Adcom GFA-555 Mk1 channel noise

Dudeman258 · 7th April 2011, 06:54 AM

Howdy good folks,

Well I just received my first 555.

It is in good shape, and inside everything appears sound.

The DC at each channel is 48mv cold.

The right channel sounds fine, while the left has an audible higher pitched hum, evident when the amplifier is turned off even.

After powering on it noticeably increases in volume, and is unaffected by volume changes in input signal. The channel still puts out sound though.

As of now I have ordered the four electrolytic capacitors for the board in the hopes that is the problem. They did not appear to be leaking or very bulged, however they are the original yellow ECI version.

Any other ideas are welcomed.

Thank you.

Walker

lineup · 7th April 2011, 07:00 AM

Could be worn out potentiometer in volume control.
Changing electrolytic caps is good.

sakis · 9th April 2011, 11:01 AM

not really sure but i think the 555 features a Dc servo i think that many problems are caused by that poor stabilzation on the IC can cause this and other problems

kind regards sakis

Dudeman258 · 10th April 2011, 12:23 AM

Thanks for the help guys!

The mark 1 555 did not have the servo DC control, it used electrolytic caps. The very ones in the mail to my doorstep.

Hopefully this will be the fix. The gentlemen I bought it from said he had a shop check it and they said it "needed capacitors".

Maybe this was correct. I sure hope so, because everything else looks great.

I'll update Monday when I get those capacitors in.

Walker

Martyn · 10th April 2011, 04:01 PM

Hi Walker,

I don't know if your problem is the same as mine but I will share my experience anyway.
I had hum in the left channel of my Adcom 535II. I took it to an authorized Adcom service center who charged me $60 to tell me the hum was within spec. Within spec was not good enough for me so I fixed the hum myself. The shielded wire from left input RCA to amplifier PCB was right beside the power cord on the inside but the wire had no shield for the last inch on either end. I replaced the input wire leaving the shield on as close to the end as possible. Hum gone.
M

Dudeman258 · 15th April 2011, 07:51 PM

Martyn,

Thank you for your response!

is this similar to what your hum sounded like??

YouTube - video-2011-04-11-16-29-12 ADCOM 555 Test.mp4

Thanks!

Walker

Martyn · 16th April 2011, 06:12 PM

Hi Walker,

My hum was 60Hz. Your hum is as you described - much higher pitched. Our problems are different. Sorry.
M

7th April 2011, 06:54 AM	#1
Dudeman258 diyAudio Member Join Date: Mar 2011	Adcom GFA-555 Mk1 channel noise Howdy good folks, Well I just received my first 555. It is in good shape, and inside everything appears sound. The DC at each channel is 48mv cold. The right channel sounds fine, while the left has an audible higher pitched hum, evident when the amplifier is turned off even. After powering on it noticeably increases in volume, and is unaffected by volume changes in input signal. The channel still puts out sound though. As of now I have ordered the four electrolytic capacitors for the board in the hopes that is the problem. They did not appear to be leaking or very bulged, however they are the original yellow ECI version. Any other ideas are welcomed. Thank you. Walker

7th April 2011, 07:00 AM	#2
lineup diyAudio Member Join Date: Dec 2005 Location: the north	Could be worn out potentiometer in volume control. Changing electrolytic caps is good. __________________ lineup

9th April 2011, 11:01 AM	#3
sakis diyAudio Member Join Date: Jan 2006 Location: nea makri athens greece	not really sure but i think the 555 features a Dc servo i think that many problems are caused by that poor stabilzation on the IC can cause this and other problems kind regards sakis __________________ SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr

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10th April 2011, 12:23 AM	#4
Dudeman258 diyAudio Member Join Date: Mar 2011	Thanks for the help guys! The mark 1 555 did not have the servo DC control, it used electrolytic caps. The very ones in the mail to my doorstep. Hopefully this will be the fix. The gentlemen I bought it from said he had a shop check it and they said it "needed capacitors". Maybe this was correct. I sure hope so, because everything else looks great. I'll update Monday when I get those capacitors in. Walker

10th April 2011, 04:01 PM	#5
Martyn diyAudio Member Join Date: Sep 2001 Location: Ottawa, Canada	Hi Walker, I don't know if your problem is the same as mine but I will share my experience anyway. I had hum in the left channel of my Adcom 535II. I took it to an authorized Adcom service center who charged me $60 to tell me the hum was within spec. Within spec was not good enough for me so I fixed the hum myself. The shielded wire from left input RCA to amplifier PCB was right beside the power cord on the inside but the wire had no shield for the last inch on either end. I replaced the input wire leaving the shield on as close to the end as possible. Hum gone. M

15th April 2011, 07:51 PM	#6
Dudeman258 diyAudio Member Join Date: Mar 2011	Martyn, Thank you for your response! is this similar to what your hum sounded like?? YouTube - video-2011-04-11-16-29-12 ADCOM 555 Test.mp4 Thanks! Walker

16th April 2011, 06:12 PM	#7
Martyn diyAudio Member Join Date: Sep 2001 Location: Ottawa, Canada	Hi Walker, My hum was 60Hz. Your hum is as you described - much higher pitched. Our problems are different. Sorry. M

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