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Another Hot Rod Deluxe Issue

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  • #1

    Another Hot Rod Deluxe Issue

    I have yet another HRD here for repair with a weird problem (imagine that). The amp looses gain and distorts badly ONLY when the reverb is turned up. Otherwise it works and sounds just fine. I have removed the board twice, looking closely for any poor solders on the foil side. None found anywhere. no loose or broken components anywhere. Power supply voltages are where they should be and they don't fluctuate. Of course, the first thing I checked were the cables to the pan and the xfmr's in the pan - all is well. I've also changed the reverb driver / recovery amp IC.

    Has anyone encountered a similar problem with a Hot Rod? I've fixed a lot of Hot Rods, but this one has me stumped.
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  • #2
    Opamp dual supply is good?

    Have you scoped U2, the Reverb IC.

    You have to see where it is failing before it can be repaired.

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    • #3
      Under steady-state conditions, that is, with a sine wave input to the amp, both continuous and burst, the performance of the U2 is fine. The second thing I checked was the + / - 15 volt supply *at the pins* of U2. That is correct also and is consistent.

      The amp looses gain and distorts badly ONLY when the reverb is turned up with an instrument (guitar) as the input signal. And the Strat along with everything else in the signal chain has been eliminated as a potential source of the problem.

      I remember having a similar problem with amps made in the 70's with 70's era 4558-type IC's. They were made considerably different then, and tended to fail in unusual ways. Todays IC's are much more "dependable".

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      • #4
        Does turning up the reverb increase the output volume?

        Maybe the recovering stage is amplifying too much.
        Open feedback resistor in that amp stage?

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        • #5
          Your description sound like it has a parasitic oscillation happening when you turn up the reverb control. Check all of the little snubber caps like C15, C28 and C30. Any of them loose or broken?

          If you have one, scope the output and see what happens when you turn up the reverb control.

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