Blown Capacitor - PSU Repair

[Trev]

I was just musing on how it might have run hot.

And that's exactly the reason that I said. "I give in"

 

[potterpg]

Ok, found the thread... (it moved)
Just by way a of a quick follow up...
After replacing the capacitors, all seemed to work fine again...
I guess only time will indicate of other problems, but so far, it seems fine!
Thanks all for your help.

 

[Black Hole]

After replacing the capacitors, all seemed to work fine again...

Great news, and a new data point. Thanks.

Ok, found the thread... (it moved)

Correct. Your initial post warranted a thread of its own, so I asked for it to be split off - but it should have been easy enough to find (click on your forum name near top right and choose "your content").

I will now link to this thread from my comments about PSU repair.

There has been a report of a failed PSU due to a burnt-out capacitor (not the electrolytic type), and caution was advised because it was not obvious whether the capacitor had simply failed, or if the failure had been precipitated due to stress imposed by a fault elsewhere (which is always a consideration at the back of every engineer's mind). The capacitor (and an adjacent one) was replaced, and the PSU restored to operation):

 

[everthewatcher]

Ok, found the thread... (it moved)

Seems you get notifications that one's posts have been moved but then none when others post to the thread, so I've only just spotted this.

After replacing the capacitors, all seemed to work fine again...

That's good news. IC1 seems to be able to look after itself but did you check D1 was still a diode?

 

[Black Hole]

Seems you get notifications that one's posts have been moved but then none when others post to the thread, so I've only just spotted this.

New thread, new "watch" required. Personally I don't watch any threads in particular, I just use the new posts listings.

 

[everthewatcher]

...was not obvious whether the capacitor had simply failed...

It simply failed due to an over-optimistic voltage rating coupled to its small physical size. It's there to deal with HF switching transients from D1 and the supply would work fine without it. As I noted earlier, in the later revision of the PSU Humax replaced it with a physically larger 100pF cap.

 

[john_parker]

Thanks for the info in this thread. My C3 has just gone up in smoke (literally). Will swap in a donor PSU board while attempt component swaps as detailed.

[Update some days later] confirming only replacing C3 fixed my PSU. Thanks again

 

[Zave]

Thanks, really helpful

 

[everthewatcher]

Thanks, really helpful

You're welcome.

 

[Hans Klopek]

Hello, I have a dead PW808. The capacitor at C3 has visibly blown and suspect the diode at D1 will need to be replaced.

Can anybody help confirm the parts I'll need? From earlier in the thread it seems that I need a 100pF 1K capacitor for C3 and for the diode in D1 I think it says 08 on the part but I'm unsure if that's all I need to know. Any help appreciated.

 

[prpr]

It's all there in post #12.

 

[Black Hole]

Summarising post 12: C3 is ceramic 68pF 1kV, and D1 is HER108 - but any 1kV 1A diode should do. It won't matter if you make C3 100pF, although that will be larger and might be a tight fit.

 

[everthewatcher]

Hello, I have a dead PW808. The capacitor at C3 has visibly blown and suspect the diode at D1 will need to be replaced

Most likely the cap as it will have failed short-circuit and the diode will be fine so just replace the cap.

That said it's always possible if unlikely the cap has gone open and the resulting switching spikes have caused the diode to give up.

 

[Hans Klopek]

Box resurrected with a change of capacitor. Thank you very much.

 

[everthewatcher]

Box resurrected with a change of capacitor. Thank you very much.

You're most welcome.

I've replaced this cap on a couple of units that have passed through my hands just in case.

 

[Haydn345]

Same for me. Seems to be working fine now. Most rewarding.