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Hum problems from a Hot Rod deluxe

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  • #31
    Well here we go...had one of these hot rod dogs on the bench last year.same problem.went through all of the usual fixes,re-cap power supply,rectifier diodes,tubes,reflow joints,check all grounding points. in the first amp the re-cap and rectifier fix solved it.this one not so lucky.I noticed in another post someone said they could kill it by switching on drive channel then turning the drive pot off. on mine, the only way to kill it is to switch to drive channel then turn master off.also if I turn up drive channel and turn on more drive the hum is unbearable.on the scope it looks like AC riding on the DC.checked cap c49 (22uf / 25v) checked good. replaced B+ caps.bias is stable.If I insert a plug into power amp input jack the noise is gone-completely.if I run signal from pre amp out to another amp noise is there.even if I remove all tubes except V1 & V2. heater AC reads normal although I haven't checked 100ohm resistors.interesting to note from the customer, when the problem first showed up it was intermittent.comes and goes. customer re-tubed.ok for a couple of days then he said it got to sounding terribly distorted. he brought it to me and one output tube was red plating. I subbed a good pair and no more red plating but still distorted and one tube sounded microphonic. swapped tubes and trouble followed the tube.after replacing all tubes,no longer microphonic but still distorted.suspected output tranny. customer insisted on trying to use it.a few weeks later he brings it in,very low power very distorted. open 100k plate resistor-pin 6 phase inverter tube. after replacing,clean channel now loud and clean,usable but with annoying hum. turn on drive channel not usable-unbearable hum. shorting pin 2 or pin 7 of V1 no change. shorting pin 2 of V2 kills it which is the same as turning down master volume.it's in the background in clean mode but in drive mode it's just wacked.any insight appreciated.

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    • #32
      I'll just mention this because it's happened to me, but it may not be your problem.
      Fixing a non-hum issue in this style of amp, problem solved but now there is a hum.
      a) Sometimes just putting the rear panel back on solves it
      b) sometimes the rear panel foil is no longer properly connecting to chassis when re-installed
      c) sometimes the ribbon cable has been moved in a way that makes it hum. The "dress" and bend of the ribbon cables can be critical.
      "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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      • #33
        Originally posted by ogeecheeman View Post
        Well here we go...had one of these hot rod dogs on the bench last year.same problem.went through all of the usual fixes,re-cap power supply,rectifier diodes,tubes,reflow joints,check all grounding points. in the first amp the re-cap and rectifier fix solved it.this one not so lucky.I noticed in another post someone said they could kill it by switching on drive channel then turning the drive pot off. on mine, the only way to kill it is to switch to drive channel then turn master off.also if I turn up drive channel and turn on more drive the hum is unbearable.on the scope it looks like AC riding on the DC.checked cap c49 (22uf / 25v) checked good. replaced B+ caps.bias is stable.If I insert a plug into power amp input jack the noise is gone-completely.if I run signal from pre amp out to another amp noise is there.even if I remove all tubes except V1 & V2. heater AC reads normal although I haven't checked 100ohm resistors.interesting to note from the customer, when the problem first showed up it was intermittent.comes and goes. customer re-tubed.ok for a couple of days then he said it got to sounding terribly distorted. he brought it to me and one output tube was red plating. I subbed a good pair and no more red plating but still distorted and one tube sounded microphonic. swapped tubes and trouble followed the tube.after replacing all tubes,no longer microphonic but still distorted.suspected output tranny. customer insisted on trying to use it.a few weeks later he brings it in,very low power very distorted. open 100k plate resistor-pin 6 phase inverter tube. after replacing,clean channel now loud and clean,usable but with annoying hum. turn on drive channel not usable-unbearable hum. shorting pin 2 or pin 7 of V1 no change. shorting pin 2 of V2 kills it which is the same as turning down master volume.it's in the background in clean mode but in drive mode it's just wacked.any insight appreciated.
        Im a little confused of what youve tried and not, so far, but have you ruled out preamptube failure or maybe leakage through the psu 22uF cap?
        Instead of shorting pin2/7 on V1, try pulling the tube. Is the hum gone?

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        • #34
          Originally posted by g1 View Post
          I'll just mention this because it's happened to me, but it may not be your problem.
          Fixing a non-hum issue in this style of amp, problem solved but now there is a hum.
          a) Sometimes just putting the rear panel back on solves it
          b) sometimes the rear panel foil is no longer properly connecting to chassis when re-installed
          c) sometimes the ribbon cable has been moved in a way that makes it hum. The "dress" and bend of the ribbon cables can be critical.
          I have had that problem with the back panel off...and I also had the problem of that foil coating on the back panel as well.....

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          • #35
            sure.if you pull V1 or V2 hum is gone. I have a laundry list of options to try so after I get back on it tomorrow I'll let you guys know what I have found.

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            • #36
              Hi ogee, welcome to the forum.

              Just a thought, it is better form to just start a new thread for your amp instead of tacking onto the end of a thread that has been dead for four years. You will get better response.
              Education is what you're left with after you have forgotten what you have learned.

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