Adcom GFA-585 problems

ben62670

I have an Adcom GFA-585, and when the unit is on all by itself with no cables connected it runs pretty hot. You don't want to leave your hand on it. If you plug in the patch cables to a source it has a hum to it. I have tried multiple sources, and patch cables. Even if you don't have the preamp on, and it is still connected it still has the hum. If you play something through it the music still sounds good. I am scared to turn it up because I don't want to blow any of my wonderful polks. Anyone have experience with an amp with similar problems?
Thanks Ben

Systems

May I ask a few questions?
Does it have good ventilation or is it in a rack that restricts air flow?

Do both channels heat sinks get equally hot?

Is the hum noticeable in both channels?Did it just start to happen or is this amp new to you?

ben62670

The amp was an ebay purchase. Both channels are hot. Right more than left. Wide open for ventalation. I was testing before installing, and the hum is in both channels.

danger boy

Adcom amps run hot. don't worry about it. mine does too. very hot.

as for the hum. that's not normal. get it looked at.

DarqueKnight

Here are some things to consider:

1. You may have a ground loop problem between the amp and some other component on the electrical circuit. Try using a "cheater" plug (three prong to two prong) adapter and see if the hum goes away.

2. Since you say you only hear the hum when the preamp is connected to the amp, there may be a ground loop between the preamp and power amp or between the power amp and some other component connected to the preamp. Connecting a grounding wire between the chassis of the preamp and the chassis of the amp may diminish or get rid of the hum.

3. Is the Adcom installed in a system that also has a cable or satellite box on the same circuit? These can induce hum. A coaxial isolation filter or cheater plug may solve the problem.

4. Are the input cables to the GFA-585 in close proximity to other component power cables? If so, the other power cables could induce currents in the Adcom's input cables which could cause hum.

5. I have no personal experience with the GFA-585. I do know that it is the stereo version of the GFA-565 mono amps, which I used to own. My GFA-565 would only run hot when connected to low impedance (4 ohms or below) speakers. The GFA-565 ran cool to slightly warm when idling or when driving 8 ohm speakers at moderate or low volumes.

Since the GFA-585 is based on the same circuitry as the GFA-565, I would think they would have similar thermal characteristics.

If your GFA-585 is running hot while idling, the bias voltage may be set too high, causing the amp to run in class A or in close to class A operation, which generates a lot of heat. The bias voltage can drift over time and may need to be adjusted every few years. The previous owner may have set the bias voltage higher on purpose. Setting a higher bias voltage can result in smoother sound, but can shorten the life of the transistors if they are not designed for "always on" operation. This is particularly true of a high power amp such as the GFA-585.

Systems

All great advice DK. The bias may need some adjustment if it is running excessively hot.It does sound like he has a classic ground loop issue.

ben62670

I appreciate the advice. I have not cheated the ground yet. It also runs hot with no speakers or source hooked up, just the power cable. The hum comes from both channels when either is hooked up, thats what gets me stumped. I think I need to bring it to a shop... The bias has been what I have heard from techs from Adcom. I hope there isn't any real damage to the unit.
Thanks Much

Ben

unc2701

The 585 just runs hot. Too much amp in too small of a box. I had one serviced for some other issues & asked about that & they said it was pretty normal- that's why the 565's are spec'd for 2ohm, but the 585's aren't.

Ground loop- it'll start for both channels as soon as one IC is hooked up. cheater plug or you can bring it down a bit by grounding the chassis of your pre if it isn't already.

ben62670

Thanks unc I cheated the ground with no results, and I tried to ajust the bias. It calls for 24mv and I can't get it lower than 30. After it warms up it raises to 60mv...

Thanks again Ben

Systems

ben62670 wrote:

It calls for 24mv and I can get it lower than 30. After it warms up it raises to 60mv...

Thanks again Ben

Thats why they run hot.:) So you can't even get it below 30mv?Is that true for both channels?
Sounds like the bias circuit does'nt thermally track very well.:rolleyes:

ben62670

yes both channels get hot.. too hot to hold my hand on with patch cables connected, and the preamp off. I hope it doesn't cost much to fix.
Thanks for checking back, Ben

venomclan

I had the exact same problem with my amp and pre amp. The speakers would hum when they were connected by RCA. When I used balanced connections the hum went away. An electrician came over and said my outlets were not grounded properly. Problem solved.

Check your house wiring. I now use a Belkin power conditioner to help.
Venom

ben62670

the balenced cable basically groun the pre to the amp correct?

venomclan

ben62670 wrote:

the balenced cable basically groun the pre to the amp correct?

I am not sure if they ground each other, usually the balanced cable has more rf shielding but maybe they did ground the amp and the pre.

ben62670

should i try to ground the amp to the pre or am i asking for trouble?

unc2701

A balanced cable usually rejects ground loop since the offset against the positive and negative will be the same (but in opposite directions)...

As for grounding the amp to the pre you will be 100% fine as long as they're UL listed and probably fine anyway. With very old appliances you could get in trouble, but I really doubt that's your situation.

ben62670

Thanks for the info I will try it later and check my bias voltage. I not shure if that would effect the bias, but I will try it anyway.
Thanks Again Ben

ben62670

I tore my adcom a part. I was looking around for any signs of an over heat situation, and I found 6 caps that had a thin black tar like looking substance on the bottom of them. They are all 25v 220uf caps. One of them had noticeable corrosion on one of the legs. When I unsoldered them they made a sizzle sound that I have never heard in my life before, and as I am typing my fingers are progressively increasing in with a burning sensation. I think I will stop typing now and go wash what ever kind of acid this is off of my fingers.

