Power filter capacitors fail by shorting or opening.

[mm]

I can't do subtle repairs, but I"m ok replacing power supply filter
capacitors when I hear the typical 120 cps hum in the speakers.

And I had always thought that the filter capacitors fail by opening,
rather than shorting, because I figured there would be sparking and
smoking if the cap was shorting.

Is this so?

In this case, I'm trying to fix up a high quality Panasonic table
radio from the 70's I think. It used to give perfect sound, but the
hum has appeared about 3 months ago.

There are 3 16-volt 2200mF capacitors in the filter, and two of them
are connected at the posive end to the +25 volt, and one is connected
at the negative end to the -25 volt location. The other ends are
connected to the chassis. Normally I cut out the old caps and replace
them, but to learn something, I used some wires with alligator clips
to connect a new 2200mf cap in parallel with the old one, or a new
4700mf cap in parallel with the old two in the other case. This did
nothing to make the hum go away. Should this have worked if one or
more of those 3 caps were bad?

!!I just found another fairly large cap, 1000mF, on the circuit board.
I'll have to take the whole thing apart to get to it. Given the
previous paragraph, is there any point to first changing any of the
first 3 caps, which are very easy to get to?


If you are inclined to email me
for some reason, remove NOPSAM

 

[[email protected]]

I can't do subtle repairs, but I"m ok replacing power supply filter
capacitors when I hear the typical 120 cps hum in the speakers.

And I had always thought that the filter capacitors fail by opening,
rather than shorting, because I figured there would be sparking and
smoking if the cap was shorting.

Is this so?

In this case, I'm trying to fix up a high quality Panasonic table
radio from the 70's I think. It used to give perfect sound, but the
hum has appeared about 3 months ago.

There are 3 16-volt 2200mF capacitors in the filter, and two of them
are connected at the posive end to the +25 volt, and one is connected
at the negative end to the -25 volt location. The other ends are
connected to the chassis. Normally I cut out the old caps and replace
them, but to learn something, I used some wires with alligator clips
to connect a new 2200mf cap in parallel with the old one, or a new
4700mf cap in parallel with the old two in the other case. This did
nothing to make the hum go away. Should this have worked if one or
more of those 3 caps were bad?

!!I just found another fairly large cap, 1000mF, on the circuit board.
I'll have to take the whole thing apart to get to it. Given the
previous paragraph, is there any point to first changing any of the
first 3 caps, which are very easy to get to?

lytic caps can go leaky, this can cause hum. But usually they go high
ESR or sometimes the capacity falls, either of which causes hum, and in
either case piggybacking them would make the hum go away. So its not
certain but most likely the problem lies elsewhere than those caps,
since piggybacking them did nothing.

Hum is more likely to come from a bad signal ground connection than bad
caps. You've some some real troubleshooting to do.


NT

 

[Charles Schuler]

I can't do subtle repairs, but I"m ok replacing power supply filter
capacitors when I hear the typical 120 cps hum in the speakers.

And I had always thought that the filter capacitors fail by opening,
rather than shorting, because I figured there would be sparking and
smoking if the cap was shorting.

Is this so?

They can short and they can also develop high series resistance.

 

[CJT]

They can short and they can also develop high series resistance.

They can even explode, spray goop everywhere, and disintegrate, at
which point their resistance is a bit moot.

 

[DaveM]

They can short and they can also develop high series resistance.

Electrolytic capacitors usually fail by
(1) drying or chemical change of the electrolyte that gives the capacitor most
of its characteristics, resulting in loss of capacitance and/or excessive series
resistance. Sometimes, this is due to age or non-use (eqpt being stored for many
years without being used). High leakage current through the capacitor is
common.
(2) exposure to excessive heat, whether external or internally generated.
Internally generated heat is usually the result of the capacitor developing
excessive series resistance. The current through the capacitor's series
resistance generates heat, and will cause rapid deterioration of the electrolyte
and failure of the capacitor.
(3) Exposure to excessive voltage, beyond the rated voltage of the capacitor.
The result of extended exposure to higher than rated voltage is usually
punch-through of the electrolyte, resulting in catastrophic failure of the
capacitor (a short).

