1974x mains fuse blown help

Discussion in 'The Workbench' started by foreverstrung, Oct 10, 2021.

  1. foreverstrung

    foreverstrung New Member Gold Supporting Member

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    I've got a 2004, 1974x reissue that I've had for about 4 months. Maybe 20 hours total of playtime on it total since I've owned it. The other day I had it in standby and then got distracted. It was there for about an hour and I smelled something electrical and it took me a beat before I realized it was the amp and when I checked, the mains fuse had blown.......I ordered some more and while I waited, I read up a bit, but online, it's all over the place. Got the fuses today, (slow blow 250v, 1.6A) popped one in and it blew straight away. Pulled the power tubes and the rectifier and it again blew right away.
    I'm sure I got the correct fuse. Appreciate a heads up if I didn't. With my online searches, it was kind of confusing as well.
    2nd, what else would be causing the fuse to blow, with the rectifier and power tubes removed?
    Also, I've been inside my fair share of amps. I realize the danger. I've set bias in more than a few other Marshall amps and done some mods on Fender Amps. Modded a few pedals and build guitars fairly regularly. Does not mean I'm so ego centered that I've got to do everything myself, but if I can, I will.
    Appreciate the help
     
  2. Matthews Guitars

    Matthews Guitars Well-Known Member

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    If you are still blowing fuses with the power tubes removed, and the rectifier tube as well, then you've got basically two chances for the problem: Filter capacitors or a transformer, most likely the power transformer. If the filter caps aren't swollen up and haven't vented, then the transformer is the likely culprit.
     
  3. Pete Farrington

    Pete Farrington Well-Known Member

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    With no rectifier in place, it can’t be bad HT caps.
     
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  4. foreverstrung

    foreverstrung New Member Gold Supporting Member

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    I read somewhere these reissues had power transformer issues and there was a warranty (recall) kinda thing which I'm sure has long passed. I'm looking at Mercury Magnetics PT's but want to make sure that this is actually the issue, (tho im pretty sure this is the culprit) and that I get the correct one.
     
  5. Chris-in-LA

    Chris-in-LA Well-Known Member

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    What would have to be happening with the PT to blow the fuse with no tubes in? An internal short? Too much voltage to the ht fuse? What could OP check?
     
  6. Spanngitter

    Spanngitter Active Member

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    In most cases it is an internal short when the Main Fuse blows w/o any tubes present.
    Only other possibility I could think of would be a short on the 6.3Vac Heater circuit which causes fuse to blow w/o tubes present as HT circuitry is complete out of the loop with no tubes. You can also ohm out the (secondary) windings with a DVM and see if anything is unlogical, I have you the datasheet of a Hammond 290PAZ attached which is a replacement for a 1974 and you can use the values given there as an approx estimate on what you should get with your tranny.
     

    Attached Files:

  7. neikeel

    neikeel Well-Known Member

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    Commonly a mains transformer internal short.
    You are quite correct that there was a warranty issue with the first batch of these.
    You could try contacting Marshall with your serial number to hand (and any numbers stamped on the PT).
    If there was a known issue they may offer to help. Depending on where you live would depend on which replacement to go for. I’m sure the MM will be great (if expensive) and the Hammond good too (often surprisingly cheap) if you are in the US or Canada. In the U.K. I’d be tempted to get a later spec beefed up one from Marshall direct.
     
  8. foreverstrung

    foreverstrung New Member Gold Supporting Member

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    I purchased the MJTM18WP Marshall replacement tranny from the Tube Amp Doctor out of Germany. The original tranny is the MJTM15WP. I'm in Texas. UPS says 6-10 days to my front door. I looked at MM tranny's and I know they've got a good reputation. A+, But I figured the actual replacement/upgraded tranny made for this amp, concerning this issue would be best. I figured installation would be a snap as, I'm assuming, everything (wires) will be matching color codes
     
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  9. TM1

    TM1 Member

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    When this model first came out they had some serious issues with the mains(power) tranny's. They would send out replacements if you had a dodgy mains. Drake had some issues recreating the original RS tranny's. Mainly the paper bobbin and the paper interleaves weren't built right and they'd work for a few hours and then crater. If you got past 20 hours use on them you were probably o.k.
     
