Original Parts Inside 1969 Jmp?

Discussion in 'Let's Talk Vintage' started by JMP MCP, Nov 29, 2016.

  1. JMP MCP

    JMP MCP New Member

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    Hi all,

    Longtime browser, first-time poster here at Marshall Forum (and excited to say that I finally am a member)!

    I am currently considering purhasing a 1969 Marshall JMP 50. The seller wants $1800. I was wondering if anyone on this site could help me identify if everything on the chassis appears to be original/checks out.

    To my untrained eye, it looks as though the power transformer (second photo) has been replaced judging by the phillips head screws used and the fact that it looks much shinier and years younger than the other components. Also, on the factory inspection slip, it looks like someone left a note in the "Assembly" line about the transformers. I can't decipher what they wrote, but maybe it has something to do with the replaced transformer??? Does anyone think the same?

    Apologies about the limited scope of the pictures. Unfortunately, I was unable to get any photos of the guts. I will see if I can get a hold of those if I decide to more seriously pursue this amp. Thanks in advance!

    IMG_1189.JPG IMG_1188.JPG IMG_1187.JPG IMG_1186.JPG IMG_1185.JPG
     

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  2. Marshallhead

    Marshallhead Well-Known Member

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    Definitely a new PT on there.

    Those green filter capacitors are more common in 1973 (and they seem to have a '73 date code on them)

    Try and get under chassis shots if you can.

    (Edited to correct my fat finger typing that said 1963 instead of 1973...!)
     
    Last edited: Nov 29, 2016
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  3. JMP MCP

    JMP MCP New Member

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    Thanks for the reply, Marshallhead.

    Unfortunately, at this time, no shots of the guts. I will see if the seller can send me some.
     
  4. neikeel

    neikeel Well-Known Member

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    It does look like a '73 era:
    choke is mid/late metal panel style
    The PT (even if replaced) is a stand up type but missing its reinforcement bracket
    as mentioned the green plessey filter caps
    need gut pics for value reports!
     
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  5. Kris Ford

    Kris Ford Well-Known Member

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    SN#?
    Is it me, or does the tag appear to be TOO clean, and I almost think the chassis has fine sanding marks under the glue...
     
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  6. JMP MCP

    JMP MCP New Member

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    Thank for the input Kris and Neikeel.

    The serial number is:

    EDIT: Serial Number has been removed.
     
    Last edited: Dec 4, 2016
  7. TAZIN

    TAZIN Active Member

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    The serial number and the tag sheet info match up so there's a good chance the chassis and rear panel are from the same amp. The Plessey filter caps may just be replacements. The PT looks like a replacement for sure. The endbell style choke appears to have been phased in around December 1969 so that seems to mesh with the tag sheet info.
     
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  8. JMP MCP

    JMP MCP New Member

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    Thanks, TAZIN. I am happy to hear some of the puzzle pieces are falling into place. Interesting. I found some photos of other '69/'70 heads online and they appear to have the same style choke (photos below). At least one in particular looks like it was signed by the same people judging by their penmanship. Also, it appears to have the same "Joyce" signature on the "Tag Board" entry of the tag sheet. It is from November of 1969, one month prior.

    My guess (without seeing the guts AND I'm obviously no expert) is that the filter caps failed and the PT blew as a result. That would explain why the caps look to be from '73 as does the PT.

    I will try to get a guts shot ASAP. The seller said he will get me some over the weekend. Thanks for all the input everyone!

    picture006.jpg

    IMG_0518.jpg

    Picturesofamps020.jpg
     
  9. neikeel

    neikeel Well-Known Member

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    Ha! That bottom picture was mine! A 1970 JMP50 Model 1989 (Organ) after I replaced the cans and the PT with a NOS -164 OT (I rewired the bias feed to the safer type)
     
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  10. Kris Ford

    Kris Ford Well-Known Member

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    Good deal!
    I may have been seeing the original marks..maybe they gave the chassis a quick swipe with sand paper before brushing the glue on for the tag?
     
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  11. johan.b

    johan.b Well-Known Member

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    What's the mess around the sticker (first pic) and how did it get under and around the sticker but not on it?
     
