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One channel on stereo amp blows filter cap on negative rail

T

Tiger Luck

Jan 1, 1970
0
William said:
I think you have problems that go fary beyond a bad cap. I suspect the bad
cap is a effect more than a cause.

I would start by pulling the output pair on the right channel and trying to
get everything else working correctly.

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 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 connect 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 riny puffs of smoke (see
schematics):

http://www.davidreaton.com/PDFs/Advent_300_newer_Amplifier_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.

Thanks all
 
T

Tiger Luck

Jan 1, 1970
0
Arfa said:
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/Advent_300_newer_Amplifier_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.
 
T

Tiger Luck

Jan 1, 1970
0
Mark said:
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/Advent_300_newer_Tuner_power_supply_schematic.pdf

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

http://home.netcarrier.com/~rstevens/advent.html
http://www.davidreaton.com/Model_300_receiver_main_page.htm
 
T

Tiger Luck

Jan 1, 1970
0
Tiger said:
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/Advent_300_newer_Amplifier_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.
 
T

Tiger Luck

Jan 1, 1970
0
Tiger said:
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.

Spoke too soon. Three of the output trannies fried and the fourth probably would
have if I had left the power on longer.

I have never given up on a piece of gear until now. It almost seems like the power
supply might be the problem seeing as how the outputs on both channels fried, but
who knows. It reads okay.

I've been at this thing for about four days straight now and I am totally fried
behind it. I've resurrected stuff the local vintage stereo store has given up on,
but this took me to my limits and left me out to dry.

If anyone wants this pup, with schematics, minus a couple of output trannies, for
the price of postage, email me at <jack6128 at gmail dot com>

Je suis fini.
 
A

Arfa Daily

Jan 1, 1970
0
Spoke too soon. Three of the output trannies fried and the fourth probably
would have if I had left the power on longer.

I have never given up on a piece of gear until now. It almost seems like
the power supply might be the problem seeing as how the outputs on both
channels fried, but who knows. It reads okay.

I've been at this thing for about four days straight now and I am totally
fried behind it. I've resurrected stuff the local vintage stereo store has
given up on, but this took me to my limits and left me out to dry.

If anyone wants this pup, with schematics, minus a couple of output
trannies, for the price of postage, email me at <jack6128 at gmail dot
com>

Je suis fini.

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
 
C

Chuck

Jan 1, 1970
0
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


If you don't have a variac, put a 60 watt light bulb in series with
the hot side of the AC in. If the light glows brightly, your amps are
still drawing too much current or there is a power supply fault. Once
you find the problem, the light will hardly glow after the initial
currrent rush and the bulb will usually keep the outputs from blowing
if there is still a problem. Chuck
 
A

Ancient_Hacker

Jan 1, 1970
0
if the -22 supply has failed, due to a blown diode or fuse, and if the
output transistors have shorted, then the -22 rail will be pulled
positive and that will blow the capacitor.

Also note that the schematic has errors on it, for instance the output
transistors have emitter and collectors reversed!

Hope you didnt wire the new ones in like the schematic!!
 
Ancient_Hacker said:
if the -22 supply has failed, due to a blown diode or fuse, and if the
output transistors have shorted, then the -22 rail will be pulled
positive and that will blow the capacitor.

Also note that the schematic has errors on it, for instance the output
transistors have emitter and collectors reversed!

Hope you didnt wire the new ones in like the schematic!!

I thought that at first too but if you zoom to 300% you'll see the
emitter marks where they belong and the 'squiggles' on the collectors
may be a 1970's version of heatsink. You'll see the same marks on the
collector of Q312 indicating it is thermally coupled to the heatsink
with the outputs.

Way back in the days when I worked on stereo gear I had a unit with
very bizarre distortion. It turned out to be a transistor with a
'leak' which on a curve tracer showed up as a transistor in parallel
with a resistor from E to C. If Tiger has a similar quirky part it may
explain why he's having such a bad time with that amp - which is
pretty straightforward and decent for it's day.

 
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