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Thread: Gibson GSS100 Tremolo

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    Gibson GSS100 Tremolo

    Hey guys, I've been working on this beast for awhile off and on, the last thing to do is get the tremolo working. I have no idea whether the optoisolator is still functional or not but the first thing is to make sure the oscillator circuit is working. I'm looking at pg 436 in the old Gibson manual (Vibrato 5055 S Set "B"). I reckon I should see an oscillating voltage on the bulb side of R-1. That is not the case at the moment so I could use some troubleshooting tips. Which components should I be zeroing in on?

    Also, is the oscillator running by default or does the footswitch tip and sleeve need to be shorted for it to run, a la Fender BF/SF?


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    Old Timer Leo_Gnardo's Avatar
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    I work on a lot more tube than transistor vibrato/tremolo circuits, trying to revive them. I'm sure they have similar failure modes - most often having to do with the capacitors in the oscillator. I'd be casting a scanny eye on those 0.1 uF caps, heck they're cheap enough why not swap 'em out with new ones & see if the wobbulator starts wobbling as it should.

    Before you try that, make sure the filament in the bulb is working, same goes for the bulb adjustment trim pot.

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    Quote Originally Posted by Leo_Gnardo View Post
    I work on a lot more tube than transistor vibrato/tremolo circuits, trying to revive them. I'm sure they have similar failure modes - most often having to do with the capacitors in the oscillator. I'd be casting a scanny eye on those 0.1 uF caps, heck they're cheap enough why not swap 'em out with new ones & see if the wobbulator starts wobbling as it should.

    Before you try that, make sure the filament in the bulb is working, same goes for the bulb adjustment trim pot.
    Thanks Leo, gives me something to start with. To clarify, the bulb and the photocells are self contained in a "black box", literally (referenced as part 7380 on the schematic). So I don't think I can verify the filament, unless it can be done with an ohmeter or something along that line. For the record I'm getting a measurement of 105 ohm from the bulb (in circuit).

    Trim pot seems to be OK.

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    Last edited by bobloblaws; 07-08-2019 at 12:00 AM.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by bobloblaws View Post
    Thanks Leo, gives me something to start with. To clarify, the bulb and the photocells are self contained in a "black box", literally (referenced as part 7380 on the schematic). So I don't think I can verify the filament, unless it can be done with an ohmeter or something along that line. For the record I'm getting a measurement of 105 ohm from the bulb (in circuit).

    Trim pot seems to be OK.
    Good so far. Having another quick blink at the circuit, I don't see any other resistive pathways that would fool us into thinking the bulb's cold resistance is 105 ohms. Of course it would be best to separate the wire to the bulb & check it directly but in this case I say don't bother, close enough for rock 'n roll. I'd have at those two 0.1 caps next. Simply testing them won't do. What I find in many old amps, although the caps may measure OK, once I've swapped 'em out for some fresh film caps, abracadabra all of a sudden the oscillator wakes up. Has something to do with "Q factor" some old boffin told me, back when I was a youngster. Maybe one of our correspondents can elaborate on that?

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    What I find in many old amps, although the caps may measure OK, once I've swapped 'em out for some fresh film caps, abracadabra all of a sudden the oscillator wakes up. Has something to do with "Q factor" some old boffin told me, back when I was a youngster. Maybe one of our correspondents can elaborate on that?
    I always measure C and Q @1kHz. With film caps a Q below 100 indicates increased leakage. With E-caps a Q value below around 3 means increased ESR and/or leakage. If caps having a Q below above limits still work satisfactorily depends on the application/circuit.

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    Last edited by Helmholtz; 07-08-2019 at 01:18 AM.
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    I will test (or simply replace those caps) and if that doesn't solve it I'll test the transistors.

    In the meantime, I'm still wondering about the footswitch question. Does the oscillator run in the default configuration (no footswitch plugged in)?

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    Supporting Member Randall's Avatar
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    I'm going to say that engaging the footswitch grounds the base of Q1 and therefore turns it off, shutting off the oscillator.

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    It's weird, because it WAS working fine.....

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    Quote Originally Posted by Randall View Post
    I'm going to say that engaging the footswitch grounds the base of Q1 and therefore turns it off, shutting off the oscillator.
    So I got the tremolo going (and the reverb, to boot), just a couple of problems left over. When I first turn the amp on operating the "rock and roll" (tremolo) switch causes a loud POP at the output. Once the first pop is out of the way it is OK, at least for a time. In other words, at that point I can toggle the switch back and forth with no popping. Is this due to some residual voltage stored in a cap? There are a bunch of electrolytics on the various preamp boards that are original and way out of spec but the two that were on the tremolo module have been changed for new ones.

