Univibe LFO problem

[MWichni]

Hi there, this is my first post on this forum. I've built Univibe from general guitar gadgets schematic, but lfo gives too low voltage on the output from it at the slowest frequencies and effect is not so audible as on faster frequencies. I've measured voltage on the lowest and fastest setting of frequency of LFO and it is 2V on lowest and 13V on highest. I thought about making some AGC (automatic gain control) but I don't know if it handle that range of changes, I mean here this 2-13V differences. Help me please.

[Zedmin_fx]

Can you get the voltages on q13,q12 and q11,(transistors)  you need the voltages from the collector to emitter, base to emitter and base to collector. Also MAKE SURE YOUR WIERS ARE CORRECT!!!! I am sure other people here will be able to help ypu more. Thoes are just a couple of things I found that I had to do when I was debugging mine. I think I would replace transistor 13 for now I have blown a few of them in my builds, just to see what it will do.

[Papa_lazerous]

DIYSB may well yield better results for a debug especially if you use the search function over there.

[Zedmin_fx]

DIYSB may well yield better results for a debug especially if you use the search function over there.

Yea I agree but do remember its kinda hard to use the search  when you are brand new here!!!!!!!

[Papa_lazerous]

DOH!! I am a first class numpty, I posted that comment that DIYSB would be a better option thinking I was on another forum!! serves me right for having 2 tabs open trying to do several things at once

[MWichni]

I think that the voltages on other transistors are correct, all is built exactly like in schematic. I've tested it, its problem of LFO only I think. But I'll check it out too. Thanks

[Zedmin_fx]

I think that the voltages on other transistors are correct, all is built exactly like in schematic. I've tested it, its problem of LFO only I think. But I'll check it out too. Thanks

I had problems with my wires they here jumpered by a litle pice of wire, so chek it out for both speed and depth knobs. Alsowere you useing the correct wiering diagram ggg has two on there site one is right one aint. post me the link and such. also I am just realizing that you said you bulb turns on and blinks right? In that cas  would just change out the transistors. I wish soone with more knoledge would get on here to help ya out.

[MWichni]

Well my vibe works, lfo is driving the bulb but on the lower frequencies phasing is not so audible like on the higher. 

[mdh]

Depending on how big the discrepancy is, this might just be the nature of the circuit.  I believe there are some comments on the Univibe LFO in the "Technology of Phasers and Flangers" article on Geofex.com.  I'm pretty sure that the bulb in my Neovibe (same project as on GGG) flashes quite a bit more intensely at high rates than low rates, and I think this is just a feature of the LFO.  Have you compared the sound of yours to the clips that are available?  The best ones are probably those at http://www.mrdwab.com/john/soundclipspage.html.

[Zedmin_fx]

Well my vibe works, lfo is driving the bulb but on the lower frequencies phasing is not so audible like on the higher. 

OOOO  Ifel dumb I thought it wasent working. Yea do what mdh says to he is the one that helped me get mine fixed (thanks again).

[MWichni]

Mr. R.G. Keen please help me. Maybe you can draw the schematic of the AGC (automatic gain control) for my LFO. Thanks for any help.

[R.G.]

The Neovibe - like the univibe - does not need an AGC for the LFO.  There is a sort of "AGC" there; actually more of a limiter than an AGC. The back-to-back diodes across one of the three electrolytic capacitors act to limit the size of the signal across that capacitor, and that limits the size of the LFO.

Of course that assumes that the LFO signal is big enough to cause the diodes to conduct, which may not be the case here.

I think you have some undiscovered defect. What transistors are you using for the LFO transistors? It may be they have a relatively low gain. It may also be that there is some other defect lowering the actual in-circuit gain.

Another possibility that I can't tell about is that the capacitor from the emitter of the LFO transistor to the hot side of the depth pot is too small, or that the depth pot is too small. Both of those would cause increased losses at low LFO frequencies. Check the value of those components.

[MWichni]

Well, I've checked values of the components and they all are just like in schematic. Transistors which I had used to the LFO in darlington is 2n3904 so it is medium gain. I'll try to replace them with BC109.