replace it. They are very cheap (possibly cheaper than replacing loads of components) and more than powerful enough to replace what you have.