When a leaky cap isn't a leaky cap !!!!!

Discussion in 'The Workbench' started by jon1917, Oct 17, 2021.

  1. jon1917

    jon1917 Active Member

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    Please see below a pic of my long suffering 20W P.A. It's had most of the usual ailments that go along with an early 70's Marshall but one has been the bane of my existence for a long time now. The buzz.
    Substituted a lot of components to track it down but still no luck. Finally tracked it down to the 0.022 coupling cap coming from pin 1 on V2. 290 ish volts DC on the pin side and about 0.6vdc on other side. Shame as it's a mustard but needs must. Popped in the cap you can see in the pic and still reads like a leaky cap 0.6vdc on the wrong side. Tried another, same result. Dug out a pulled Mustard, same result.
    Went to bed.
    Woke at about half 2 in the morning and started thinking about conductive Fender boards (as you do). Must be watching too many youtube tech videos, and had a thought.
    Remembered the jumper wires across the board on this amp are a waxy cloth rather than pvc insulated like on many others.
    Anyhow, to the point. As you can see in the pic, there are two brown cloth covered wires running left to right along the centre of the board, underneath the 0.1 cap. One is for the plates on V1 (290ish vdc) and the other comes from what should be the 0vdc from my cap.
    I put the meter from chassis to the plate wire cloth and got about 9vdc. Tried the same thing on the other wire and bingo, 0.6v.
    After giving all the cloth covered jumpers a good soak in isopropyl and a lot of cotton bud rubbing, i separated the two in question as they were touching and i fired up the amp and tentatively stuck the meter on the end of the 'leaky' cap. 20mv dc, it worked and the buzz is gone.
    Feeling very happy and a bit techy. Oh, i will be putting the mustard back.
     

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  2. StingRay85

    StingRay85 Well-Known Member

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    Great fix congrats
     
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  3. Pete Farrington

    Pete Farrington Well-Known Member

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    Well found!
    Sound City used the same stuff, I’ve encountered that issue a couple of times with them.
     
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  4. PelliX

    PelliX Well-Known Member

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    Good stuff, well found! If you want to skip the restless nights, get an ESR meter. There are cheap kits available nowadays. "Component tester" is what you're looking for. You can also get them assembled, but you don't strike me as someone who cares for that. ;)
     
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  5. Pete Farrington

    Pete Farrington Well-Known Member

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    It requires a certain degree of scepticism / lateral thinking to consider wire insulation which looks ok to be dodgy.
     
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  6. Old Punker

    Old Punker Well-Known Member

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    Damned lazy electrons, always taking the easiest path!

    [​IMG]
     
  7. PelliX

    PelliX Well-Known Member

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    I'm very happy with the way this works, as long as I'm not that path :thumb:
     
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  8. Gene Ballzz

    Gene Ballzz Well-Known Member

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    You sir seem to have a fair amount of experience with and knowledge of the early SOUND CITY stuff! :cheers: In the mid '70s my best friend "Biff Spliff" Came to rehearsal with a brand spankin' new SOUND CITY 50 (maybe PLUS not sure) on a stack of SOUND CITY 4x10s loaded with Dallas Arbiter speakers. What an absolutely glorious sounding rig! :naughty: I knew eaven less about amps then (hard to believe) than I do now. I'd love to know some hands on info and discussion about what appears to be a very unique preamp!
    Thank You,
    Gene
     
  9. StingRay85

    StingRay85 Well-Known Member

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    Sound City AKA Sound Shitty Mark 4. The 50 Plus was a 2x EL34 with active tone stack. It's a small chassis compared to the big brother L120 (6x EL34). Indeed all these amps used this wiring, so far I haven't had any issues with them. I have:

    PA120R Mark 4 converted to 2204 preamp
    L120 Mark 4 converted to 2204 cascade V1 + active L120 preamp
    PA50 Mark 4 stock (future project)
    50 Plus converted to Bassman (not by me)
    B100 Mark III stock
    L120 Mark 4 converted to Hiwatt 4 hole mid 70s preamp
    L120 currently gutted and going for a Soldano SLO100 preamp

    The active tone stack basically separates the signal into three separate triodes, where each focus on either bass, mid or treble. Afterwards they are joined in a resistive mixer and further amplified. These tone volume pots also interact with eachother, if you open up treble, then you already notice that mids are going down again. The bass is not so responsive IMO. The mid is really like a volume control.

