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Started by FenderDeluxe112Plus, April 12, 2022, 04:39:01 PM
Quote from: phatt on April 24, 2022, 09:39:20 PMA scope may not help much if it's ground hum.I'm assuming the amp is working and passing signal except for the hum?Well as there is no ripple on the supply and voltages are within spec then the most likely issue is one of 2 possibilities.1/ open ground connection somewhere.If the star ground node of main supply is not connected back to chassis correctly then it can cause hum issues. (Which is the node between C47 & C48 or Conn CP4)You can test simply by shorting the main Common back to chassis with a wire. If the hum stops then you found the issue,, if hum increases then you just created a ground loop,, which brings us to,, 2/ Something is grounded that should Not be grounded, which would cause a ground loop hum.Ground plane issues can be a nightmare to track down.sometimes in design phase one has to use an alligator clip to find the best ground path before committing to a layout.Often components like input jacks, FX loops and speaker outputs, even the PCB need to be *Isolated from chassis*This will depend on how the circuit common was designed which is hardly ever noted on schematics.Tiss easy to loose these little isolation washers or forget to reinstall them.If you acquired this amp with issues then someone may have already worked on it and if small parts have been lost then you have no idea why it hums.*I would be researching ground path issues long before replacing parts in hope.*I got caught once, I did not notice an isolation washer dropped off a PCB post and the hum drove me nuts for a few days.finally found it hiding in the chassis corner. Phil.
Quote from: FenderDeluxe112Plus on April 28, 2022, 11:45:12 AMI am reading around 24ohms resistance from cp4 to chassis ground.
Quote from: phatt on April 29, 2022, 02:23:11 AMJust turn the amp on and with a length of wire with alligator clips on the ends and just probe the Known ground points of the pcb and related ground points back to chassis. **Just make darn sure you don't short power nodes back to ground.**Use the schematic to verify test points and ground nodes.If hum increases then you have created a ground loop.If it reduces hum then you know there is a ground missing somewhere.BTW the speaker Neg terminal is NOT Ground. This is a current feedback system and spk NEG is lifted from circuit common via that big 10 Watt resistor, R76. You can visually work out the ground path but often not obvious.Hence the wire probe can help define the problem.Maybe post some circuit pictures and we might be able to help more?Phil.