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  1. S

    '77 JMP 100W MKII Red Plating

    OK.. I didn't know which version of the Bassman was used to create the original marshall.
  2. S

    '77 JMP 100W MKII Red Plating

    Yes of course. I know that, except Bassmans had a 5U4 instead of solid state rectification. It's exactly what you said: Layout. Unfamiliar territory. The amps work the essentially same way.
  3. S

    '77 JMP 100W MKII Red Plating

    OK all. I did the best I could with my limited knowledge of Marshalls. I am a Fender amp guy and know my way around them pretty well. My friend wanted to get the red plating to cease. I managed to do that. I gave it back to him on the premise that he would test it and watch it to see if the...
  4. S

    '77 JMP 100W MKII Red Plating

    According to the owner this problem began years ago and hasn't ever been "solved" by others before me as well. I told him to keep an eye on it and if the problem returns we will delve deeper. I suspect the previous techs also were baffled why the voltage is so off and bumped up the bias...
  5. S

    '77 JMP 100W MKII Red Plating

    I did and got a bit under 400V
  6. S

    '77 JMP 100W MKII Red Plating

    Well, I don't understand why I don't see more than 400V? I returned the amp to it's owner and I'll see what happens later. Many thanks for all of your help!
  7. S

    '77 JMP 100W MKII Red Plating

    I haven't measured current again since getting things more or less calmed down. The plate voltage is pretty low at just under 400V. The filter caps were replaced a few years ago & the diodes are good. When I measured the secondary (if I have the correct points!) the voltage was just below 400...
  8. S

    '77 JMP 100W MKII Red Plating

    I don't see any available. Those voltages would be great though. thanks!
  9. S

    '77 JMP 100W MKII Red Plating

    I think I figured out which wires coming out of the PT are the secondary...whew! Yes it looks like he really should replace it.
  10. S

    '77 JMP 100W MKII Red Plating

    It says T4074 I guess also Dagnall. I am trying to test the secondary & all of these schematics suck. What colors ate the secondary..green, black & yellow?
  11. S

    '77 JMP 100W MKII Red Plating

    So, I have gotten this amplifier to work without red plating. My friend wants to know about replacing the PT as it seems to be a bit light on secondary voltage. What is the current replacement make/model, so I can price it out? Thanks everyone for all of your help!
  12. S

    '77 JMP 100W MKII Red Plating

    I was experimenting with different resistor values and combinations from 72K to 110K ohms
  13. S

    '77 JMP 100W MKII Red Plating

    The caps on the bias circuit seem to be ok. I experimented a bit with different resistive values and was able to get the bias range from -25V to -50V and the red plating is gone, running much cooler & actually still sounds pretty darn good. I am not finished yet though.
  14. S

    '77 JMP 100W MKII Red Plating

    Yeah I agree. I expected to see a higher plate voltage. I think the caps were changed not too long ago, but I don't know when. I am much more knowledgeable with fenders but I would have expected 450 or more. I am looking at the schematic suggested and it is definitely different than the one I...
  15. S

    '77 JMP 100W MKII Red Plating

    Thus far I have measured the same range of -18V to -25V on all four pin 5s. What I am curious about is the resistor on each pin 5. They are 5.6K. I cannot find them on the schematic. I thought they were supposed to be 1.5K. I will check OT today as well. Jeez, if this was a Fender I would have...
  16. S

    '77 JMP 100W MKII Red Plating

    Well it's been a US amp for its entire life I am to understand, so I doubt it's ever been set up for 240. The plate voltage is just a hair under 400V (394). I plan to measure the PT voltages today.
  17. S

    '77 JMP 100W MKII Red Plating

    OK, so as I consider all of this, I began thinking the same thing. Apparently the bias voltage isn't sufficient. So, how should I get it to what's needed? One thought is to check the bias supply voltage at the transformer.
  18. S

    '77 JMP 100W MKII Red Plating

    Hmmm, interesting. The bias resistor value of 47K is silkscreened onto the board in series with an adjustable 25K. The 2203 schematic I am working from (albeit blurry) shows that arrangement. I measured -20V at pin 5 and thought that would suffice. Apparently not. I cannot find any 150K...
  19. S

    '77 JMP 100W MKII Red Plating

    Hello All. I am working on a Marshall 1977 JMP MK II 100 watt. It was brought to me by a friend who said it was running hot and melted the plastic grill above the tubes! It was definitely running hot. Red plating the power tubes (6550s). He told me he had it worked on by another tech years ago...
  20. S

    MHZ15 No filament voltage on V1

    yes, an unusual design, but it actually sounds pretty decent.

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