Adcom GFA-585 Problems/Explosions

[ben62670]

I was refereed to this form from the Polk audio forum. I was having problems with my 585, noise, bias at 60mv, and running real hot at idle. I did notice the 220 caps were leaking, and nothing looked like it was on the board, so i replaced all of the lytic caps. When I powered up the amp there were a couple small explosions followed by smoke! I thought caps only blew up with an over voltage situation? voltage measured at the power suply caps is a solid 81.7. Any help would be gratefully appreciated.

Thanks Ben

[RetroAudio]

maybe check for lytics that were installed backwards? just comes to mind.

[ben62670]

O boy you mean the are sensative to polarity? Like LEDs. The long leg would be + or -

Thanks much Ben

[pinkmouse]

Yup.

The cases are usually very well marked with a stripe or similar indicating the negative pin, just hope that your PCBs are as well so you know the correct orientation.

[ashok]

Long leg is usually +

- ve leg ( short leg ) is marked on the body.
+ ve leg has no markings but is longer.
See picture

[ben62670]

OK they are polarized! The negative is the marked leg, and on the pcb there is a dot which would also signify the negative also?

Thanks much Guys, I was ready to send my amp out to a shop, and I really don't have the money to do that.

Thanks again Ben

[anatech]

Hi Ben,
Those boards need to be washed thoroughly. See other Adcom threads for details on this.

You might be further ahead to save up and send the amp into a really good shop . One that is known to be good and probably does not repair TV's and VCR's.

Once done properly, you will have a long lasting, very good amplifier.

-Chris

[ben62670]

Thanks Anatech I had recieved lots of advice about installing new caps to recitfy the original problem I had with my amp.
heres a copy of my original post at polk audio

"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 plays but it doesn't sound good. The bias voltage is up around 60mv. I am scared to turn it up because I don't want to blow any of my wonderful polks. "

The power suply is putting out a constant 81.7 volts.

Thanks again Ben

[anatech]

Hi Ben,
You need a good digital multimeter to set the bias currents. There is a procedure for this that involves dummy loads and an audio oscillator + time.

If the original caps leaked, or your new ones, the wash procedure can not be omitted. The shop must know this. An older Adcom warranty shop would be a good bet (I hope). Also, if a couple outputs blow, you need to replace the entire channel with matched sets. I don't wish to see that happen to you. Please take the safe road.

-Chris

[ben62670]

Yes I have read lots about washing the board. I do a lot of PC work, and I am good at soldering. I have 5 multi meters, and the Adcom service manual. So I don't see setting the bias as being a big deal. I really can't afford a shop to do it now, and I live in South Florida. It seems like a stupidity contest down here, and most people are tied for first place. It would cost about $100 round trip to send it somewhere I trust plus parts and labor. I can see, and appreciate your concern especially after I did something as dumb as installing some of the caps back wards. I am still kicking my self for that one.

Thanks much, I will reply with a progress report.

Ben