Fender Pro Reverb Ultralinear - Distortion on Reverb channel

[Shawnobi]

I'm having distortion issues with my ultralinear pro reverb.
Schematic here: http://www.thetubestore.com/lib/thet...-Schematic.pdf

Distortion is most noticeable on the lower notes, quite pronounced in fact. Only happens on the Reverb channel. Normal channel is completely unaffected.

Things I've tried (Aside from guitar/ cable/ speaker swaps)

Tube substitution - no change
Reverb disconnection - No change
Vibrato select/ deselect - no change.
Reverb controls also have no affect.

Preamp Votages are:

1 2 3 4 5 6 7 8 9
V1 253 0 1.87 0 - 0 249 0.02 2.11 0
V2 245 0 1.98 0 - 0 242 0 2.12 0
V3 399 0.07 5.7 0 - 0 399 0.07 5.7 0
V4 263 0 2.1 0 - 0 227 0.03 2.1 0
V5 220 62 96.7 0 - 0 212 67 97 0
V6 -0.02 63.5 97.6 0 - 0 212 68 97.6 0

I've popped a signal gen' into the guitar input for the reverb channel & scope on V2 pin 7 and the sine wave looks fine.
If I pop the scope on pin 2 of V3 I get this:


Putting the scope probe back on the other side of the 500pF cap connected to V3 pin 2 and the signal looks fine again.

Turning the bass to zero cleans that wave up. Turning the treble all the way up improves it slightly. Mid control has no effect.

I'm a bit lost as to where to go next. Not quite sure if that scope photo is showing actual distortion or out of phase harmonic?
Any advice appreciated.

[TimmyP1955]

My first guess would be a leaky capacitor.

[The Dude]

Normally, when I hear that symptom, the first thing I check is filter caps.

[Justin Thomas]

If it didn't do it before but does it now, something failed. Use the order of suspicion. Tubes, check. Power supply - filter caps, etc - check those? Leaky/failed coupling/signal cap, drifted (like, WAY off) resistors? Bad ground, maybe in the tone stack somewhere? Wiggle the RCA jacks for the reverb cables - it's free and easy, and is a common source of problems...

Why do you have ZERO volts on pin 1 (I presume) of the phase inverter? That should be pretty darn close to pin 6...

Justin

[g1]

Can you double check the voltages you listed for V5 & V6, not sure if they are mixed up or what, but they are not good.

[Shawnobi]

Can you double check the voltages you listed for V5 & V6, not sure if they are mixed up or what, but they are not good.

Will go back over them tonight.

[Shawnobi]

Seems I'd measured V6 twice! (it was getting late...)

1 == 2 == 3 == 4 == 5 == 6 == 7 == 8 == 9
V1 253 == 0 == 1.87 == 0 == 0 == 249 == 0.02 == 2.11 == 0
V2 245 == 0 == 1.98 == 0 == 0 == 242 == 0 == 2.12 == 0
V3 399 == 0.07 == 5.7 == 0 == 0 == 399 == 0.07 == 5.7 == 0
V4 263 == 0 == 2.1 == 0 == 0 == 227 == 0.03 == 2.1 == 0
V5 253 == -1 == 2.1 == -0.2 == -0.2 == 338 == -0.7 == 12.6 == 0
V6 220 == 62 == 96.7 == 0 == 0 == 212 == 68 == 97 == 0

Measurements taken with Vibrato on - speed & depth at Max, as specified on schematic.

[nickb]

I think the distortion you see on V3 pin 2 is just because you are driving it too hard. I would say your vertical scale of the scope shot is 10V/div. In any case, as that is the input to the reverb driver it doesn't matter***.

With the signal still applied to the Vibrato channel, reverb at min, reduce the volume to get 150mV at the jctn of the 0.022uf and 3.3M and then scope the top of the master volume pot. Don't forgot to specify the vertical scale of your scope image.

Try slowing the timebase to 10mS/div and see if you can see if the signal is riding on any hum.

***Edit: I overlooked the MVS switch so that statement is not quite true. Try it in both positions when doing the measurement.

[Shawnobi]

I think the distortion you see on V3 pin 2 is just because you are driving it too hard. I would say your vertical scale of the scope shot is 10V/div. In any case, as that is the input to the reverb driver it doesn't matter***.

With the signal still applied to the Vibrato channel, reverb at min, reduce the volume to get 150mV at the jctn of the 0.022uf and 3.3M and then scope the top of the master volume pot. Don't forgot to specify the vertical scale of your scope image.

Try slowing the timebase to 10mS/div and see if you can see if the signal is riding on any hum.

***Edit: I overlooked the MVS switch so that statement is not quite true. Try it in both positions when doing the measurement.

The MVS has been bypassed on this amp.

Not sure what hum would look like, but it looks like a nice clean wave when I look on the scope.

[nickb]

The MVS has been bypassed on this amp.

Not sure what hum would look like, but it looks like a nice clean wave when I look on the scope.

Please set scope to 10ms/div, take a photo and post it.

Also, post a sound sample showing the problem.

[Shawnobi]

Please set scope to 10ms/div, take a photo and post it.

Also, post a sound sample showing the problem.

Well this is kind of annoying. I've been poking about inside checking resistor values. I plugged it back into to record it and it sounds fine again... Hmm. I'll give it a thrashing at rehearsal next week and see what happens.
Thanks for your help, Nickb. In the mean time I'll carry on checking connections, wires and resistor values incase anything comes up.

[nickb]

Well this is kind of annoying. I've been poking about inside checking resistor values. I plugged it back into to record it and it sounds fine again... Hmm. I'll give it a thrashing at rehearsal next week and see what happens.
Thanks for your help, Nickb. In the mean time I'll carry on checking connections, wires and resistor values incase anything comes up.

I hate it when that happens

[Shawnobi]

On the plus side I found a few wayward resistors. V3 cathode was over 100R off spec.