My Newest Marshall 1978 Jmp MKII Super/lead/bass

Discussion in 'Marshall Amps' started by chilipeppermaniac, May 15, 2019 at 12:19 PM.

  1. chilipeppermaniac

    chilipeppermaniac Active Member

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    OK So far I have that batch of pics loaded. I will try to sort through things to see if I missed some vital shots.

    Anyhow, I will tell the story as best I know it. I am still in the dark on how the seller got the amp to work long enough for me to buy it.

    SO, I saw his ad, we conversed and arranged a day to try it as I was en route to my mom's apartment in Delaware. I got to his house and he almost began to fire up the amp. I asked if I could see inside it first.

    He agreed and set it on his kitchen table for me to look over. The things I noticed were NO Ohms cap selector, the mismatched quad, the replaced screen grid resistor, likely original filter caps, a few changed resistors, the mismatched nuts on the 4 input holes, the missing control knob gold marker caps, discoloration stains on the top of chassis, obviously the BASS over Lead with S/L serial. Apparently original trannies and choke.

    The things I missed were that V1 wire Ivan pointed out and apparent Red plating in V5 but I did see V4 was a Sovtek and not a Ruby like the other 3.

    So, once home, I expected to at least be able to hear the amp thru the seller's speaker cab. But NO.
    As soon as I took it off standby, I got a nasty surge sound and then no sound thru the speakers. Blew the Mains fuse. SO, I took it out and found it to be a 3 A SLow Blow. Not a 4 or 5 A as the amp calls for. Got another 4 A and this time I took the chassis out of the cab. Saw a flash in V7 and same noise in speaker. Turns out this time HT fuse blew. Replaced the HT fuse and tried again. Same deal.

    SO, I inquired with my more experienced brothers elsewhere who conferred I would be wise to have my tubes tested. One spotted the Red plating evidence in the V5 tube close up shot and of course the switched resistor was in V5. This is when I found a super nice Ebay tube seller who also happens to be local. Bought the Svetlana set off him and then he also agreed to test the 6550's and 12 ax7's that came with this amp for me. 5 of the 7 were below spec and the V7 that flashed, actually shut his Amplirex 1000 tester down to protect itself. He told me that tube was dangerous.

    Now that I got the new Svets, and 2 more 12 ax7's, the amp never blew a fuse yet.
     
    Last edited: May 15, 2019 at 11:47 PM
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  2. chilipeppermaniac

    chilipeppermaniac Active Member

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    As a side note, LIKE Pedecamp who got a damaged amp the guy claimed was mint, I got this amp the guy claimed a really good tech serviced every year for him.

    Needless to say I called bogus and asked him if he would issue me a full refund, or at worst pay for me to go to a tech to get it tested and to pay to get it fixed. He turned into a jerk and basically said, " NO WAY< How do I know what you did to it once you took it...?"

    I told him thanks for selling me his amp that clearly was NOT as he represented as being serviced regularly. Then told him I saw what kind of person he was to do that.
     
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  3. chilipeppermaniac

    chilipeppermaniac Active Member

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    Also, just because of the red plating, the mis matched quad, and the blowing fuses occurences, I figured I better check the OHM on his cab. Instead of getting 8 ohms Stereo and 16 ohms Mono, I was getting like 32 ohm in the stereo setting and 64 in the mono.

    Rewired the cab like my 1960A and now it is working as it should, 8 ohm Stereo, 16 in mono.

    MY 1960A is already verified to be in proper spec and all testing I did on the SUPER was done through the 1960A to ensure proper OHMS and no damage to the amp from any chance of running a 16 ohm Amp into an 8 or 4 ohm cab.
     
    Last edited: May 15, 2019 at 2:21 PM
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  4. chilipeppermaniac

    chilipeppermaniac Active Member

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    Governors wiring 2.jpg

    This is how he had it wired.
     
  5. chilipeppermaniac

    chilipeppermaniac Active Member

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    Governor Polarity.PNG

    Marshall 1960A 2.jpg

    Now the Redcoats are wired like my G12 T 75's
     
    Last edited: May 15, 2019 at 2:15 PM
  6. chilipeppermaniac

    chilipeppermaniac Active Member

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    Superbass 6550's.jpg

    This is where my friend saw signs of red plating in V5
     
  7. chilipeppermaniac

    chilipeppermaniac Active Member

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    Superbass Resistor weird 2.jpg

    I also told the guy AFTER he balked at refunding me anything, that if I were him, I would think twice about using that tech, if he did stuff like this and if he was the one who wired up his cab.

