One channel on stereo amp blows filter cap on negative rail

[Tiger Luck]

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/Advent_300_newer_Amplifier_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?

 

[Tiger Luck]

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.

 

[[email protected]]

Tiger Luck wrote:

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.

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.

G²

 

[Tiger Luck]

Tiger Luck wrote:

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.

G²

My mistake to refer to them as filter caps.

 

[Tiger Luck]

Are you saying that *ONE* capacitor has failed? Not
a series of replacements? The thing's 30 years old - *all*
of the electrolytic cap's are bad now. That's life.

Replace *all* of the electrolytics and move on. They're
*all* bad. Parts are cheap.

I replaced all the electro caps less than two years ago. Amp worked fine up until
now. Polarity was and is correct. After cap blew today, new replacement cap over
heated. I shut off power before it blew out.

BTW, if your question had been about repeated failures,
the diagnosis would be: the cap got too hot. It got too
hot because it was defective, or it was exposed to too
high a voltage,

I checked voltage across cap terminals with cap out. It read correctly.

or it was exposed to reverse voltage,

Trust me. Polarity was correct.

or it was cooked by ESR.

Equivalent series resistance? Is that a measure of quality?

Caps are no-names, both from the same parts house. hmm...

 

[Kevin McMurtrie]

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/Advent_300_newer_Amplifier_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?

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.

 

[[email protected]]

At work we buy 100uF 25 volt caps in bags of 1000 and have so far
installed 2500 in 20 year old gear, mostly Sony. But we also have some
Ampex 2" quad machines built in 1978 with _all_ the orignal
electrolytics and the machines are in cherry condition. 30 years is
not necessarily the end point for caps.

I replaced all the electro caps less than two years ago. Amp worked fine up until
now. Polarity was and is correct. After cap blew today, new replacement cap over
heated. I shut off power before it blew out.

What do you see with a scope? Something definitely funny going on. Is
the transformer heating up too?

I checked voltage across cap terminals with cap out. It read correctly.

or it was exposed to reverse voltage,

Trust me. Polarity was correct.


Equivalent series resistance? Is that a measure of quality?

Oh yeah, ESR is a big deal.

Caps are no-names, both from the same parts house. hmm...

For little ( 1-4700 uF) caps we were using Panasonic FC series but
after reading some Hydrogenaudio forum posts, I found that the FM
series caps are even lower in ESR, longer llife (2000 vs 1000 hrs at
105 C) and they cost a little less. ($57.54 vs $61 / 1000 for 100uF
25V)

Definitely want to hear how this shakes out.

G²

 

[DC]

Caps are no-names, both from the same parts house. hmm...

Well, this may not be your problem, but things have deteriorated in the
electronic component area over the last few years. Many manufacturers have
out sourced production to China. Most if not all capacitor failures I have
seen in recent years have made in China - or no brand. Try to find something
that says made in Japan!!

Cheers

Dave

 

[Franc Zabkar]

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/Advent_300_newer_Amplifier_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?

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

 

[Tiger Luck]

At work we buy 100uF 25 volt caps in bags of 1000 and have so far
installed 2500 in 20 year old gear, mostly Sony. But we also have some
Ampex 2" quad machines built in 1978 with _all_ the orignal
electrolytics and the machines are in cherry condition. 30 years is
not necessarily the end point for caps.


What do you see with a scope? Something definitely funny going on. Is
the transformer heating up too?

Transformer has stayed cool. The 3 amp fuses on the positive and negative rails are
not blowing at this point. I don't have a scope, but have a line on a Philips PM3050
for $85. There is more going on here than I can read at the moment. I have the
output pair on the left channel yanked at this point because they shorted out. Now
the output pair on the right channel are heating up when the power is on. The
electro cap that blew is on the right channel. Part of the 15 volt regulator
circuitry failed and I replaced some parts there. But that's it for tonight.

 

[Arfa Daily]

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/Advent_300_newer_Amplifier_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?

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

 

[Mark D. Zacharias]

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.

 

[Eeyore]

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,

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

 

[Eeyore]

Well, this may not be your problem, but things have deteriorated in the
electronic component area over the last few years. Many manufacturers have
out sourced production to China. Most if not all capacitor failures I have
seen in recent years have made in China - or no brand. Try to find something
that says made in Japan!!

Panasonic are reliable. But the op says C311 and C313 have their negatives
connected together AIUI. That's plain WRONG.

Graham

 

[Eeyore]

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.

An HF fault causing high local ripple ?

Graham

 

[Eeyore]

It's remotely possible the replacement was defective. If the voltage across
the cap's location is correct in level and polarity, then that can't be the
problem.

Question... How safe is it -- other than the cap blowing -- to operate the
amp? I think you need to put a 'scope on a number of points and see what's
happening.

I don't mean to hassle you on the question of polarity, but... Japanese caps
generally have a black line on one side pointing toward the negative lead.

Or a gold one etc etc.

Graham

 

[Tiger Luck]

It's remotely possible the replacement was defective. If the voltage across
the cap's location is correct in level and polarity, then that can't be the
problem.

Question... How safe is it -- other than the cap blowing -- to operate the
amp? I think you need to put a 'scope on a number of points and see what's
happening.

I don't mean to hassle you on the question of polarity, but... Japanese caps
generally have a black line on one side pointing toward the negative lead.

These caps have a line of pluses down one side pointing to the plus lead. The CB has
a plus printed next to one hole for the ECs. I installed several of these caps in
this amp. They are foolproof. The amp worked perfectly for a couple of years with
these caps installed.

I don't have a scope, but I may buy a used Philips PM3050 for $85 just to see what
I'm missing.

 

[Dave]

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

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.

 

[[email protected]]

These caps have a line of pluses down one side pointing to the plus lead.The CB has
a plus printed next to one hole for the ECs. I installed several of thesecaps in
this amp. They are foolproof. The amp worked perfectly for a couple of years with
these caps installed.

I don't have a scope, but I may buy a used Philips PM3050 for $85 just tosee what
I'm missing.

You need to put a scope on it. I think you're going to find ac or a
lot of ripples. If you isolate the power supply don't forget to put a
load on it.

 

[Franc Zabkar]

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

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

- Franc Zabkar