Tiger Luck wrote:
Arfa Daily wrote:
"Tiger Luck" wrote in message
...
For the record, the amp is in an Advent 300, which I refuse to let die.

Here's a PDF of the schematics:

http://www.davidreaton.com/PDFs/Adve..._schematic.pdf

The cap that blows is C313. It is a 100mfd 35volt cap It is the filter cap
connecting the negative rail in the lower channel to ground. It is on the
far lower right of the schematic, just above the separate, isolated
schematic of the 15 volt regulator circuit.

The voltage on the negative rail is as it should be. The polarity of the
cap was correct. The corresponding cap in the other channel does not blow.
Both caps have the negative connected to the same source from the power
supply and both have their positives connected to ground. In other words,
the negs of both caps are connected together to the negative rail from the
power supply and both positives are grounded, yet one cap blows and the
other doesn't. The cap blew with a sharp crack and the rubber plug at the
bottom blew out. The cap that blew is not shorted. It still acts like a
cap should when measured with an analog ohm meter. I installed another,
new cap of equal value and it started heating up, so I cut the power.

I'm not a pro tech, but fairly knowledgeable and good at logic and such.

Any insights?



--
Einstein forgot to carry the two
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

Arfa, an update as requested and as promised.

First, a repost of my reply to William Sommerwerck posted today, 9/11/08 at 2:11pm
so that everything is in one post:
----------------------------------

Both output pairs are pulled.

I am reading 20 volts AC to ground on each of the stepped down leads from the
transformer and 40 volts AC between them.

The positive rail is reading 26.3 volts DC to ground and the negative rail is
reading 26.7 volts DC to ground after going through the bridge rectifier and the
filter caps. They are reading 53 volts between them. The replacement for the electro
cap that blew is reading 26.7 volts across its terminals. It connects the negative
rail to ground. Positive side is grounded. It is no longer heating up when power is on.

With power on and about to take some voltage readings across the empty output tranny
terminals on the CB, I noticed some tiny sparking going on at the edges of the trace
leading to the collector terminal of Q317, with occasional tiny puffs of smoke (see
schematics):

http://www.davidreaton.com/PDFs/Adve..._schematic.pdf

I removed as much solder as possible from the traces around all output terminals
with a solder wick and cleaned up the board with switch cleaner. Sparking appears to
have stopped.

Began taking various voltage measurements at same points in right and left channels.
Discovered huge variation on voltage across C-E terminals with output transistors
removed. Right side had about 26 to 27 volts across the C-E terminals of each
output transistor. The left side had no voltage across C-E terminals of one output
transistor and about 52 to 54 volts across the C-E terminals of the other output
transistor.

Also had different readings across C-E terminals of the two pairs of differential
transistors at the input. Right side had about 26-27 volts across C-E terminals of
each transistor and the left side had about 7 volts across the same terminals.
Disconnecting the base of Q302 resulted in both differential transistors on the left
reading identical to the right side across the C-E terminals. I traced it down to
where D304 connects to the base of Q302 thru a 20K resistor, R336. I reconnected the
base of Q302 and disconnecting the cathode side of D304 and got the same results as
disconnecting the base of Q302. Now it's just a matter of going through the same
procedure until I find the faulty component.

Disconnecting the base of Q302 had no effect on the voltage measurements of the
output transistors in the left channel, but disconnecting D304 did. Reading across
the C-E terminals of one output tranny on the right channel with D304 disconnected,
the digital meter read about 27 volts and then began drifting steadily lower,
similar to reading a large cap with an ohmmeter. When I switched the meter to the
C-E terminals of the other output tranny on the right side, it read higher than 27
volts by as much as the first tranny's voltage diminished when it was read.

And that's where it stands. Sorry for not reading and replying to every post, but
when I got enough insight from what I did read to see where I needed to go, I
stopped reading and went to work. Will report all findings and methods used.
--------------------------------------

Update:

Narrowed down the search to the upper section of the right channel as seen on the
schematics at the link above. Following the logic outlined above, I reconnected
everything and disconnected the base of Q308 and turned the power on to check
voltages discrepancies outlined above. With the unit on its side and probing the
underside of the circuit board, smoke began rising from the top side opposite me. I
cut power and found the following fried components:

Q308
Q310
R326
R328
R324

Pulled all the fried components and checked. Q308, Q310, R326 and R328 were toast
and I replaced them. R324 was still good, but replaced it. Everything except the
output trannies are in place and the voltage discrepancies between the two channels
have disappeared. I will now install the output trannies and give it a go.

Output trannies are back in. Speakers hooked up. Tuner tuned in.

Fingers crossed.

And here goes... power up... power down, got smoke.

Output trannies on the left side got hot, but held.

The drama continues. Will pull output trannies on left side and check right side for
okayness.

--
Einstein forgot to carry the two