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Fender Hotrod Deluxe Circuit Board Buzzing

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  • #16
    I take it back, if that is what you are hearing and where you are hearing it, it is those green ceramic caps. Ceramic caps have a tendency to be piezo electric and your probably hearing the effects of that.
    If those filter caps where the source of this noise, it would most probably be audible in the output of the amp as well
    Last edited by SoulFetish; 03-18-2019, 03:39 AM.
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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    • #17
      VG -

      A cap might not show physical signs of failure, but it could be bad - especially the IC brand!! If you have a cap tester, you can lift one end and measure it. I have a regular Cap Tester meter and the "Peak Atlas ESR60" meter. The best test is to apply a high voltage and check for leakage. For that, I use my trusty ole Sprague Tel-Ohmike Cap Analyzer. Anyway, be careful not to assume components are ok just because they look ok.
      Last edited by TomCarlos; 03-18-2019, 06:20 AM.

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      • #18
        So maybe the row of green caps? FIne. Take a piece of foam rubber or a soft sponge, and press down over them. Does it stop or stifle the noise?
        Education is what you're left with after you have forgotten what you have learned.

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        • #19
          Taking a second look at you're board brought a couple of other things to mind, independent of the issue you are dealing with now. Firstly, that board is in remarkably good shape for being just over 16 years old, with no real visual signs of heat stress which I see all the time in these amps.

          **Once you get this particular noise issue all sussed out, there is some preventative maintenance you can do while you have the amp open which could go a long way in keeping your amp from experiencing some common failures which are problematic for these particular amplifiers.

          1. - re-solder pcb connections on those two 5W, white cement resistors and zener diode in LV supply. These components dissipate a lot of heat and will dry out and crack their solder joints. Add enough silicone under the resistors to prevent them from moving under vibrations, but still allow enough airflow to dissipate their heat.

          2. - re-solder all the filter cap solder joints on the pcb. Or you can replace all the filter caps if you choose, it's your amp. We only replace the filters if the customer specifically requests it, or they show any signs of failure. Again, add some silicone between the caps and pcb to prevent the caps from moving due to vibration and breaking their solder joints. Neutral cure silicone is best to use on electronics, but you can probably use regular RTV silicone for this job and be fine. Just leave the cover off and maybe put a fan on it while it cures.

          3. - remove the existing 470Ω screen resistors and replace them with 3-5W metal oxide or power film resistors (If it were my amp, I would use 5W MOX resistors and sleep well at night, knowing they are up for diming the amp and should even survive a dead short in the output tubes long enough to blow for the fuse to blow). The ones in there are under-spec'd and commonly fail. Solder the new ones on the top/visible side of the board. This will give you easy access for checking/servicing and provide a little more insulation from all the heat rising off the tubes, while also allowing the heat to dissipate better than if it were mounted under the board.

          4. - remove and replace the plate resistors in the phase inverter as well for the same reason as above. In these amps, I've seen these fail more often than the other preamp tubes. My working hypothesis is that these resistor failures are the combined result of high ambient temperature surrounding the resistors, being mounted right against the PCB, and being physically mounted under an insulating pcb, which will prevent adequate hot air convection and heat dissipation. All these particular resistors are mounted physically much closer to the output tubes. The hot air generated by the tubes will want to flow physically up, due to convection. Hot air generated by tubes often increases the temperature inside an amp chassis.
          Because of the physical design of these amps, I suspect that the area between the tubes board and the chassis traps some of this heat, leading to premature component failure. Ambient temperature is the greatest variable which factors into a resistor being able to dissipate the heat necessary for it's rated power. High ambient temperatures can significantly de-rate the power handling for any resistor, and this point is either consistently ignored or completely disregarded in many commercial tube amp designs - particularly in combo amps.

          5. - go around and re-tighten all the screws in the baffle board and speakers.

          6. - listen to the other techs who may want to add something to this list which I may have overlooked.

          When I tell people that we see these amps in all the time, at first it may seem like a bad thing. But it only means that Fender must have sold a shitload of these amplifiers, and that these amps are very repairable.
          It's one of the endorsements I would really give these amps. No amp design is perfect, and these amps can hold up to a lot of abuse.
          Last edited by SoulFetish; 03-19-2019, 11:47 AM. Reason: I misspelled “MOX”. go figure.....
          If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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          • #20
            I took Enzo's suggestion and systematically covered all components in the highlighted area. The sound remained constant (didn't even change pitch). I then tried an old car engine troubleshooting trick and touched each component in the area with the paper towel roller. Interestingly, when I touched the ceramic capacitors, the sound got slightly louder and more pronounced. Some caps were louder than the others. So, it seems we may have found the culprit.

            I plan to change the snubber capacitors with polypropylene film/foil capacitors -- any thoughts here? And, while I have the amp open, I'll change the large filter caps and do the stuff SF suggested.

            -VGuitarist

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            • #21
              I think the ceramic caps transfer their vibrations to the board and make the whole board radiate the buzz.
              This effect can also be observed in dimmers and SMPS, even with some film caps.

