Adcom GFA-555 Issue

Phish56466

Gents,

I'm currently using an Adcom GFA-555 to power my M12's. It puked DC on the outputs this morning and smoked my RDO's. I need someone to PLEASE point me to the thread about re-capping these(if one exists).

billbillw

Sorry to tell you but DC on the output is usually more than just re-capping. If you are not amp savvy, you probably need to take it in for service. Recapping usually just gets rid of hum and/or low level noise.

Glen B

Sounds like your Adcom suffered an output stage failure. That's the problem with such amps that have no output relay. When output transistors fail, they take your speaker with it. When the problem is fixed, you should look into having DC protection installed. I've done so with my Phase Linear 400 Series 2 amp. Any DC of 500mV or more causes the output relays to open.

Phish56466

I'm fairly fluent in electronics. From what I've been reading(since starting this thread), there is an electrolytic cap in the feedback
loop prone to puking. I've actually been a CP member for a while(forget what my other profile was). I've got the service manual for
this Adcom, so it's just a matter of money and time at this point. Thanks for the responses guys!

PreCd

The 1st generation 555s were not prone to leaking caps. There are only 4 electrolytic caps on the PS board that exist so if you want to replace those it will not be a big deal. Two should be 4.7uf 63v and the 2 others 47uf 100v. If you are fluent in electronics then you must know that the big caps can knock the piss out of you. Caution should be used when entering the inside of the amp.

As someone stated above the problem is more than likely not a cap. If it has leaked then you are in for a long ride. The proper way to clean electrolytic off the board is by using a vibrating cleaner. That entails removing various components from the board before submerging it in cleaning solution. If you have high DC offset then more than likely it is something else like a transistor failure.

Phish56466

Yeah, I'm aware the main rail caps pack a punch(more than I'd like to get bit by). I've read in several places there is a cap on the input PCB prone to doing this. I have yet to open it up and see what's up though! Here's how it went down :I was watching TV, and the power went out. It did this with the Adcom under load. My M12's made a GAWD AWFUL noise and the top end was GONE! I'm guessing that when the power went out, the finals went into oscillation due to this cap going south. Going to order more RDO's Friday and hopefully have a fix on the Adcom shortly there after.

leftwinger57

I also have a 555 and in the manual it does not directly say it does not have output relays for speaker amp protection. What it says is it has "internal and external fuses to protect your loudspeakers" thermal protection for the amp if it gets to hot from pushing it to hard. Now I did read from either another Adcom manual or audio magazine review that it does not have output relays because they wanted a very smooth instant on and no thump and that's exactly what I get and adding the relays in their thinking was not the cleanest way to get that quick signal out. This caught my attention because there were many threads here of amps thumping on power up and making sounds on spooling down. There is a warning to techs ''burnout of the output stage because of failing to observe the following procedures would void your warranty". So even after the fact they warn you.. I agree if you checked all your fuses which clearly would be the cheapest fix then it might be alot more than the caps which is more than likely a send out to tech proposition. Oh guys, you'll get a kick out of this, they recommend using 16gu. lamp cord as your speaker cable for short to moderate runs and then go down to 14 gu. for 50' or so. I guess they didn't want to get into the great boutique spkr cable debate either or at least play favorites.

PreCd

What does your DC offset measure at the speaker out on your 555?

Phish56466 wrote: »

Yeah, I'm aware the main rail caps pack a punch(more than I'd like to get bit by). I've read in several places there is a cap on the input PCB prone to doing this. I have yet to open it up and see what's up though! Here's how it went down :I was watching TV, and the power went out. It did this with the Adcom under load. My M12's made a GAWD AWFUL noise and the top end was GONE! I'm guessing that when the power went out, the finals went into oscillation due to this cap going south. Going to order more RDO's Friday and hopefully have a fix on the Adcom shortly there after.

Phish56466

Haven't made it that far to be honest gents(measuring DC offset). It still plays(just sounds like total crap due to no tweeters). It's not made any odd noises powering up or down to my knowledge(the wife hasn't said anything either).

soundfreak1

Before u spend any money on that amp ck to be sure exactly which componets are bad, as some of the ic's cannot be replaced (propriety parts were used and can not be found or replaced) just telling you what ive been told on that early modle.

PreCd

1st step. Check DC offset. If it looks good then the power going off under load may have been the culprit. I hope so.

