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Thread: Music Man 120 2247 Low Output

  1. #1
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    Music Man 120 2247 Low Output

    Hey All,

    I'm trying to fix in amp in one day for a touring band coming through. It's a music man 2475 HD-120 (with 12ax7 phase inverter). Any shots in the dark would be helpful seeing as I'm on a time limit.

    http://www.thetubestore.com/lib/thet...-Schematic.pdf

    Came in pulling lots of current from my limiter, no signal. Turns out one of the 'snubber' caps (plate to ground) was shorted out, when I removed it the signal returned and the current stopped being pulled. It had destroyed the flyback diode and the -16v supply (1n4003 diode and a cap on the bias board was fried, the 150uf electrolytic) The filter caps are out of spec but the voltages are pretty strong, high is about 550 on the limiter. (Can't remember exact and I'm not at the shop...)

    Preamp signal seems strong, crank it up and the tubes saturate the way you'd think. Just low overall volume, like a stereo on medium/low output.

    16v and -16v have been repaired, snubber caps and flybacks out of circuit for now. Suspect phase inverter, replaced all film caps one by one and measured resistors on the driver board. At this point, I'm guessing OT??? what else could cause low volume, possibly something I missed from working a 12 hour day in here with a fuzzy head??

    Thanks so much.

  2. #2
    Senior Member Enzo's Avatar
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    You can test the transformer. Go over to RG Keen's Geofex web site and look up his transformer tester. very simple, it uses a battery and neon bulb. I'd disconnect it from the circuit first to be sure.

    In my shop, I have a old Fender output transformer that I use as a sub. it doesn;t matter what impedance really, all we want to know is if the existing transformer is bad, even a wrong one will still make plenty loud. I'd disconnect the three primary wires and clip in my tranny. Then clip the secondary of mine direct to speakers - ignore the NFB and stuff. Loud now?

    I see no reason why your power amp failure should afffect the 16v supplies. But how much signal is going into the power amp? Since the 16v rails crapped out, you could have an IC in the preamp that was damaged causing low signal level.
    J M Fahey likes this.
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    Hey Enzo, You're the best.

    I think i can salvage a tranny off of a dead amp here, I'll see what I can do about the test.

    I don't have a scope but I signal probed the grids coming off the PI circuit and it's pretty healthy and loud sounding,, louder in my probe-connected amp on 1 than the music man is at 10/10. Grids / Plates are both healthy across the board now, so I will begin to go through the output circuitry of the amplifier. Weird question, it looks like the tip and sleeve of both output jacks show continuity to ground. Is this a normal occurrence, perhaps through low resistance OT wingdings?

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    Supporting Member Jazz P Bass's Avatar
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    Quote:"Weird question, it looks like the tip and sleeve of both output jacks show continuity to ground. Is this a normal occurrence, perhaps through low resistance OT windings? "
    Yes. Normal.

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    Quote Originally Posted by Mr_bibbles View Post
    It had destroyed the flyback diode and the -16v supply (1n4003 diode and a cap on the bias board was fried, the 150uf electrolytic)
    I'm guessing this is a typo as neither the flyback diode nor the zener should be 1N4003.
    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

  6. #6
    Senior Member Enzo's Avatar
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    Please also do look at the tester at Geofex. it is a handy thing and real simple, and even if you don't use it here, you will know of it the next time. There is a lot of other great stuff there as well.
    Education is what you're left with after you have forgotten what you have learned.

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    Thanks Jazz!

    So doing resistance readings from OT B+ to the plates, i have two very mismatched signals, which could account for a phase differential and therefor low volume?

    B+ to plate 1 - 37 ohms
    B+ to plate 2 - 103 ohms

    Can anyone verify that this could be the issue?
    Last edited by Mr_bibbles; 05-14-2017 at 07:44 PM.

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    Senior Member Enzo's Avatar
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    Ah 1N4003. I was assuming he meant the rectifiers over by the transformer. There is 1N4003 and 150uf there.

    Mismatched transformer resistance. SOme is normal for certain transformer, but that seems like way too big a diffence to me, so transformer becomes suspect.
    Mr_bibbles likes this.
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    Not a typo, the diodes near the two 150uf caps on the rectifier board we're existing 1n4003's (correct as according to the non-12ax7 schematic) and had been working inside this amp for a while. Replaced them with 1n4007's I had on had to get it working at least temporarily.

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    You're measuring DC resistance effectively from CT to tube plates on either side of the OT. It's usual to find that these readings aren't symmetrical due to the way a transformer is wound. It transforms on turns ratio, not resistance. as you wind further from the core it requires more wire per turn.

    What are the output tube voltage readings for screen and plate?

  11. #11
    g1
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    Ok, sorry for the misunderstanding. I was unclear what you meant by 'bias board'.
    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

  12. #12
    Senior Member Enzo's Avatar
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    1N4007s are fine, I never stocked any 1N400x other than 007s.

    I don't recall how the thing is laid out inside, then again I barely remember breakfast.
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    On low,

    459 Plate voltage
    -28v Grids

    Across the board.

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    OK So following the instructions, i hooked up one of the taps (8 ohm I believe) to the heater from the output tube, and disconnected all other primaries and secondaries.

    The voltage reading from winding to ground was .028 v AC on the connected winding....

    AC voltage,


    8 ohm to plate+ 1 (where the flyback diode blew) - .63vac
    8 Ohm to plate+ 2 : .35
    8 ohm to 4 ohm: .01
    8 ohm to center tap: .01
    8 ohm to B+ primary.4

    So this to me looks like the first plate winding has shorted??

  15. #15
    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Mr_bibbles View Post
    The voltage reading from winding to ground was .028 v AC on the connected winding....
    Impossible.
    Winding supplies 3.15V relative to ground, in series with 100 ohms.

    AC voltage,
    8 ohm to plate+ 1 (where the flyback diode blew) - .63vac
    8 Ohm to plate+ 2 : .35
    8 ohm to 4 ohm: .01
    8 ohm to center tap: .01
    8 ohm to B+ primary.4
    All those voltages are so low as to be useless.

    So this to me looks like the first plate winding has shorted??
    It might, but once 1 winding is shorted, all others around same core behave as shorted too,because they are all magnetically coupled.
    And Im worried about :
    disconnected all other primaries and secondaries.
    For a transformer to work, you must connect both ends of a winding, so if you connected one end of filament winding to 8 ohm tap, tou must also connect other end of said winding to the common tap on the OT secondary.
    Juan Manuel Fahey

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    Ok, let me see if I understand, I have almost no experience with these tests.

    8ohm tap (orange)---> filiment.
    Common tap (black) ----> filiment
    All others disconnected, measure the AC voltage from 8ohm to 4ohm tap, also from 8ohm to both plate windings and B+ winding?

  17. #17
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    Quote Originally Posted by Mr_bibbles View Post
    Ok, let me see if I understand, I have almost no experience with these tests.

    8ohm tap (orange)---> filiment.
    Common tap (black) ----> filiment
    Yes. All other wires disconnected. Now double check that there is 6.3VAC between those 2 wires.
    If so:
    1) What is AC voltage between wires that go to plates?
    2) What is AC voltage from B+ winding to each of plate windings.
    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

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