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Old 19th May 2005, 05:12 PM   #1
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Default amp hums on start then fades

I am working on one of my old amps, a Gibson GPA50(50 watt PA amp). I pulled it out and turned it on and it had quite a bit of hum(60Hz). So I replaced the power supply 2000uF/35V caps with new 2200uF/35V caps. Now when I turn it on you hear hum that lasts about 5 seconds then fades to a soft hum( you have to be 2ft or so close to the speaker) that never goes away. The hum is unaffect by the volume. Otherwise, the amp seems to work normally. I really want to clean up the hum as much as possible.
Any ideas? I have included the schematic.
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Old 19th May 2005, 06:31 PM   #2
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Replace all the other electrolytics as well, and see if that helps.
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Old 19th May 2005, 07:07 PM   #3
HDTVman is offline HDTVman  United States
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I agree with pinkmouse. C22, C23, C24 and C25 are in the power supply too and the most likely target. They provide the filtering for the low level stages at reduced voltage set by the 12 volt zenors.

Later BZ
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Old 19th May 2005, 07:41 PM   #4
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Default replaced C22 - C25

I replaced these 4 50uF caps. However, the hum did not change.
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Old 19th May 2005, 07:45 PM   #5
Netlist is offline Netlist  Belgium
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Can you measure the ripple on the PSU caps?

/Hugo
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Old 19th May 2005, 08:52 PM   #6
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Default power supply ripple measurement

Unfortunately I don't have a scope or a true RMS meter.
Measuring accross the 2200uF caps with my VOM I see 33V DC and .06V AC.
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Old 19th May 2005, 08:57 PM   #7
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And what voltages do you get at the output?

/Hugo
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Old 19th May 2005, 09:15 PM   #8
johnnyx is offline johnnyx  United Kingdom
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I've had this in valve amps., but I think you have a different problem.
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Old 19th May 2005, 09:20 PM   #9
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Default output voltage

DC .1 V
AC .002V
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Old 19th May 2005, 09:22 PM   #10
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Default Defective bias circuit also produce that sound when switch on

Switching on, and having short time high current, will overcharge supply and the transformer hum can be heard during around half second...then bias goes reasonable and the amplifier goes silent.

I also think electrolitic condensers, the supply filters are the first and main suspect ones.

regards,

Carlos
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