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Old 10th May 2009, 07:39 AM   #1
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Join Date: Mar 2009
Default Adcom GFA-555 MKii blown channel checklist?

Hi,

I have a GFA-555 MKii that I was hoping to do some work on. I have worked my way up on a couple of broken NAD 3020s, event/tannoy active monitor amps, and have worked on different recording consoles & related studio gear for a few years and was hoping I could learn something from this. Over the past few years my troubleshooting has gotten better. I've learned the value of replacing anything that might go bad based on the experience of what might get blown when X device goes, instead of just replacing X device. The parts are cheap new and I prefer to work quickly and not have to open it twice.

This amp's right channel blows a power supply fuse when it starts up, after a child switched the rear switch on the amp from regular mode to bridged mode while it was on and powering a low impedance speaker. I assumed bad output devices.

My procedure was going to be as follows.

a) Obtain new output devices, buy 12 new MJ15025G and MJ15024G for power transistors, 5 of each MJE15030/31 for driver transistors and get good matches with a heathkit IT-121, and put the rest on the shelf for the next project.

b) Get new mica insulators and thermal grease, clean off surface of transistors and heatsink with 91% isopropyl alcohol, apply new grease on transistor and heatsink and put new mica insulator on.

c) Replace all emitter 0.22 ohm resistors with wirewound cement resistors of the same kind as the originals on both boards.

d) Replace all electrolytic and mica capacitors on output PCB with ones of equal values.

e) check if board has god awful electrolytic smell after touching it with the soldering iron, and if so, vacate its parts and find an ultrasonic cleaner.

I very rarely, if ever, have this occur. I did have a tantalum capacitor from an old neve module explode in my face and it made me very sick for a week.

I have searched and read numerous very helpful threads here which have helped me build up the confidence to work on the 3020s before deciding to tackle this project, I have read a lot on the adcoms and very much appreciate all of the information available on this board. I was wondering if there was any advice one might have prior to me diving into this new project.

This will be powering a low impedance, low sensitivity load with pretty linear response to 27 hz, 25 in my room, that my 3020 & parasound hca-1000a currently cry attempting to drive. I am hoping if restored properly, it might prove a more capable amp.
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