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Odd M60 Problem

My M80 had one rail always "high" because the transistor driver was bad (TR159, 151). You can also check the voltages in this area with the channel that works well if you are sure that the small transistors and diodes in the APS circuit are fine (TR159, 151, 153, 161. D135, 137, 141, 143, 133, 145 ...)
 
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I'm not at home now to check but I believe I've checked all of the driver transistors for the APS. It's definitely possible one is failing under load. I'm pretty sure I've got most/all of the correct subs so can replace one at a time in the APS circuit and see if I can shake the problem.

As far as tracing voltage, so far I know the proper high and low voltage is coming from the rectifiers. The blue and red twisted pair that pop out from between the filter cans have +/-35v on the good side and +58/-41 on the insufficiently good side.
My brain was a bit fried from staring at the amp and the scat all day yesterday so I didn't go further than that.
If shotgunning devices doesn't have the desired effect then I will go point to point comparing with the good side and see what's what. If that becomes irritating I've got another M60 in the closet I can pop the bottom off so I can have all the test points in the exact same spot.

Even though I spent considerable time and a bit of money on what was supposed to be a parts amp I'm enjoying the detective work. Thanks for the tips.
 
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Interesting that you are getting one rail at -42V like mine +/-42V. I suspect the voltage rails go up if not under load (ie failed/open circuits). Thinks mine is more Chernobyl....

Ah might have misread my 42v is the low side and 72 high side.

While I have schematic I cant find a full parts listing. Would make it a bit easier when shopping for bits.
 
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Stop! The APS needs a minimum load if the amp is not biased (i.e with the dummy resistors) You can wire a 1K/5W resistor between the collector and the emitter of the power transistor pads, to simulate the bias current
 
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I do have resistors from C to E on the channel I'm working on. I think from what I see and from what you guys are saying it sounds like something is definitely bad on the + side of the APS causing that rail to stay high.
I ordered some 12V zeners which I'll blanket replace when they get here since I've replaced all of the other diodes in the bad channel already. If that doesn't do it I'll replace all the transistors one by one and see which is causing my issue.
 
What is the value of the resistor? I'm not talking about the dummy resistors across B-E. These "new" resistors simulates the bias current on the rail that loads the APS. You have to use two, one per rail.
 
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Something like this.

Rd = Dummy resistor that loads the VAS
Rl = Dummy resistor that loads the APS

In my tests the APS doesn't work well without Rl.

IMG_9685-s.JPG
 
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I don't have resistors as you drew out there.
I can series two 470ohm 3w to get close to 1k. Then go C to E on the output device pads?
 
Yes, between C-E or C-Ground. Only in one output of each side, to load the LP rails (+LP and -LP)
 
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