Repair Advice Univibe Clone

[johngreene]

hoping to hear from someone who has had experience with any sort of univibe troubles/repairs and or suggestions as to where to start

Thank you

The LFO and bulb driver are the main source
of trouble in both clones and stock units

hope you have access to a scope
otherwise, lots of tail chasing

You need to see what your LFO is doing on its own first
ie., against speed pot, and then go east from there ...

—-

ps. lookup my bulb driver offset mod while you're at it

http://www.lynx.net/~jc/pedalsUnivibe.html

best ...

The trimmer pot R48 is the offset adjustment (also called 'SYM' for symmetry). By adjusting this trimmer you can adjust the offset of modulation and by adjusting R46 (intensity limit) you can adjust the maximum brightness. The bulb will never get very bright as the LDRs are very sensitive and most of the modulation happens in the higher resistance range.

All the transistors are the same except for the input preamp transistor and the output buffer transistor. MMBTH10LT1G

You will not see any signal on the transistor to the right of the speed control (Q14) as that is the sink for the vactrol control.

The intensity control should change how bright the bulb gets with modulation. With the intensity knob all the way up the modulation should be severe.

Remember that if you have the cover removed the ambient light is going to severely reduce the amount of modulation because the LDRs never see 'dark'.

The opto-couplers can show a lot of sensitivity to ambient light as well. The black plastic doesn't completely block the light, which makes working on the pedals that use it difficult at times. But you need to keep this in mind when doing a final adjustment as things will change once you put the cover back on!

It sounds like the LFO amplitude is really low for some reason. For reference, R52 and R53 are in series with the LDR side of the opto-couplers and restrict the minimum resistance, thus the maximum speed. R49 and R50 are in parallel with the LDR side of the optos and restricts the maximum resistance value, thus the slowest speed, and keeps the LFO from stalling if set too slow. So the LDR have a fixed resistor across them so even if they are not balanced, at the slowest and maximum speeds their value should be swamped by the fixed values. You mentioned that the modulation seemed to increase significantly at high speeds which could imply one of the opto-couplers isn't going high in resistance. You could try (carefully) lifting the LDR side of the optocouplers and see if it works better at this very slow speed. This will also give you the opportunity to change the speed control and measure the resistance of each opto. The LEDs of the optos are wired in series so they are guaranteed to see the same current.

Hopefully this info isn't too late to help you.