Univibe repair advice needed

Discussion regarding early stompbox technology: 1960-1975 Please keep discussion focused and contribute what info you have...
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Post by idy »

If it sound right it is right. The bulb isn't to read by, so as long as you like the sound, no need to go trying to brighten it. Though there are ways to do that . Your unit seems to be missing the light shield, the little (shiny) metal box that encloses the bulb+LDRs. That will deepen the sound a bit. The position of bulb/ldr does not seem super ciritical. Though you are free to tinker.

RG's article has set of updates on the unity gain fix. Just changing the resistor to ground in the input mixer is a good idea. From 47K to over 1M.

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Post by ppluis0 »

Nice to read that your unit is working again !

Remember to place the cover over the four cells and the incandescent lamp at time to test it.

If you use this pedal opened and exposed the board to ambient or artificial light the ldr's present a fixed value of resistance and the phasing is nulled.

Cheers and happy 2.019 !!!!

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Post by Isittooloudoutthere »

Oh well, the light shield is there, just took it off before I shot the pics.

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Post by Dirk_Hendrik »

Congrats!! :applause:
Sorry. Plain out of planes.


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Post by Isittooloudoutthere »

One more thing... One reason I always prefered the Univibe over the K.R. Megavibe - which I use as a replacement - was that there's no true bypass and the signal runs through the unit, no matter if chorus or vibrato are on or off. So I get identical sounds shaped by the circuit of the Univibe - and a certain treble loss, yes... At first this may seem like a disadvantage, but playing through my shared cathode JTM45 or JMP50 I am able to compensate this treble loss by dialing in a little more presence and treble on the amp. This, at least for me, is one the key ingredients if you are trying to nail f.e. Hendrix/B.o.G. And it sounds great with my old SG too.

That's why I hesitate to change the 47k input mixer resistor right now to 1M or more. I will do some testing with different resistors here when I can crank up the amp, since the change of this resistor, as far as I understand it according to RG Keen's article, will definitely affect the volume of the effect and treble of the unit. Is that correct?

That said, I would still like to compensate the slight volume drop when I switch on the chorus. Now RG offers two kinds of mods here. So I'm wondering if the mod about changing the resistors around the third transistor in the preamp also will affect the treble of the unit?

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