    If you want to see what I did to one, check here: https://www.marshallforum.com/threads/cascaded-2204-style-sound-city-l120.122405/
     
  10. Pete Farrington

    Pete Farrington Well-Known Member

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    Yes, they are great sounding amps, kinda one trick ponies, but what a trick!

    I’m not sure that the preamps had much to do with it, rather it was the great transformers and very low sag HT supply, no choke etc, so no screen grid node sag. Resulting in the EL34 being hammered really hard, delivering a rather different, harder hitting response to Marshalls, especially the lower HT voltage JMP 50 watters of that era.
    Those tone controls had a bigger than normal range of control, but not so much that the knobs were a hair trigger. The main issue with them was hiss, though a master volume might have made it less noticeable.
    They look to have been a development from the Marshall Pig, a similar concept but arranged differently, and adding a midrange control.
    Despite what Merlin and other super brains told me, I still can’t regard them as active. Rather I see them as passive filters followed by recovery stages, which then feed into a passive mixer.
    In themselves, the passive filters operate just the same whether there’s a recovery stage or not.
    Whereas for ‘active’ to be meaningful, the active element should be essential to filter circuit’s operation.
    Plus https://el34world.com/charts/Schematics/files/Soundcity/Soundcity50plus.pdf
    Pig https://ampgarage.com/forum/download/file.php?id=64181&mode=view

    I think it would be pretty difficult to make 3 band passive filters any less interactive than those in the Plus range. I suspect that it’s their wider range of control that may lead to the perception of interaction, when compared to a regular tonestack.
     
    Last edited: Oct 18, 2021
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  11. StingRay85

    StingRay85 Well-Known Member

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    I'm sure it's not just a perceived interaction, with treble completely cut, volume set with mid, and then slightly opening up treble causes a volume drop including in the mids.

    On the other side of the spectrum, the bass control is not very responsive.

    It's on my to do list to scope it to obtain all data, but right now I'm glad to have closed it up again and play it a little.
     
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  12. Pete Farrington

    Pete Farrington Well-Known Member

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    Hmm, that seems strange, I don’t recall them acting like that. I’ve not got a stock one knocking around anymore to check, unfortunately.
    Maybe something isn’t working right?
    Let us know if you find anything when the spirit moves you to commencing the investigation.
    I recall they tended to use a few rather fragile polystyrene caps, very thin leads, which could break etc.
     
  13. StingRay85

    StingRay85 Well-Known Member

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    When I open it up again to scope, I'll check again. But in any case I already cross checked the schematic and found a few deviations. It was a bone stock amp though, surely it left the factory that way. I'll keep you posted.
     
  14. Matthews Guitars

    Matthews Guitars Well-Known Member

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    I always ask the question when chasing a gremlin: What can go conductive in 50 years, that wasn't meant to?

    Fiber boards. Cloth insulated wire. Other insulated wire depending on insulation type. Even standoffs. Nylon can go conductive if it absorbs enough moisture.

    I rely on physical spacing for leads, components, and wires to keep them from unwanted conduction issues, when practical. I don't care how well made the parts are, I don't even allow epoxy dipped low voltage capacitors to touch each other.

    A lot of consumer electronics have been assembled with types of adhesive (usually holding capacitors to the PC board so they stay in place when wave soldered) that become conductive after a few years. When I see that brown crud on a PC board around a capacitor, I KNOW that's suspect. It is often part of the problem. Usually the capacitor it's been slopped all over is bad, too,
     
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