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Sizzling & Crackling in HR Deville Problem

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  • #16
    Most likely you need to re-solder the power tube sockets.
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


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    • #17
      Originally posted by g1 View Post
      Most likely you need to re-solder the power tube sockets.
      I'll reflow those tomorrow morning. What do you think about the ON &/or STANDBY switches? Would you know how to test the switch?

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      • #18
        wire across it.

        So by design, putting in a 1 ohm resistor, allows TP30 to measure the plate current. Why then does the schematic state TP30 in mV?
        Because the current through the 1 ohm resistor is causing a voltage across the resistor - Ohm's Law - so you measure in VOLTS. Since the current and voltage will be the same in a 1 ohm resistor, we INFER the current by reading that voltage. We COULD measure current directly, but then you would need to open the circuit and insert an ammeter in series. The 1 ohm was chosen precisely for that reason, the numbers are the same for volts and current. We COULD have used a 17.9 ohm resistor, in which case 60ma would read as 1.07v. And then the schematic would simply say adjust for 1.07v at the TP. But that offers no convenience. (I picked 17.9 at random) So we are not MEASURING current, we are inferring it from a voltage reading.

        I agree with g1, resolder all the pins on the power tube sockets. I don't care how good they appear to be.
        Education is what you're left with after you have forgotten what you have learned.

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        • #19
          Originally posted by Perkinsman View Post
          POWER AMP IN jack, turn it down-Noise still present
          PREAMP OUT jack to some other amp-Noise still present in Deville, not other amp speaker
          So this seems to narrow it down to the power amp section, or possibly the effects loop jacks/circuit. I would pull the PI tube and see if the noise persists. If there is still noise with no PI tube it has to be the power tubes. If the noise goes away it narrows it down to the loop jacks or the PI. I hate the 9 pin PCB jacks that they use on these, they are quite fragile internally and sometimes they behave like they need a good cleaning yet cleaning doesn't help. You could do a bridge past the power amp in jack pretty easily to rule it out.

          edit: since you said the reverb level doesn't affect the noise I would guess it isn't the power amp in jack, as anything that happens at the jack goes straight into the reverb circuit.
          Last edited by glebert; 11-17-2019, 08:42 AM.

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          • #20
            Originally posted by glebert View Post
            So this seems to narrow it down to the power amp section, or possibly the effects loop jacks/circuit. I would pull the PI tube and see if the noise persists. If there is still noise with no PI tube it has to be the power tubes. If the noise goes away it narrows it down to the loop jacks or the PI. I hate the 9 pin PCB jacks that they use on these, they are quite fragile internally and sometimes they behave like they need a good cleaning yet cleaning doesn't help. You could do a bridge past the power amp in jack pretty easily to rule it out.

            edit: since you said the reverb level doesn't affect the noise I would guess it isn't the power amp in jack, as anything that happens at the jack goes straight into the reverb circuit.
            I resoldered all the power tube pins but still had noise. I had previously plugged directly into the power amp jack and still had crackling/sizzling....BUT i pulled the PI and it went dead quiet. I resoldered those pins and put the tube back in...so far, so good but since it's intermittent I'll play it awhile and see what happens. Meanwhile, I've got questions......I noticed some "cap stuff" on one of the caps C34 and it's only measuring 10.5V. If I'mreading the schematic correctly, it should be up over 450. Is it appropriate to change out just one cap if it's bad? Also, why are they rated at 350v when the voltages are much higher, wouldn't 500v be more appropriate?

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            • #21
              I noticed some "cap stuff" on one of the caps C34 and it's only measuring 10.5V. If I'mreading the schematic correctly, it should be up over 450. Is it appropriate to change out just one cap if it's bad? Also, why are they rated at 350v when the voltages are much higher, wouldn't 500v be more appropriate?
              The voltage divider R72/73 takes care that with good caps the DCV across C34 is 50% of the voltage at point Z, i.e. about 240VDC. If it's much lower C34 is leaky and must be replaced. What is the Z voltage?
              - Own Opinions Only -

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              • #22
                I will add that the IC branded capacitors in modern Fender amps are notoriously famous for causing a range of problems. When I worked at a Fender authorized shop and needed to call support, it was the first thing they often said, "did you change those caps?"
                It's weird, because it WAS working fine.....

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                • #23
                  I have seen problems with the 82K and 100K PI plate resistors so often in the Hot Rod/Blues Deluxe/DeVille series I automatically change them in every amp I see.
                  With the PI pulled out the noise disappears because there is no DC current though the resistors, I suggest using 1 Watt resistors for their higher breakdown voltage spec.
                  Symptoms are: crackling when they start to break down, distortion when they go open.

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                  • #24
                    Good point, could be 2 separate issues, solder joints on power tube sockets causing bias problems/redplate, and PI plate resistors making crackling.
                    Originally posted by Enzo
                    I have a sign in my shop that says, "Never think up reasons not to check something."


                    Comment


                    • #25
                      Originally posted by Helmholtz View Post
                      The voltage divider R72/73 takes care that with good caps the DCV across C34 is 50% of the voltage at point Z, i.e. about 240VDC. If it's much lower C34 is leaky and must be replaced. What is the Z voltage?
                      HH, I'm just learning to read schematics. I see that point Z is at the intersection of R72 & R74 & whatever SP2 means but they don't actually connect on the top of the board that I see so is point Z on the + side of C33 or C34? I measured 480dc on the + side of C33 (R72) and 8.5dc on the + side of C34.

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                      • #26
                        Originally posted by g1 View Post
                        Good point, could be 2 separate issues, solder joints on power tube sockets causing bias problems/redplate, and PI plate resistors making crackling.
                        Probably is multiple issues, including a leaky cap C34. The bias current has stabilized and there hasn't been any more crackling since I resoldered the V3, PI pins but I'm going to replace R57 & R58 and the filter caps tomorrow. Several of the smaller caps are pretty loose on the board so I'll resolder those too.

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                        • #27
                          SP = solder post, or something like that. Look at the schematic, they are simply places on the board to connect the wires from the choke, which is not on the board itself. Look at the parts layout drawing, or for that matter look on your board to where the choke wires go, SP is probably silk screened on the board.

                          I see that point Z is at the intersection of R72 & R74 & whatever SP2 means but they don't actually connect on the top of the board that I see so is point Z on the + side of C33 or C34?
                          I think you are trying too hard to see it as complicated. I am looking at the schematic and I see: SP2, R72, R74, and C33 ALL connected to node Z. C34 connects from ground to the BOTTOM end of C33, not node Z. Although they are not drawn directly above one another, the two caps are in series, and the two parallel resistors (parallel to the caps, not to each other) are ther to balance the voltage across the two caps so each has about half. 8.5v indicates a problem.
                          Education is what you're left with after you have forgotten what you have learned.

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