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Superlead help

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Roodboy, Jun 6, 2005.

  1. Roodboy

    Roodboy Supporting Member

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    I recently bought a 74 Canadian Superlead, which promptly stopped working after 5 glorious minutes.
    Heres a link to the auction:
    http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=38075&item=7321316050&rd=1

    These Canadian Marshalls are a bit unusual due to more stringent electrical standards than US and UK and resulting modifications for the Canadian market.

    I received a repair bill as follows for this "nonmodded' amp:
    -replace wrong fuses
    -replace shorted tubes and rebuild bad bias supply
    -clean pots and jacks
    -Note: PC board has damaged traces from previous repairs and mods. many traces damaged & patched with wire.
    Parts included:
    1-4A s/B fuse
    1- 1A F/B fuse
    4 EL34 Winged C's
    2- 1 mega cts pot
    1- .0047/600v sprague cap
    2- 8wt/150v sprague cap
    1- 47k 1/2w bias resistor
    1- 25k bias pot
    I got the amp back from the tech and played it for 1/2 hour and the amp dies, no power no nothing. I turned it on after 5 minutes and it worked fine, then after a 1/2 hour same thing and it went on again after 5 minutes. This time I didn't play it again.
    I have a few questions:
    1)Any idea what my problem could be?
    2)In one of the photos there is an outline for two parts on the board in white that don't appear to be present, any idea what thats all about?
    3) Any suggestions as to any additional work that should be done froma preventative maintenance or tone standpoint?
    4) Any observations or comments on the amp from the pics would be appreciated.
    Thanks in advance for your help.
    Links to pics:
    http://tinypic.com/5n4sxh
    http://tinypic.com/5n4t2b
    http://tinypic.com/5n4suc
    http://tinypic.com/5n4t4h
    http://tinypic.com/5n4t8p
    http://tinypic.com/5n4tbm
    http://tinypic.com/5n4ti1
    http://tinypic.com/5n4tjq
    http://tinypic.com/5n4uuq
    http://tinypic.com/5n4tg8
    I have taken the amp back to the tech since as the power shuts itself off after 1/2 hour. I can turn it back on again after 5 minutes. The story I got from the tech is the power cord is too loose in the electrical jack. This is obviously incorrect as I have used different jacks since and have had the same problem. The only other thing it could be he said was a thermal breaker inside the power transformer. if that is the case i would think something is causing this to activate.
    Anyone ever hear of this or have any ideas?
    Thanks for the input.
     
  2. John Phillips

    John Phillips Member

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    Don't worry about the 'missing' parts positions, they are for a different version and were never used on this amp.

    I don't think there has ever been a thermal breaker in a Marshall power transformer, but it could be a transformer fault. In fact, if the power dies completely and then can apparently restore itself with no other damage showing up, there is a problem in the primary circuit. It must be a bad connection somewhere in either the power cord, mains fuseholder or mains switch - or the transformer. There are no other components on the primary side since this amp doesn't have a voltage selector (which I'd suspect first, otherwise).

    It does sound like it could be a transformer problem since it seems to be due to heating, but here are some things to check:

    The power cord wiring just inside the chassis. At some point the cable has been broken at the clamp - probably just the outer cover, but it's possible the internal wires have broken too. You can tell this because there is old crimping on the inner wires just inside the chassis, where the whole thing has been moved about 1" into the amp, the last section of the original outer having been removed. Try carefully moving the wires around here and see if you can get it to cut out.

    The mains fuseholder has been changed. Check the quality of the wiring job on it, and the fit of the cap into the barrel. It's a low-quality type anyway and you would be better getting your tech to re-replace it with the correct one (he should have done that, actually).

    It could also be the power switch - they are well known for failing, which is why you see so many old Marshalls with mismatching ones.


    I read the seller's description, and although it is not strictly accurate, it's possible that he didn't know about all this stuff, and it's worded loosely enough that you probably don't have a case against him even if he did. Proving that he definitely knew about this fault would be hard. The amp isn't too damaged (unless it's the PT, which could cost a fair amount) so if you like it, it's probably best just to live with it and get it fixed properly, although you could always try sending the repair description to the seller and asking him to explain it...
     
  3. Roodboy

    Roodboy Supporting Member

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    Thanks for the reply, John. I really appreciate it. I will post a follow up after it is all resolved. The amp has the tone of the gods.
     
  4. Roodboy

    Roodboy Supporting Member

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    As a followup it turns out that the problem will only occur when my hotplate is used. I am waiting for a diagnosis from the tech.
    Has anyone ever heard of an issue with a canadian marshall not liking a hotplate?
     
