JTM 30 Low Output

Discussion in 'The Workbench' started by Goldguitarguy, Jun 14, 2019.

  1. Goldguitarguy

    Goldguitarguy Member

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    Hey folks

    I have another JTM 30 here plagued with low output that a friend bought from EBay. My personal one went through the same thing a while back but it was just the effect loop jacks needed cleaning.

    All voltages were good through out the amp that I know to check. The schematic for these is terrible by the way. I cleaned all the pots and checked their resistance. Cleaned all the jacks & I now have some volume through the amp but it's definitely not 30 watts sounding. I also jumped the effects loop to see if it would make a difference but it didn't.

    I removed the leads from the output transformer and measured the DC resistance but something seems weird on the primary side compared to the specs I found for the Hammond replacement.

    Pin 18-17 measures 12.3r
    Pin 18-14 measures 87.2r
    Pin 17-14 measures 74.8

    The specs of the Hammond 1750NA are

    Pin 18-17 measures 74.41
    Pin 18-14 measures 147.4
    Pin 17-14 measures 73.2

    Would this mean the primary side of the output transformer is toast? Should the resistance be double across both ends? Is there another way to test this to confirm?

    Just looking for someone else input before I make him spend $65 on a transformer. I know this is a pretty common issue with these amps too.
     
  2. Goldguitarguy

    Goldguitarguy Member

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    Judging by the price tag I found on the Drake output transformer I'm gunna say this is not the first one for this amp...
     
  3. ampmadscientist

    ampmadscientist Well-Known Member

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    We need to figure out what "pin" is what actual wire. "Pin" doesn't mean anything.
    Transformer burn out will be caused by speaker impedance mismatch, most commonly....

    Leads of transformer are completely disconnected from circuit?

    Hammond1750NA.png
     
    Last edited: Jun 14, 2019
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    "Pin 18-17 measures 12.3r
    Pin 18-14 measures 87.2r
    Pin 17-14 measures 74.8"

    OK well if transformer is completely disconnected from circuit and 18-17 measures 12.3 ohms, then the primary is shorted.
     
  5. Goldguitarguy

    Goldguitarguy Member

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    It was completely disconnected when I measured the resistance and just wasn’t a 100% that it was toast given this is the first one I’ve ever had to test or replace.

    Told my buddy the bad news and he didn't seem to mind. Said he bought it cheap because it was broke hoping I could fix it in the first place. Nice to be needed I suppose...
     

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