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Adcom GFP-1A loud hum
Adcom GFP-1A loud hum
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Old 4th August 2020, 05:24 PM   #1
junket is offline junket
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Join Date: Aug 2020
Default Adcom GFP-1A loud hum

wanting to put back into service my gfp-1a along with my gfa 555 both of which I purchased new in mid 80's.

Both units worked perfectly for decades, then put into non climate controlled storage for ~10 years. Temperature range while stored was 40 to 90 degrees F.

Hooked up to test speakers and using a CD audio input there is no discernible source audio... only a hum from both speakers.

Disconnect the 555 and with headphones on the preamp there's nothing but loud hum (volume setting makes no change) even with no source and even with the signal processor loop open.

Will open the case and look for obvious, but any advice for this neophyte repair fogey?
Only testing eq on hand is digital and analog volt meters
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Old 4th August 2020, 06:24 PM   #2
junket is offline junket
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Join Date: Aug 2020
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Old 4th August 2020, 07:14 PM   #3
duncan2 is online now duncan2  United Kingdom
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Join Date: Mar 2013
Location: Scotland.
Just going by the information you supplied and not going into --check this/ check that and the fact it was stored at up to 90 degrees F I would check the signal capacitors if they are electrolytic that would be step #1.

At the moment power supply capacitors are secondary if you are getting the correct voltages from the power supply .

Hum from them can be fixed later unless as I said they are a cause of lowered voltage .
A S/C would blow fuses but a hum tells you at least some voltage is reaching the active devices .
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Old 4th August 2020, 08:12 PM   #4
junket is offline junket
diyAudio Member
Join Date: Aug 2020
Thanks for helping. How do I identify and test those caps?

Also, perhaps noteworthy, the two transistors marked seem hot for unit switched on, no signal, volume at zero. Temps of 125* & 140*F respectively
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Old 1st December 2020, 01:15 PM   #5
tubebob is offline tubebob  United States
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Join Date: Mar 2020
This happened to me several years ago, it was in the PS. Note I no longer have the pre-amp. That said, I had to replace some transistors too, as they break down the chain as the PS goes off. The service manual for this unit is readily available, and I used the parts and schematic to guide. I also recall that this unit has a separate headphone amp. You may want to check what part of the circuit those hot transistors participate and check those areas first.
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