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Old 26th March 2003, 01:48 PM   #1
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Default cleaning my volly pots

I just gave my amplifier volly pots a clean by hoovering them and then squirting in a short blast of Servisol Super 10 switch cleaner/lubricator. I also squirted the source selector whilst I was at it.

The sound afterwards was slightly clearer and a little louder (perhaps 5 to 10 percent louder). In fact, I can hear slight sound when the volume is at 0, and it takes a smaller movement of the volume knob to kick in both channels; and as soon as they kick in full-frequency sound fills the room. Have I increased gain slightly by providing a better contact between the two sliding surfaces?

Also, I have just damped the large caps with blu tack and dynamat equivalent. Dare I tell you that this seems to have improved overall performance also?

Can I also recommend that a good clean of the heatsinks in an amplifier every 3-6 months is a good idea. My amp used to get very hot after a few hours. Now it just gets very warm...
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Old 18th November 2006, 02:55 PM   #2
gni is offline gni  United States
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Default Hot Amp

Check the Bias current. . .that will usually set how much heat is
dissipated. . .bias may have drifted lower. . . .

Damping caps can be important. . . they can be microphonic. . .there
are plates that can move. . . .and change capacitance.

I would say you have restored the gain lost by years of grim and gunk.

Let us know any new developments.

Chris
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Old 18th November 2006, 03:17 PM   #3
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Default Re: Hot Amp

Quote:
Originally posted by gni
Check the Bias current. . .that will usually set how much heat is
dissipated. . .bias may have drifted lower. . . .
Chris
Is Bias drift only caused by resistors changing value with age?
If I check that all my resistors are within spec then can I assume that the bias current is correct? This is especially important to me since I do not own a service manual.
Thanks,
Martin.
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Old 18th November 2006, 03:51 PM   #4
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Sonusthree;

If your resistors really had changed so much in value, that your bias are drifted outside the range, I would recommend you to scrap your amp

However the bias is normally set by at small pot, and it could be a good idea to check and maybe readjust the bias
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Old 18th November 2006, 10:01 PM   #5
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Thanks for the reply ACD.
My amp has no bias pots at all so.......

What would cause a bias shift in my case?
Do transistors properties change over time?

I haven't checked the bias in my amp yet but I'm interested in the mechanism that causes this 'drift'.

Do people only need to worry about drift in their amplifiers because of these unreliable presets (often open to dirt and dust)?

Many Thanks,
Martin
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Old 19th November 2006, 06:05 AM   #6
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If you have no pots to adjust your bias, then your amp has fixed bias (a fixed resistor or diodes instead of a pot)

This shouldn't be able to drift very much over the years.
However there may be a minor degrade in the bias transistor and in the thermal junction between the bias transistor and the heatsink.

I build a fixed bias amp more than 20 years ago, and last time I measured the bias, it was drifted less than 0.5 mV.

Power up your amp, and let it be turned on for at least 30 minutes without any input signal and no load, and then measure the voltage across the emittor power resistors in the output stage. The voltage across should be in between 10 to 23 mV if your emittor resistors are in the normal range (0.22 to 0.47R)

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Old 19th November 2006, 07:23 AM   #7
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Quote:
Power up your amp, and let it be turned on for at least 30 minutes without any input signal and no load, and then measure the voltage across the emittor power resistors in the output stage. The voltage across should be in between 10 to 23 mV if your emittor resistors are in the normal range (0.22 to 0.47R)
I should have pointed out that the above are generelly speaking for normal Class A/B amps, and that your amp bias may differ from this
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Old 19th November 2006, 08:41 AM   #8
es44 is offline es44  Denmark
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Default Re: Re: Hot Amp

Quote:
Originally posted by Sonusthree


Is Bias drift only caused by resistors changing value with age?
If I check that all my resistors are within spec then can I assume that the bias current is correct? This is especially important to me since I do not own a service manual.
Thanks,
Martin.
You could tell us the name and model of the amp, maybe someone does have the SM for it.
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