My Amp is Broken - Not Sure What to Do - Any Recommendations?

Hm, The output section is right out of OnSemi's application notes.

Devices like this give me the willies, since so much relies on them. If it were me, I would be tempted to test the transistor portions to see if they are still good, and if so, perhaps see if a diode applied the transistor could sub for the diode portion of the device. If the amp heats up or has crossover/notch distortion issues, then a bias scheme would have to be employed.
But of course outfitting it with new devices seems the preferred method. This just doesn't seem like it would be that hard to sort out.

Not hard for a guru like you Ralph!  But I have had the pleasure of dealing with botched jobs of those $25/h audio repair experts who couldnt figure the difference between emitter and collector that I think dealing with a 5 pin output device would be a hair-pulling experiment.  I think I know why Audio Research doesnt want to service these things.... Figure the parts cost and the need to keep inventory of matched triplets for SD135 and matched Sextets for HD220 can be a cost they may not be able to recover.  After engaging in this thread curiosity got me to open up my HD220 and BOY AM I LUCKY.... my HD220 actually has 1302DGs and 3281DGs!   I know from the seller that the unit went back to Audio Research once in 2012/13.  Maybe those were the days when Audio Research still cared.   And yes each device has markers that indicated they were hand matched.

Do you guys think it would alter the sound to replace the 3281D and 1302D Thermal Trak transistors with NJL3281DG (NPN) NJL1302DG (PNP) Thermal Trak transistors, and do you think the work to match the output devices would be difficult or highly involved? I know of a good audio technician in my area who makes certified repairs for McIntosh and Arcam equipment, but he gets $50/hour.

I am also concerned that even if the repair is successful, the new Thermal Trak transistors will fail at some point and I might be in the same boat I am now. Do you think that could be why Audio Research stopped using them in their new equipment and won't service older devices with them?

Thanks again.

I would look at the spec sheets of the new and old parts. The things to look for are maximum current, breakdown voltage, hfe. If those three things are right, then its likely no worries.

Obviously there's a little bit of engineering to do all this, but its not rocket science. Any tech (not engineer even) that actually went to school should be able to do it. Matching shouldn't be a huge deal.

If you get it right, the sound won't change. This is mostly a reliability issue. BTW $50/hour sounds cheap!

Regarding reliability of the replacement parts, ARC may have the same concerns as you; if this were my amp I would share that concern!

Be sure you don't kill yourself when you short a filter cap to the chassis (seriously).

I used to be a do-it-yourselfer, but as I got older and wiser, I realized the experience, knowledge, and training involved was not worth the expense.

Those of you who enjoy such things, by all means keep on soldering.  Most of us just want to push a button and have the stereo come on.  Asking me to match output devices is about the same as asking me to carve an amplifier out of a billet of aluminum.