GFA-555 parts upgrade/substitutions?

[Mac_Hath]

Hi,

I've got a GFA-555 that has had water spilled on it (looks like), and it's pretty old anyway, so it looks like the repair will require replacing a number of the transistors. After looking at it, it appears that this has been done in the past as well, as there are several transistors with parts numbers that don't match the schematic I have (downloaded from...can't remember, but there were two different schematics available, and it wasn't the factory) and several that don't match from the left channel to the right channel. Specifically, many of the 2SC2240's have been replaced with 2SC3000's, and the 2SA970 looks like it's been replaced with something marked AMP A43 4G.

I'd like to bring this back to factory spec, or better. Are there any known substitutions which will improve the sound? Should I assume these random part variations came from the factory (doesn't seem likely, but maybe transistors are like resistors, and the brand doesn't matter too much...)?

If someone has the "real" parts list from a GFA-555 (and I understand there are at least two revisions of the 555 board, not counting the 555II...), that would be helpful, too. I downloaded two different schematics from the same source, but they don't match each other, and Nelson Pass commented that they didn't look familiar, so the factory may have changed his design a bit, for whatever reason....

Any help is welcome. Also, one of the output transistors is shorted out, letting 70V DC through to the output. Any clever tricks to determine which one, without pulling each one off the heatsink, unsoldering, etc...?

Thanks!

Mac
Maynard, MA
mhathaway@_nospaaaam_netway.com

[Mayank]

Mac:
I know I am responding to an old post, but I was wondering if you ever got the amp to work?
A few months ago I had picked up a GFA-555 off eBay with a dead channel and incorrect driver substituitions. This was carefully restored to its former glory and now proudly drives my Def Tech speakers. The sound is really outstanding.
I can send you the parts list from the schematic used if you are still interested.
Mayank

[1/137]

I'd be interested in the schematic and parts list...maybe it matches the 2 555's I have.

Thank you,
rt

[anatech]

Hi Guys,
Firstly, when one output goes, change the lot in that channel. Change the driver for cheap insurance. I think the signal transistors are still available. Match the diff pair. Match the outputs if you can. Use new mica and grease. Do not over tighten the screws.

I tended to get better matches with MJ15024 and MJ15025 than the newer MJ2119X parts. Don't worry about the SOA too much, the originals were Toshiba 2SD424 and 2SB554.

Mayank, I just have the info off the web these days, could you send me what you have? bhomester at gmail dot com . I used to have the original manuals (don't any more )

-Chris

[Macrom]

Hi Mayank

Is it possible to get the documents for the GFA555 from you.

I own one pair of them too and I´m very interested in the schematic.

My email: student_fhc@web.de

Thanks
Stefan

[beppe61]

Originally posted by Mac_Hath
Hi,
I've got a GFA-555 ... and it's pretty old anyway....
Mac
Maynard, MA
mhathaway@_nospaaaam_netway.com

Dear Sir,

just a question: your GFA 555 has fuses on the voltage rails?
Have you kept them or replaced with something with better specs?
I ask you this because I have a GFA 545 1st series and after cleaning the fuses ends the sound improved evidently.
Thank you very much indeed.
Regards,

beppe

[anatech]

Hi beppe,
The amplifier should have local bypass capacitors on the output PCB's. Check them to make sure they are good. You can try to bypass them with a good film or foil capacitor. Do not up the value of the fuses. If you cleaned the oxides off the ends of the fuses and holders, you've done as much as you can. There are some expensive fuses I've heard about available, but I'm not sure they will help in this situation.

With regard to the capacitors on the output boards. I wouldn't install the large value caps as I've seen. Be aware that almost any part change will generate positive reports that are usually not supported when objective people compare the changes.

-Chris

[beppe61]

Originally posted by anatech
Hi beppe,
1) The amplifier should have local bypass capacitors on the output PCB's. Check them to make sure they are good. You can try to bypass them with a good film or foil capacitor.
With regard to the capacitors on the output boards. I wouldn't install the large value caps as I've seen.

2) Do not up the value of the fuses. If you cleaned the oxides off the ends of the fuses and holders, you've done as much as you can.
There are some expensive fuses I've heard about available, but I'm not sure they will help in this situation.

Be aware that almost any part change will generate positive reports that are usually not supported when objective people compare the changes.
-Chris

Dear Mr. Chris,
I am attaching a photo of the 545 interiors.

1) I cannot see any local bypass capacitors. Do you think it would be wise to add them ? and where ?

2) I have already decided not to bypass the 4A fuses on the voltage rails, nevertheless and maybe a replacement of them with something with better specs could be beneficial ?

Thank you very much indeed for your always kind and valuable advice.
Regards,

beppe

[anatech]

Hi beppe,
Just shows you how good a memory can be. No bypasses on that one, I've been working on 565's and 585's lately. They do have them.

You can add something like 100uF on the circuit side of the fuse. My only concern is that it has to be done neatly and returned to the correct (noisy) ground. Some later Adcom amps like this did have those caps.

You can try to "tack" solder some in to the fuse clips and return the grounds to the power supply. This will prevent you having to pull the boards until you are sure you can hear a difference. Mount them close to the outputs if you want to install them on a permanent basis.

Just make sure the capacitors will not short to anything and that they do not get too hot.

-Chris

[Mayank]

Chris/Stephan:

Here is the schematic from the Service Manual. Hope it helps in your project.

Good Luck
Mayank