VOX AC30C2X - Top Boost extremely low and distorted - Normal channel is ok

[spunko]

DEJA VU... Some time ago I posted a similar topic for another amp.

With this one the Normal channel is ok, but the top boost is extremely low and distorted if you crank it. New tubes didn't fix it.
I compared voltage readings I have in my notes against the other VOX amp I had and they all are basically the same. Checked voltages on the oamps (NJM2147) and all of them have -27 / +27 VDC.

With my signal tracer I noticed that when plugged into the top boost channel the oamps sound low and distorted, but when using the Normal channel they sound good so I assume the ICs are ok.

As far as I understand the diagram, one half of V1 is for top boost, and then it goes to V2 both halfs. On V2 I have strong signal out of pin 1, but none out of pin 6.
Also tested all the resistors from the PCB with the 12ax7 tubes and they all read good against the diagram. Unfortunately the diagram doesn't show voltages or signal test points and I'm lost  .

 

[shooter]

With my signal tracer I noticed that when plugged into the top boost channel

compare the junction of R7/R8 to the junction R18/R20
sound the same?

[spunko]

Yes they sound the same.
Signal on Normal ch R18/20 sounds the same as R7/R8 with signal on Top Boost CH

[shooter]

put a signal in the norm out the FX send into any amp, listen, then send then Top boost out, same?

[spunko]

put a signal in the norm out the FX send into any amp, listen, then send then Top boost out, same?

Not the same, that was the first test I did without power tubes. Norm CH sounds ok, TB CH low and distorted. Then I started to check with the signal tracer.

[AlNewman]

Have you tried a different tube in v2 position?
Looks like the normal channel bypasses it.

[shooter]

New tubes didn't fix it.

V2 I have strong signal out of pin 1, but none out of pin 6.

check the jumper at J14C J16C, that would kill 1/2 the tube
WITH power OFF, ohm from cathode to R21 C84 junction

[spunko]

check the jumper at J14C J16C, that would kill 1/2 the tube
WITH power OFF, ohm from cathode to R21 C84 junction

Yes, there is continuity between those points. I have checked all the jumper cables and all passed the continuity test. Also the pcb traces that goes to the tube sockets.

I wonder if there should be signal on pin6 of V2, looking at the diagram I think it should, but I dont understand why is not present.

[AlNewman]

I wonder if there should be signal on pin6 of V2, looking at the diagram I think it should, but I dont understand why is not present.

It's a cathode follower stage.  The signal should be present at pin 7 and pin 8. 

[Maddo]

I’ve had two AC30CC's and on both mysteriously volume pots (can’t remember which channel) went bad.
Did you check the pots resistance while continuously turning them? On mine the resistance rose to a point and then they either shorted or stayed low in value.

[spunko]

I’ve had two AC30CC's and on both mysteriously volume pots (can’t remember which channel) went bad.
Did you check the pots resistance while continuously turning them? On mine the resistance rose to a point and then they either shorted or stayed low in value.

I totally forgot to post. Yes it was the Volumen pot  I was loosing my mind because I had already cleanned the pots checked voltages and all that stuff, but suddenly it worked ok again, full output, so I turned the pot down and it went silent again. I ended up changing the three A500k volumen pots, the other 2 were noisy even after cleaning.

Thanks everybody for replying this topic.