Guitar amp re-capping

[Bearded Occam]

An old guitar amp is headed my way, solid state, maybe 50 Watts, maybe
a couple decades old. One reads about the finite lifetime of
electrolytic capacitors, so I have some questions:

1. What are the actual failure mechanisms?

2. Is there a convenient way to check an electrolytic?

I don't want to replace more caps than necessary. One website suggests
simply following the signals with a scope and verifying that the
functioning of the circuit is consistent with functioning capacitors,
and I will certainly do this, but am wondering if there are some
tricks of the trade to know.

Thanks in advance!

 

[Phil Allison]

"Bearded Occam"

An old guitar amp is headed my way, solid state, maybe 50 Watts, maybe
a couple decades old.

** But how many hours has it done??

Bet it is only a tiny percentage of what a TV set usually does in 20 years.

One reads about the finite lifetime of
electrolytic capacitors,

** 99% pure bollocks.

Forget it with a SS guitar amp.



....... Phil

 

[Eeyore]

An old guitar amp is headed my way, solid state, maybe 50 Watts, maybe
a couple decades old. One reads about the finite lifetime of
electrolytic capacitors, so I have some questions:

1. What are the actual failure mechanisms?

The electrolyte dries out.

2. Is there a convenient way to check an electrolytic?

Measure the capacitance.

I don't want to replace more caps than necessary. One website suggests
simply following the signals with a scope and verifying that the
functioning of the circuit is consistent with functioning capacitors

Not really adequate.

and I will certainly do this, but am wondering if there are some
tricks of the trade to know.

Power supply caps are likely to be the main problem. Check for abnormally
high ripple voltages. Also check for bulging cans and signs of leaking
electrolyte.

Graham

 

[Phil Allison]

"Eeysore"

Bearded Occam wrote:


Measure the capacitance.

** Wrong.

One measures the ESR and compares the reading with a similar, known good
electro.

A high ESR value indicates serious loss of electrolyte.

The capacitance falls only much later in the failure process.





......... Phil

 

[Eeyore]

"Eeysore"

** Wrong.

One measures the ESR and compares the reading with a similar, known good
electro.

A high ESR value indicates serious loss of electrolyte.

The capacitance falls only much later in the failure process.

I've certainly seen examples of capacitance seriously degraded as a result
of the 'bad caps' issue.

Also, not everyone has an ESR meter by a long way. Any other suggestion in
the absense of one ?

Graham

 

[Phil Allison]

"Eeysore"

I've certainly seen examples of capacitance seriously degraded as a result
of the 'bad caps' issue.

Also, not everyone has an ESR meter by a long way. Any other suggestion in
the absense of one ?

** You can fabricate a simple ESR meter using an sine generator ( set to
50kHz to 100kHz ) and monitor the output with a scope set to max
sensitivity.

Then just connect a co-axial output lead across the electro under test.



........ Phil

 

[Bearded Occam]

"Eeysore"






** You can fabricate a simple ESR meter using an sine generator ( set to
50kHz to 100kHz ) and monitor the output with a scope set to max
sensitivity.

Then just connect a co-axial output lead across the electro under test.

....... Phil

Nice and simple. Good call. Lacking a real signal generator, I will
try it tomorrow with a frequency in the audio band, using my PC as the
generator and analyzer.