600 Supreme bias drifting

igiveup

Member
I'm working on one of these now and I can adjust the bias to 25mv, but as it continues to warm up one channel will drift down to 15mv with 18mv as high as it will adjust too. The other channel is perfect and stable. I'm going to replace Q5 and Q6 and the diodes but I don't have the 2SC1746 or 2SC1452 in my stock of old parts. What is the best subs you guys have been using as I can't seem to find a good replacement. Hopefully it's not the thermistor in the bias circuit as I bet it's going to be impossible to track down.

I will be re-capping this amplifier as well and plan on keeping it as my daily driver in our basement.

Thanks in advance
 
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Qe5 is not likely the problem, and changing it will bring on a host of other issues. That transistor needs to stay unless you definitely discover that it has a problem.

First things first...you gotta get rid of the stock bias and offset trimpots, and swap out the 2SA810's and the 2SC1452's. Replace with the Sanyo 2SA1209 and 2SC2911 (B&D Enterprises).
 
Thanks Glen, The trim pots are on the list. I had 2 of the 470 Bournes here and have the others on the order I'm working on. I really like the multi turns pots in these old amps.

I will replace all of the transistors you mentioned, in both channels. I did use the search and saw that they were common problems.

I just found out my wife threw away a Tupperware storage container that was full of old semiconducters. I had new output packs for M2 and the M1 amplifiers, Original 2sd287/ 2SB539's plus all of the normal everyday stuff. 3 gallon size. It also had a tone generator and some service manuals. She needed the space on a shelf.
 
Glenn, I got the Sanyo 2SA1209 and 2SC2911 in from B & D and I'm glad I checked the leads on these replacements as the originals are CBE and the replacements are ECB. Just a heads up to others not all subs are drop in and always need to be checked.
 
The PC board has the holes in a semi-circular config, so installing the replacements should not be a big deal.
 
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