If the capacitor is used as a power supply filter, and the capacitor develops a
short, then the most likely result will be overheating of the power transformer.
If not caught in time, the transformer will be destroyed, usually accompanied by
a large volume of pungent smoke. If the equipment has a power line fuse, the
transformer may be saved.
A failed rectifier can absolutely cause a capacitor to fail. Since the capacitor
should be exposed only to DC voltage, if there is an AC source connected (caused
by a failed junction in the rectifier) it will fail in short order. Violent
explosion is possible in this case.
If the capacitor fails with high series resistance, the bad effects will be more
subtle. As indicated in your post, excessive 120 Hz hum might result.
Sometimes the capacitor develops high leakage current (1). Bridging a good
capacitor across the bad one can sometimes reduce the hum.
There are other things that can cause hum in audio equipment. Open shields in
the signal path can certainly allow induced hum to be amplified. Poor
connections of circuit components can allow hum to pass. Poor grounding of
components can cause hum. Missing cover plates or other chassis components can
allow hum to be picked up.

If this is a tube amplifier that you're asking about, then it's not uncommon for
one of the tubes to develop a heater-to-cathode short. That will allow the AC
filament voltage to be superimposed on the signal. A tube tester can easily
identify defective tubes. Find a friend who has a tube tester and test all the
tubes. Alternatively, substituting the tubes with known good tubes can quickly
identify the bad tube.

Cheers!!!!

-- Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Some days you're the dog, some days the hydrant.

 

[mm]

Thanks to all and thanks to you Dave for the detailed answer.

This is a background project, and new things came to the fore today,
so it's going to take me several days at least to do all the testing I
see I need to do, and to read this thread thorougly as well.

So I plan to get back to you all in a few days.

Thank you.

Electrolytic capacitors usually fail by
(1) drying or chemical change of the electrolyte that gives the capacitor most
of its characteristics, resulting in loss of capacitance and/or excessive series
resistance. Sometimes, this is due to age or non-use (eqpt being stored for many
years without being used). High leakage current through the capacitor is
common.
(2) exposure to excessive heat, whether external or internally generated.
Internally generated heat is usually the result of the capacitor developing
excessive series resistance. The current through the capacitor's series
resistance generates heat, and will cause rapid deterioration of the electrolyte
and failure of the capacitor.
(3) Exposure to excessive voltage, beyond the rated voltage of the capacitor.
The result of extended exposure to higher than rated voltage is usually
punch-through of the electrolyte, resulting in catastrophic failure of the
capacitor (a short).

If the capacitor is used as a power supply filter, and the capacitor develops a
short, then the most likely result will be overheating of the power transformer.
If not caught in time, the transformer will be destroyed, usually accompanied by
a large volume of pungent smoke. If the equipment has a power line fuse, the
transformer may be saved.
A failed rectifier can absolutely cause a capacitor to fail. Since the capacitor
should be exposed only to DC voltage, if there is an AC source connected (caused
by a failed junction in the rectifier) it will fail in short order. Violent
explosion is possible in this case.
If the capacitor fails with high series resistance, the bad effects will be more
subtle. As indicated in your post, excessive 120 Hz hum might result.
Sometimes the capacitor develops high leakage current (1). Bridging a good
capacitor across the bad one can sometimes reduce the hum.
There are other things that can cause hum in audio equipment. Open shields in
the signal path can certainly allow induced hum to be amplified. Poor
connections of circuit components can allow hum to pass. Poor grounding of
components can cause hum. Missing cover plates or other chassis components can
allow hum to be picked up.

If this is a tube amplifier that you're asking about, then it's not uncommon for
one of the tubes to develop a heater-to-cathode short. That will allow the AC
filament voltage to be superimposed on the signal. A tube tester can easily
identify defective tubes. Find a friend who has a tube tester and test all the
tubes. Alternatively, substituting the tubes with known good tubes can quickly
identify the bad tube.

Cheers!!!!

-- Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Some days you're the dog, some days the hydrant.

If you are inclined to email me
for some reason, remove NOPSAM