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Vox Cambridge 30 Problem with Tremelo Roach Circuit

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  • Vox Cambridge 30 Problem with Tremelo Roach Circuit

    So I picked up a used Cambridge 30 some time ago, and it came to me with a low volume issue. I assumed it would be an easy repair, as there are numerous posts online about failures with the opto coupler used in the Trem circuit, and when it fails the amp loses volume. I did some troubleshooting to validate that this was the problem by jumping across the leads of the photoresistor side of the roach and got full volume out of it, so that indicated to me that the problem was a failed roach. I read that a Fender style tremelo bug can be used to replace the stock coupler, so I dug through my parts and found a new roach I had purchased years ago and replaced the failed coupler with this one, but the problem with low volume remained. OK, maybe that coupler was no good. So I the next time I was ordering parts I ordered several more, and last night my bench was clear so I got the amp up on the bench and opened it up, removed the pcb and swapped out the optocoupler again and put it back together, with the same results. Again, I jumped across the leads of the photoresistor side of the roach and got full volume (it actually sounds pretty good). I checked for voltage to the hot side of the roach and I'm getting around 6.5 volts dc there, that's a little low according to the schematic, not sure if it's too low. But the on the ground side I'm getting no continuity to ground, which if I'm reading the schematic correctly there should be continuity to ground, am I right? (see attached schematic) I've considered just replacing the coupler with a jumper and do without the Tremelo, but I'd like to have it, I do use Tremelo, and if this tremelo is close to the Vox Pathfinder 15R Tremelo, I really like it, it's one of the nicest trems I've used. So I'd like to get it working. I thought I would check in here and see if you guys had any words of wisdom for me.

    Here's the optocoupler I used, is there a problem with trying to use this style of coupler?

    I have the side with the dot going to the 9 volt power, it is supposed to be non-polar, so I don't think it matters which direction it's installed, but maybe that's wrong.
    Attached Files

  • #2
    FYI, the original part is available from the same company you bought from.

    (Note: You have to select the exact part in the "Model Number" box.)

    The original is an LED and the ones you bought are not. You may still be able to make it work with some circuit modification, but if it were me, I'd just order the original part.
    "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22


    • #3
      I don't know who said a Fender bug would work. The Fender has a neon lamp in it. This circuit runs on 9vDC and uses an LED. The neon needs several times that voltage just to light. Fender bug will not work here.

      Look at your schematic, it does not tell you 9v is on the opto, it just says 9v is what the circuit runs on. There is a 22k resistor between 9v and the LED. And continuity to ground? Not on the schematic. The low end of the LED is connected through VR11 and R60 to the output of IC6b pin 7. Not to ground.

      This is a trem bug. If we just connected the top to a power supply and the low end to ground, the LED would just sit there glowing steady. It needs to pulse to make trem.

      The schematic calls for a LT9914. I found as a warranty center that originally a lot of them - like every single one the store had - had bad trem bugs. But once I replaced them, the new bugs seemed reliable enough. I think the factory got a bad bunch. You can still get the LT9914. The schematic also notes VTLC5-3/2 SO while I have not done that, apparently you can use the VTL parts.

      You will have to adjust the trim pots.
      Education is what you're left with after you have forgotten what you have learned.


      • #4
        Ah, thank you very much guys. I do have a VTLC5-3 optocoupler as well, so I guess I will swap it out and see if that helps. I have gigs the next couple of evenings, so I won't get a chance to get back on this for a few days, but I'll report back and close the loop on this thread if all is well. Thanks again, appreciate the help.


        • #5
          Don't forget that LEDs are diodes, i.e. polarized.
          - Own Opinions Only -


          • #6
            Originally posted by Helmholtz View Post
            Don't forget that LEDs are diodes, i.e. polarized.
            10-4, thanks


            • #7
              OK, I wanted to come back and close the loop on this thread in case anyone finds it in a search later. I got a little time yesterday between doing honeydo projects and heading out to an early gig, so I replaced the Fender style roach opticoupler with the VTLC5-3 and that did the trick. I had to adjust VR11 & VR12 potentiometers afterward, as I had already screwed around with them when I was working with the wrong coupler, but that was not difficult, and now it appears to be getting full power output, though I haven't checked that yet. This is a pretty nice sounding amp, though the stock 10" speaker seems to get overwhelmed when cranked up much beyond 5 on the volume, unless I stick to the bridge pickup. I may have to play around with a different speaker to see if I can get that sounding a little better. This is all in an attempt at getting a decent Vox amp at a decent low cost and that doesn't weight a ton. Even the AC15's are pretty hefty IMO. The older I get, the more I'm drifting towards lighter amps. I've had one of the Pathfinder 15R's for a number of years, in fact I had 2 and recently just sold 1 of them. I really like that amp, but it's not very practical for gigging with, though I've done it several times. I picked up one of the Mini Super Beetles not too long ago, and while I love the tone from it, it's too small and low powered to gig with. I took it to a blues jam and had to crank it all the way to just be heard. So this was my next attempt, and I'm not sure it's going to work out either. I may have to break down and build something to get what I want. Anyway, thanks for the help guys, this is still the best Amp related forum on the net because of all of you.


              • #8
                If you think the AC15 is heavy, don't go near an AC30. The one my friend played was 75 pounds. I had the same experience as Hasseri with the mini Beetle (bass) and quickly sold it.