Bias issue on TSL yes I know but this is weird

Discussion in 'Marshall Amps' started by John T, Sep 20, 2019.

  1. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    yeah, funny, this is my 3rd TSL. 1st 2 were stolen. When I bought this one, I wasn't aware of all the issues, as I never had any issues w/ mine. My last one got stolen in '04. So, I hadn't even thought about 1 for a long time. When I saw this one for sale, I didn't think to go looking for caveats, because the 2 I had were fine & I had no reason to look for warnings...

    Once I started looking, after the issue, I noticesd a bunch of footswitch issues, then I found that TSL site that describes the nightmare, before Marshall started issuing replacement boards. Now, the last one I guess is ISS 21. If things start to go downhill after this, I might just get ISS 21 & see if I need the bias board mod. If so, I'll assemble them before installing & go...

    I might then also do the Merc Mag trannies/choke too & it'll be a high performance TSL.
     
    Last edited: Sep 28, 2019
  2. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    ok, so, I'm waiting on the resistors for R6 & R9 to come in, & am getting the amp ready to swap those out.

    I clipped the wire retainer straps, so I could move the output board out of the way & I find this...

    r9.jpg

    Couldn't see it before. I was just thinking of clipping the wires & soldering the new leads to the ones sticking up, like piggy-back, but removing the resistor. This looks like there's some carbon, from something on the underside.

    I'm not a tech, so I was hoping to just getting it to the point of troubleshooting, then I could either go to the tech, or if I have to, start pulling connector cables & pulling boards, just buy ISS21 board & the bias kit, then I'd have a new amp.

    Admittedly, I used to be good at soldering, but, since the newer solder concoctions, I have a hard time getting it to melt, w/o putting a lot of heat to it, so if I have to start soldering on this, I'l leave it to someone who can do it...
     
  3. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    Ok, so, I got a brush out & brushed it off, & noticed that it seemed to brush, so I got a toothbrush & some 90% alcohol & brushed around & it appears to have cleaned up.

    r9-3.jpg

    I'm guessing the carbon is from the resistor when it went?

    I don't see any residue on the resistor it's self...
     
  4. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    So it was from a previous burnout?
     
  5. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    I didn't take the board out. It cleaned up (2nd pic) using a brush & alcohol. I'm wondering if it was from the resistor in there that isn't any good. I don't see anything on that though. I'm thinking when the resistor failed it shat on the board?

    I was supposed to get the resistor today, but UPS failed me...
     
  6. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Check for spots or cracks underneath when pulled.
     
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  7. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    ok, got resistors today...

    all 4 measure 1.4 ohms on pin 8 from chassis ground...
     
  8. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    there was a black blowout hole on the underside of the resistor, that blew the carbon on the pcb...
     
  9. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    I checked everything out, got it to match -42 on all tubes pin 5.

    When I power on, it drops to -41.5

    When I install the tubes, the bias points read 52mVdc w/o adjustment.
     
  10. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    here's the underside of R9:
    PAUL1226.jpg
     
  11. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    I think the bias pots might be bad though...

    I got it to bias at around 79.5mVdc/side.

    It moves around a bit though, like it climbs up to 80mVdc & drops to around 78.8mVdc

    or... the dreaded bias board issue.

    It doesn't seem to drift. I plugged it into an 8 ohm cab to bias & play for about ½ hour, before testing it again. It stayed stable, w/in that same climb/drop range.
     
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  12. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    All seems normal, but replace bias pots recommended.
     
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  13. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    thanks you guys!

    @mickeydg5 @ampmadscientist @marshalltsl @jmp45

    I attached the resistors to the old legs, by making a small loop on the new legs, then making a solder pad. Then I melted the pad & pushed the old leg through. Tugged on it & seemed solid/stable.

    Eventually, I'll have someone solder new ones to the board pad direct (I bought a pack of 10 of the R6/9 variety & 100 of the R21 - same cost :shrug: ) later, but, it should be good enough for jamming around making noise for now...

    I didn't do anything to the ground on the Cliff Jack, but, it seems to be ok for now, as I'm using an 8 ohm cab & it runs ok...

    Looks simple enough, since that board comes right out anyway...
     
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  14. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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