[sdiy] Diagnosis Help Please

[patchell]

    Wild Ass guess here....if you have a scope, check your power supply
rails.  It sounds like your power supply is going out of regulation.  If you
are using 78xx/79xx type regulators, this is pretty common if you don't have
enough headroom going into them (they require that the input voltage be 3
volts min above the output).

    If you have any spare filter caps, double them up.  If this reduces or
eliminates the problem, you are probably on the right track.

    As a rule of thumb...for +/- 15 volt supplies, I use at least a 30 VCT
transformer with 4700uF 35 volt filter caps.  For +/- 12 volts, a 24 VCT
transformer with 4700uF caps.  Of course, I am assuming that the power
supply is your problem.


Mark Melvin wrote:

> Hi all-
>
> First of all, let me say that I'm still new at this
> stuff... so be patient with my ignorance.
>
> I'm almost finished with my first synth project and
> almost everything is working... and what isn't working
> I believe I can fix.
> HOWEVER, I have this annoying feature that I hadn't
> planned on incuding.  Perhaps if I describe what I am
> hearing someone could make a guess as to what is
> happening.
>
> What I am experiencing is best bescribed as "beating"
> in the VCO.  To me it seems like there is a signal
> that has bled into the VCO.  It seems to be constant.
> If I sweep the VCO over it's frequency range I will
> hear the beating (two frequencies sounding together)
> changing and then disappearing at a few given points
> across the range of the VCO.  I believe these points
> are one octave apart.
>
> This goes away completely when power the VCO alone.
> However, when I add one or more additional modules
> from my PSU the beating returns and becomes worse with
> each additional module.
>
> Any ideas about what this could be?  Thank you in
> advance for any help!
>
> -Mark
>
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--
 -Jim
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