Adcom GFA-555 - Need troubleshooting assistance.

randar · 8th February 2009, 11:56 PM

I'm trying to help a friend troubleshoot a problem with his GFA-555 (not MKII, the one with the flat faceplate and handles).

The amp powers on and you can hear music on both speakers, but the sound is very soft, even with the volume turned up to what would normally be very loud. The output also sounds like low end frequencies are being filtered out.

Is this a symptom that anyone has seen before with this amp? After reading through all of the postings on this forum relating to the 555, I didn't see anything with these symptoms.

We do have the service manual and the output voltage of the power supply corresponds with the voltage indicated on the schematic. The caps were replaced a couple of years ago for what it's worth.

What other components would cause both channels to have the same symptom. My first thought was the power supply, but that doesn't appear to be the case. Maybe something on the input stage?

I'm by no means an expert, but taking measurements and replacing components is well within my grasp, but troubleshooting components is a little over my head.

If you guys have any ideas, that would be grate!

Thanks!

Tony

cristal63 · 29th March 2009, 02:05 AM

Hello, I have the same problem with my adcom gfa 55 power amplifier.
I have buy used from my friend.
Have you solved this problem?
can you send me service manual ( I haven't!) ?
Let me know please. Kindly Roberto

djk · 29th March 2009, 05:27 AM

Check and see if the cap in the feedback loop to ground is open.

cristal63 · 29th March 2009, 12:23 PM

Hello, can you explain better please ?
Kindly Roberto

SY · 29th March 2009, 01:02 PM

If it's in both channels, it's unlikely to be electrolytic in the feedback loop.

Maybe S1 (the stereo/mono switch) has developed a problem? If you don't use the strapping (mono) option, you can just remove that switch.

cristal63 · 29th March 2009, 05:00 PM

Yes , soft sound in both channels and first model with the flat faceplate and handles ( no MK II)

I haven't never use the stereo/mono switch, but it's unlikely to be the switch mono options.
With service manual guide I can control voltage parameters.
There is a member that send me service manual please?
I will keep informad.
Kindly Roberto

SY · 29th March 2009, 05:03 PM

The schematic is quite available. With that, you should be able to trace things down pretty easily.

http://www.diyaudio.com/forums/attac...amp=1142458830

cristal63 · 29th March 2009, 08:09 PM

Thank you but I need manual for complete information of the power amplifier. My email is:

blobbulobis@inwind.it

Kindly Roberto

djk · 29th March 2009, 08:27 PM

You look at the board, you look at the schematic. What do you need that isn't in the schematic? Not that much to this circuit.

cristal63 · 29th March 2009, 10:00 PM

Ok, thank you.
Kindly Roberto

8th February 2009, 11:56 PM	#1
randar diyAudio Member Join Date: Jan 2009	Adcom GFA-555 - Need troubleshooting assistance. I'm trying to help a friend troubleshoot a problem with his GFA-555 (not MKII, the one with the flat faceplate and handles). The amp powers on and you can hear music on both speakers, but the sound is very soft, even with the volume turned up to what would normally be very loud. The output also sounds like low end frequencies are being filtered out. Is this a symptom that anyone has seen before with this amp? After reading through all of the postings on this forum relating to the 555, I didn't see anything with these symptoms. We do have the service manual and the output voltage of the power supply corresponds with the voltage indicated on the schematic. The caps were replaced a couple of years ago for what it's worth. What other components would cause both channels to have the same symptom. My first thought was the power supply, but that doesn't appear to be the case. Maybe something on the input stage? I'm by no means an expert, but taking measurements and replacing components is well within my grasp, but troubleshooting components is a little over my head. If you guys have any ideas, that would be grate! Thanks! Tony

29th March 2009, 02:05 AM	#2
cristal63 diyAudio Member Join Date: Mar 2009	adcom gfa 555 Hello, I have the same problem with my adcom gfa 55 power amplifier. I have buy used from my friend. Have you solved this problem? can you send me service manual ( I haven't!) ? Let me know please. Kindly Roberto

29th March 2009, 05:27 AM	#3
djk diyAudio Member Join Date: Feb 2001 Location: USA	Check and see if the cap in the feedback loop to ground is open. __________________ Candidates for the Darwin Award should not read this author.

29th March 2009, 01:02 PM	#5
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	If it's in both channels, it's unlikely to be electrolytic in the feedback loop. Maybe S1 (the stereo/mono switch) has developed a problem? If you don't use the strapping (mono) option, you can just remove that switch. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

29th March 2009, 05:03 PM	#7
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	The schematic is quite available. With that, you should be able to trace things down pretty easily. http://www.diyaudio.com/forums/attac...amp=1142458830 __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache


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29th March 2009, 12:23 PM	#4
cristal63 diyAudio Member Join Date: Mar 2009	Hello, can you explain better please ? Kindly Roberto

29th March 2009, 05:00 PM	#6
cristal63 diyAudio Member Join Date: Mar 2009	Yes , soft sound in both channels and first model with the flat faceplate and handles ( no MK II) I haven't never use the stereo/mono switch, but it's unlikely to be the switch mono options. With service manual guide I can control voltage parameters. There is a member that send me service manual please? I will keep informad. Kindly Roberto

29th March 2009, 08:09 PM	#8
cristal63 diyAudio Member Join Date: Mar 2009	Thank you but I need manual for complete information of the power amplifier. My email is: blobbulobis@inwind.it Kindly Roberto

29th March 2009, 08:27 PM	#9
djk diyAudio Member Join Date: Feb 2001 Location: USA	You look at the board, you look at the schematic. What do you need that isn't in the schematic? Not that much to this circuit. __________________ Candidates for the Darwin Award should not read this author.

29th March 2009, 10:00 PM	#10
cristal63 diyAudio Member Join Date: Mar 2009	Ok, thank you. Kindly Roberto

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