Do you think I may have found the problem?
Ben

billbillw

Sounds like those caps were leaking and that most likely a major component of your problem. Replace them, readjust bias. Also check and adjust the DC Offset on your output (there is a lengthy discussion on this over at AudioKarma in the vintage electronics subforum.)

ben62670

Thanks so much Bill. I am on my way out to get som caps. While I'm in there I am just going to replace all the electro caps I can get at. I figured out how to ajust the bias but not the dc offset yet. I am jugling kids today so I don't know when I can get them done.
Thanks again, Ben

billbillw

Not every amp has adjustable DC Offset. Check it to see if there is a problem. If there is, then worry about how to fix it.

ben62670

Replaced the caps, and some of them exploded. Time for a pro!

ben62670

Ahhhhh caps are polarized. I installed some reverse polarity. Oops I am kicking myself now.

WilliamM2

Sorry to hear that. You may have caused other damage now.

ben62670

I hope not! I will replace the caps with new ones today. Hopefully things test out OK. I will also hook them up to a dummy load first. Then run it on cheap speakers for a while, and then hopefully things work out back to my Polk's.

Thanks Ben

Are NTE caps any good?

heiney9

If you are asking what caps are good you are already in over your head. Yes caps are polarized again if you didn't know this you are in over your head. The GFA 585 is a very nice amplifier and if you 'eff it up it's going to cost some money to get it right again. Plus that Adcom can put out enough current to arc weld so.................BE CAREFUL messing around with it.

We all like to get our learn on but an Adcom GFA-585 is too nice and rare of a piece to just start swapping things in and out trying to hunt down a problem. Plus if you have no idea about how/where to set DC offset you really need a pro to handle it.

The 585 and 565's were a different design than the 535, 545, and 555. They are a bit more complicated. As I stated in your other thread check DIYAUDIO.COM there have been some threads over there and especially pay attention to either ANATECH or K-AMPS posts/discussions as they are the Adcom guru's over there and have much correct information.

If you've messed the amp up it's a damn shame because those are limited and quite nice amps and get ready to pony up some cash to get them fixed properly by someone who knows what they are doing, not your typical TV/VCR repair shop.

Good luck

H9

heiney9

I'm serious

www.diyaudio.com

There are many on there that will be more than happy to give you correct, solid information if you still insist on trying to do the repairs/upgrades yourself. Sign up & Check it out . Do you have the service manual for your amp?

danger boy

this is something i would never attempt.. but that's just me. i'd leave it up to someone who knows what they're doing. as much fun as DIY is.. it's to expensive to fix if you screw it up.

heiney9

Good call ben, I see you've posted at DIYAUDIO and are getting some solid help. If I were going to send my Adcom for repair I wouldn't hesitate to send it to Anatech (Chris) but he lives in Ontario and the shipping would be a deal breaker.

http://www.diyaudio.com/forums/showthread.php?threadid=90059&perpage=10&pagenumber=4

If you haven't looked at this thread yet, take a look. On page 4 Walt Jung posted some very good info. FYI, Walt Jung and Victor Campos are very well known amp designers and were repsonsible for the most part of the design of the Adcom 565-mono's and 585. One is getting advice straight from the horses mouth and that my friend is very cool :cool: :cool: .

Mr. Nelson Pass also has commented in the past on some Adcom and Nakamichi designs he's responsible for in addition ot his Pass labs forum. DIYAUDIO is one of the better forums around and most of those guys know what they are talking about.

Again good luck with you repair

H9

ben62670

I have been reading a lot of forums at DIY AUDIO and Chris has been a great help. I have the unit apart now and I am thoroughly cleaning the boards to get rid of the electrolyte that has leaked. I do have the service manual, and I have repaired a lot of laptops, and PCs. I know this is different. I do have the wires documented as to their proper where abouts. Things are going well. When I do Finnish assembling it I will hook it up to my SDA2's because they can handle the most power out of my speaker collection. I'm just kidding, and I only said that because I know you guys can't slap, and sterilize me so I don't reproduce via the INTERNET. On a serious note I will be very cautious when I power it up the first time. Then I will power it up with the 10 ohm 25watt resisters I just bought, and then a pair of cheap jvc bookshelf speakers.
I was an electrician so I do appreciate the concern of charged caps that are holding 81.7 volts (ouch). I will keep you posted. Thanks much for your concern.

Sparky

And Kids if you are reading this DON'T TRY THIS AT HOME, OR ANYWHERE ELSE FOR THAT MATTER!

heiney9

Sounds like you have some working knowledge and now that you've made contact at DIYAUDIO if you get into a situation they might be able to help. Anatech is a former Adcom/Nak/Carver, etc. warranty tech and he is top notch in giving factory correct help.

Something I just recently learned is how well Simple Green works for cleaning PCB's. As you probably know cleaning the PCB's is perhaps one of the most important steps. I had forgotten that one of the weaknesses in the 565 and 585 were those Elna caps leaking. My brother has a 585 and it might be time to look inside and see if he's having any leakage issues.

With the repair manual, your knowledge and the offerings from Anatech, Walt Jung and others on DIY you might end up with a like new amp .