  10. foreverstrung

    foreverstrung New Member Gold Supporting Member

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    Th PT I purchased is as follows (spread sheet)

    Product information "Mains transf. for Marshall 18W Plexi / TAD Amp-Kit"
    Can be used with Rectifier tubes EZ81 (6.3V) and GZ34/5AR4 (5V).
    Vintage correct slotted lamination. Made in Europe to Dagnall specs.
    For 2x EL84 and up to 4x ECC83, lay-down type: Lamination 3,75"
    Perfect replacement/Upgrade for the Marshall 1974X or 2061X, replaces D4969 and D5323

    Mounting: 79x64mm

    Primary:
    240V Red
    120V White
    0V Black

    Secondary:
    [email protected] Red
    0V Green/Yellow
    290V Red

    3,[email protected] Green
    0V Green/Yellow
    3,15V Green

    6,[email protected] White
    [email protected] Orange
    0V Yellow

    PT-Secondary: incl. 5V winding (for Rectifier)
    Stack: EI96(3.75 Inch)
    Additional winding to 230V (primary): 110V-120V

    I don't think this is what I want. On the 18watt forum I've been getting some schooling on this and I guess the HT @ 120mA is 30 mA less that the Dagnell tranny.
    I've emailed Marshall and Dagnell. See what they say. I think I'll just return this one arriving today. I don't want to put a POS in there or something subpar.
    Anyone know anything about the HT being 120mA being weak? will it run well despite?
    Appreciate the feedback
     
  11. neikeel

    neikeel Well-Known Member

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    120mA in an 18w is fine
    The original RS Heavy duty as used in the JTM45 is 150mA.
    Perhaps the people on 18w forum are confused (not been there in a while)?
     
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  12. foreverstrung

    foreverstrung New Member Gold Supporting Member

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    Thanks neikeel. I emailed Marshall yesterday. They sent a replacement today. LOL. did not expect that, but I'm real happy about it.
    I'd appreciate any info on installing a new PT. Anything easily confused? this is my first.Thx
     
    Last edited: Oct 14, 2021
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  13. neikeel

    neikeel Well-Known Member

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    I’m delighted Marshall are seeing to the problem
    Yes we’d be happy to help (not Royal ‘we’) but plenty here to guide.
    Have your camera and ‘studio’ ready as pictures will be invaluable.
     
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  14. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Take plenty of pics of the old PT wiring before you unplug any of the wires.
     
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  15. foreverstrung

    foreverstrung New Member Gold Supporting Member

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    What's the simplest way to upload pics?
     
  16. neikeel

    neikeel Well-Known Member

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    any free web host. I use IMGUR now.
     
  17. Heiko Jakob

    Heiko Jakob Active Member

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    There is reason why fuses blow up, that's their job. And they will keep blowing up until you solve the problem why they keep blowing up.
    First, be sure to understand electric safety rules and precautions. -> No work on device while connected to mains and while capacitors are still charged.
    Then grab the schematics and a multimeter and you're ready to go ....
     
  18. Chris-in-LA

    Chris-in-LA Well-Known Member

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    I have an iPad that I take pics with. I resize pics by emailing them to myself, that usually takes them down below 1meg. I then copy the pics from the email and paste them into the reply window. I don’t even need to use the “Upload File” button using this process. Some guys with PC’s use MS Paint I think to resize picture files.
     
  19. foreverstrung

    foreverstrung New Member Gold Supporting Member

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    Does anyone know where I might find a Dagnall 4909 PT drawing? I can't find one anywhere and Dagnall has not responded to emails. Marshall is sending a replacement, but IDK when and I'll likely have the same challenge identifying primary and secondary wire locations as the colored wires will likely be different as they are with this Hammond 290PAZ. And, the Hammond is here now.
    Pictures coming
     
  20. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Are you planning to put in the Hammond while you wait for the PT from Marshall?
     

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