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Adcom GTP-350 PreAmp/Tuner with 60 HZ Hum

Garry777

Garry777

Active Member
I've had this Adcom GTP-350 solid state pre-amp for about 7 years and the 60Hz hum has gotten a little louder in the past year. When listening with headphones the hum is noticeable (both channels equaly) between songs and increasing the volume does NOT increase the volume of the hum. I opened the unit today and there are numerous electrolytic caps associated with the power supply and don't want to replace everything unless necessary. Does anyone know which cap(s) would likely be responsible for the Hum? I have downloaded two GTP-350 PDF schematics from online but can't read the component part numbers on either, even after enlarging the drawings. Does anyone have access to a readable schematic. Regards, Garry
 
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Let's see a pic of the power supply. It is usually two main capacitors in the power supply while any others are usually for smoothing. You might have to do all of them in the power supply. Can't be that many. :no:
 
There's a +5.5V, +15V and -15V dc power supply. BTW the caps are not leaking, that brown stuff on ckt brd is cement/glue from the factory. IMG_0609.JPG
 

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At minimum replace the 2 large caps and the one next to the heat sink, heat kills parts. Replace with 105 degree C caps.
 
IIRC, and maybe i don't so well, didn't some of the glue Adcom used, become conductive over time?

Didn't this become a significant issue in some of the amps as they aged?

I don't like the looks of the stuff that appears to have gotten a little darker than the rest. It's also contacting a resistor and some jumpers from what i can see.

I'd start by removing that gunk as best i could.
 
bowtie427ss said:
IIRC, and maybe i don't so well, didn't some of the glue Adcom used, become conductive over time?

Didn't this become a significant issue in some of the amps as they aged?

I don't like the looks of the stuff that appears to have gotten a little darker than the rest. It's also contacting a resistor and some jumpers from what i can see.

I'd start by removing that gunk as best i could.
Click to expand...

Thanks, I will look into the Adcom glue issue further. However, I checked the DC Volts and AC Volts on the +15v & -15v supplies and they look good. The ACV is 0.000ACV on my Fluke 77 DMM on both +15v and -15v. There are some wire wrap connections in the unit. Wondering if one of the wire wrap grounds has become resistive.
 
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