Amp was heavy but small( compact). It is filled with parts with no space inside. It looked like my suit case for week long vacation ( Small but full ). When some on designed circuit board and wiring, he never thought about some one needs to open up the case and replace some parts in some day. This amp is one of not repair friendly amps out there. It is worse than Mark Levinson 331 for repair.
This amp has analog module 1 and 2 as buffer and voltage gain section. Modules are impossible to repair because it is molded in epoxy or some plastic. This amp contains no adjustable bias control circuit. I was praying for amp module to be OK. I disconnected amp module board ( voltage gain section) form outputs and pre drivers and hook up +-15v as voltage source and scoped. I saw beautiful sine wave on scope on both channels. I am safe on this purchase.
Amp has one of rail cap ( 300uf 75v) with shrunken plastic sleeve . This is tell tale sign of old cap failure. I removed all 4 rail caps from both channels. I hook up my +-15 to see if output section is working. Found right channel impedance of pre-driver is extremely low. This means probably pre-driver is shorted but out put devices checked OK(?) in circuit. So it was blowing fuses because of failed TO-3 case devices. Hopefully, it did not wipe out all 6 or 12 output devices. That can cost some unless if you have Some Crown amps to salvage the output devices from them. Crown is known to match their output devices in their amps ( they wrote numbers and now colors).
Bad thing about Audio Research D-100 repair is that getting access to parts. All the boards that contain emitter resistors and other parts are Soldered on to output device board. To check the output devices these boards need to be removed. If output devices need to be replaced all 4 emitter resistor boards need to be de-soldered. Each emitter board has around 10 pins that are soldered through. It can be very messy and slow process because of this design.
This amp has way too many TO-3 case devices. It brings back the memory of Caver amps or Crown amps with so many TO-3 case devices. For 100 watts, it has 24 TO-3 compare to ML331 has 16 total.
Largest repair bill comes from four 30000uf 60v can caps. Lowest price I saw on Mouser ( United Chemi) was 70 per piece. This amp uses total of 120000uf for filter section!!!.
(update)
I found 2 dead TO-3 devices on left channel. pre-driver -2sb555 and M 300026.
Mean time I went through some junk Crown amp to see if I can find some matched TO3's. On Crown with 16 TO3 all dead but one!! As if some one plug in speaker terminal in to AC outlet!! I have few MJ TO3's on hand from last repair project. I do not recall these are matched or not. I need to set up an BJT matching circuit or buy a semiconductor analyzer. There are not much choice on TO-3 devices because Onsemi is only one manufacturer still make them. I need to replace with new from On Semi devices or get some old matched pairs (impossible task).
I decide to replace all TO-3 of the channel to give max safety and match them to keep integrity of amp. When one of parallel devices short, you can not trust device next to it and complementary side for safety.
All four pre drivers and output devices are matched sets. All 12 silicon's will get new Onsemi matched set. I have 8 ( 4 pairs ) on hand and start working on matching them.
I liked the way amp was made and I am sure for keeping this amp in my collection.
