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the ol' hot rod deville...more drive not functioning

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  • #1

    the ol' hot rod deville...more drive not functioning

    Hey all, I have a moment here for a breather and decided to pull out my hrd for a bit of much needed service
    FWIW i did a full cap(electro)job on it, 'fixed' the taper of the master volume pot, replaced a couple burnt parts around a shorted power tube. And actually found a break in a trace leaving the first plate of V1 before the couplers. this may have just happened pulling that dang board in and out.

    Well everthing I addressed worked out and all seemed well until I realized that the "more drive" button, although triggering the indicator to red, is not kicking in the more drive. It just seems to stay in normal drive in both positions. I'm not sure what to look at here as I am not really that hip to the whole switching crkt. I am getting slightly high dcv (+17v and +16.5) across the two 5watt cement resistors though I wouldn't think this enough to cause a problem here.

    I'm just not real sure on troubleshooting this so any help for me to find it quicker is great. Since as i said i did find one broken trace I'm leaning toward maybe another one around this area. (>?_)
    Last edited by doctorfate; 10-10-2010, 11:44 PM.
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  • #2
    " I am getting slightly high dcv (+17v and +16.5) across the two 5watt cement resistors though I wouldn't think this enough to cause a problem here."
    You should be getting +16VDC & -16Vdc across the cement resistors.

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    • #3
      Oh sorry that was a little mind slip there in my typing, of course I meant +/- on those readings. 17+ and 16.5-. again if there is any way this small discrepancy could cause such a symptom I will give a shot replacing those resistors
      also i seem to recall the more drive working fine upon a quick test before I messed with the pcb which if I'm right is a pretty good indicator of a broken connection or the like. I'm just not familiar with that little crkt
      thanks

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      • #4
        ACROSS the resistors? Or from one end to ground?
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          Simulpost...


          Those resistors don;t set the voltages, the zener diodes do, you xcan change resistors all day and not affect the voltages. And no, that is not your problem.


          The light turns red, so the switching circuit is working. Look at the schematic, there is a control signal MORE_DRV coming from U3A. Now look up under V2, see Q1, Q2? And CR16 diode to their gates? The MORE_DRV comes through that diode to those gates. Q1,2 thus switch cathode bypass caps in and out to afect gain.

          As you switch the more gain button off and on and the red light goes off and on, watch the voltage at CR16. Does the voltage at each end of it go off and on with the switch? And does it get to the gates on each transistor?

          Next to Q1,2 are R23,R24. Q1,2 basically short acrosss those resistors when on. SO with the amp running, measure resistance across each of those resistors. Does it toggle between about 100k and about 100 ohms with the switch? The resistors I feel are a lot mor convenient to take the reaqdong on than directly on the Q1,2 legs.
          Education is what you're left with after you have forgotten what you have learned.

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          • #6
            first off thanks for explaining a bit about how that little fet crkt works, I can see it just not perfectly. so here it is hopefully this will explain what's amiss

            MD switch ON / OFF

            CR16(K) - 1v / +16v
            CR16(A) 0v / -4.92v

            R23 20ohm / 45ohm
            R24 20ohm / 21.5ohm
            also that CV coming from U3 reads -14.9 OFF and +16.2 ON , that's taken directly off pin1 before the 100ohmR
            thanks again

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            • #7
              JFETs are by their nature "ON" until turned "off" at their gates. Yours are stuck on. I'd suspect you have a bad JFET, and since they are wired together, one of them makes both stop working. Since the voltage is toggling up to the diode, I have to think the whole control circuit is OK. JFETs are cheap, so rather than guessing which might be the bad one, just replace them both, and since those diodes cost a nickel, replace it too.
              Education is what you're left with after you have forgotten what you have learned.

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              • #8
                I actually can't stand the more drive setting and never use it and was thinking of leaving it be( also not to risk "uneccesary" further abuse of the pcb getting in and out)but can't stand to not know why something isn't working. also I am just wondering what could cause one of those fets to go bad during the work I was doing when I'm sure the drive was working previously, I wonder if I somehow broke a trace around there. I'll check it out and also what fets I have around, is this crkt picky about the on off specs?

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