Adcom GFA-555 Left Channel Buzz

Dudeman258 · 11th April 2011, 08:54 PM

Hello good folks.

Well I am currently looking for some help with a 555 I purchased recently.

It powers up alright on the DBT, the bulb dims to nearly off.

DC offset is reading 49mv right channel, 46left channel. Turning the adjustment pots is very finicky, and I really cannot get it much better (this is on through the DBT).

With NO inputs connected, left channel has a significant buzz (like a very loud bee).

Right channel is fine.

Both channels play music.

All fuses look good. Everything looks in good shape, nothing is burned. I have replaced the electrolytic capacitors on the board with exact replacements no change. I also changed the driver transistors with high capacity ones (suggested). No change after that alteration.

I am uploading a video of the symptoms, should be on here in a little while.

Any help would be great. I called the adcom support and the guy was completely no help at all....

Thank you.

Walker

Nelson Pass · 11th April 2011, 08:59 PM

Make up a pair of RCA connectors as shorting plugs (not connected to
anything else) and put them in the inputs. What noise do you get?

Zen Mod · 11th April 2011, 09:04 PM

one thing more - is it MkII , or "plain" one ?

Dudeman258 · 11th April 2011, 09:26 PM

It is a "plain" one. Funny when I told the Adcom guy it was a "Mark 1" he said "those don't exist".

At any rate, when the shorted RCA plug is inserted there is the typical scratch/small thump, however the noise is overall unchanged. Perhaps it shifts a few Hz.

Walker

Zen Mod · 11th April 2011, 09:34 PM

please - post some pictures

I don't know it's physical layout , is reg common for both channel front ends ,etc.

Dudeman258 · 12th April 2011, 01:51 AM

Walker

Dudeman258 · 06:44 AM

Ok good people, here is the video I made today.

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

Please excuse the videography, I was using my phone.

Walker

Dudeman258 · 15th April 2011, 03:54 PM

Last plea bump.

Walker

Dudeman258 · 2nd May 2011, 05:40 PM

I switched the main power caps from the right side to the left. Yes I maintained the correct polarity, and started it up on the DBT.

Now in stereo both channels hum VERY loudly, and I am getting 140mv DC at each output.

When switched to bridged only the right channel makes noise (also very loud), and the left is silent.

Someone tell me to take this to a shop please.

Walker

PreCD · 6th October 2011, 01:16 AM

Quote:

Originally Posted by Nelson Pass

Make up a pair of RCA connectors as shorting plugs (not connected to
anything else) and put them in the inputs. What noise do you get?

Nelson,

Does that apply to the 1 Gen 555s. Mine is below:

11th April 2011, 08:54 PM	#1
Dudeman258 diyAudio Member Join Date: Mar 2011	Adcom GFA-555 Left Channel Buzz Hello good folks. Well I am currently looking for some help with a 555 I purchased recently. It powers up alright on the DBT, the bulb dims to nearly off. DC offset is reading 49mv right channel, 46left channel. Turning the adjustment pots is very finicky, and I really cannot get it much better (this is on through the DBT). With NO inputs connected, left channel has a significant buzz (like a very loud bee). Right channel is fine. Both channels play music. All fuses look good. Everything looks in good shape, nothing is burned. I have replaced the electrolytic capacitors on the board with exact replacements no change. I also changed the driver transistors with high capacity ones (suggested). No change after that alteration. I am uploading a video of the symptoms, should be on here in a little while. Any help would be great. I called the adcom support and the guy was completely no help at all.... Thank you. Walker

11th April 2011, 09:04 PM	#3
Zen Mod diyAudio Member Join Date: Jan 2003 Location: ancient Batsch , behind Iron Curtain	one thing more - is it MkII , or "plain" one ? __________________ my Papa is smarter than your Nelson ! tnx to clean thread ; Cook Book ; PSM LS Cook Book ; Baby Diyaudio FORUM ; Mighty ZM's Bloggg;I'm dumb

12th April 2011, 06:42 AM	#7
Dudeman258 diyAudio Member Join Date: Mar 2011	Ok good people, here is the video I made today. YouTube - video-2011-04-11-16-29-12 ADCOM 555 Test.mp4 Please excuse the videography, I was using my phone. Walker Last edited by Dudeman258; 12th April 2011 at 06:44 AM.

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11th April 2011, 08:59 PM	#2
Nelson Pass The one and only Join Date: Mar 2001	Make up a pair of RCA connectors as shorting plugs (not connected to anything else) and put them in the inputs. What noise do you get?

11th April 2011, 09:26 PM	#4
Dudeman258 diyAudio Member Join Date: Mar 2011	It is a "plain" one. Funny when I told the Adcom guy it was a "Mark 1" he said "those don't exist". At any rate, when the shorted RCA plug is inserted there is the typical scratch/small thump, however the noise is overall unchanged. Perhaps it shifts a few Hz. Walker

12th April 2011, 01:51 AM	#6
Dudeman258 diyAudio Member Join Date: Mar 2011	Walker

15th April 2011, 03:54 PM	#8
Dudeman258 diyAudio Member Join Date: Mar 2011	Last plea bump. Walker

2nd May 2011, 05:40 PM	#9
Dudeman258 diyAudio Member Join Date: Mar 2011	I switched the main power caps from the right side to the left. Yes I maintained the correct polarity, and started it up on the DBT. Now in stereo both channels hum VERY loudly, and I am getting 140mv DC at each output. When switched to bridged only the right channel makes noise (also very loud), and the left is silent. Someone tell me to take this to a shop please. Walker

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