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

You can still get thermaltrak chips on mouser or digikey last I checked.  

Ralph the OP was referring to thermaltrak output chips made by OnSemi.  Can’t be bypassed.  They were discontinued for a while. Now back in
the market.  

Here’s the problem with ThermalTrak devices that supposedly was solve by Ayre but not AR https://www.stereophile.com/solidpoweramps/407ayre/index.html
ON Semi only made 1 set of thermaltraks, and yes I just confirmed that Mouser has them in stock.
By the way the SD135s are only 10 years old.

I have a Thermaltrak equipped HD220 myself and Audio Research would ABSOLUTELY refuse to return my repeated calls AND repeated emails.  The good old days of Leonard and Kalvin are gone.

A $25/hr tech... GOOD LUCK finding one who knows what the 5 pins of the thermaltrak devices are about.   

The parts did not go obsolete.  Check OnSemi, aka ON Semiconductor who manufactures the parts.  Audio Research EOLed the SD135 and HD220.  

OP lucky you the schematic is HERE:

https://elektrotanya.com/PREVIEWS/63463243/23432455/audio-research/audio-research_sd135_amplifier_sc...

I bet not everything is toast likely 1 phase of 1 channel.

Get a tech WHO KNOWS HOW TO MATCH OUTPUT DEVICES.
Buy a dozen of 
NJL3281DG (NPN) NJL1302DG (PNP)
and match them and repair the blown channel!

https://www.mouser.com/ProductDetail/ON-Semiconductor/NJL3281DG?qs=sGAEpiMZZMvFp%252byPHbnZYyVkaBWrH...

3281D was INDEED obsolete.
Original collateral dated June 2006
http://www.onsemi.com/pub/Collateral/NJL3281D-D.PDF

replaced by 3281DG  Same application notes per Mouser.com and ONSemi.  Available and NOT discontinued.

My HD220 has not blown (yet) so I have not attempted a repair myself.
I suspect it means you need to change out ALL the devices for matching sake.  That's likely why Audio Research doesnt want to deal with it.
Boy if they could just respond to my email and get me the schematic!

Even the manual found online says the original devices are 3281D and 1302D
Replace the defective channel's output devices with MATCHED
DG versions and you are good to go.

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.

3 matched devices per phase per channel on the SD135.
Usually you can get a matched triplet every 10 devices AT LEAST.
Now if the SD135 had say 10 devices per phase per channel that’s a completely different story.
Getting 3 devices to match is much much easier than say 5 or 6.
So figure even if BOTH channels are shot,
Buy 12-15 of each type and you can get 2 sets of each from mouser.com.
Unlike tubes, esp. those snake oil KT88 and WE300B replicas,these "sand" devices only cost a few bucks each. 2 hours of the tech’s bench time cost more than the components.
When done, sell the surplus on eBay for 1/2 the acquisition cost. heck you might even get 1 or 2 extra matched triplets good for the next hobbyist!
I am not advocating a DIY approach, just find a competent tech who knows how to match devices and bring up an  amplifier using a variac instead of a crow bar.