Blow Output Transistors Identified...What Next?

[DreadPirate]

Per spec refers to the procedure, I believe I'm doing everything as described in the manual. The power in is 52V both channels.

Now, I'm only turning the pot from its 5 oclock position (max--300 ohm) to about 1 oclock and getting a dead on zero reading throughout the range, both channels. The pre-rebuild setting was about 10 oclock (90 ohm or so). I'm afraid to go further as I was expecting at least some kind of reaction to adjusting this pot. Is it possible I'm not turning counter clockwise enough and at some point it will "turn on" and start showing a reading? My expectation was that any adjustment of this pot would get a reaction. The outputs remain cold, but I have only run it for about 10 minutes at one time.

Only one channel was dead when I started, the other channel appeared to be in good condition and I verified all transistors (I did both sides equally) worked as I pulled them.

If the bias transistor is Q708, then it was replaced with identical new transistor on both sides, no subsitution.

The diode stack I tested on both sides and got strange result. I used the diode checker on my meter and both of them gave me a reading in both directions. I realize that one would expect no reading in one direction, but both tested identically and the other channel was working, so I assumed they were ok. I will re-check this in a bit. Isn't it unlikely that this would be bad on both channels?

I only used one meter, the best one I own the Fluke, but have a couple of others to make sure it is not the problem.

The transistors I changed are circled in the diagram of Post #12, pre drivers, drivers, outputs, etc. I am using all the subsitution devices mentioned herein previously, MJE15xxx for drivers, MJE21xxx for outputs, etc.

 

[djk]

Measure the voltage across the bases of the driver transistorsat different pot settings. It should be about 1.5V and change as the pot is changed.

 

[EchoWars]

Per spec refers to the procedure, I believe I'm doing everything as described in the manual. The power in is 52V both channels.

I C...you said 'adjusted per spec', you made it sound like you were somehow adjusting to get 10mV with a music signal, but had zero with no input.

Now, I'm only turning the pot from its 5 oclock position (max--300 ohm) to about 1 oclock and getting a dead on zero reading throughout the range, both channels. The pre-rebuild setting was about 10 oclock (90 ohm or so). I'm afraid to go further as I was expecting at least some kind of reaction to adjusting this pot. Is it possible I'm not turning counter clockwise enough and at some point it will "turn on" and start showing a reading? My expectation was that any adjustment of this pot would get a reaction.

Yes...if you are measuring bias correctly, but reading nothing, you have to confirm that you're adjustment doesn't affect the bias current regardless of the position of the pot. When in doubt with a unit where it is difficult to measure bias directly, I use a dim-bulb tester (a 100W incandescent in series with the AC line) to give me a visual indication of the current draw (and protects the outputs from getting too frisky with the bias adjust). Once I've confirmed that the bias adjust is doing something, I'll set the adjust pot to minimum, power the unit up directly from the AC line, and proceed with the proper adjustment procedure.

 

[DreadPirate]

I removed and checked the diode string Q711 STV3HY and it checks out ok. Power in is at 52V, as in the schematic.


These are the parts I used and what's in the manual:


Digikey Part No. Original Spec

Q706 2SA09140R-ND 2SA914 (R,S)
Q707 2SC19530S-ND 2SC1953 (R,S)
Q708 2SC15680R-ND 2SC1568 (R,S)
Q709 MJE15031GOS-ND 2SA913 (Q,R)
Q710 MJE15030GOS-ND 2SC1913 (Q,R)
Q001/Q002 MJ21194GOS-ND 2SD427
Q003/Q004 MJ21193GOS-ND 2SB557

Have I used the correct parts? I'm thinking because I see the same phenomenon on both sides, and one side worked fine before, that it could be the parts I used, as that is common to both sides. Unfortunately, I do not know if the bias was adjustable on the good channel before I replaced the devices there, did not check.

 

[EchoWars]

Unfortunately, I do not know if the bias was adjustable on the good channel before I replaced the devices there, did not check.

According to your posts, you don't even know if it's adjustable now, since you're apparently afraid to turn the bias control beyond a certain point.

 

[DreadPirate]

Still no readings at J741 or J742 with pot fully turned counter clockwise (0 ohm). I'm getting 325mV at the base of Q708 (is this the "bias" transistor?) and it does wander upward as I adjust the pot counterclockwise to 360mV or so. Same with Q709 (driver). I'm reading -935mV and upwards to -980mV as I adjust fully counterclockwise. Manual has -1.1V indicated at this point. Q710 (other driver) is buried and I can't get to the base.

I tripled checked the polarity of each electrolytic as I replaced them, possible I put one in the wrong way on both sides?