Capacitor blown... any advice please!

Here's a copy of an email that I sent to Croft (please read!), they've determined without looking that the transformer has blown. Is there any chance that it's just the capacitor? I don't want to be spending money for fun!

Thanks, Tim


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I have a Croft 'Vista' power amplifier (not very old) and the other day it started smoking. On examination I found that the only visible fault seemed to be a burnt out capacitor, the one connected across the mains input wires to the large 'Mains transformer' (ie burn-out damage on its side).

The transformer has stamped on the side "transformer 120V+120V 50/60hz".
The capacitor has printed stampted on the side: 0.047 (micrfarads) 400v/220v 40/0

In case it is relevant I checked the fuse across the mains input and it has 250 V T2AL on it, and is not blown. The other fuse (across the two smaller transformers is a 250v F500MAL and is not blown either. (The amplifier was protected by a qualtiy 'surge protection' unit so I doubt whether a surge was to blame for the burn-out.)

 

That cap sounds like its part of a filter on the power input if its across the mains. The cap going should not affect anything as they're designed not to (X class caps are designed to blow closed circuit not open, hence the use in mains filtering). I'd have thought that tx has gone too but hopefully someone else more experienced than myself can confirm

 

sounds like a bit more than just a tx to me, they don't normally just die for no reason, I suspect another dodgy component that has drawn too much current, ie a valve. did you hear any nasty noises from the speakers just before the breakdown?

check the valves for white spots on the inside of the glass (looks like the printing on the outside sometimes just in dots) this indicates a valve that has arced.

will they not look at it for you ?
sounds like this may be a common problem or weak point, if they said tx straight away ?




good luck

 

If that was the case though, why haven't the fuses blown?

Without looking at it I wouldn't like to suggest anything more, but the tech guys on the end of the phone know their amps and know what they do.

 

yes you would think so.
is it valve rectified?
I have had a rectifier pop and the fuse didn't go, it was a quickblow of the correct size too. ?

I would probably just get it apart and check it out, but then I am silly.

best to probably just give it to them and get them to quote for repairs.

 

Thanks all, just trying to save some cash and check a few opinions before I go signing it away... would it be dangerous to change the cap and see what happens?

Appreciate your advice.

 

test the cap is it a short circuit?
if not then I would get it sent off.
if it is, try changing/removing it (at your own risk, this isn't advise, merely what I would do)

if the cap short circuited before the fuses this would explain a lot.

start with the resistance test.

 

It won't be the cap and it won't be short circuited. If it was short circuited you would be shorting the mains and blow the breakers in the house- see this line in the email 'the one connected ACROSS the MAINS input wires to the large Mains transformer'. The cap isn't needed for the amp to run, its just a filter.

If it started smoking the ABSOLUTE last thing you do is turn it back on again to see what happens. Don't try to replace the cap etc. the amp has a serious problem and needs to be checked out properly.

 

As Anex said, it's an X-type cap (if it wasn't it would not be legal) and can NOT fail short-circuit.

Unless you're very confident with what you're doing (and that means you really know your onions, not a have a go hero), do NOT mess about with mains stuff.

 

I would just get it checked out far easier.

Anex - silly question how do you think the cap got burnt ?
just interested.

cheers.

 

Croft (and I) suspect the windings somewhere in the trans have shorted, bridging the + and - and wrenching much badness through the cap which wouldn't have stood a chance (and is designed not to). As to the cause of the trans short, that is another question entirely.

 

Shoddy manufacture?

 

I have never really come across these xtype caps before as I only really mess about with really old stuff that hasn't got any, do you think I could benefit from fitting some in my valve amps ?

 

Possibly, depends on the amps and mains really, I stick them in my gear (RS sell in bulk), now and again I've noticed an improvement, certainly never done any harm. Use X2 class caps, one across Live and neutral and stick a VDR (Voltage Dependent Resistor) across there too. Then another cap between Neutral and Earth. Be careful though! The X2's are basically designed to break in a way that won't destroy you and the equipment but you still need to be careful you keep the leads as short as poss and out the way etc. etc. I can't remember the cap values off the top of my head aside from the .068uF on my phono stage input, I think possibly I stuck those on my CD player and pre too. The power amp has a copy of the Russ Andrews silencer thing on it (which I never found to make a difference anyway ).