[Juergen]

This isn't exactly headphone related but it is audio related...

I have a Carver C500 amp that recently stopped working. The main line fuse is blown and one of the secondary ps fuses has blown. I have minimal troubleshooting skills but I have learned a few things building various head amps (mint, PPA, Dynahi). Since this amp is early 80's vintage I suspected the caps might have gone bad. I checked and one of the 5900uF ps filter caps is a dead short. I am picking up a new pair today. The original caps are rated at 85V but the only ones I could find locally are rated at 75V. Will this be a problem? Also, the secondary fuse is an AGX type (as opposed to the more commonly available AG type). I did find an AGX fuse at the local hardware store. It was packaged as a christmas light type fuse. Anyway, I am considering replacing the AGX fuse holders with AG type. Is there anything special about an AGX fuse other than its' smaller form factor?

 

[FallenAngel]

The caps being rated at 85V vs 75V, you might want check the voltage that *should* go through that section. A schematic would be best, but if not available, you can probably just check the transformer wirnig and try to trace the amp to figure it out.

AGX vs AG sizes shouldn't matter, just make sure it's the same type of fuse.

 

[Pars]

I would probably try to find some 100V caps just to be safe. I would not think that the value of 5900uf would be that critical, anything from 4700uf or so should be fine. You could ask over on diyaudio... there is a mod there (anatech) who used to own a repair facility in Canada and knows alot about vintage amps. It also wouldn't surprise me if someone there could help you out with schematics, etc.

 

[ericj]

It's unusual for an electrolytic cap to fail into a short circuit. You might want to check caps in parallel with it as well.

 

[Juergen]

I forgot to mention that I do have the service manual w/ schematics. If I remember correctly, according to the schematic each side of the ps secondary saw 63V. 75V is over that but is it enough of a safety margin? Anyway, I just got back w/ a pair for $8 each so I'll give it a shot.

 

[AndrewB]

Just a word to the wise: you should never down-rate (the voltage of)electrolytic capacitors. other than the voltage they work at, there is also what's known as 'inrush' which is a peak voltage surge that occurs at power-up. Under rated component's can fail, sometimes quietly, sometimes spectacularly. Either way the fluid inside them is corossive/conductive and if it leaks onto the circuit board can cause other problems.

Of course YMMV,

Andrew

 

[Juergen]

I replaced the cap last night and it works! Unfortunately the 75V rating is going to bother me till the end of time unless I replace it. So I guess I'll shop around and get a proper one soon. Thanks to all for the advice.

 

[mono]

Don't worry, MEASURE. Measure the voltage they see.

Also, what is the specific specs on the caps you bought? Often in power amps, higher voltage caps are rated as "WV" working voltage. If you see 75WV, it doesn't necessarily mean their peak voltage is any lower than a 85V (with no WV rating) cap.

Time, heat, and original cap fitness are also factors, if your peak voltage is under 75V, there is no need at all to worry about it. When the cap manufacturer spec'd 75V, they really meant, I mean guarantee, it. That doesn't mean it isn't a good engineering practice to spec above by as much as reasonable, but given a peak voltage under 75V, if they did eventually fail I wouldn't attribute it to that.

 

[star882]

Quote:

Originally Posted by AndrewB /img/forum/go_quote.gif Just a word to the wise: you should never down-rate (the voltage of)electrolytic capacitors. other than the voltage they work at, there is also what's known as 'inrush' which is a peak voltage surge that occurs at power-up. Under rated component's can fail, sometimes quietly, sometimes spectacularly. Either way the fluid inside them is corossive/conductive and if it leaks onto the circuit board can cause other problems. Of course YMMV, Andrew

The oil inside capacitors is not conductive. It will, however, make a mess. Mineral oil is pretty unreactive but some synthetic oils can damage plastics.