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  12. m1989jmp

    m1989jmp Active Member

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    Glue was applied without much care, every 60s Marshall I've seen looks like that
     
    Last edited: Dec 2, 2016
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  13. JMP MCP

    JMP MCP New Member

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    Nice, Neikeel! Yes, that makes sense. I pulled the photo off of another thread on this site. Cool amp!

    Kris, that definitely seems possible.

    EDIT: I've definitely come the point where I need to get my hands on gut shots. I will hopefully be able to post some this weekend.
     
  14. JMP MCP

    JMP MCP New Member

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    Hi all,

    Alright, here are some pics of the inside. Also, after giving it a whirl for ten minutes, I decided to pull the trigger on it. Even if some of the components are non-original, the amp sings!

    What circuit is this? Any modifications?

    File_009.jpeg File_006-2.jpeg
    File_001-2.jpeg File_002-2.jpeg File_003-2.jpeg File_005-2.jpeg File_004-2.jpeg File_007-2.jpeg File_000-3.jpeg File_000-2.jpeg
     
  15. neikeel

    neikeel Well-Known Member

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    The circuit board is typical for 1970 (so the 69 date would be ok)
    The diode block, snubbers, PT, filter caps, presence cap on/off and standby switches are all replaced.
    The caps are dated June '73 and the PT dated March '87.
    It looks to have been a bass circuit originally (the rest of the circuit board has not been messed with).
    Unfortunately the wiring mistake that caused the PT to go (maybe the snubbers and diode block went with it) has been replicated by the repairer.
    The ac feed to the bias circuit needs to be hooked up to the ac from the PT before the standby. Unfortunately they have removed the DPST standby switch and are switching the rectified DC with the standby - likely to take down another PT in the same way.

    Glad it sounds good, but you should get a DPST standby and wire it properly before your 'new' PT goes the same way as the other. The amp in the pics of mine you see did just that. The original standup PT (Drake 1202-164) died and was replaced with a Drake 1202-324 which also died until I found an NOS -164 and wired the bias feed correctly. Of course the best thing to do would be to buy a Marstran or Merren lay down PT that is correct for your chassis and wire it in correctly...........................
     
  16. stickyfinger

    stickyfinger Active Member

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    Hope not to derail the thread.

    Is Ceriatone's layout the correct way to wire the mains/standby?
    http://www.ceriatone.com/ceriatone/wp-content/uploads/2016/10/Ceriatone-68-Plexi50-Lead.jpg
     
  17. Ron W

    Ron W Member

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    Here's a couple of shots for comparison of my July '69 JMP50 s/n 1972A that show the PT/rectifier area of the board, as restored for me by Greg Germino:

    [​IMG]

    [​IMG]

    [​IMG]
     
    Last edited: Dec 4, 2016
  18. neikeel

    neikeel Well-Known Member

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    The Ceriatone diagram is not correct - same issue, you hit the valves with HT same time as the bias voltage. Not a problem with the valve rectifier amps as the B+ rises slowly but with these ideally you want the bias voltage to come on whilst on standby then you apply the voltage.
    Would love to see Gregs work on Ron's amp but the message I get tells me Error that I do not have permission to view media in this forum:hmm:
     
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  19. Ron W

    Ron W Member

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    I changed the links to Photobucket...should be displaying now.
     
  20. TAZIN

    TAZIN Active Member

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    'neikeel' summed up the changes nicely. I'll add the correction that the ON/OFF switch is original, but the Standby switch is a replacement. In addition, you'll notice that the NFB resistor is a 47K rather than the "traditional" 27K found in model 1986 (Bass) amps...That's because around this time frame (beginning of 1970) Marshall started to phase in this change. Also, you can see that this amp has two 10K B+ dropping resistor in series instead of the "typical" two 8K2 resistors. I've seen a decent amount of 50w amps throughout 1970 that had the two 10K's and some with one 10K & one 8K2 so it's just a factory oversight.
    Here's an amp that was produced around the same time as your amp (Tag sheet date: 8/12/69; Serial No. S/A38xxA).

    http://www.amparchives.com/album/Ma... JMP 50W - Paul Wright/P3180007_jpg_orig.html
     

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