  5. Roodboy

    Roodboy Supporting Member

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    Its fixed! It turns out there is some kind of thermal resistor or somethng in the power transformer which is completely unnecessary and has been removed.
    Problem solved.
     
  6. Swarty

    Swarty Member

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    hmm, very wacky. I wish I could get a good explanation for the reason late '70s thru JCM800 Canadian Marshalls do not have a 16ohm tap on the transformer.
     
  7. Roodboy

    Roodboy Supporting Member

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    I think I have a 16 ohm tap on the OT but the ohm selector is the kind that is a two prong knob that gets pulled out and depending on the postion it gets pushed back in and is shown in the window is the ohms.
    Anyway one option is 16.
    My concern is that this thermal gizmo was there for a reason other than the canadian electrical authority (or whatever they were called), giving Marshall a hard time.
     
  8. John Phillips

    John Phillips Member

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    Probably because the maximum voltage which can be developed at 16 ohms from a 100W Marshall is over 50VAC. The continuous distorted power can easily exceed 200W, which gives 56V at 16 ohms - though only 40V at 8 ohms. That probably qualifies as a 'hazardous voltage' which is above the rating of 1/4" connectors. I also wonder if this is the reason why all newer Marshalls don't put out quite such brutal amounts of power - ie JCM900 onwards, and even the reissues don't seem to.
     
  9. Swarty

    Swarty Member

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    Thanks for reply John. That makes sense. It makes one wonder if they ever shipped Majors or SVTs to Canada.... What about high power PA amps?
     
  10. John Phillips

    John Phillips Member

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    Don't SVTs use a 4-pin connector and cable for the speaker which acts as a secondary standby circuit and stops the amp powering up until the cab is connected? I thought it was intended to protect the tubes and OT, but it would work just as well for preventing high voltages at the end of a loose cable.

    I could be wrong, but I think XLRs (commonly used in higher-power speaker systems) and Speakons are rated higher, too... maybe 100V, I'm not sure.
     
  11. Swarty

    Swarty Member

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    SVTs have 1/4" jacks. The older portaflexes have XLR connectors that act as a standby.
     
  12. Roodboy

    Roodboy Supporting Member

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    Heres a link to a picture of the thermal breakerthing that was removed from the power transformer.

    http://tinypic.com/6741w1
     
  13. Roodboy

    Roodboy Supporting Member

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    Thanks for the help. This has really stressed me out, I have over $450 into this amp in repairs and it sounds great, but I am concerned about why this themal thingamabob in the link above was tripping. My tech said it was just old and that these things are unnecessary in a Power transformer.
    I guess my concerns are:
    1) Why would the power transformer get more stressed and hotter than the OT when using the hotplate?
    2) Could the PT be having issues due to "hazardous voltage" issue Mr Phillips described to Swarty?
    3) Has anyone ever seen this thermal breaker before and is it really OK to have it removed without increasing risk of damage to the PT?
    I really want to keep this amp it has girth to the tone unlike anything I have ever heard. I had one @ 25 years ago but was unable to acheive this tone, probably because I was a dopey teenager.
    I really want the amp to be stock, I have tried the modding route in amps and have decided the best mods are speakers & tubes.
    However, the bad kharma from the inaccurate auction description and the subsequent repairs have caused me a lot of aggravation.
    Any input would be appreciated.
     
  14. John Phillips

    John Phillips Member

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    I've worked on at least two SVTs with the 4-pin cables as well (though not more modern ones, I don't think). Maybe it was a requirement under British rules too...


    It's hard to say whether the PT is at risk or not, without knowing what has and hasn't been checked in the amp. Running flat-out into a too-low impedance load would draw excess current though, and may possibly overheat the PT if it's on the edge anyway. It may well be overheating the OT too, but you won't know about it since it doesn't have a thermal breaker...

    UK Marshalls do not have the thermal breaker - but equally, blown PTs are not totally unknown either... though rarer than blown OTs, for reasons which as I said I don't think are really the fault of the transformer - just the crappy impedace selectors, bad speaker cables, and the 'X' factor... musicians ;).

    The 'hazardous voltage' regulations would only apply to the speaker output voltage BTW, nothing to do with the PT, although I believe Canadian amps do not have external voltage selectors either for the same reasons. Having mentioned that, have you checked that the amp is set to run at the correct voltage? If it's wired for 110V and it's now getting more like 120 or 125 it will cause extra stress on everything in the amp.
     
  15. Roodboy

    Roodboy Supporting Member

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    I don't know what the volatge is on my outlets in the USA but if it really is 120 or 125 in the USA I guess I would need to have the amp reiwred or would a variac do the same thing?
     

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