exxos's Atari, Amiga & retro forum

I thought it was about time to recap my STFM PSU after 33 years of service. It still boots fine when cold, but after warming up can randomly bomb. It identifies as ASP34-2 rev 5, so I grabbed the suitable replacement cap kit.

Cap replacement was all going to plan until I removed one capacitor only to find it appears to have been originally installed with the incorrect polarity!
I'm no electronics expert but I know that capacitors can explode and ideally I'd like to avoid that.

So..
- was this a manufacturing defect which somehow has not resulted in "bad things" happening in all that time (current best guess)
- was the board incorrectly printed (doubtful)

I'd appreciate any advice from the experts before I proceed.

BEFORE:


AFTER:



Maybe it didn't look like the happiest capacitor in there too. Top starting to bulge?

Wow. I would measure the 0v black wire to the cap and see which side is really 0v.

Looking at my images https://exxosforum.co.uk/atari/last/psu/index.htm#ASP34 looks like negative is always at the top.

exxos wrote: 18 Dec 2022 12:16 Wow. I would measure the 0v black wire to the cap and see which side is really 0v.

Good idea!
My continuity tester beeps when I connect the black wire to the negative terminal (as printed on the board). The negative terminal also seems to be directly connected to the big heat sink next to it.

So combined with this info and your images, does this confirm I've had a manufacturing defect all this time and the sensible thing to do is to trust the printed board?

SteveC wrote: 18 Dec 2022 12:30 So combined with this info and your images, does this confirm I've had a manufacturing defect all this time and the sensible thing to do is to trust the printed board?

Yeah the PCB looks correct. Could be manufacture defect, or someone else has messed with it in the past.

Looks like the cap is starting to "blow" the top off. So its not happy being in backwards for sure. Amazing it hasn't blown up totally.

Thanks!

I'll install the rest of the caps and stand well back when turn it on :lol:

SteveC wrote: 18 Dec 2022 12:41 I'll install the rest of the caps and stand well back when turn it on :lol:

Good plan! 8-) :hide:

Recap completed, powered on and no explosion :)

Unfortunately I still get bombs so it's time to reseat ICs and have another check for dry joints I guess.

Thanks for the help :)

Oooh, looks a little bulgy on the top...

SteveC wrote: 18 Dec 2022 12:14 I thought it was about time to recap my STFM PSU after 33 years of service. It still boots fine when cold, but after warming up can randomly bomb. It identifies as ASP34-2 rev 5, so I grabbed the suitable replacement cap kit.
....
Maybe it didn't look like the happiest capacitor in there too. Top starting to bulge?

Hi Steve, for the record, i noticed that the caps in the ST psus i worked on were prone to be like ready to explode, but from under.

Just check the external coating on the top of the caps, it's all thin on your pictures. This means that one day or another, they would have went kaboom......

Good idea to replace them. Your ST will thank you for that.