Fender Pro Reverb (Blackface, but w/Ultralinear OT). Output Tubes Matching with adjustable bias.

[g1]

vintagekiki , your translator has betrayed you and you have clearly misunderstood several posts in this thread.

[vintagekiki]

Reviewed this thread. It is a machine translation. Everything is possible.
Please specify which posts are misunderstood (#number)

[SoulFetish]

Or we could all look back on this and juuuust laugh... and move on. There are more pressing concerns now!

So, in the particular Pro Reverb I'm currently working on, I've ran into an issue this afternoon which has me stumped so far. Here the symptoms:

After performing the modification specified and photographed earlier, I powered up monitoring mains current draw to ensure no catastrophic faults, and measuring bias control voltages and secondary voltages at the reservoir capacitor node with everything reading out according to expectations.
Set the bias voltage, and plugged it into a speaker to adjust the OTM control, and I was able to dial it in to a near inaudible power supply ripple at idle. Everything going great.
Plugged my guitar into the vibrato channel and noticed low output and distortion on the notes. (kind of similar to the sound of one when one side of push pull circuit drops out, but not as drastic).
Hmmm. So I plugged into the normal channel and that seemed to function normally with plenty of output.
So, another strange behavior was that without activating the vibrato, the depth control was acting as a "cut" control, attenuating higher frequencies as the control was turned up.
First thing - swapped out V2 to eliminate the possibility of a bad tube.
same behavior.
pulled the reverb drive tube and recovery tube - no change.
swapped out all the Preamp tubes - no change.

All resistors measure within 10% (which is prettty good for aging carbon comps).
Checked the voltages at the cathodes, plates, and grids on V2 and everything looks normal.
Pulled the Neon/LDR bug out of circuit, and the strange cut effect is gone, but otherwise the channel has the same low/distorted output.
reflowed all solder joints on the board, and I'm pretty sure I touched up all the pots when I was started the initial work.

dudes, one thing I definitely noticed when touching up and just looking at the board is that it is waxed to high hell... so I'm wondering if I'm fighting a board which has gone conductive and resistive?
would be the first time I personally had one on my bench so I'm not sure what is symptomatic of that.

So, here is a couple of points to note:
I made the dumb mistake of not turning it on and playing it to get a baseline before I opened it up. I have no excuse. I know better.
the second thing is I'm in the middle of breaking down my own shop because the landlord (my father, who I share a shop with) decided to be a bastard and he and my mother are selling their house and building a van to tour the country with.
What the hell is it with these people feeling so entitled to their own stuff like that?
I kid of course. They're the best. But, I'm consolidating shops with my real job, and my scope is already at that shop. So, at the moment I can't signal trace (go figure)

What do you think?

[Enzo]

As soon as I hear the depth control acts like a volume, I think "bulb stuck on" in the opto bug.

[SoulFetish]

As soon as I hear the depth control acts like a volume, I think "bulb stuck on" in the opto bug.

So it was on, actually. But its now lifted out of the circuit

[SoulFetish]

I think I just found it! I accidentally grounded the 820 ohm cathode resistor to the junction of the 1M-1M-2.M Resistors in the oscillator circuit. Shhh Don’t tell anybody

[Enzo]

Oh well, I am sitting here with my second vodka...

Sorry, my next suggestion would have been scope or signal trace a signal through the channel. You may not have official test gear with you, but do you have some other amp? A little practice amp is perfect. Plug a cord into it, connect a cap to the tip of the free end and probe the circuit with that cap. You have a signal tracer right there. Make sure to ground the tracer to the amp chassis.

I can't count the times I grabbed a couple clip wires and a cord and threw together a signal tracer on the spot.

[SoulFetish]

[vintagekiki]

#39

#20

[g1]

I think I just found it! I accidentally grounded the 820 ohm cathode resistor to the junction of the 1M-1M-2.M Resistors in the oscillator circuit. Shhh Don’t tell anybody

The V2B cathode resistor?

[vintagekiki]

The V2B cathode resistor?

Yes probably the end of the cathode resistor that is connected to pin 8 (V1B, V2B).
The other end of the cathode resistor is grounded to the PCB.

[SoulFetish]

The V2B cathode resistor?

No. It was the shared cathode resistor in V4.

The one in this photograph, actually. Weird, right?



Normally it is grounded at the Vibrato Footswitch RCA jact (at the chassis) in this model. But it's such a bad physical grounding choice for two sensitive input stages where one is the reverb recovery, and the other is the mixer of a heavily padded signal.
In earlier SF models, Fender began grounding the bypass cap to this point, separating it from the cathode grounding point. so I generally move cap ground point on those to parallel the cathode resistor on the board when I replace them.
I notice an anomaly, when checking voltages and comparing them to the corresponding schematic voltage. In this stage the measured cathode voltage was >+4V, and I pause for a sec and thought "that ain't right..."

And when I followed that black/white wire to the board location, I quickly saw what I had done. There were so many wires running over this point that I thought the solder point of the 1M/1M/2.2M was the eyelet right next to it, which was the ground location of one of the grid leak resistors (If I remember correctly). I wonder if I would have noticed the condition if I inserted a shorted RCA to activate the vibrato at the time, because that point would have been shorted to the chassis ground.
In any case, after I corrected it and moved to the solder point I intended to, if roared to life with plenty of volume.