![]() ![]() Work backwards, it is easier and more logical as you can hear what is happening. Remove C6 (the power amp input) if the hum stops then the hum is coming from a previous stage, if the hum is still there, it is power amplifier/ground born. When op amps fail, it is usually catastrophic and the output goes to a rail voltage with no AC output.ĭon't allow yourself to get confused between DC and Hum. Well, that's what I'm facing, any suggestions are greatly appreciated, as always.Ī slight DC off set is normal. Should I be looking at he caps in that section? REG1 in particular shows signs that it's been cooking pretty good. ![]() The two regulators and two resistors get pretty hot. Meanwhile on the other side of R21 and R20 I have +40V and -40V, respectively. On the input of REG1 and REG2 I would have expected an approximate mirror image as well but in fact REG1 is +21V and REG2 is -28V. Is this a problem? F1 and D2, D3, D4, and D5 seem OK.Īccording to the schematic I should have +15V and -15V at the outputs of REG1 and REG2 but in fact I'm seeing +12V and -12V. Does that make a difference?įor the HT voltage, if I measure AC volts with both DMM probes on either side of the xfrmr tap it measures 235 VAC, but if I measure AC wrt ground I get 132V on one side and 125VAC on the other. Heater voltage looks a bit high, more like 7.3VAC on pins 4 and 6 as opposed to the 6.3 that I would expect. I replaced the 12AX7 with a Chinese tube and that got rid of some of the background hum, but not all, sounds like the part that got eliminated is of a lower frequency. I've looked at all 3 circuit boards for bad solder joints and re-soldered a couple that seemed dodgy but it seems to have been of no consequence as the symptoms remain. ![]() In this scenario holding the guitar cable and moving it around affects the amount of noise and the "reception", like the cable is an antenna. If I plug a guitar cable in there is additional hum and RF (local country station) especially if I turn the volume pot to zero or turn up the reverb pot. ![]() I'm thinking maybe filter caps but I haven't gotten to swapping them out. It has a steady background hum that is not affected by any of the controls. Hey guys, I'm working on a Marshall VS 265, stereo power amps with chorus which has potentially a number of issues (different symptoms at any rate). ![]()
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