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Bassman 100 sloppy repair attempt

Discussion in 'Amp Tech Center' started by tizianod, Nov 27, 2020.

  1. tizianod

    tizianod Tele-Meister

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    Currently I have a Bassman 100 on my bench and I really want to share this experience.

    The owner complained for a loud hum. At first sight it seemed that a tube was shorted out. After a close inspection I replaced the tubes, re-biased the amp, changed some grid resistors and replaced all the electrolytic caps. I had also to replace the rectification diodes, one was even floating... speaking of bad solder skills. Bust most of all... I wondered why some heater wires were melted. So I found out the hum balance pot was shorted and instead there is an artificial center tap installed. Two 100 ohms resistors, check.. but the wires were touching each other. So the safest to do is to check the heater wiring circuit and redo the wiring. Urgh :(

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    Wally and That Cal Webway like this.
  2. ReverendRevolver

    ReverendRevolver Tele-Afflicted

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    What would the benefit of that had been? Like, is it modded otherwise? Sloppy work/elements touching doesn't imply the actions of someone who should be inside an amp, but what would was the purpose?
    "Dude, this SF Bassman will be SO MUCH BETTER with a nerve grating ambient hum..... I'm gonna make a killing doing this mod on hundreds of amps for years to come......"
    I get circumventing hum balance, I get artificial center tap, but sloppily doing it and not checking your work? I have no business inside an amp. And apparently neither does the cat who got in this one prior to you.
     
  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Unfortunately, even if it functions, you have to correct all of that old work or your reputation will be damaged.
     
  4. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Exactly this. This is also why I won't do a partial repair. "Oh, yeah, can you just do a grounded plug on my 50s/60s/70s/80s amp? I don't wanna do all the caps."

    Nope, because I'm not gonna be the last one to touch it before the inevitable BOOM happens.
     
  5. tizianod

    tizianod Tele-Meister

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    Indeed that’s the situation I encounter now... I’m trying to find all the previous attempts to fix the amp and undo the mistakes. I don’t want anybody see this mess and think I did it.

    I don’t stock hum balancing pots and I’m not going to order just one pot and pay tons of shipping. The tubes are pretty good balanced so I’m going for a more safer and neater implementation of an artificial CT. I don’t think something else shorted. Tubes, sockets and the rest seems to look ok. But who knows after all...
     
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  6. BobbyZ

    BobbyZ Doctor of Teleocity

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    Did you put the new artificial center tap off the pilot light? That's how Fender did um, don't have to worry about them touching together that way.

    The last guy's mess makes on otherwise simple job a pain in the arse, don't it? ;)
     
  7. tizianod

    tizianod Tele-Meister

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    Haha Yes. I couldn’t handle it anymore on a Friday evening o_O so I need a little weekend break... I am indeed going to mount the resistors off the pilot light. But first rewire everything and double check if the filament tap from the transformer didn’t short out as well. Don’t want to reword it twice
     
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  8. Wally

    Wally Telefied Ad Free Member

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    It looks as if that rectifier circuit was never really correct from the get go, doesn’t it? I would think it was a bad/questionable connection at the factory.
     
  9. tizianod

    tizianod Tele-Meister

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    Wally, none of the leads were inserted through the holes. They were just soldered on top of them. Seems odd no?
     
  10. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I may be wrong but, I do not think those diodes and the solder job for the rectifier are original.
    A look at bassman 100 gut shots on the internet show all of the diodes look like the silver colored diode of the bias circuit.
     
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  11. Wally

    Wally Telefied Ad Free Member

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    Thanks, Lower. More informed now I am.
     
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  12. tizianod

    tizianod Tele-Meister

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    Indeed I confirm Lowerleftcoast previous comment. I rewired the heater wires and gave the amp a final check today. No more surprises from previous jobs. This amp is up and running.
     
    LudwigvonBirk, BobbyZ and corliss1 like this.
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