O-scope - When all else fails

[stratomaster]

Been chipping away at a 67 Super Reverb.  Was in better shape internally, at least visually, than the external appearance would have lead me to believe.  But as I worked on it the gremlins emerged--drifted resistors, weak tremolo, noisy vintage RCA tubes, arcing tube sockets, the whole works. 

I thought I'd had them all sorted as voltages were checking out, the noise floor was sufficiently low, there was minimal DC where there was supposed to be none (thanks vintage fiberboard), the controls all seemed to do what they were supposed to, and nothing was scratchy.  Power draw at idle was good indicating no shorts or ultrasonics.  Even a preliminary power measurement into 2.1 ohm dummy load with a 1kHz @ 175mv input to the normal channel yielded a respectable 41.8W.  But the vibrato channel didn't quite sound right.

In lieu of blindly swapping parts I traced the wave through the first gain stage without issue, but the issue arose after the tonestack.  The sweep of the treble pot yielded the following change in the waveform.


I swapped the 250pF cap and the problem was resolved.

There were no other indicators this cap had failed besides the sound not being "quite right".  Voltages, function, and DC were all as they should have been for a functioning cap.

I will be adding a basic waveform check to all vintage Fenders I service from now on--especially if they still have their original tonestack caps.  Not doing so would have left me with a suboptimal result.

[acheld]

Good pick up!

[shooter]

basic waveform check

once bias n DC is solid, I do a signal-path run-through, fix oddities, then do a final screenshot with max clean signal and max signal as part of the "package" I leave with the owner and future tech

[mresistor]

was it a ceramic cap or a film cap? I'm thinking a ceramic cap.

[stratomaster]

was it a ceramic cap or a film cap? I'm thinking a ceramic cap.

Ceramic treble cap in the tonestack. 250pF

[Lectroid]

I will be adding a basic waveform check to all vintage Fenders I service from now on--especially if they still have their original tonestack caps. 

Nice case study, and you thought up an elegant way to ID the problem.  That's definitely a keeper.        Well done!

[acheld]

Question about the ceramic cap:   any clue as to the dielectric?   

I've been using C0G caps, less often X7R, at times, without issues.  I do try to keep them out of the signal chain, however, that's not always possible.

Film caps, polypropylene if possible, are my preferred go-to.

[stratomaster]

Whatever Fender was using in '67.  I'm not sure as to the dielectric--but I bet there are historians here who would know for sure.  For reference, the two ceramic bright caps hadn't drifted much and measured in spec as did the 250pF in the normal channel. I haven't had cause to pull the vibrato channel 0.02μF ceramic coupler nor the PI input 0.001μF.  Both ceramic.

The 0.02μF ceramic cap in the tremolo oscillator drifted low by quite a bit, so it was replaced. 

Out of the circuit the faulty cap measured 221pF with normal characteristics on my M tester (low voltage) unit.