[sdiy] Korg MS-20 VCO gives up below 3.5 kHz

[Tim Stinchcombe]

Hi Aaron,

> more I think, with an 850 mV peak-to-peak swing .

Where's this measured at, as it seems very low? Quick simulation of the
circuit suggests the swing across C2 is going to be around 6V pk-to-pk.

> R16 gets toasty (but not too toasty), as one might expect 
> with 12 volts dropped across such a small resistor. It's sort 
> of at the edge of a 1/4 watt  rating.

Most of the time the junction of those two 427s (R16, R17) should be at
mid-rail, i.e. 7.5V, so I suspect a wiring error...? It does _spike_ up to
around 13V or so when the cap resets, which will be the rail minus the D4
and Q2 diode drops. 

> Juergen Haible suggested that the original transistors might 
> be important;

I haven't quite got my head round how it is working completely, but it looks
like an 'SCR' type of thing, so as C2 charges, Q3/Q4 are off, and the
sawtooth is falling; when it gets to around mid-rail, the SCR 'fires', i.e.
Q3/Q4 come hard on, discharging the cap, sending the sawtooth back up to the
rail and switching the transistors off. I suspect the relative values of the
hfes and things of Q3 & Q4 will be crucial to its correct operation, to make
the 'SCR' effect work at the right point (is it an SCR-thing going on here?
- too lazy to look it up..). Thus I agree with Juergen's observation, and
even if you have it wired correctly, if the transistors aren't close to what
they should be, then I don't think I would surprised if it didn't work
across the full frequency range expected!

Tim
__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk
www.timstinchcombe.co.uk