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Old 1st February 2006, 05:28 PM   #1
beppe61 is offline beppe61  Italy
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Default Dried out electrolytics in an amp.

Dear Friends,


I would like to know if there is a way to test the condition of the electrolytics caps in an old amp without desolderind and measuring them.
I have heard of a phenomenon called "electrolytics drying out" or something like that.

Thank you so much indeed.
Kind regards,

bg
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Old 1st February 2006, 05:58 PM   #2
glen65 is offline glen65  United States
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You can test them in circuit using an ohm meter.
The only problem is that you may run into occasions
where the surrounding components cause a false reading.
Then you will have to disconnect one lead in order to
check them. If this amp is fairly old you may end up having to reccap the unit. (electrolytics only) This is something I have
had to do many occasions to older equipment to get rid of
all the little problems that weak caps can cause.

Good Luck.
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Old 1st February 2006, 06:01 PM   #3
anatech is offline anatech  Canada
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Hi beppe61,
The only true way to test a cap is out of circuit with an LCR meter that also measures the real resistive portion too. You can get an idea by using an ESR meter. If the cap goes short, an ohmmeter may help. In any case, you need to lift one leg to be sure you are measuring the cap and not the circuit.

No completely easy way out, but why should there be?

-Chris
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Old 1st February 2006, 06:30 PM   #4
beppe61 is offline beppe61  Italy
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Thank you so much Sirs for the very kind and valuable advice.
To be fully honest I have to declare my almost complete ignorance in electronics.
Nevertheless, established that old (and of bad quality) electrolytic caps are weak points in old equipment I would like to kindly ask you the following other question:
- Which are the effects of a dried electrolytic cap in the two situations
1) in series with the signal (coupling cap) and
2) working as a filter cap in a power supply?
I need you an explanation anyway.
I read that a dried cap in the feedback loop between the resistor to ground and the ground could cut the low frequencies (i.e. weak bass response).
I read that a dried filter cap in the power supply could lower the peak current delivery from the amp itself.
Are these senseless statements?
Thank you so much because these are quite interesting topics for me from the moment I decided to play with some vintage amps.
I hear you: "Go to school and study !" eh, eh.

Thank you sincerely again for your great support.
Kind regards,

beppe
Torino
Italy
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Old 1st February 2006, 06:42 PM   #5
anatech is offline anatech  Canada
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Hi beppe,
Electrolytic caps can fail with effective resistance in series and / or parallel with the capacitance. In coupling applications, thedefective cap may allow DC current to "leak" through. This can affect the DC bias of the following stage. If it's a filter, this may lower the DC voltage (as in bias supply).

A failing capacitor across the DC supply may conduct current, causing the part to run warm and draw power. The reduced capacitance and increased resistance act to make the capacitor become less effective, or "disappear" from the circuit.

Electrolyte may leak out of the part in both cases.

-Chris
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Old 1st February 2006, 07:02 PM   #6
blmn is offline blmn  Brazil
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beppe61,

In very old equipment I recommend you to replace all the electrolitic caps. Save a lot of time and pacience to find the problematic ones.

Best regards,
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Old 1st February 2006, 07:50 PM   #7
anatech is offline anatech  Canada
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Yup
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Old 1st February 2006, 08:05 PM   #8
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A dry capacitor will loose it's capacitance or loose it at not so high frequencies (> 10 kHz) => high impedance. One way to notice a dry capacitor is if it's used as a smoothing cap. The smoothing will be much less or nothing at all.

May I ask why you wonder?
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Old 2nd February 2006, 07:09 PM   #9
beppe61 is offline beppe61  Italy
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[QUOTE]Originally posted by peranders
[B]A dry capacitor will loose it's capacitance or loose it at not so high frequencies (> 10 kHz) => high impedance.
One way to notice a dry capacitor is if it's used as a smoothing cap. The smoothing will be much less or nothing at all.

Thank you very much Mr. Peranders,
For smoothing do you mean the ripple reduction?
In this case if the filter caps are dried out I would listen a 120Hz noise ?
And what in the case if the dried-out cap is in series with the signal (coupling cap)?

> May I ask why you wonder?

- Of course. I have at hand a 15 years old amp that sounds "thin", without body.
I think that this means weak low frequencies response.
Someone advised me to look at the electrolytic cap in the feedback loop between the gain (?) resistor and the ground.
What happens if this resistor dries-out ?

Thank you so much.
Kind regards,

beppe
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Old 2nd February 2006, 07:29 PM   #10
anatech is offline anatech  Canada
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Hi beppe,
Quote:
Someone advised me to look at the electrolytic cap in the feedback loop between the gain (?) resistor and the ground.
This will cause the low frequency response to roll off higher. To test this, apply a 1kHz tone and measure the output level, change this frequency to 100Hz and go down from there while watching the level. When you reach about 70%, that indicates your -3 dB frequency.

This is a very quick an easy test. If you remove this capacitor, the gain of the amplifier should fall to unity (X1).

-Chris
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