Adcom gfa-555II problem/question - diyAudio
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Old 1st January 2010, 08:13 PM   #1
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Default Adcom gfa-555II problem/question

First I'd like to thank you for taking the time to read my post. I became a member here today because my Adcom gfa-555II has recently broke and I need advice as to what I should do with it.
I have enjoyed this amp for years and it always sounded great. A couple years ago the left channel seemed to kinda fade. It worked but the sound was very low. About that time I moved and the amp has been in storage untill recently. When I hooked it up this time I heard a pop come from the left speaker and then nothing. The left channel is now dead. I could not get one of the fuses out of the back of the amp. I guess it was cross threaded. Since I have no knowledge of electronics I took it to an audio store near me. They specialize in high end equipment and have their own tech/repair business. I recieved a call yesterday telling me that the fuse that was "cross threaded" was a 20amp fuse and that it was a big problem. They told me that the amp needed a new fuse holder installed (forgive me as I have no idea of the proper terms) and a new power supply. Just for starters. There could me more problems once the power supply is installed. I was quoted $350-$450 for the power supply and fuse holder.

I have no idea if I'm getting ripped off or if this is reasonible. I'm torn as to wether or not I should fix this amp that I love or try to buy another one. After reading a lot of posts on here it sounds impossible that nothing else could be wrong. Would the power supply make the left channel play at a lower level than the right? And then finally blow? I did take the top off the amp before I took it in and everything looked fine. No burn marks on the board, no leaks any where and the big blue capacitors looked fine. I know you can't visually diagnose the problem. I guess I feel like I might be getting ripped off and I'm not sure what questions to ask the tech. What should I be asking? Is it impossible to check the other parts before putting in a new power supply?


I hope I don't sound to scrambled here but honestly this stuff is way over my head. The price they quoted me just seems high considering that there could be other problems. I could sure use some advice.


Thanks,

Brian
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Old 1st January 2010, 08:59 PM   #2
SQLGuy is offline SQLGuy  United States
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A new "power supply" sounds unlikely for that amp. The power supply is an 800VA potted toroidal transformer, a pair of pretty heavy duty bridge rectifiers, and four large filter capacitors. The total cost of those would be be more than $350 installed, and they're also not likely to go bad suddenly. It's possible that the filter caps are getting a bit old and dry, but I think it's more likely that the shop is not presenting an accurate story of the issues with the amp.

What's common for these amps is:

1) Bad transistors on the input board (mainly the 2SA1210's and 2SC2912's, which overheat and die).

2) Bad fuse holders (as you've seen).

I replaced all the fuse holders on mine, which is a bid tedious as there's not much room to work easily. And I replaced all the 2SA1210's and 2SC2912's. The amp's been working fine for a couple of years since I did this work.

For a shop to do the work I did on my amp, I'd expect about $150 - $200 parts and labor.

At the least, I'd get a second opinion on what your shop is telling you, and more specifics about what "power supply" parts they are suggesting to replace.
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Old 1st January 2010, 10:05 PM   #3
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Thank you for the info SQLGuy. I do have a couple questions in regards to your post. Are the filter caps you speak of that could be getting old and dry the big blue capacitors? Those looked fine to me. No swelling, no leaks. What are the transistors that heat up and die responsible for? Is it possible that they were the reason for low output in the left channel before it blew? And lastly, what could go wrong with having a 20 amp fuse in one of the fuse holders below the speaker output? Isn't that there to protect how many amps are going to my speakers? Would it really effect the electronics inside the amp like they are claiming?


For some reason I feel like I'm being "taken" because I have zero knowledge about amps and they know it. I'm trying to educate myself a little here in order to make the right decision and avoid getting ripped off. I really apreciate you taking the time to reply to my post.

