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Marshall JTM 30 Issues | OT Testing

Discussion in 'Amp Tech Center' started by Huddy, Oct 18, 2020.

  1. Huddy

    Huddy Tele-Holic

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    Alright I got a fresh problem - I got a Marshall JTM 30 from a fella for free with something else I bought. He said he thought it needed an output transformer and that he had already replaced it once. I get it home sit it in the corner for a month. Today I pull out the chassis. It's got both a Hammond replacement PT (290QX) and OT (1750NA). When I check the leads going to the primary they're super loose crimp connectors. I touch the secondaries... also super loose. This may or may not have been the cause of the supposed failure.

    So I pull out the OT and start poking around the multimeter. Each side of the primary from the center tap is even - 71 Ω. Open Circuit from the Primary to the secondary. I check the specs of this transformer and it's supposed to be a 6k primary w/ 8 / 16 Ω secondary taps.

    I use my variac and put 5.5 v into the secondaries. I get 80.1 volts and 110 volts.

    With that math @ the 6k primary I get an impedance of 15Ω and 28 Ω. Which isn't right, obviously.
    I can't help but feel like that numbers would be all out of whack and not proportional or there'd be a short between the primary and secondary if the transformer was damaged. Is it possible this thing was just wound wrong? Or is my math wrong? Maybe I'm skipping a step? Any other way to test an output transformer? Thanks!

    Screen Shot 2020-10-18 at 6.33.30 PM.png
     
  2. elpico

    elpico Tele-Afflicted

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    Where did you place the probes to measure "voltage out"?
     
  3. Huddy

    Huddy Tele-Holic

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    either end of the primary. The center tap is not touched when taking the readings.
     
  4. elpico

    elpico Tele-Afflicted

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    Good. Check if CT to each end gives equal voltage
     
  5. Huddy

    Huddy Tele-Holic

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    it does - I just checked as I had a feeling that’s where you were going.
     
  6. YYZman

    YYZman TDPRI Member Silver Supporter

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    If this is the JTM30 combo produced in the mid 1990s (1x12 or 2x10) they are known for running hot and for having issues with the board as a result.

    IMHO I’d test for an open circuit/failed connection before suspecting the OT.
     
  7. Huddy

    Huddy Tele-Holic

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    Yeah I was looking at the board and it looks good. No dark spots that look like they’ve gotten warm. Solder joints look clean.

    I wasn’t really suspecting the OT - more so the weak crimp connections. but I wanted to check it quickly check and make sure that somehow it wasn’t damaged as a result of the weak connections - maybe from it not seeing a load from the speakers intermittently.

    that’s when I noticed the impedance ratio seemed off.

    I’ve got it hooked back up now with solid connections and will try it without tubes and make sure everything is getting proper voltage where it should be. (Crosses fingers)
     
  8. elpico

    elpico Tele-Afflicted

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    A real mystery then. Cool. I can't think of a failure mode that could cause such proportional but incorrect voltages.

    Could the secondary pins be mislabeled? I'm thinking if the 8ohm tap were used as the com in this test then you might get these halfway-there voltages.

    You could apply a voltage to the primary to check. Or if your meter has a low ohms mode you could sus it out from the dc resistances.
     
  9. Huddy

    Huddy Tele-Holic

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    I had a 2x12 JTM 60 that I bought broken. I got lucky with that. The connector put the speakers in series had come loose flopping around in the back of the cab. Probably won’t be as lucky this time
     
  10. Huddy

    Huddy Tele-Holic

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    We got smoke! All voltages are OK but C108, the B+1 second node of the power supply that goes to the screens, is about to pop and has smoke coming out of it. Not coming out of the transformers though which I'll consider a good thing.
     
  11. Huddy

    Huddy Tele-Holic

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    This is what I was thinking as well but the diagram from Hammond matches up with the physical locations of the pins. The DCRs all match up. There aren't any other pins to be able to test. It's just 8 or 16 ohm taps. And youre supposed to plug in two 16 Ω loads to each one and the switching jacks puts them speakers in parallel and selects the 8Ω tap. Tested all of that as well.
     
  12. elpico

    elpico Tele-Afflicted

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    It would be easy enough for a worker to solder the wrong lead to the wrong pin so the label matching the drawing doesn't tell you much, but if the DCRs match the drawing I'd say that's ruled out.

    Sounding a lot like a manufacturing error then. Email hammond, see what they think.
     
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  13. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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  14. Huddy

    Huddy Tele-Holic

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    I think you're right - So trying to remember back to when I got it, the previous owner said he replaced the output transformer and it was fine for about a year and then one day it started smoking, he cut it off and just used it as a speaker cabinet from there on out.

    I don't know if you saw a few posts back, while taking a few voltage measurements with the standby in "play" mode and after 2-3 mins the second filter cap started emitting smoke/steam out of the top. I got some new filter caps on order. Should be back on it Wednesday.
     
  15. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    It doesn't seem to match your winding ratio numbers. Odd. ???
     
  16. Huddy

    Huddy Tele-Holic

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    Yeah it's about double the output impedance. Tempted to get a replacement and see if they're the same. The OEM OT is only $36 with the hammond model being a few bucks more. Decisions... decisions... decisions...

    They usually say 100% mismatch is okay but I'd be hesitant knowing how finicky these amps can be.
     
  17. YYZman

    YYZman TDPRI Member Silver Supporter

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    I’m sure you’ve already checked this, but often the colour codes of the transformer wires on the Hammond replacements are different than the OEM (even though the Hammonds themselves are of a high quality). Hammond has also updated wiring colours over the years, so it’s worth double or triple checking the actual layout.

    I learned this the hard way by doing an OT replacement on my Marshall 18W reissue.
     
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  18. Huddy

    Huddy Tele-Holic

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    So on these Marshall style transformers they don't have leads. They just have solder lugs that are numbered. 3 Lugs on each side. Primary has a center tap and each end. The secondary has a common and what's supposed to be the 8 Ω and 16 Ω taps. They all measure out totally normal for the stated DCR from the spec sheet directly from hammond.

    I don't know enough (or anything really) about OT construction to say this with any authority but I'd think if all the DCRs were correct the impedance ratio would be correct. The only thing I can think of is that one of the wire gauges is incorrect which permits it get the same DCR but the turns ratio would half-ish. I think I'm reaching there though. That's a lot of accidents that gotta line up. I just don't think it's possible.

    I've got the caps on the way and once they're installed I'll just monitor it closely and cross my fingers. If the OT is broken it's not gonna get any more broken by using it - there's no such thing as sorta pregnant... unless that analogy is not cross compatible. :lol:
     
  19. Huddy

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    Alright - so I replaced all the filter caps. All is right with the world as far as I can tell. Here's my best guess as to what might've gone wrong...

    I think the poor connections from the on the primary to the plates caused the voltage to spike on the filter caps... without the tube load was a bit over 500v - between time and probably about to wear out naturally and the excess voltage the cap burned out causing the smoke to emit from it.

    The clean channel sounds good - the drivel channel isn't much like my JTM60. Very dark, kind of a 1-trick pony and really only sounds decent on a bridge pickup. Maybe single coils are different. I'll be playing on it for a bit before I go to sell it... cross your fingers for me. ;-)
     
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