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

[Dave Magnuson]

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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