1973 Jmp 50w Circuitry Help?

Discussion in 'Let's Talk Vintage' started by Steven Lee Waugh, Feb 8, 2019.

  1. ampmadscientist

    ampmadscientist Well-Known Member

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    The screen resistors are missing.
    This will cause the fuse to blow.
    This is not the capacitors.

    The design of the output tube was changed in the early 70s.
    The design change cause the tubes to draw more screen current.
    This is why the screen resistors were added to the circuit.
    If the resistors are not installed, the screens will draw too much current and the fuse can blow. This is not caused by any capacitors.

    1. Put the amp back to stock.
    2. Install the screen resistors.
    3. Verify the speaker load with an ohm meter.
    (make sure the output tubes are good)
    4. Set the output tube current.

    Then the amp should not blow fuses.
     
    Last edited: Feb 11, 2019
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  2. boola1

    boola1 Active Member

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    If you want something done properly (and you really care about your vintage Marshall), do it yourself....

    Working on PTP Marshalls really isn't very hard. You have to take care not to kill yourself but really, most other jobs are quite simple. We all started somewhere, I couldn't imagine it any other way for me now.
     
  3. AndyD

    AndyD New Member

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    I was only offering another potential cause. The HT fused is designed to protect the mains transformer from drawing too much current. If the filter capacitors are dead, this could be the cause of the extra current draw. Plenty of mains transformers have been taken out by filter cap failure. If the amp has never had screen resistors, why would the amp suddenly be blowing fuses. Without seeing the amp, I am only offering possible causes.
     
  4. neikeel

    neikeel Well-Known Member

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    I have had a look at your amp, the bias feed is of the correct type. I do not routinely ‘shotgun’ Filter caps.
    If it hums - yes replace
    If the caps are venting powder- yes replace
    If they have a very high ESR - yes replace
    You are in the UK, Amy where near Suffolk? If you buy the bits I will set it up to stock FOC.
     
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  5. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Was this ever answered?
    Prior to getting it serviced how long ago was the last time it was turned ON or played?
     
  6. Steven Lee Waugh

    Steven Lee Waugh New Member

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    You fellas put a smile on my dial, knowing that you know whats ahead if I get find all that parts to equation all together.
    I'm really optimistic towards my amp now - giving the level of input from such good folk - a lot more than before.
    Ultimately it's down to finding the right man for the job who looks at my amp & follows the original schematics.
    I'm from Newcastle on Tyne if anyone knows of any 'salt of the earth tech's' in the area?
    Getting tough having to spend more money tho!... man.
    Glad to to know that it hasn't been completely re-arranged & what's required is nothing too drastic.

    Many thanks too you all for your input!!
     
    Last edited: Feb 11, 2019
  7. Steven Lee Waugh

    Steven Lee Waugh New Member

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    Thank you kindly sir, if I was local to you, I'd most certainly bring it to your door.
     
    Last edited: Feb 11, 2019
  8. Steven Lee Waugh

    Steven Lee Waugh New Member

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    Quite a while to be honest - I'd say about 5yrs without regular use but turned on for the odd occasion. Would anything have degraded over time? It's been kept well for sure.
     
  9. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    The fact is that it is old and may have the original power supply capacitors (we have not seen them). Those old capacitors will dry out, get brittle with time especially if not used once in a while. Others have mentioned it.
     
    Last edited: Feb 12, 2019
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  10. ampmadscientist

    ampmadscientist Well-Known Member

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    "Newcastle on Tyne"

    A. Take it to the factory in Milton Keynes...
    B. Send us some of that excellent beer. (burp.)
     
  11. stratburst

    stratburst Active Member

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    I strongly urge you to NOT send it to the factory. They will not perform a period correct restoration or use period correct parts which will likely devalue your otherwise largely unmelested amp. Find a recommended tech in your area who will perform a vintage correct restoration. You may have to ship it but you will be better off.


    Good luck!
     
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  12. neikeel

    neikeel Well-Known Member

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    Postage would be cheap!
    I actually have the correct parts.
     
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  13. Steven Lee Waugh

    Steven Lee Waugh New Member

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    The amp has been modded before I bought it - for the kettle power lead.
     
  14. Steven Lee Waugh

    Steven Lee Waugh New Member

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    I'm a little dubious about posting tbh not to mention posting to someone I've met only through an online forum.
    Absolutely no offence towards you, it's just a thought that comes to my mind. That what if scenario...
    Thank you very much for your kind offer, & as I've mentioned, I'd be at your door if only I lived closer.
     
  15. neikeel

    neikeel Well-Known Member

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    No problem, your call, your amp and your money.
    Hope you get it sorted properly.
    :cheers:
     
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