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Hiwatt custom 20 tube amp hum problem

Discussion in 'Repairing Electronics' started by frank57, Nov 8, 2009.

  1. frank57

    frank57 New Member

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    Would it help with the high end static or buzz and the hum?
    There were low pass filters of some kind in there before, and there was no real buzz problem.
    Tech took them out for some reason.
    Other thing I remember was some squealing when the amp is turned up.
    Good or bad idea? There are no resistors anywhere on the tube pcb.

    I'm hoping to get a new jack this week so I can test the amp better .
     
  2. Chippie

    Chippie New Member

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    Well rather than second guessing where the low pass filter components were connected, would it be a better idea to ask the tech for the parts he removed and ask him to show where they were connected?

    If the filter was added to attenuate the buzz then the amp is badly designed imho
     
    Last edited: Feb 8, 2010
  3. frank57

    frank57 New Member

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    They threw the parts out so I have no clue.
    They also said the amp "was fixed" and it wasn't, so...

    Just trying to figure out where to put the grid stopper to see what it does, if anything.
    Almost every amp has one, but where would I put it? On the tube socket on the tube pcb?
    Aikenamps and PaulRuby amps seem to like them.
    Right now because it's biased at -24, the buzz isn't a big issue.
    It's pretty much gone.
    I'm getting a bias plate voltage tester from Amp-head,
    so at least I can tell what the voltage is and what's what with the bias.

    Might it be a good idea to simply replace the filter caps or have them ready to go?
    Not sure if they are snap in or radial leads. What specs should I look for in a filter cap?
    c23 was replaced with a 100volt cap. I've updated the schematic with the voltages.
    How to I get rid of the glue they used to glue them down?
     

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  4. dave

    Dave New Member

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  5. frank57

    frank57 New Member

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    Anybody know exactly which connectors they used?
    These are 13mm long and 2 mm wide with a tab on the back.

    Here's a photo. The wire is 22awg 105 degrees or 90 degrees.
     

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  6. frank57

    frank57 New Member

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    Marshall and fender jacks are too small.
    Can I take a cliff solder lug s2-1220a and cut the legs to fit?
     
  7. frank57

    frank57 New Member

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    Here's a great suggestion I got:
    "Move the blue standby wire to the negative lug of the main filter cap C22 100uFd 450V. I mean TO that lug, even though this will mean a blobby butt-solder joint.

    There are many other PCB problems, but when I started trying to work it out I got a headache. Combined with general low-cost "quality", I'd be real tempted to bulldoze the chassis and start over with proper turrets and sockets.

    Getting the massive ripple-garbage localized within 1/8" of ground bus, instead of spread-out over two inches in the middle, may help a lot.

    If you do not understand exactly what to do, you can smoke the whole amp.'"
    What do you guys think?
     
  8. frank57

    frank57 New Member

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    Anybody have any troubleshooting ideas for this amp?
    I've narrowed it down to tube 2.
    Shorting pin 2 kills all the hum and buzz.
    There are some voltages and stuff here:
    Would this make my Hiwatt go kaboom?
     
  9. gary350

    gary350 Well-Known Member

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    If you have a lot of hum and buzzing then you have a bad power supply capacitor.

    Lets see the circuit drawing.
     
  10. frank57

    frank57 New Member

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    Those caps are all new.
    Although I'm not sure how to check them.
    Maybe the filtering is wrong for the amp?
    The voltages are too high on the heaters so that's a known issue.
    They're 3.82 and 7 .6 I think it was.
    There is a plan to move the diodes off the board and see if that's the cause of the problems.
    The problem seems to be coming in from the input jack to pin7 of v2.
    There I don't quite short out all the noise.
     

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    Last edited: Oct 28, 2010

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