Electronics > Repair

Commodore C128D Power Supply - Capacitor Issue

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Hi all!!

Okay so I recently began work on my old C128D which had no power.  I'm not a fan of working with power supplies due to the high voltage in.

The PSU has 2 transformers and 3 outputs

* 9v AC
* +5v DC
* +12v DCAfter taking out the PSU, I found the 12v dc (yellow wire) was shorting to ground.  After removing a few things I finally removed a capacitor which alleviated the short.

The bad capacitor has the following written on it:
470 /16
S 9

So I took that to mean it is a 470uf 16V.

I put in an Electrolytic replacement, which physically is much smaller (but I assumed this is just because newer components are smaller), and wired up the PSU to the switch.

It looked good, the fan started up (Note: the Fan is on the +12v rail) and I was happy for about 5 seconds then a huge bang happened.  I've taken a photo of the caps.  The newly installed cap exploded!!

So my question is that did I install a capacitor that was too weak?  Did I misunderstand the writing on the old cap?  Could something else cause the cap to blow up, which may have also caused the old one to fail too?

In the screenshot the gold cap (left) is the original one, which is shorted (I have also tested on the cap itself, and confirm its definitely shorted).

The black cap (bottom right) is what I had replaced it with, and the other (top right) is the exploded cap,  It was the same as the black one.

Any ideas?

Thanks :)

On the old cap, you have marked + on the new one you have marked - pole. Do you know this? If yes, check bridge rectifier.

Well I built up the courage to give it another go.. maybe I did put it in backwards??

I double checked the orientation, and this time I connected the output to the C128's mainboard.  This time the power is staying on and no explosion yet.  Maybe it needed to be connected before switching it on?

Chances are the fact that that the only capacitor that I replaced was the one to blow up most likely points to me being an idiot and installing it backwards

Anyway just going over it again seemed to help so thank you for taking the time to respond!

If there's IEC power inlet filter can, remove it. Those tend to blow up and stink

I still wondering why to chance bad quality caps ?.
Waist of time.


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