Cool, thanks for the info. I will get after Q308 asap. Happy Thanksgiving to you all

  Here's the values you asked for:
TP 301 to ground- 23.2 mV
TP 302 to ground- 24.0 mV
TP 301 to TP 302- 0.00 mV
You guys nailed that one, eh. Does this mean only one half of the power amp is working? Or, that both halves work, just not together? Why the fuzz sound?

  When you say between the TPs, are you referring to the halfrail voltage?

   Hey guys. I finally got to try the 3500 on a known working cab.
Its still a little sick.
When I play, the sound is fuzzy/distorted. Otherwise, its silent, no buzz or hiss or hum. I ran a tone through the  FX return with the same result. What do you think?
Will I be able to fault find with DMM, or will I inevitably need a scope or some other test equipment?

  Well guys, I'm happy to report, the B+ preamp supply voltage is clocking in at a cool 14.75V. The hum is no longer.
  I'd like to thank everyone who responded to my call. I couldn't have gotten this far without you. And a special thank you to Roly for your patience, and walking me through step by step.
  So after sorting out the supply voltage, I hooked up the speakers and fired her up for a victory lap. Plugged in my daughters bass and nothing. Had signal in, no sound out.
WHAT?!
Unfettered, and with a newfound electrical prowess, I began to poke and wiggle components on the board. When performing this technical troubleshooting, I found CN404 ( a plugged connection) was loose where it fastened to the board. Not sure how it ties in,but a closer look revealed another broken trace. I tightened her up and tried again.
   Excelsior! Self induced noise and no hum. That's one for the good guys.
  My victory was short lived,I regret. The sound was fuzzy,.particularly when playing the high strings. That's with the EQ on or off. I'm wondering if it might be the cab,but I haven't been able to try another yet. Thoughts?

  Cool, I'll get after it and let you know the results.

    Yup. I checked continuity to ground and got OL.

   Here are current supply voltages:
Half rail     23.1mV
Main +    IN  87.2V  / OUT  33.44V
Main --  IN  87.2V  / OUT  33.29V
Pre +   23.07V
Pre --   15.34V

  The B+ value on the pre amp is pretty high I see. I took that value from IC401( the voltage reg 7815). Is this the cause of my hum?
Earlier on I saw it was really loose. Could it have sustained damage to the board?
  I had tested it for output,and it checked out. When I resoldered it, I made sure to not run puddles together,so nothing is touching. As loose as it was though, could it have cracked,or otherwise stressed out, the trace in the board? Then, by me applying heat and solder, a bridge was created across the stressed areas.

   OK, so when I send a signal through the FX return, I get a fuzzy/ distorted sound,and the hum is still present. When I short the inputs it is still hummy.
   I also put the ohm meter on R411,like phatt suggested,and got 0.00.
With power off, I touched the leads to either side of the resistor while still in circuit

   Roly, sorry but,I don't know what o.n.o means

Quote from: Roly on November 03, 2014, 01:54:25 AM

Quote from: js1970There was no change in the bulb brightness.

Now that the output half-rail is balancing to (near) zero volts correctly, have you tried powering the amp directly from the mains?  It could be that you hum is due to the voltage reduction of the lamp.

    I did take the plunge and fire it up off of mains power. Everything functions as it should, but with that incessant him. I removed the tube, with no change. When you gave me the run down of signal flow I started to wonder. Hear me out on this. The flow was from the instrument inputs, through the compression,through the EQ, then out the master volume.
What got me thinking, was that tbe amp hums with no signal in. So I followed the power input. What I found was a resistor ,R406 inline with the EQ switch, that appeared discolored. I will check its value when I get a chance.
    Also, when using the light bulb limiter, the bulb glows steadily even after power up. It is my impression that that means there is a short somewhere. I ve run other amps through the limiter to see how it worked. They were SS guitar amps and upon initial start up the bulb lights and fades out. Is it because of the preamp tube the bulb stays lit or is it signifying a problem? It is my theory that the limiter bulb and the hum are connected. If I find the problem, the bulb will go out and the hum will cease.
    Does this have any merit,or am I looking at it the wrong way?

  I pulled the tube with limiter engaged and no speaker load before. There was no change in the bulb brightness. I didn't know I could pull the tube and run it through the speakers. I will do that this weekend for sure.
   As for the ICs you mentioned, again I see the +/-15 off of pins 4and 8. Is that how I would check that they were functioning correctly?

    The hum is present, whether the EQ is on or off. It makes no difference. However, with the EQ on, I can filter the hum down to some degree.

   Adjusting the solid or tube knobs makes no difference. Niether does the compression / EQ. All ground connections to the chassis showed good for continuity. I don't know what the twin triode is.
   I'll go ahead and give the board another good look for bad joints.
   Could a bad pre amp tube give me that hum?

   Ok, so I plugged direct,no limiter. My offset voltage is around 20 mV and the supply is 87 V.
   I connected speakers and still have that hum.  I also noticed the offset increased with volume. I assume this is normal, as its driving the speakers?
  The hum is affected by ,and increased with, the master volume, and the contour knobs  ( low pass and high pass). To some degree the EQ will filter the hum by turning down the 64,125,250,500 Hz,and 1 KHz knobs.
   
    It would seem as this is a different issue unrelated to the blown transistor. If so let me know and I'll start another thread. Thanks to all who responded for your helpful guidance

   Cool, the power amp section seems to be working as it should. With a 100 watt bulb in the limiter, the relay kicks over and it will play.
   However, I think I have a bit more to look over. With the amp at idle, no instrument and no load,my limiter glows a bit.
   Speaker load hooked up, bulb stays lit. No change.
  Speaker and instrument hooked up, bulb still lit. No change
With volume up, the blb will get brighter when I hit the strings
  When playing, there is a hum,and a slight distortion in the sound.
  The hum  increases with volume
  So that's it in a nutshell. With power amp disconnected, the bulb still glows. Ground wires checked OK. Unplugged the tube and no change. What do you think?  Is there a place you think is a likely culprit or should I just check all the components value out of circuit? As always, your guidance is appreciated.