Fixing or I should say trying to fix a Fender Bandmaster AB763 for a friend at no charge. The Vibrato circuit doesn't seem to be functioning correctly. I fixed the foot switch which was shorted all the time ( wire shorted on RCA Jack). Anyway the vibrato circuit when engauged seems to do very little with a program/music input into the guitar amp input #2. Is there some type of opto senser under a heat shrink on the circuit board or is it some type of light? Any help would be appreciated. I'm 90% done with the amp with everything else working and sounding correctly. I just don't know the symbol on the schematic and or what to replace it with. Any help much appreciated.
Vibrato jack must be shorted across for the trem to function.
There is a 12AX7 for the trem, left center on the schematic. The left triode is the oscillator. Ground the pedal jack and see if there is a low freq oscillation at the plate of that triode. The speed control should affect the frequency. We call that the LFO.
The LFO signal is tapped off the oscillator and fed to the grid of the other triode just to give it strength. The symbol in the oval is a opto-coupler. A neon lamp and a photoresistor cell are put together, yes, in black heat shrink. The part should be visually obvious on the part board. The neon bulb is pulsed by the LFO at the plate of the right triode. it should blink each time. Does it? You should be able to see it flashing out the ends of the shrink tubing. And monitoring the plate of the drive triode should show if it is operating.
The pulsing light controls the photocell, which is simply a varying resistance across the signalpath in the upper channel.
So first determine if e LFO is oscillating, then is that signal amplified by the second tride, then is the neon bulb flashing. The 50k intensity pot: measuer across it with the control all the way up. Do you read lower than 50k substantially? With the trem running, do you see the reading unstable or moving? (indicative of operation)
There is a 12AX7 for the trem, left center on the schematic. The left triode is the oscillator. Ground the pedal jack and see if there is a low freq oscillation at the plate of that triode. The speed control should affect the frequency. We call that the LFO.
The LFO signal is tapped off the oscillator and fed to the grid of the other triode just to give it strength. The symbol in the oval is a opto-coupler. A neon lamp and a photoresistor cell are put together, yes, in black heat shrink. The part should be visually obvious on the part board. The neon bulb is pulsed by the LFO at the plate of the right triode. it should blink each time. Does it? You should be able to see it flashing out the ends of the shrink tubing. And monitoring the plate of the drive triode should show if it is operating.
The pulsing light controls the photocell, which is simply a varying resistance across the signalpath in the upper channel.
So first determine if e LFO is oscillating, then is that signal amplified by the second tride, then is the neon bulb flashing. The 50k intensity pot: measuer across it with the control all the way up. Do you read lower than 50k substantially? With the trem running, do you see the reading unstable or moving? (indicative of operation)
I found the problem shortly after I posted the thread. Someone put the wrong tube in a 12ay7 instead of a 12aX7. The cathode resistor was open. Replaced the tube and the cathode resistor and it now works.
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