Adcom GFA-555II with DC Offset - diyAudio
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Old 16th February 2009, 09:25 AM   #1
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Default Adcom GFA-555II with DC Offset

Hi,
I have an Adcom GFA-555II that I picked up as a repairable. I'm a hobbyist and have been collecting some test equipment, so I have a scope, transistor testers, capacitor testers, variac and multimeters. I've read through most everything that I could find in this forum, but I'm a little stumped.

Initially, the amp had about 67V of offset in one channel and 45V in the other. I tested all of the transistors and the outputs seemed fine, but I did find the differential input pairs of both channels to have dramatically different gains, so I replaced them (after beta matching them). After that, the offset was around .5 - 1 volt. I then replaced the next stage of transistors and the offset now varies between +/- 20 - 100mv. Also, there is quite a bit of offset when the amp is first turned on. I have another identical working 555II that I'm using as a reference, and the offset is a rock solid 3-5mv. What really has me confused though is that when I hooked the amp up to a pair of test speakers, the woofers seem to be periodically sucked in and out when playing music loudly (maybe every couple of seconds), like the amp isn't doing a good job of controlling the woofer cone. I tested the amp with a sine wave and the wave form seems to look the same at the input and output. Also, I tested the 100 ohm input ground resistors, and they are good.

Any ideas what is causing the offset to vary, and the strange movement of the woofer? Also, this may be a stupid question, but what is amplifier oscillation, and how do you test and measure it?

Thanks!
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Old 16th February 2009, 05:22 PM   #2
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Try replacing the electrolytic caps in the feedback. I would suggest a bipolar type for a replacement.
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Old 16th February 2009, 07:28 PM   #3
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Default Electrolytics

Thanks Steve for the reply. I forgot to mention that I did replace all of the electrolytic caps on the driver board and the electrolytic caps on the output boards tested good.
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Old 17th February 2009, 12:46 AM   #4
CBRworm is offline CBRworm  United States
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I believe that on those amps offset should be controlled by IC opamps 101 and 151, labeled Adcom 3A, if everything is working properly. I don't remember the specifics but you should be able to search this site to find out how to tell if those IC's are working, and if not, how to source AD820s which I believe are a suitable replacement.

You might want to play the same music through your other amp to make sure it isn't a low frequency signal that is recorded being played back. Since you have a scope you could watch the input signal and the output signal during music for a huge wave of speaker pulling low frequency on the output that isn't on the input.

Not knowing very much about very much I would be suspicious of the driver transistors and the outputs themselves even though they tested good. I would also try to figure out if whatever is happening is happening to both channels or just one side. It is unlikely (IMO) that a problem on one of the amp modules would cause a problem on both sides, whereas a problem on the input card (power source?) could cause the same issue on both channels.

The suckout you describe does sound like what I would imagine would happen if the amp lost it's DC balance and the Adcom 3A tried to compensate by adjusting the offset in the opposite direction, then the balance was regained and the 3A adjusted back the other way. What would cause this? I don't know.
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Old 17th February 2009, 01:53 AM   #5
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Quote:
Originally posted by CBRworm
The suckout you describe does sound like what I would imagine would happen if the amp lost it's DC balance and the Adcom 3A tried to compensate by adjusting the offset in the opposite direction, then the balance was regained and the 3A adjusted back the other way. What would cause this? I don't know.
Thanks for the help CBRworm. That's a great way to describe what is going on. It sums it up perfectly. Also, both channels behave the same way, which is odd since they are isolated on the driver board. I'm not sure if both channels were subjected to something bad at the same time which caused the same problem for both or if the problem lies elsewhare. I'll hook the amp back up to the scope and play music and see if I can confirm the big wave that you were referring to, and I'll also recheck the transistors.
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Old 17th February 2009, 03:26 AM   #6
SQLGuy is offline SQLGuy  United States
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I have one of these as well, and had to repair it. I would suggest you check, and probably replace, all of the 2SC2912's and 2SA1210's.

They run very hot and have a tendency to short. Once they short, you get full or near-full DC at the affected channel.

IIRC, I got my replacements (original Sanyo) from MCM.

I suggest replacing all of them because, when I first repaired mine, I replaced just the one shorted 2SA1210. A week later one, in a different circuit position, shorted in the other channel. I replaced all of them and it's been running good, 24/7, for over a year now.

By the way, what "electroytic in the feedback"? I am looking at the schematics now, and feedback is coupled direct from the output, through R123 (22.1K), to Q102 of the differential pair. No coupling caps involved. The input coupling caps are film types.

Cheers,
Paul
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Old 17th February 2009, 04:45 PM   #7
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Quote:
Originally posted by SQLGuy


By the way, what "electroytic in the feedback"? I am looking at the schematics now, and feedback is coupled direct from the output, through R123 (22.1K), to Q102 of the differential pair. No coupling caps involved. The input coupling caps are film types.

Cheers,
Paul

My mistake. I was speaking of the 555. The 555II must be completely different. I am not familiar with this version.
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Old 17th February 2009, 09:41 PM   #8
marley is offline marley  United States
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I have an adcom 555mk2 that has been doing something similar. When powering up, the amp will have 7 and 9 volts on the L and R channel. This offset drops to .5mv with in 3-5 seconds and remains there. I also notice that when I do power it up, it sucks in the woofers on my kappa 7 speakers. I have contacted adcom, told them of my dc offset and they said that was normal behavior and that the amps were just stabalizing. I am the original owner of this amp and it has never been serviced.
Is this something the 555mk2 does over a period of time, should I have this looked at? Thanks, Brian
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Old 17th February 2009, 10:15 PM   #9
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Thanks everyone for your help on this one. I'm going to replace the driver transistors (waiting for the shipment to come in) and possibly the 3A op-amp to see if it makes a difference. I'll report back when I get the parts.

Marley, based on what I've read on this forum, I'd suggest inspecting and replacing all of the electrolytic caps on the driver board due to age and due to the know problem with the original caps leaking. If yours are already leaking, there is lots of documentation in other threads on how to clean it up (doesn't look fun). I have a second, working 555II and it does not suck in the woofers nearly as much as the one I'm working on.
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Old 17th February 2009, 10:19 PM   #10
CBRworm is offline CBRworm  United States
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I think it is normal though for the 555ii to suck in the woofers on power-up, but within a second or so the offset should settle to very close to zero.

I just scoped mine - at warm powerup the offset is -2.5 volts on each channel, rises up to +1.7 volts then settles to right around 0 within 1.5 seconds of power up.
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