Background, I’m a hobbiest, but working on electronics since the seventies. I recapped all electrolytic and bumble bee caps and retubed the amp. It has a slight 120 hum. When I plug a guitar Into the mic input it sounds good, breaks up when volume is maxed. When the projector is turned on, it gets emf from the motor. The noise follows the volume. I unplugged the connector that connects the projector and amp and it’s exactly the same noise, so it’s microphonic. I added rubber pads to the mounts to the chassis. I tin foil tented the transformer just to see if I modified the sound. No change in volume or tone. I replaced the mica audio input cap from the PEC no change and removed the PEC, no change. When I remove the input tube of course it all sound is dead. The projectors motor noise is being induced into the audio path, running out of ideas. The optical circuitry does work and it reads the film well. The noise is the same volume as the sound. What am I missing?
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@ Chris M, i,m no expert in vacuum tubes world but when changing those non-polarised caps did you verify the foil side of each new ones ? Fitting them wrong way around can cause some unexpected noise. On the other hand (hopefully you have the schematic diagram or the service manual) i would do a bit by bit comparison with the schematic just bin case you have (by mistake) soldered things in the wrong place etc. On the other hand I would thoroughly verify the wiring and grounding path, certain kinds of metal shielding etc.
Thanks for the heads up, I have the schem. I had a solder drip on a resistor and a foil cap backwards on the input side of the amp, all corrected on multiple scans of my work to see mistakes. I have several sound projectors, this has never happened on other models and this one didn’t work when I bought it. Fully aware if I had the proper test equip. I’d be able to get myself out of this…