Distortion in vibrato channel '81 super reverb

[sharaga]

normal channel sounds fine, i tried replacing v4 and v2 with known good tubes and no change, pulled v3 and no change. all the tubes are new JJs, the screen grid resistors, e-caps, and power tube resistors have all been replaced recently. tried the chopstick method with no luck. any ideas?

[Tom Phillips]

Does the Tremolo effect still seem to operate properly even though the sound may be distorted?

[sharaga]

Does the Tremolo effect still seem to operate properly even though the sound may be distorted?

It does, though when the intensity is turned up it has a slight motorboat effect. I added the fender tick cure when i replaced the e-caps

[Helmholtz]

Please post schematic and check all DC voltages in the vibrato channel.

[sharaga]

I can't find the schematic for this current year but it's identical to this one.
cbs_70w_mstrvol_pullsw_super-pro-bmstr_rev.pdf
I measured
V2a at 242v
V2b at 245v
V4a at 254v
v4B at 260v

[The Dude]

Do you have a scope? If so, run a test sine into the amp and trace through finding where distortion starts.

[pdf64]

Has the amp had any mods (tremolo tick?)?

Has the fx channel ever worked right in the time you’ve owned it?

[Chuck H]

It might help to know the nature of the distortion. Does the amp "break up early" as it were? Is there a lingering buzz behind notes? Are notes crackling? Does the amp do this full time? When played clean? Only when turned up loud or only on loud notes?

[sharaga]

It might help to know the nature of the distortion. Does the amp "break up early" as it were? Is there a lingering buzz behind notes? Are notes crackling? Does the amp do this full time? When played clean? Only when turned up loud or only on loud notes?

The distortion is only on the vibrato channel, only with the channel volume cranked. at 2-3 channel volume with cranked master volume it sounds clean. Bass notes are much more distorted than treble notes (I'm using a 4 string bass to test) Some notes crackle but not all. I can upload a video if it would help.

Has the amp had any mods (tremolo tick?)?

Has the fx channel ever worked right in the time you’ve owned it?

The fx channel has worked before, i think right after i recapped it but before i replaced resistors. as for mods there aren't any (the tick cure came stock in this model with a blue turd coupling cap)

[pdf64]

Is the pull boost switch working properly?

[Tom Phillips]

The distortion is only on the vibrato channel, only with the channel volume cranked. at 2-3 channel volume with cranked master volume it sounds clean. Bass notes are much more distorted than treble notes (I'm using a 4 string bass to test) Some notes crackle but not all...

Those symptoms can be caused by bad connections due to such things as bad solder joints, dirty or loose mechanical connections, components with bad lead connections at or inside the component, something touching another point in the circuit that it should not touch etc. Since you already tried the chopstick method,
another way to test is to play a sustained note and thump on the cabinet with your fist to determine if the mechanical vibration causes the sound to change. You can also inject a clean signal and tune the frequency to a value that causes the problem. I suspect that you do not have test equipment available so you can use a phone app signal generator or download a test tone file to play into the amp.

If thumping on the case causes the issue, especially crackling, then a careful inspection of the un-powered amp guts after the filter caps have been discharged can sometimes find the cause of the problem.

[sharaga]

another way to test is to play a sustained note and thump on the cabinet with your fist to determine if the mechanical vibration causes the sound to change. You can also inject a clean signal and tune the frequency to a value that causes the problem. I suspect that you do not have test equipment available so you can use a phone app signal generator or download a test tone file to play into the amp.

Thumping had no change on the signal. I'll take another look at the leads and reflow all the joints.

Is the pull boost switch working properly?

Curiously, it works for the vibrato channel but not the normal channel. Normal channel bright switch isn't working either.