Axtman 4/7/2018 3:50 PM
Tremolo ticking but only when a note is played.
My late 70's Vibrosonic reverb still has a ticking or thumping in the tremolo. I have done the Fender service bulletin #9 fix and put a 0.01 uf capacitor across the 10M resistor to the neon bulb to ground. This cap eliminate the ticking when the amp was not being played but the amp thumps when a note is played. The thumping increases and decreases with volume change on the guitar and/or amp.

Any ideas?
Jazz P Bass 4/7/2018 7:21 PM
It may be a bad decoupling capacitor.

80uf/ 75 volt on the schematic.

Axtman 4/7/2018 7:56 PM
I did replace all the electrolytic capacitors. I believe I replaced that capacitor with a 100uf/100v cap. I will check under the "dog house" for the cap and connections. Thanks.

BTW, my am is the 135 utralinear version.

tubeswell 4/7/2018 8:35 PM
+! what Jazz P Bass said. Could be a bad/open cap or a bad ground connection at one of the filter caps. If any of the filter caps aren't decoupling the different HT-supply nodes from one another, you'll get inter-stage feedback happening through the power rail under signal conditions. Since the LFO stage takes its supply from the screen supply node, I'd check there first. Meter-out the ground lead of each cap with the amp's chassis to check you have a continuity in the ground return path.
g1 4/8/2018 1:33 PM
And just to verify, this is a thump, and not a tick you are dealing with?
Axtman 4/8/2018 2:10 PM
Yes. It is hard to describe but the noise is more of a flubby "thumping" than a sharp "ticking" sound. This thumping is noticeable when the strings are hit hard and decays as the string vibration decays. The thumping is most prevalent on the bass frequencies. I find if I set my signal generator to 100 Hz the thumping is very prominent. If I turn the frequency up to 1K Hz the thumping is less prominent. This thumping sound is affected by the intensity, speed, volume and master volume knob. If an input signal is removed then the thumping sound disappears and is not affected by controls. The sound is only present when there is an input signal.

Before I installed the 0.01 uf capacitor across the "roach" there was a definite ticking sound coming from the amp without input and when the vibrato (tremolo) pedal was engaged (or input was shorted). After I installed the capacitor the ticking sound went away.
Axtman 4/16/2018 4:42 PM
I have given up on repairing the "thumping". I can remove the "tick" but not the thump.