              The parallel caps are used to suppress electrical noise and RFI produced by slow switching standard diodes. If you replace the diodes with fast switching types (UF4007), you should not need the caps.
              - Own Opinions Only -

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              • #22
                I like Helmholtz’s suggestion. Replace the 1N4007s with the UF recovery diodes he recommended. Those with probably be cheaper than 1000V film snubber caps and are a better fix.
                If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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                • #23
                  Parts are ordered as suggested. I'll report back when installed. Fingers crossed...

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                  • #24
                    I was reading some articles on Rod Elliot's site (so much great info), and he had a write up on snubbers and their use in audio.
                    I remembered this particular thread, and resurrect it only to show some interesting measurements recorded from EMI caused by standard 1N400* and UF diode switching with/without snubbers.
                    http://sound.whsites.net/articles/psu-snubber2.htm


                    For those interested, the whole opinion piece is written up here:
                    http://sound.whsites.net/articles/psu-snubber.htm
                    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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                    • #25
                      Those gray IC (Illinois Capacitor) caps are junk. I replace them whenever possible. I've had so many strange problems disappear after replacing them.

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                      • #26
                        All,

                        The noise issue is fixed -- the snubber capacitors C62, C63, C64 and C65 were definitely oscillating. I replaced them with 8200pf @ 1.6KV metal film capacitors. In addition to the capacitors, I replaced the rectifier diodes D6, D7, D8 and D9 (1N4006) with UF4006 fast switching diodes. The circuit board is ultra quiet now.

                        While I had the amp open, I followed your recommendation and replaced a handful of other components.

                        1. Purchased a capacitor replacement kit from Fromel Electronics. It covered the larger capacitors C31 (47uf/500v), C33 (22uf/500v), C35 (22uf/500v) and C36 (22uf/500v) and a few smaller capacitors: C1 (47uf/25v), C8 (1uf/100v), C9 (22uf/35v) and C56 (47uf/25v). To be honest, a few of their caps were under-rated, so I backfilled them with parts from Mouser electronics (C1, C9 and C56).
                        2. Replaced R61 and R62 screen resistors (470 ohm) with 2-watt metal film (flameproof) resistors.
                        3. Replaced plate resistors R11 (100K), R4 (100K), R16 (100K), R22 (100K), R57 (82K) and R58 (100K) with 2-watt metal film (flameproof) resistors.
                        4. I ordered a tube kit from Tube Depot and replaced all 5 tubes (3x Tung-Sol 12AX7 and 2x Tung-Sol 6L6GCSTR)

                        Beyond that, I own a nice Pace solder/de-soldering station, so I de-soldered/re-soldered every connection. I also lifted the two 5-watt resistors (R78 and R79) off the board. This job required me to purchase neutral cure silicone (Amazon) to glue the large filter caps to the circuit board. All of the component parts came from Mouser Electronics. If you want part numbers, please let me know.


                        Thank you very much for your feedback and guidance throughout this event. I plugged in the amp for the first time today and it sounds great! Hopefully, it will last for many years.

                        With Deepest Appreciation,

                        -VGuitarist

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                        • #27
                          Nicely done and thanks for the details you provided. I'm gonna bookmark this one!

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                          • #28
                            Hello,

                            I am having similar issues with my Hot Rod Deluxe. Can you please provide the part# information? Thanks.

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                            • #29
                              Originally posted by nothing2fear View Post
                              Hello,

                              I am having similar issues with my Hot Rod Deluxe. Can you please provide the part# information? Thanks.
                              Issues? Do you mean you’re hearing audible buzzing coming from the main board?
                              If your issue is caused by the ceramic snubbers (like rhe OP), I’ve never know it to cause any kind of problem in these amps, other than a little annoyance or concern if you don’t work on these much.
                              But if you wanted to get rid of it, you could just replace the 4 diodes in the HT bridge rectifier with MUR2100E diodes and leave out the snubber caps altogether. This would be the simplest improvement.
                              If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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                              • #30
                                Thanks for the reply. Yes. I am hearing an audible buzz from the main board. I am also having channel switching issues. I have replaced the C31 (47uf/500v), C33 (22uf/500v), C35 (22uf/500v) and C36 (22uf/500v) with new F&T caps and the resistors in R61 and R62 with identical equivalents (5W 470ohm cement type). The traces around the R61/R62 resistors were badly damaged. I ran jumper wires and confirmed continuity, but something is still not right. I have ordered replacement diodes and resistors for around the R61/R62 resisistors. I stumbled onto this forum after I did that work. I want to proactively replace as much as possible to make this amp reliable. Since i will have access i think it may be a good idea to replace C62, C63, C64, C65, D6, D7, D8 and D9 . I can find the Diodes easy enough but I am not sure which capacitors to get with the spec "8200pf @ 1.6KV". I know how to solder and follow a schematic but i get a bit lost when sourcing parts. Do you have recommendations for a suitable capacitor replacement? Any help is greatly appreciated.

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