Yet another Adcom GFA-555 question - diyAudio
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Old 3rd February 2017, 04:59 PM   #1
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Default Yet another Adcom GFA-555 question

Greetings:
I have an Adcom GFA-555 that I am working through, and had some questions. I started a post on it in the Pass forum, but was hoping to reach a larger audience, as most of the Adcom 555 threads are here. The main board is the 72-2174-0-1 version. There are no 'initial symptoms' as it was found by my brother at a salvage store and powered up (no load, no source) without letting any smoke out. Just trying to bring it up in as safe and sane a manner as possible.
I wanted to put R4 back in (factory inserted a jumper wire) to help balance Q2 to Q1, but that requires getting the main board out. The wires are fairly taught and all are soldered in. I was wondering what the best practice was for getting the board out, and if that included unsoldering then resoldering the wires, or if putting connectors in on reassembly was the better way to go?
So far, I've done a basic powerup with the power boards de-fused (all went well) and a diode test (in circuit) for the power transistors (all seemed well). Next up is the basic bias setting (per manual). I think I'm in pretty good shape for that part of the testing.
After that, I'm looking at the usual replacing of the capacitors and adding the R4s in place of the wires (between Q2 & Q3, same place R3 is between Q1 & Q3), using better/higher precision (and hand matched) resistors for R3/R4 and for a couple key places. While several people have mentioned upgrade/rebuild parts lists, I haven't seen much that's recent (in case a transistor or two should need to be upgraded). Does anyone have a list of key parts to replace (beyond the electrolitic caps)?
One last question - Does anyone have a basic DC analysis of the board, so that I can check basic settings (bias voltages for transistors, voltages at other key points in the circuit)? Just want to give the circuitry on the main board a sanity check before going to the trouble of putting the fuses back in and setting up a load cell for the speakers? I don't have speakers that can handle the power, much less a set of sacrificials in case something goes wrong.
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Old 3rd February 2017, 10:16 PM   #2
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With this much effort on this amp might as well integrate dc relay.. I blew few speakers with adcom amps and know a few who bought adcom new and had their 2000-2000$ speakers blown all because no dc.. and quality controll I guess.. I am scared shitless of this "transformer" Hum at power up. It may be start up hum.. or it may be dc volts blowing up your speakers
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Old 7th February 2017, 03:51 PM   #3
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Thanks, Richard. I have some 1237-based kits on the way to catch DC bias and to delay turn on.
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