Shin-Ei Uni-Vibe rebuild question

[Jimmyguitar]

Hi, I'm new here so please give me a bit of a break if that's possible as far as maybe not having posted this where it belongs etc.! I have an original Shin-Ei Uni-Vibe that hasn't worked in years. I am doing my best to revive this sucker. I am having a few problems currently. My first is that the original transformer suddenly started putting out about the same voltage for the pilot lamp as it does to power the unit. It rather abruptly burned out the 6.3 V bulb I had in there. I bought some higher voltage bulbs so that isn't a big concern, today it seems to be powering a 14V bulb with no issues. My other issues are that I have the bulb for the LDR's lit, and I can easily adjust the brightness by turning the variable resistor. However, when I short the green wire from the PCB to ground  (to CANCEL) the effect, it turns that bulb to extremely BRIGHT. My last problem is the fact that I have yet to get the LDR bulb to pulsate. I have some good ideas on that one, like using some different transistors in the LFO that I am awaiting delivery on. I'm also a bit miffed as far as settling between a 100K dual Rev. Log Pot., or a 250K dual Rev. Log Pot (which seems to be extremely hard to find). I have the original one in my pedal which I have not yet measured but intend to soon. Any help would be much appreciated! Thanks, Jim

[Mark Hammer]

First, welcome.  I'm sure you'll be glad you signed up here.
Here's the schematic you are, or should be, working from.  The "cancel" footswitch simply grounds out the LFO output.  The trimmer on Q13's emitter adjusts the "default" brightness of the bulb when there is no LFO modulating its brightness.

[R.G.]

+1 for Mark's comments - welcome; and what the cancel does, and use that schematic.

If you haven't yet, you might want to read "The Technology of the Univibe" at geofex.com.

I've been trying to think of why grounding the cancel wire should make the bulb go bright, haven't come up with a good theory yet. Connecting the cancel switch to something more positive than a volt or so surely would do that, though.

Good intuition on the LFO transistors. The Univibe's current-mode phase shift oscillator circuit requires that the LFO transistors have both high enough gain and high enough impedance at the base of the two transistors Q11 and Q12 to oscillation. They might be "tired".

It also requires that the three oscillator caps from the base of Q11 to ground be in good condition. In restarting an old, disused pedal, I always replace all of the polarized capacitors first, without necessarily bothering to test them. Electrolytics go bad over time; they just do. If it were mine, I'd order a complete new set.