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Marshall lead 30 burnt opamp issue

Discussion in 'The Workbench' started by sandro70, Apr 16, 2021.

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Keep original MC1458 or go for modern opamp?

  1. Yes, keep the old buddy!

    100.0%
  2. Are you kidding? Waste that '76 garbage chip!

    0 vote(s)
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  3. Choose another hobby, guitars are too noisy and neighborhood unhappy...

    0 vote(s)
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  1. sandro70

    sandro70 New Member

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    Hi,
    I have a Marshall lead 30 burnt opamp issue, some months ago I swapped the MC1458 opamp to RC4558 and it sounded good, IMHO better; then I burnt it playing an overdrive, surely a strong input signal or I guess so. Opamp tester blinked red led all the line.

    Substituted with LM308 playing clean and good. But again I inserted a Boss FDR (a fender twin-reverb emulator) and craaack, craaaack (loud noise of dirty pots)...the end, no sound at all. Speaker working, checked with a battery.

    I assume it's a voltage issue, I'd like to use a RC4558 again but not sure which components check first to avoid overLOAD.
    Bad resistor, diodes, capacitors at +V opamp input? Bad negative feedback?
    Not an expert so...Any clue please?
    thanks
     
  2. sandro70

    sandro70 New Member

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  3. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Go back to the stock MC1558 part and strap 14 volt zener diodes across the inputs to ground as a device protection measure.

    Put a DIP socket on the board so you can swap the part without having to solder it AGAIN.
     
  4. thetragichero

    thetragichero Well-Known Member

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    are you sure you used an lm308 in place of a dual op amp? i definitely would expect the chip with a different pinout to fry (your amp is putting v+ on pin 8 while the lm308 expects v+ on pin 7; pin 8 is one end of the compensation capacitor)
    the zener suggestion is a good one
     
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  5. sandro70

    sandro70 New Member

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    Thank you guys,
    ok back to MC1458 with socket (already soldered) and zener 14V (just 12V in stock, maybe fit well the same?) ...for inputs you mean jack IN or/and op-amp #3 V+ input?
    PLease check picture, A or B?
    ZENER.gif

    Not sure about fried LM*** code, I'll check tonight.
    ciao
     
  6. thetragichero

    thetragichero Well-Known Member

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  7. sandro70

    sandro70 New Member

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    Good! Appreciated!

    U right not a LM308 but well fried LM1458!
     
    Last edited: Apr 17, 2021
  8. thetragichero

    thetragichero Well-Known Member

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    over voltage protection is my guess
     
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  9. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Even better, strap 14 volt zener diodes to ground thru 47K resistors in series. That should make the zener clamping effect much less harsh sounding. But if you're clamping via the zeners, your input signal is too hot anyway. Lower it so the clamp doesn't trigger.
     
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  10. mickeydg5

    mickeydg5 Well-Known Member

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    A Boss FDR is nominal output -20dBu.

    What are your voltages on pins 4 and 8 of the opamp?
     
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  11. Pete Farrington

    Pete Farrington Active Member

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    10k input impedance on a guitar amp - what nonsense is that?!
     
  12. thetragichero

    thetragichero Well-Known Member

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    isn't op amp input impedance pretty much infinite?
     
  13. Pete Farrington

    Pete Farrington Active Member

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    For the device itself, yes, basically it’s defined by the circuit, eg R2 (10k) in this case.
     
  14. thetragichero

    thetragichero Well-Known Member

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    shouldn't it be r1 + r2 in this case?
     
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  15. Pete Farrington

    Pete Farrington Active Member

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    Oops, yes, I saw the bypass cap C1 and misread it as being a full bypass :rolleyes:
    Whereas it is very, very partial.
     
    Last edited: Apr 18, 2021
  16. johan.b

    johan.b Well-Known Member

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    1458 or possibly 4558 are the only ones that belong in there... if you want better protection, antiparrallell diodes across the inputs, like the old musicman amps, Is probably the best you can do on an opamp input.
    J
     
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  17. johan.b

    johan.b Well-Known Member

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  18. sandro70

    sandro70 New Member

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    MC1458 in/out values 1458 input.png

    Local store got only 13V in stock.
    4558 sound better in my opinion;
    sounds good!
     
  19. sandro70

    sandro70 New Member

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    I'm on the right path?
     

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  20. mickeydg5

    mickeydg5 Well-Known Member

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    No, I do not require a datasheet. I pull one of those up myself.

    What I want to see and what you need to find out is part of troubleshooting, getting voltage readings from the power supply to the opamp.
     
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