Marshall 5212 Solid State Amp Hot Burnt Resistor R60

Discussion in 'Building the Classics' started by Timothy L Manning, May 10, 2018.

  1. Timothy L Manning

    Timothy L Manning New Member

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    marshall 5212, trying to find resistance of R60 on circuit board, so hot burnt in middle, red left end and gold on other end of resistor.
    any guide to this will be great,
    Thanks,
    Tim
     
    Last edited: May 10, 2018
  2. anitoli

    anitoli Well-Known Member

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  3. johan.b

    johan.b Well-Known Member

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    [​IMG]
    220Ohm and probably 2w. if it's burnt, there is a big chance the zener (ZD3) is fried too
    j
    EDIT: appearantly we need to know what year your amp is from...
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    If the resistor is burned, that's the symptom. Not the cause.
    In these amps, the 15 volt zener diode can short out and burn the resistor.
     
  5. Timothy L Manning

    Timothy L Manning New Member

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    sorry it has taken to get back. Thanks for your input.
    It's a 1988 Marshall 5212
    a bit of a close lightning strike took my amp, printer and garage door opener just a bit of troubleshooting information any help or guiding to repair would be really cool.
    Thanks for your help
    Tim
     
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    Lightning Strike:
    Probably replace, do not bother to try repairing it. It's toast.

    Lightning is like 5000 times normal voltage.....now that amp is FUBAR.
     
    Last edited: Aug 3, 2018
  7. Timothy L Manning

    Timothy L Manning New Member

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    load side of transformer is Red to black 20.7 vac & red to red - 40.5 vac
    Any one know if this is correct? This is still the 1988 5212 amp
    Thanks
     
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    Sounds OK, surprised that it still works.
     
  9. ampmadscientist

    ampmadscientist Well-Known Member

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    It's a good amp to learn on, but try not to be disappointed if everything shorted.
    Surge protector...
    Maybe you were lucky, lightning usually destroys everything.
    It's hard to say, because the output transistors are right there in the path of mayhem.

    Lemme see...
    Take out R59 and take out R60. (or unsolder one lead of each resistor)
    This disconnects the low voltage supplies (plus minus 15 volts) input power.

    Now put a 500 ma fuse in, and turn it in...
    does the fuse still blow?

    Yes - the fuse blows = output transistors are shorted.
    No - the fuse does not blow, output power amp is still (might be) probably OK.

    What's the verdict? Are the output transistors toasted?
     
  10. Timothy L Manning

    Timothy L Manning New Member

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    Thanks for the information !! I already cut r-60 out thinking it was bad because it was burnt in center. I will try your suggestion and get back.
    Thanks for your help!!!
     
  11. ampmadscientist

    ampmadscientist Well-Known Member

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    1. Cut the resistor close to the body - leave the lead long.
    2. straighten out the cut off lead.
    3. Heat the connection, and push the lead straight out of the hole in the board.
    4. DO NOT try to un-bend the lead, where it is soldered to the board.

    It is better to cut the lead and push it straight out of the hole.
    If you try to remove the entire resistor - it can damage the circuit tracks.
     
  12. Timothy L Manning

    Timothy L Manning New Member

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    just to be clear the fuse you are referring to is an inline fuse to test the zener ?
    or the main 500ma fuse built to protect everything on the board ? Because that fuse is good, 1step and all, that's when I found burnt and loose resister.after cutting the r-60 out the pins were loose just pulled them out with no iron.
    Just want to make sure before beginning to heat up the iron.
    Tim
     
  13. ampmadscientist

    ampmadscientist Well-Known Member

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    Take out R59 and R 60 (disconnect at least 1 leg of each resistor)
    This disconnects the zeners Z2 & Z3 entirely and shuts off the low voltage supplies.
    (we are not testing the zeners)
    Put a 500ma fuse in, this is the mains fuse.
    Turn the amp on.

    Now you can measure with a DC volt meter across the speaker output.
    Is there any DC across the speaker?

    No - the power amp may be OK.
    Yes - the power transistors are shorted.

    Disconnect one lead of Z2, and one lead of Z3.
    Set the meter for diode test.
    Measure across the 2 zener diodes to see if one is shorted.

    We are suspecting that Z3 is shorted. (because R 60 burned up)



    You must disconnect at least 1 leg of each diode to check the diodes.
    The diode cannot be tested when it's connected into the circuit.
     
  14. Timothy L Manning

    Timothy L Manning New Member

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    Thank You very much!!! it passed with flying colors !!! I am guessing that R-60 resister saved the amp or it finished the loose R-60 off the board ???
    It looking like I just need R-60 & r-59 resisters and Z-3 & z-2 diodes !!! Great News !!!! THANK YOU.
    Are these common ? easy to find ? Or should I contact parts dept @ Marshall ?
    Any wattage soldering iron or certain temp. ?
    Thanks for you help You really are A MAD SCIENTIST ! I lots of time and thought this would be fun and I love it. retired electrician so I have aptitude and drive, where to from here but ....
     
  15. ampmadscientist

    ampmadscientist Well-Known Member

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    Did either diode test shorted?

    Looks like R59, R60 is 220 ohm 2 watt and zener diodes are 15 volt 2 watt.
     
    Last edited: Aug 7, 2018
  16. Timothy L Manning

    Timothy L Manning New Member

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    the diodes tested with a fluke 23 series II
    OL one direction switch leads .681 - Z3
    OL one " " " .684 - Z2
    so far so good, right ? or did I jump the gun in my excitement ? more to come...
    no shorts on diodes, open OL closed .681 - .684 each
     
    Last edited: Aug 7, 2018
  17. Timothy L Manning

    Timothy L Manning New Member

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    Having a hard time finding 2 watt 220 ohm carbon composition resistors. any ideas ?
    i'm guessing this 1988 5212 has to have these type or will another type work ?
     
  18. anitoli

    anitoli Well-Known Member

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    Mouser.
     
  19. ampmadscientist

    ampmadscientist Well-Known Member

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    Resistors are wire wound, not carbon.
    Carbon resistors are obsolete.

    Diodes did not test shorted...something seems to be shorted, maybe it's the filter caps.
     
  20. Timothy L Manning

    Timothy L Manning New Member

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    Thanks
    so as long as it's a 220 ohm 2 watt resistor. any type ?
    What seems shorted if both diodes test okay if .681 ohms one way and open the other way ? any idea what the test values should be or does .681 & .684 indicate good or bad ?
    Do you by chance know the circuit board numbers for the filter caps. like C46 & C47, close to the friedR60 r. ?
    Any way this just might be repairable. best way to learn is just doing it, right ?
    Thanks for your help.
     
    Last edited: Aug 20, 2018

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