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Fender hot rod deville 410 high hum problem drive modes

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  • Fender hot rod deville 410 high hum problem drive modes

    This amp came in with High Hum in the Drive/More Drive mode. The amp came back after being out a couple weeks, with the same complaint : Normal Ch crackling. I never did hear it exhibit that, though went digging for it. What I DID hear was significant HUM when I engaged the Drive/More Drive mode. I pulled the main PCB out, which the previous time I didn’t, having found solder fractures (on the tube PCB) on the two screen resistors and an input grid connection on the driver tube, which took a jumper to restore and re-soldering / seating of the screen resistors.

    Once I got the main PCB lifted out, I put on the surgical loupes and headlight, and went looking, finding loads of solder fractures across the front pots…mostly the support frame connections, but a few pot terminals as well. Then, found more across the back…ribbon cable connections for V2, the power resistors for the +/- 16V supplies, as well as the zener diodes for that section, one of the bias pot leads, and some other parts here and there. Seemed like all that might contribute to what I was hearing.

    Nope…made no difference with the hum. So, moved to the scope probe , and looked at the plates of V1A, V1B, V2A & V2B. Only at V2B was I seeing the culprit. On V1A & B, all I was seeing was low 60Hz hum from the heaters. On V2A, a tiny bit of 120Hz, but when I engaged the Drive mode, that came up loud and strong! I’m not used to hearing that. I swapped preamp tubes around. I found this one didn’t have the 12AY7 that I know I had installed when I made the mod a couple years ago. That tube didn’t make any difference with regards to the hum. Swapped V1 out, no difference either, so I put the two 12AX7’s back in to V1 & V2, and pulled the main PCB back out.

    This time around, I discovered a jumper that had been previously cut by some hack before, and tack-soldered back together from the top of the PCB, as well as C36…filter cap for V1 & V2 plate circuits. It’s (+) lead had been cut and then tack-soldered back into place…which had failed, so there was NO cap in place. Ahah…..found the culprit. Wrong. After replacing that cap, it didn’t make any difference…and, just after switching out of s/b, smoke trails greeted me. Shut down, and found I had installed the replacement 22uF/450V cap at C36 backwards! Pulled the main PCB back out again, getting it right this time. And, found R75 dropping resistor had overheated in that episode, so replaced it.

    But….still yielding high hum when I engage Drive Mode! Probed around some more, all the ripple down thru the supplies drop way down as they should, the choke is working, no issues on the +/- 16V supplies nor on the bias supply. Pulled the main board back out, found a few more solder joints to repair, pulled the relay K2 out and checked it, which was ok. Put it back in. Put the main board back into place, powered back up, still had High Hum. Feeling like I was licked, I tried one more thing. Removed V1 again. Now no High Hum. Put a new 12AX7 into V1, and still, no more High Hum. Huh???!!! I had already done that before. Though….that was prior to all the surgery on the main PCB, as well as on the tube PCB, which did come out in the process, finding a couple more connections on a ribbon cable.

    I put the original 12AX7 back in, just to hear the high hum again. Not so loud this time. Ooooooo……I hate it when they do that. Nevertheless, into the trash with that input tube, put the new one back into place, still have the quiet results. Double-checked it again with the 12AY7 in V2, which restored the moderate increase in gain in the More Drive mode, but I had to pull that tube, it being one of my vintage NOS 12AY7’s. I’ll have to order more new 12AY7’s for these amps with that Drive/More Drive mod.
    Last edited by nevetslab; 09-20-2018, 02:36 AM.
    Logic is an organized way of going wrong with confidence

  • #2
    Never trust a new tube anymore. I've had it happen where I've replaced a hummy tube with a new tube and still have hum. Then try a third tube and the hum goes away. These would be new tubes out of the box as purchased from a common on line vendor. Not out of some crate of untested tubes. This was on a new build so after replacing the tube the first time with a brand new one I assumed there must be an error in my build work. I spent about half a day trying to find what I did wrong before trying the third tube. Now I test newly purchased tubes in a known working amp before using them. And I purchase an extra couple of tubes for each build just in case I need to cull noisy ones. It's a shameful situation.

    And I've also found myself in a time crunch without a new (quiet) tube and had to dig into my vintage, but good or NOS tubes. Fortunately I only pick up such tubes when I stumble onto a bargain so I'm not losing tens to the dollar when it happens.
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "If you're not interested in opinions and the experience of others, why even start a thread?
    You can't just expect consent." Helmholtz

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