CurtPalme.com Home Theater Forum

rabies_70 · Posted: Fri Dec 26, 2008 8:18 am Post subject: Will ray electrocute himself?

Anybody want to have some fun and see if they can keep me from electrocuting myself??? Ive got a couple of amps I would like to trouble shoot and see if I can repair, at the board level is I think the correct terminology?

Adcom GFA-555. Ive got the service manual, but is there an electrical theory I should read about first Any pointers other than the obvious, Ray shut up you cheap prick and pay a tech to repair them. I've got a multimeter and an O scope. Lemme guess, I need a signal generator. I'll shut up now...thanks o wise ones Twisted Evil

emdawgz1 · Joined: 14 Mar 2006 Posts: 7949

Can i have your G70 when your "finished" by.... i mean with your amp troubleshooting????

rabies_70 · Posted: Fri Dec 26, 2008 7:13 pm Post subject:

You'll have to fight the wife for it...she's kinda scrappy. Twisted Evil

I think she would actually miss the projector. Although the cool thing about her not being too technically savvy is that when I add in the new VP50 or the algolith mosquito or the new f/x1000 side channels or the 2 new to me GFA-555's I'm betting she won't even notice Twisted Evil

rabies_70 · Posted: Fri Dec 26, 2008 7:48 pm Post subject:

Uh oh, I found thishttp://sound.westhost.com/articles.htm
Now the fun begins

rabies_70 · Posted: Fri Dec 26, 2008 9:05 pm Post subject:

Before I give my money to some anonymous choad on the bay, anybody got the following for sale?

Variac

Audio oscillator

Dummy load

macgyver655 · Joined: 22 Aug 2007 Posts: 8508

Ray. Before you buy a lot of worthless equipment that really wont help you trouble shoot your amp, what is wrong with it?

Tom.W · Joined: 09 Mar 2006 Posts: 6637

Good place to start... Wink

rabies_70 · Posted: Fri Dec 26, 2008 10:45 pm Post subject:

I've got two Adcom GFA 555 each with one channel not working. Fuses were all good on one unit, the other one had two fuses blown and a bad power switch. Replacing switch did not correct missing channel, neither did replacing 6A fuses with 20A
(just kidding I just wanted to give you a chuckle I replaced them with same type and amp rating. They did not blow upon powering up.

I've been reading around on the above link and on diyaudio.com. These are basically just parts amps so I don't mind too much tinkering on them especially if two things happen
1. I learn something
2. I don't electrocute myself in the process.

A working amp would of course be a bonus Twisted Evil

Also a bonus would be an excuse to buy some more tools.

So I need a variac, sound oscillator and a dummy load right Twisted Evil

ecrabb · Posted: Fri Dec 26, 2008 11:29 pm Post subject:

Ray,

Not sure how you've tested the amps to this point, but just a word of caution... I had an Adcom 545 that I bought off fleabay in "as-is/working" condition. I get it, spend an hour cleaning every square inch of the case, button, front and rear panels, and cord of years of dirt, grime, fingerprints, etc. It had a few scratches, but was in really nice shape.

Anyway, I connect the speakers to the banana plugs, connect the pre-outs on my super-nice receiver to the pre-ins on the Adcom. I hit the power button on the Adcom, and hear this HORRIBLE WICKED-ASS LOUD POP come from one of my nice M&K speakers. The AVR immediately goes into "protect" mode. It takes me another 1/2 a second or so and I kill the power on the Adcom and AVR and disconnect everything, freaking out the whole time thinking I just destroyed input stage on the AVR and probably the speaker, too.

I go get a VOM and find out there's a bunch of DC voltage on one of the pre-ins. Like, 25 or 30 VOLTS, IIRC. There's a thread here where I asked for advice. Curt had an idea about where to start, I think.

Somehow the AVR and speaker both survived. I'm just saying - test with non-critical stuff - i.e. junk car speaker, junk signal generator or worthless source equipment so you don't kill something you'd hate to lose.

Oh, and don't electrocute yourself, mmmmkay?

SC

EDIT - Found the old thread. Curt did have some ideas and I never even got around to thanking him. Rolling Eyes

FYI, I sold it to somebody here to tinker with. If we can figure out who, and he's repaired, maybe he can give you some ideas on your 555.

http://www.curtpalme.com/forum/viewtopic.php?t=3228

macgyver655 · Joined: 22 Aug 2007 Posts: 8508

macgyver655 · Joined: 22 Aug 2007 Posts: 8508

Start with Q13 to Q20 on the channel thats not working.

It was F1 and F2 that was blown, correct?

**Curt Palme** · Posted: Sat Dec 27, 2008 12:15 am Post subject:

If you give up on it , lemme give it a try.. Smile

WOn't charge you the going rate either, it's been a while since I've gotten into a transistor amp.

macgyver655 · Joined: 22 Aug 2007 Posts: 8508

rabies_70 · Posted: Sat Dec 27, 2008 12:40 am Post subject:

Now were talking. Thanks guys.

I used to not know better and would just hook a preamp, amp & speakers and go for it. Then I read a story like yours mr crabb and I got a junk setup to test on now. It would SO suck to kill something that didn't need killing. I

You guys are definitely getting me going in the right direction. I surely appreciate it.

Curt, I would love to send this for you to fix, if I give up I will. Alternatively I am re-writing my will so if I get electrocuted I will have it shipped to you.

macgyver...thanks big time for the pointers. I am going to do a wee bit more reading then dig into these beasts. If y'all don't hear from me in a few days it just means Curt's getting a free killer amp Twisted Evil

CRT_Ben · Joined: 28 Aug 2006 Posts: 1684 Location: Northern Virginia

Two things -

SC, I bought that amp. I haven't fixed it yet, what is junk for besides taking up space Laughing

In all seriousness though, I hope to fix that amp soon and use that along with my current 545 to biamp my mains.

Mac - Unfortunately for Ray, the Adcoms have separate power supplies for each channel, they're a bit more complex to troubleshoot.

macgyver655 · Joined: 22 Aug 2007 Posts: 8508

CRT_Ben · Joined: 28 Aug 2006 Posts: 1684 Location: Northern Virginia

Hey Mac,

I'll confess that I'm not the EE of the family (my brother is), but when we opened up one of my 535s to take a look, there were two large power transformers and my brother remarked that it looked like each channel had its own power supply. When I get back in town, I can post pix of the inside of the busted 545 and we can take a look.

macgyver655 · Joined: 22 Aug 2007 Posts: 8508

rabies_70 · Posted: Sat Dec 27, 2008 5:45 pm Post subject:

Thanks again guys...when I get back in later I'll send some photos as well. I stayed up last night til like 3am reading all kinds of theory and practice on amp repair, Transistors, diodes, zener diodes, drain, capacitors, resistors, gate, npn, pnp, signal generators, audio oscillators, how to finally use the oscope I bought last year, diode testers, variacs.

Here's what I learned so far in a very condensed format. Wholly crap you electronic repair guys know a LOT. When I asked my simply ignorant blissful question about repairing an amp, a can of worms I didn't even see coming opened up all over the place. I really only wanted to save a few hundred bucks on fixing a couple amps I have collecting dust. Now there are more tools on the way, more testers and more reading than I truly expected.

Tools and equipment on the way is always good.

macgyver655 · Joined: 22 Aug 2007 Posts: 8508

Ray. One more little hint. After looking over the diagrams I see there is a thermal switch in the input section. Its identified as T1. I would check it for continuity. It is designed to open if overheated. I also see a T2 at 1k which is a thermistor. Its in the output section.Check it also.