Marshall Valvestate 8200 issues

Discussion in 'Amp Tech Center' started by Tube Life, Dec 12, 2019.

  1. Tube Life

    Tube Life TDPRI Member

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    So my pops bought this head for me brand new back in the early 90’s when I was a kid and so it holds some sentimental value to me. Because of this, I am up for the challenge to try and fix it myself. When I first got it I used it for 3-4 years problem free then it suddenly stopped producing sound. Never blew fuses, no hum, always powered up but just silent.

    So decades later after recently building a MojoTone Super Reverb, and recapping some McIntosh power amps, I’m facing my greatest challenge to date, trying to fix this. I brought it up on a variac with no speaker hooked up and everything seemed ok. Replaced the one 12ax7 in it. Then hooked up a speaker and the speaker pulled in and out repeatedly and had a loud hum. So ordered up the 4700uf 50v filter caps and a few others on the power amp board and replaced. Nothing changed, as soon as I get to about 5 volts on the variac I can hear an electronic hum coming out of the speaker so I have not put full power with a speaker attached since, as I’m nervous about ruining the speaker. Before changing the filter caps I hooked up a signal gen from my phone and put the oscilloscope on the speaker output with a dummy load and was able to get an ok wave but was inconsistent or intermittent.

    So I’m basically asking anyone out there that may have worked on this particular era of amps for any insight or direction to try and get her singing again. Thanks in advance!!
     
  2. fraser

    fraser Tele-Holic

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    I have one of the early 90s British made valve state 8080.
    Bought new, returned 3 times due to issues while under warranty.

    The problem with mine is the pots, they are board mounted, then they stick out the front of the panel, waiting to get bumped around.
    The solder joints to the board get jiggled and break.
    It’s a design flaw.
    And the pots themselves seem really prone to going bad.

    ive had the problem with no sound a number of times, it was always the pots.
    I don’t recall how many I replaced in the time I used that amp.

    I stopped using it about 15 years ago.
    It currently doesn’t work due to several pots.
    It still makes a good speaker cabinet.
    One day if I get bored I’d like to tear it down and try to find a way to replace all the pots with decent ones that solder off the board.
    Not a bad sounding amp, not worth enough to bother selling as is,
    Not going to toss it out, and I dislike large broken things cluttering my apartment.

    anyway, check the solder pads on the pots.
    Also, work the pots back and forth with the amp powered up and see if yu can get a reaction
     
    Last edited: Dec 13, 2019
  3. brokenbones

    brokenbones Tele-Meister

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    It sounds like maybe a transistor has gone bad. Those amps were notorious for bad transformers on one of the channels but i don't think that's the case here. I would find the schematic, hook up a dummy load, and start measuring voltage according to the schematic.
     
  4. Tube Life

    Tube Life TDPRI Member

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    That’s good to know regarding the pots. I just placed another order with Mouser for the 5 radial caps on the preamp board which I should have changed initially so I’ll have access to check the pots more closely and their solder joints.

    I’m a tube guy all the way, but I’m up for the challenge with this amp, parts are cheap and my labor is free. Thanks for your response, more to come!
     
  5. Tube Life

    Tube Life TDPRI Member

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    Yea I printed out the schematics for each section, at this link:
    https://drtube.com/library/schematics/69-marshall-schemas#VS1

    It’s actually the 8280 schematic which apparently is close to identical to the 8200. I also ordered three VN2410L transistors for TR-1, TR-2 and TR-3 which are know to be problematic.
    Unless I’m mistaken, there’s no voltage readings on the schematic to go off of? I’m new to the troubleshooting game. Any info greatly appreciated!! Thanks again
     
  6. Tube Life

    Tube Life TDPRI Member

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    Did some part changing last night.
    •Replaced all the higher voltage caps on the preamp board, 5 total.
    •Touched up all the pots solder joints.
    •Replaced the input jack while I was in there.
    I still have two more caps on the power amp board to change but the weird electronic type noise is still there. I will video it and post later tonight.
    I did run a cable to my mixer from the amps line out and I am getting clean sound so I believe the preamp is ok. I did this due to the noise and fear of damaging a speaker.
     
  7. Tube Life

    Tube Life TDPRI Member

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    Also, if anyone has any pointers on using a o’scope to try and find this issue I’d appreciate it. For example, for something like this am I suppose to use the AC setting on the scope? When I send a 400hz signal through the input of the amp, hook the scope to the output of the amp I get a pretty nice looking wave. As soon as I hook up a speaker there’s this wild hum, so I’m looking to find out the cause.
     
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