Thanks again,

Brian
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Old 1st January 2010, 11:13 PM   #4
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just joined here and am having the same exact problem with my right channel .....sounds like somebodys stuffed a sock in my speaker....very low and muffled ...any ideas where to start ??
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Old 2nd January 2010, 12:52 AM   #5
tvrgeek is offline tvrgeek  United States
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Cheaper, get another off e-bay. But any older equipment has the high probability that all the electrolytics have dried out, not just the main power supply. Unfortunately, with typical temperatures, about 7 to 10 years is life. That was a very good amp in its day, noted for solid bottom end. You may find current amps even better.
Ripped off? no. If you add up what it takes to have someone just take a unit in, do the paperwork , let alone fix anything, you will find out why we are now in a disposable society. Liability insurance, helath insurance, business documentation.
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Old 2nd January 2010, 01:16 AM   #6
SQLGuy is offline SQLGuy  United States
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Yes, the filter caps are the big blue Coke cans. I'd put their typical lifespan at more like 20 years. There are some smaller electrolytics on the input and output boards (not in the signal path); these are more likely to see the 7 year lifespan that tvrgeek mentioned. Come to think of it, I replaced a couple of those as well when I repaired my amp.

Those transistors are part of the voltage amplification section of the amp. The main indication that they've failed is (+ or -) 80V DC on one of the output channels. I don't know whether low output would also be a symptom.

The fuses are to limit how much current the output transistors can draw. This does indirectly limit how much goes to the speakers, but it's unlikely you would blow them during normal operation unless running very hard into a fairly low impedance load. They are supposed to be 7A, not 20. However, if the amp is driven into a short, even the correct 7A fuses may not blow until after the output transistors do.

You still didn't say whether there were any interesting cirumstances surrounding the failure of the amp.

Paul
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Old 2nd January 2010, 05:08 PM   #7
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I really can't think of any circumstances that would contribute to the failure of this amp other than I play it LOUD. It pushed my Infinity RSII speakers perfectly for years. I have an Adcom GTP-500II tuner I use because it has phono capabilites and I play a lot of vinyl. For awhile I had my Counterpoint SA1000 hooked upto it aswell. Never had any problems at all. None. Other than playing my system loud I never abused my amp. I always left it on though. Maybe I should have turned it off everday. Never thought about that till now.

Brian
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Old 6th January 2010, 06:19 PM   #8
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bump for any more info
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Old 6th January 2010, 07:05 PM   #9
SQLGuy is offline SQLGuy  United States
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Quote:
Originally Posted by nodoubt45 View Post
bump for any more info

What have you already checked that's mentioned in this thread? Have you checked the 2SA1210's and 2SC2912's? What was the result?
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Old 6th January 2010, 11:41 PM   #10
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Default I fixed it

I have the same amp and I had some problems that I was able to fix myself. The 555 uses a 10amp mains fuse, not a 20 amp. Changing a fuse holder is actually pretty easy on that amp. The problem though though could be one of many items, but for starters check the voltages across the filter caps, should be 74.5 volts. Check the bias voltage also on the driver boards. Those should be 1.2 volts and 0.6 volts respectively. There are fuses on EACH of the drive boards that need to be checked as well, they could have blown if you overdrove the amp. There are also thermal cutout switches mounted to the heat sinks that may have gone bad.
My amp had a problem with the resistors that are on the small boards sitting on the filter caps. One of them overheated and partialy burned out, but instead of going high resistance, they went low. They are 2w 3.9k resistors that bleed off the filter cap voltages, but if the resistance goes low, the main voltage to the output transistors goes under spec and the amp starts sounding bad, the + voltage and the - voltage arent equal then, causing waveform distortion.

That is a very simple amp to fix, once you open it up theres almost all empty space in there. Compared to a Yamaha M2 which is just the opposite, crammed full of circuit boards.

Do you have any test equipment yourself to conduct some simple voltage tests? First step is to check the power supply volages at the caps. Then the drive boards. The schematic for the 555 is a very simple one, easy to trace where stuff went wrong.- As someone else said here, its unlikely the power transformer went out, but then again if someone overfused the amp, it IS possible to destroy a transformer.
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