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

I went to turn on my Audio Research SD135 amplifier the other day, but it will not turn on. According to the Audio Research service technician I spoke to, the problem is a broken Thermal Trak module chip. He said that it is a temperature sensing component, and that replacements are no longer being manufactured. He said that it is a known problem, and that Audio Research will give me credit to purchase an Audio Research amplifier from an Audio Research authorized dealer. 

I was wondering if Audio Research makes anything powerful enough to drive Vandersteen 3A Signature speakers (which need between 100 - 200 watts of power)? I am using an Audio Research SP8 Mk II as a pre-amp.

I don't really have much of a budget and am not sure what to do. I would appreciate any advice. Thank you.
distortions
johnsonwu
469 posts
 
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!


johnsonwu
469 posts
 
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.

atmasphere's avatar
atmasphere
11,834 posts
 
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.
johnsonwu
469 posts
 
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.
distortions OP
9 posts
 
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.
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