[sdiy] EFM VCO 4e is singing a fixed frequency

[cs e]

I replaced the TL072 CV input mixer and it's getting power, checked
for shorts around there, and I'm pretty sure the 50k pot is fried.
So, I still need to replace the pot (on the panel so it's going to be
a pain in the..)

But the VCO is still oscillating at a fixed (high pitch) frequency.

Could there be any other troublesome spots besides the CV input mixer section?

chris

On 2/28/08, Dave Magnuson <resonant at hoohahrecords.com> wrote:
> I don't have a copy of the VCO4e schematics, so I'm hoping it's similar to
> revision 4D.
>
> You mention "magic smoke" coming from your freq pot - sounds like you may
> have cooked it. Check voltage at the wiper to see if it still swings from
> v- to v+.
>
> One possible cause would be a short in the CV mixing section - look for a
> solder bridge in there somewhere. If the mixer was shorted to ground, for
> example, you may have just connected V+ to ground through your pot - hence
> the smoke. You may have also dripped some solder in the back of the pot...
> or you may have a frayed wire. You could also have a cracked trace / bad
> joint... or a bad opamp.
>
> Start your troubleshooting at the mixer inputs - see if you're getting a
> swing in voltage from the pot and CV inputs (check at the op amp input).
> If you're getting a voltage swing at the input, check the output... you
> should also see the voltage swing here. (You should be able to use your DMM
> for this).
>
> Make sure you have V+, V- and ground at the appropriate pins of the opamp.
>
> If the FM input is working, it's likely that the expo pair is doing
> something... so start with the opamp... reply with what you've found, and
> the list could probably help you get that thing working.
>
> Dave Magnuson
>
> ----- Original Message -----
> From: "cs e" <modulemania at gmail.com>
> To: <synth-diy at dropmix.xs4all.nl>
> Sent: Thursday, February 28, 2008 7:29 PM
> Subject: [sdiy] EFM VCO 4e is singing a fixed frequency
>
>
> > I've got the EFM VCO 4e singing, but at a fixed frequency.
> > The problem is the frequency can't be adjusted - by knob or any CV inputs.
> >
> > It hard syncs to ext VCO, FM-in works, PWM and all waveforms look OK.
> > What would be stopping this thing from letting its frequency be
> > adjusted? I hope the CV inputs OpAmp isn't bad - that would be my
> > luck though, because I didn't use a socket on that one :-(
> >
> > Little side note - while looking at some voltages with the scope and
> > the DMM, I saw a little magic smoke coming from the "FREQ" pot (the
> > one with +/-15V and the wiper going to the pcb).
> >
> > Any troubleshooting tips here?
> >
> > Chris
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