GFA Firestarter

Rev. Hayes

At our dump there is an electronics recycling "trailer" that I usually stop at to scavenge. I've found some good stuff there and today was looking extra good when I found an Adcom GTP 500 and a GFA 535. CLean too !!!!

Got them home and did a quick hookup with some Boston A40's. (great little bookshelf speakers) Right channel was out so I cracked the amp open and found that some fool (:o) had neglected to replace one of the 4 A fuses on that side. Empty holder, it just wasn't' there. Dug one out of the junk drawer and hooked it all back up.

30 seconds later a humming noise out of the right speaker and then.....SMOKE!!!

So I guess that I toasted the voice coil. Clearly it was getting some heinous power not intended for accurate audio reproduction.

I would like to try and repair the amp as it seems to be a well regarded piece of equipment. (and you can never have too many 2 channel rigs)

Iguess I need some advice as to what the next step to troubleshooting this would be. A visual inspection only reveals a very clean amp with no "burnt" looking parts. Plugged in it does make a slight vibration. What might that indicate?

Any help here would be greatly appreciated.

Rev. Hayes

I posted this in the wrong place.

Sorry, it should be in troubleshooting.
Mods, any chance you could move this for me?

dpowell

Did you have a pre hooked up to the power amp or was the speaker getting full power from the amp?

heiney9

dpowell wrote: »

Did you have a pre hooked up to the power amp or was the speaker getting full power from the amp?

A speaker doesn't get full power from a power amp w/o a preamp. Sound like very high DC offset in one or both channels. Not something you want to mess with unless you have knowledge and proper measuring gear. Take it to have it serviced, it's not a simple fix. Maybe Ben can fix it for you.

H9

dpowell

heiney9 wrote: »

A speaker doesn't get full power from a power amp w/o a preamp.
H9

Doesn't an amp have the potential to ouput at max wattage without the presense of a pre-amp acting as the potentiometer? In other words, couldn't a malfunctioning amp possibly send a noisy output signal at full strength if nothing is connected to restrict it?

heiney9

dpowell wrote: »

Doesn't an amp have the potential to ouput at max wattage without the presense of a pre-amp acting as the potentiometer? In other words, couldn't a malfunctioning amp possibly send a noisy output signal at full strength if nothing is connected to restrict it?

If you don't have a pre, there is no signal present to amplify. If someone plugged the source directly to the power amp inputs that could potentially cause what you describe.

Even if the amp malfunctions it wouldn't result in full power being applied because again there is no signal present if you don't have a preamp hooked up with a source attached to the preamp.

The malfunction in the amp in this case sounds like there is a high amount of DC present at the output, so it is a malfunctioning amp, just not in the way you describe it as having full power output amplifying a signal.

dpowell

Good to know. Thanks for the explanation.

Rev. Hayes

We have a winner!!!

heiney9 wrote: »

Sound like very high DC offset in one or both channels.
H9

Broke out the multimeter and found the left channel to be running a charming 10.2 mV.

The right channel was running at a charing 49.6 V

So how would I track this issue back to the source? This must be worse than bad input differential pairs. What might it be?

Edit: I realize this fix may well be above my head but I like to learn hands on. I don't have any money in the amp so I don't mind if I only make it worse.

heiney9

Just be careful not to ruin one of the dual transformers. They have long been out of production and I've read other places that it's very difficult to find an iron core transformer that will fit in the case. Many equivalents are to tall.

H9

FTGV

Rev. Hayes wrote: »

What might it be?

The first place you'll want to look is for a shorted output transistor(s).

Rev. Hayes

Ok after in circuit testing I think I can say with some certitude that two of the transistors in the right channel are bad. (I am assuming these are the output transistors: D 1047 E 6M)

If this is the case why would the output fuse not have blown. Or for that matter why didn't the clipping light (which adcom claims is also triggered by DC offset) come on?

Would it be worth it to just try replacing the transistors or is there clearly something more sinister causing them to short?

FTGV

Rev. Hayes wrote: »

If this is the case why would the output fuse not have blown.

Ideally it should have assuming the correct amperage fuse(s) was/were used.

Would it be worth it to just try replacing the transistors or is there clearly something more sinister causing them to short?

If you can source some replacements then it is likely worth repairing but yes there is the possibility of other components upstream that have also failed.Therfore it would be prudent to be sure that any circuit blocks preceeding the output stage are good before installing new outputs,or you risk having them fail again.There are some real Adcom savvy techs(Anatech) on Diyaudio that can give you some good advice trouble shooting.

heiney9

The other wonderful thing about Adcom products, especially the early ones, is they used all their own part numbers so cross referencing them to the "real" part is a PITA. It can be done but you have to hunt around the internet for people who have done repairs or talk to a qualified professional. Adcom part numbers are used on the schematic as well.

Good luck

H9

dudeinaroom

Listen to H9, he is a very credible ADCOM source of information.

Rev. Hayes

heiney9 wrote: »

The other wonderful thing about Adcom products, especially the early ones, is they used all their own part numbers so cross referencing them to the "real" part is a PITA.
H9

No doubt.

I spent hours last night trying to decipher suppliers websites and match what I have to what they have. I found something that seemed about right but I guess I need to download the data sheets for both and really compare those numbers.

Anyone else done this that can lend some direction?

Possibly a NOS connection?

heiney9

dudeinaroom wrote: »

Listen to H9, he is a very credible ADCOM source of information.

Rev. Hayes wrote: »

No doubt.

I spent hours last night trying to decipher suppliers websites and match what I have to what they have. I found something that seemed about right but I guess I need to download the data sheets for both and really compare those numbers.

Anyone else done this that can lend some direction?

Possibly a NOS connection?

For any and all technical advice, listen to Fred (FTGV) his electronics knowledge and skills are excellent.

I know about Adcom history or about some of the glitches in getting them repaired, etc but I am not an EE like Fred.

Good luck

H9

FTGV

heiney9 wrote: »

... an EE like Fred.

I wish I were an EE Brock,but I'm just an ET.

heiney9

ET = Excellent Technician.

H9