    The other similar problem is an equally loud noise when plugging/unplugging the footswitch jack. Unlike the RnR switch issue this happens all the time.

    Any suggestions would be most welcome, thanks.

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    Quote Originally Posted by bobloblaws View Post
    So I got the tremolo going (and the reverb, to boot), just a couple of problems left over. When I first turn the amp on operating the "rock and roll" (tremolo) switch causes a loud POP at the output. Once the first pop is out of the way it is OK, at least for a time. In other words, at that point I can toggle the switch back and forth with no popping. Is this due to some residual voltage stored in a cap? There are a bunch of electrolytics on the various preamp boards that are original and way out of spec but the two that were on the tremolo module have been changed for new ones.

    The other similar problem is an equally loud noise when plugging/unplugging the footswitch jack. Unlike the RnR switch issue this happens all the time.

    Any suggestions would be most welcome, thanks.
    Welp, upon further investigation I figured out the cause of the noise when plugging/unplugging the footswitch. When the plug is all the way into the jack one tab is connected to the tip (for tremolo) and the other to the ring (for reverb) But on its way in the tip momentarily makes contact with both the reverb tab and tremolo tab simultaneously. If the footswitch for the tremolo is OFF there is no noise because the tip is grounded when it makes contact with the two tabs. Probably needs a better jack where that situation doesn't occur.

    I changed out a bunch of old and way out of spec electrolytics and I was hoping the loud pop from the RnR switch would be solved as a bonus but the problem persists. Again, it happens when the amp is first turned on, after that I can toggle back and forth to my hearts content with no noise. However, I discovered something that might be a clue for you guys that are more electronically minded than I. Out of curiosity I was using my DMM to measure voltage on the connections of that switch. Here's what happens. If I take a voltage reading to ground on the terminal connected to the output of the tremolo module while the switch is open and then close the switch I get the same loud pop as when I first turned on the amp and operated the switch. And then no more pops until I repeat the process. I'm wondering if the switch itself could be faulty. I guess it's easy enough to test that theory.

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    Don't forget the joker g1's Avatar
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    Both those 1.5Meg resistors at the R&R switch are good?
    Attaching schems for interconnect (R&R switch) and other 3 boards that R&R switch connects to.








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    Quote Originally Posted by g1 View Post
    Both those 1.5Meg resistors at the R&R switch are good?
    Yes, the resistors are good.


    I noticed a couple of things further. In order to make it pop I only need to connect the tremolo side terminal to ground momentarily and then operate the switch (taking the DMM out of the equation). Actually the opposite of what I thought might happen. Also, if I leave the switch closed when cycling the power there is no pop when I subsequently operate the switch. But if the switch is open when I turn the amp off and then back on I do get the loud pop when I go ahead and operate the switch.

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    So I looked at the fact that the one side of the switch has the two 1.5M resistors but the other side doesn't. I tried adding a similar series connected pair of resistors (1.2M each) from the tremolo side lug to the reverb input. That seems to have eliminated the popping problem for the most part. But is it the best solution? I'm also curious why Gibson used two identical 1.5M resistors in series instead of a single 3M resistor. I can't imagine it was cheaper to use a pair of 1.5M in place of a single 3M, unless maybe they were already buying 1.5M's in bulk. But there is no difference electrically, correct?

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    Well I spoke too soon, I still have the basic problem that if I turn the amp on with the RnR switch closed (tremolo ON) there is a loud pop when flipping the RnR switch to the opposite position. I tried a few different things such as I mentioned in my last post above with resistor or cap on the tremolo side of the switch but to no avail. One thing that has me wondering is that the signal output of the treble pre-amp has 7VDC on it. I find it curious that the output of the bass pre-amp has a .47uF cap between Q-3 and the output but on the treble pre-amp there is no cap in series and signal output #2 is connected directly to Q4.

    Edit: I went ahead and put a .1uF cap in series between treble pre-amp signal output #2 and the RnR switch. This appears to solve my popping problem with no significant effect on the tone that I can hear. So I think I'm ready to rock (and roll)!

    Out of curiosity, does anybody know what Gibson's thinking was for the naming of the switch "Rock and Roll"? When it is in the position marked on the panel as "OFF" the tremolo is actually on and stops running in the "ON" position. So did they think of the tremolo mode as being Country and Western or Blues and lack of tremolo being Rock and Roll, or something like that?

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    Last edited by bobloblaws; 07-17-2019 at 10:51 PM.

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    Member patlaw's Avatar
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    Maybe I missed it, but what did you do to fix the tremelo?

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