Phish56466 wrote: »

Haven't made it that far to be honest gents(measuring DC offset). It still plays(just sounds like total crap due to no tweeters). It's not made any odd noises powering up or down to my knowledge(the wife hasn't said anything either).

leftwinger57

Here's an ad I found that does go into some detail and does talk a little about output. I do not think this is the one that firmly states they do not have out put relays for dc protection but might gain something from this.

http://www.stereophile.com/solidpoweramps/678/

Phish56466

NONE of the 555 scats I've seen have ANY output protection relay or anything other. The only thing I've seen so far are fuses for the actual final stage off the rails. I'm opening this one up this weekend, so I'll know more soon(as will all of you).

tonyb

I would keep in mind the current value of that amp, around 300 bucks, as to your decisions of how much you want to spend fixing or diagnosing it. A tech will most likely charge at least 60-100 bucks to just look at it.

Monster Jam

If my 555 takes my LSI F/X speakers, I will be very upset.

deronb1

tonyb wrote: »

I would keep in mind the current value of that amp, around 300 bucks, as to your decisions of how much you want to spend fixing or diagnosing it. A tech will most likely charge at least 60-100 bucks to just look at it.

Was thinking the same thing. You could list it as "for repair" and see what happens

Phish56466

OK, cracked it open and right off I noticed the bleed resistors and bypass caps had been removed from the main rail caps. It has the PCB's in place and the solder looks like crap! I did the the offset test. NO INPUT, NO LOAD right channel was -.026Vdc, left was -.039. Same when loaded with my M12's. WITH the input in place right was -.011 and the left was -.019. Same loaded or unloaded. Shouldn't these numbers be positive or am I looking for consistency? I did observe polarity and I'm using a Fluke 87V meter.

PreCd

The numbers you quoted are ok. Do you have a Gen 1 555 or a 555 II?

Phish56466

Gen 1. SUPPOSEDLY has MKII input mods done. I'm finding a LOT of ****ty work and questionable stuff in here! It's actually an amp from a friend that's letting me audition it(he has another one with the consecutive serial number). He said it popped fuses once and he sent it to a 'friend' to repair it. It's actually playing right now. My M12's sound like CRAPOLA, trying to get a little heat in the sinks for further testing.

PreCd

Good luck. Frankenamps are creatures of different kind.

Phish56466

I've got service manuals for both, so it shouldn't be a big deal. Sounded wonderful before the tweeters went south. A LOT warmer than I thought it'd be(had an M-501 that was a bit harsh when pushed, but it wouldn't drop current for crap). I'll get it lined out. Just wished it'd have been bone stock.

Phish56466

Any first(OR second) gen owners that have been inside have ANY point to point stuff done on the input PCB(it's NOT factory-and on the bottom of the board)? I noticed it earlier when I had it opened up, thought it kind of strange. There appears to be active silicon here, but I'll have to pull the board and remap it to figure out what's going on. AND the wife FINALLY tells me she's heard 'strange' noises when powering down(SHEESIMUS)!

Monster Jam

tonyb wrote: »

I would keep in mind the current value of that amp, around 300 bucks, as to your decisions of how much you want to spend fixing or diagnosing it. A tech will most likely charge at least 60-100 bucks to just look at it.

My local authorized Adcom repair shop charges a flat $75 for repair without knowing what the problem is - even in the event there is NO problem. Had my 555 serviced this way, and they also repaired my Sunfire for a reasonable amount as well.

Doesn't hurt to ask what the rate is.

Phish56466

So there is an unusual twist. I replaced the bleed resistors and bypass caps in the power supply today. The tweeters came back to life! I CANNOT EXPLAIN IT EITHER! This should NOT have had that much effect. I've got another set of RDO's coming anyway. I also noticed the mods on the input PCB. It's a first gen amp with second gen mods done. It's a first gen with a triple Darlington configuration(explains the tight mid range). I'm STILL baffled about the tweeter thing though. Any thoughts?

CinderLyon

i know this is a very old thread. Just hoping someone can enlighten me.

I have a Adcom gfa 555 version one amp. Pretty sure I bought it 25 years ago.

I just noticed that the big coil in the center is making a low level humming sound, that cycles from not hearable to just hearable in a quiet room.

looking inside every thing looks clean. It still sounds great. Only humming only bothers me when the room is quiet.

Should this be fixed? Can this be fixed? Any idea on the cost? Do you recommend i just sell it and move on?

I need to update my signature. I now am using a denon 3600 AVR.

invalid

If the hum is just coming from the amp and not through the speakers, it might, I mean might just be the transformer bolt has gotten a little loose. Also if there is a small amount of DC on the AC line this could also cause this.

CinderLyon

I tightened down the bolt. I seemed to lower the volume a little but, but still humming.

I tried plugging in the amp into several different sockets in the house, same resulting sound.

Primarily I have it plugged into a belkin power conditioner. That is full up, all sockets being used.

The buzzing sound might have been there for a very long time, I just did not notice it till I built a new high end tower PC. The PC is now dead quiet.

My Adcom seems to run just fine. Just noticeable low level audible hum in a quiet room.

What can i do to test for DC?