    Poor resistor replacement soldering from top of board, Geeesh
     
  8. neikeel

    neikeel Well-Known Member

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    I see no really big deal there. But it depends on what you think you bought.
    The amp started off as a stock Superlead.
    Someone partly converted it to Superbass, ie made it shared V1 cathode to SB spec. They replaced the 33k slope resistor in the tone stack to 56k (SL to SB), they also replaced the 0.0022uF coupling cap on the lead channel to SB 0.022uF.
    Things that they did not do was to change the PI output couplers from 0.022 to 0.1 or the tone stack cap to 250pF.
    The hard wiring of the OT selector is quite common on working amps, the jack thing is easy to do as well.
    More irritating is the mismatched output tubes and fuse blowing, that should be factored in to any purchase price.
    Will the guy shift on price? I guess not if he said that you came, tested and took it away? What are your buying rights in the US on used goods?

    Oh and you were lucky not to blow your OT with that cab!
     
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  9. charveldan

    charveldan Well-Known Member

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  10. ampmadscientist

    ampmadscientist Well-Known Member

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    1. What do the power tube sockets look like, is there any burn marks?
    2. Measure all 4 screen grid resistors w/ the tubes removed.
    3. Ohms "cap," replace that with a selector switch. Do this without modifying the chassis or screw holes.
     
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  11. pedecamp

    pedecamp Well-Known Member

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    Thats Jeff Bober right? He's local to Baltimore.
     
  12. chilipeppermaniac

    chilipeppermaniac Active Member

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    YES, Pede, Jeff Bober. I have contacted him by email and he replied a few weeks ago.
     
  13. chilipeppermaniac

    chilipeppermaniac Active Member

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    Will do, Ampmad. Thanks

    1. As far as I remember, No burn marks in tube sockets but will get bright light and Magnifying visor out for a good look.

    2. I had measured all the Screen grid resistors. Can't recollect if I did with tubes out. They measured at Spec though. Will Recheck.

    3. I will show you the Ohms Selector I want to use when I find it. Yup, it was you who posted the one I am glad I saw you did. Now to locate some.

    [​IMG]
     
    Last edited: May 15, 2019 at 9:50 PM
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  14. chilipeppermaniac

    chilipeppermaniac Active Member

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    In My opinion and that of a few other excellent builder friends of mine, the work was very sloppy and could have been done right with a little more attention to authenticity and not half assed shortcuts like the Ohms selector jumper wire thing and V1 yellow wire cut and jumper mod. All while leaving 40 year old filter caps in the amp to live way past their recommended replacement span.
     
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  15. ampmadscientist

    ampmadscientist Well-Known Member

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    The o
    The old ohms plug is unreliable and can cause the transformer to blow. If you are able install a good switch instead of the plug-in type.
     
  16. chilipeppermaniac

    chilipeppermaniac Active Member

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    courtesy of your post in another thread

    [​IMG]
     
  17. pedecamp

    pedecamp Well-Known Member

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    Does Jeff do general amp repair, I feel like I spoke to him and he couldnt help me for some reason, I think he said he only works on vintage amps. I used a guy named Leroy Fitch in Glen Burnie, High Voltage Repair, have you heard of him? He was good but he smokes and stunk up my amp.
     
    Last edited: May 16, 2019 at 8:13 AM
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  18. Mitchell Pearrow

    Mitchell Pearrow Well-Known Member

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    :slash:
     
  19. charveldan

    charveldan Well-Known Member

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    My point was top side in general not your specific amp. The ohms jumper on your wasn't the correct gauge wire and your amp as a whole has alot of issues.
    Im sure once you're squared away it will be a nice amp.
     
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  20. chilipeppermaniac

    chilipeppermaniac Active Member

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    Pede, I too, used Leroy as he was a friend of a friend who has played guitar at least 20 years longer than me.

    Unfortunately, Leroy did me wrong and I would never use him again. He basically said my Factory Output Trans was bad and that was why I was blowing fuses and losing signal. Then when I provided a Marstran and he installed it and I went to pick up my 1972 JMP, he claimed he did not keep my original Trans when I asked for it back.

    In the end, it turned out my Ohms selector with the wonky removable Cap style selector was losing contact and I, myself diagnosed and fixed this because the original issue I took it to Leroy for was never fixed and it cost me a Vintage Tranny that was likely still plenty good.
     
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