One channel on stereo amp blows filter cap on negative rail

Take a look at the points where the + and - caps are grounded. If that part of the ground bus is intact, but the bus is broken on the way to the supply ground, so that it is isolated, that point could float up towards +22 volts, making the total voltage over the cap that blows 44 volts. Which cap blows is whichever leaks less, just a matter of chance. So check that ground bus very carefully for integrity.

Bob Morein (310) 237-6511

Hi,Bob and thanks

The grounds are intact. By that I mean to say that point in the CB where electrolytic caps C312 and C313 are grounded is continuous with the ground at the power supply filter caps. Chassis is the same. All points which are supposed to be grounded are continuous as read by an ohm meter.

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Einstein forgot to carry the two

Those little 100uF caps are _not_ the main 'lytics. The mains would likely be 1000uF to 10000uF (or possibly even more) and be located near the main bridge rectifier - which is on the _rest_ of the schematic along with the power transformer.. I think you'll find the main negative bus 'lytic to be open or dried out so that you're putting LOTS of ripple current on those littlle 100uF caps. Those

100uF caps are just local 'clean up' caps and can't handle the load.

G=B2

My mistake to refer to them as filter caps.

--
Einstein forgot to carry the two

Every piece of gear I have that has + and - power buss has the positive rail cap with the + to the rail and the - lead to ground. HOWEVER the negative rail has the - to the rail and the + to ground. Things are not always as they seem dnw

Part of the circuit might have failed so that it oscillates or pulses. For example - R313, C305, D301, C309, R335, or any of that bias circuit. Drag the leads of a big foil capacitor across the leads of C313. It should spark once to charge it. If it keeps sparking (or smokes), there's AC current there.

The great thing about stereo amps is that you can put a voltmeter between the matching parts of each side to find faults.

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Google is a pro-spamming service.  I will not see your reply if you use Google.

On Wed, 10 Sep 2008 18:29:26 -0700, Tiger Luck put finger to keyboard and composed:

lower

volt

was

have

their

connected

grounded,

the

acts

new

Maybe the trace from that channel to the -22V rail is partially open (ie resistive), allowing C313 to be reverse charged from the +22V rail via Q317 ???

Just for testing purposes, try running two hookup wires between the caps in both channels.

- Franc Zabkar

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Please remove one \'i\' from my address when replying by email.

In many years of repairing this stuff, and replacing electrolytics that have failed any way from passively to explosively, I have only ever found three things which cause their demise in the way you have described.

1) Excess voltage across them 2) Incorrect polarity of voltage across them 3) Excess current through them

3) is obviously the most difficult to diagnose, as it really requires a 'scope to see what's going on. However, given that this cap is effectively in parallel with the main filter cap for the output stage negative rail, and the one for the other channel, which is also connected to the same place for all intents and purposes, it's hard to see how there could be excessive ac across one of them on its own. Have you tried running the amp without the offending cap in ? It should almost certainly run normally without it, as small caps in that position are usually just for 'belt and braces' decoupling purposes.

I know that you have stated several times that the cap is in correctly, but are you *absolutely totally completely and without question* sure ? Noted that you replaced the caps a couple of years back and have not had trouble in the intervening period, but that is not necessarily an indication that all was well. I have seen equipment where caps have been fitted backwards by the manufacturers, and have lasted a long time before finally failing. If you are basing your latest replacements which are failing, on the way you took out the first failed one - or even based on the way that the board is marked, because I've seen them silkscreened backwards as well - I would say please check very very carefully, go away and have a cup of coffee, then come back and check again. It's easy to convince yourself that you are seeing something you actually aren't. It has happened to me many times over the years ...

I too would like to know what you eventually find, as the circumstances you are describing are a bit of a conundrum. If it does come down to something you have done, please tell us. I'm sure no-one will think any the less of you. If we're honest, we've all been there d;~}

Arfa

The matching cap on the other channel may now be open, thus not exploding from a shorted rectifier in the power supply putting AC across the cap, which would explain the original cap failure also, and would also go a long way toward explaining the heating on the other channel.

The multiple failure does suggest to me that you are getting AC to that negative rail. This would explain the voltage regulator failure, and I'll bet the main transformer is getting hot as well.

More than once I've been misled by DC voltages only to find that it was an AC problem, either line-related or HF oscillation.

Wouldn't be surprised if the main filter on that negative rail were bad also (or maybe it just hasn't exploded yet from the AC !).

Too bad the schematic is not complete - I would have liked to have a full PDF on this model.

Mark Z.

their

connected

grounded,

Which is WRONG ! Read the diagram !

The negative of C311 should be grounded together with the POSITIVE of C313

Possibly a pcb legend error. I've known it happen.

Graham

An HF fault causing high local ripple ?

Graham

Then you must not have any pieces of gear that are in working order. On the negative rail, the - lead of the cap is on the rail and the + lead is to ground. Can't be any other way.

On Thu, 11 Sep 2008 13:51:27 +0100, Eeyore put finger to keyboard and composed:

their

connected

grounded,

The OP wrote that "the corresponding cap in the other *channel* [C312] does not blow", not the other *rail*.

- Franc Zabkar

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Please remove one \'i\' from my address when replying by email.

Mark, here is the other PDF. It includes the power supply and the tuner:

Here are two sites devoted to this receiver. Schematics came from the latter:

--
Einstein forgot to carry the two

ground.

the

As I read it, that's what he said.

I was partly right once, so perhaps the luck (ahem) will continue.

Barring a bad part, it sounds -- as someone else suggested -- that there's something wrong with the board, or a solder joint.

Lemme tell you a story...

Back in 1980, I assembled an Integrex kit and had problems with one of the op amps overheating. I spend almost an hour checking voltages, resistances, etc, trying to figure out exactly what was causing the problem.

It then occurred to me to turn the board over and look carefully. Viola! Two traces had a solder bridge. (Integrex was cheap and didn't provide a solder mask.)

Be patient. Put the unit aside for few days and come back to it. Don't expect to solve every problem instantly. When your mind is fresh again, you're likely to figure it out.

I do sympathise old boy. In years past, I never gave up on anything either, but I must admit that these days, sometimes I do, out of both a need to be commercial and keep the wife off my back for not earning any money, and out of sheer frustration. I think that latter condition is something that creeps up on you with age. Suddenly, some things are more important than not letting some inanimate object beat you ...

That said, I reckon that this baby has got to be worth one more go if - and

*only* if - you can lay hands on a variac, so you can wind up the input power to make the rails just high enough to take sensible measurements, but low enough to stop the ouput trannies releasing their magic smoke ...

As William says, give it a break over the weekend, then come back to it with fresh eyes (and a variac !!) on Monday. If you do, carry on keeping us updated. It's an interesting saga, if a little frustrating for you.

Arfa

either,

creeps

and

One of the reasons I've enjoyed helping is that you've explained exactly what you've done, which makes the process fun -- and educational, too!

PS: I have several working Advents. Let me know if you want me to check the voltages on a "good" one. I'm willing to do it "live", in real time, if we can find a time suitable for both of us.