[sdiy] Question for MOTM300 UltraVCO owners...

[Jean-Pierre Desrochers]

Hi,

I read some web pages about the MOTM300
UltraVCO assembly & HF tracking modification.
I have a MOTM300 vco's to fine tune myself.
After a couple of hours of tweaking trimpots
(1v/oct, HF tracking resistor values swapping)
I ended up with the HF tracking resistor R50=3.3M, 
a bit high for a resistor to me 
but the vco was tracking around 8 octaves
very nicely ! I was very happy about that.
I used a very accurate Korg tuner and a 5.5 digits HP voltmeter
for all the tweakings. 
Some days later measured back the overall tracking
and found the top octaves were flat and getting
worse in the higher freq..
I found that 'blowing' on the R50 resistor was
changing the HF tracking accuracy.
This is not suprising since 5% carbon resistors
like this are 1000-1500ppm / degC sensitive !
And the resistor is series with a 1N4148 so..

So here is my question to MOTM300 owners:
Did your VCO's stay stable days after your HF tracking tuning ?
Mine went flat at high frequencies..
I'm considering to use metal film type resistors
but again, in the megohms spectrum they tend
to have high ppm/degC deviation.. (around 1000-1500ppm)
compared to lower resistance values,
but I found some 50-100ppm parts at Digikey
for values from 1meg to 2.2meg.
Your precious advices are welcomed.
Thanks
J-Pierre