[sdiy] Modding an old analog - Puzzle part II

[Vitreous Flux]

Thanks for helping me on this project!

OK - found the right cap and tacked on a few different caps in parallel.

The schematic calls for a tantal electrolytic cap, but the actual cap used
appears to be a paper or ceramic (not sure - glossy epoxy looking, and bulb
shaped).

The original cap was a 3.3u 16v. The best match I had lying around were some
10u 35v (electrolytic) caps. Tried one singly, then two in parallel. It sure
does work, it slows the LFO way down, to a nice desirable speed. Is the cap
voltage rating difference going to matter?

Problem now, is that the longer the LFO time gets, the closer it distorts
away from a sine wave (some sort of mutated sawtooth is best I can explain
it) - probably the issue with the cct that you mentioned? Larger cap values
make this worse, and seem to make the LFO depth weaker (that's pin 52, near
the 33u cap). Any reasonable way to fix this? Notice the 500K trimpot there
too - I believe that is for adjusting waveshape, but it doesnt do much once
the new caps are in the circuit.

Thanks again!

----- Original Message -----
From: ">>>marjan<<<" <urekar.m at EUnet.yu>
To: "Vitreous Flux" <vitreousflux at wwnet.net>
Cc: "Synth DIY Maillist" <synth-diy at node12b53.a2000.nl>
Sent: Thursday, February 08, 2001 5:00 PM
Subject: Re: [sdiy] Modding an old analog - can someone assist? A Puzzle...


> > The mod I am currently working on is to adjust the 'travel vibrato' time
> > (that's early Korgspeak for LFO adjusting the filter).
> >
> > I have a close up of the schematic on this link:
> >
> > www.wwnet.net/~midibear/700.gif
> >
> > Pin 51 goes to a 50K pot on the panel, then to ground, for LFO speed
adjust.
> >
> > What I want to do is slow it way down, so LFO speed can be upwards of 10
to
> > 30 seconds max, instead of 1 second max.
> >
> >
>
> Try 33u to 100u across that original tantal 3.3u cap in the middle.
> Mind the polarities.
> I'm not sure how'd the cct handle such large cap but try some 10u
> values first than increase.
>
>
> marjan
>
>