Mark D. Zacharias wrote:
"Arfa Daily" wrote in message
...
"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


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.

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

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

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

http://home.netcarrier.com/%7Erstevens/advent.html
http://www.davidreaton.com/Model_300..._main_page.htm



--
Einstein forgot to carry the two