[sdiy] Question for MOTM300 UltraVCO owners...

[blacet at blacet.com]

Avoid breathing on the PCB when you are doing a calibration. There ARE
some components that can effect accuracy if they are exposed to
differential heating, such as your 98 degree BREATH!

The design of most VCOs rely on the expection that temperatures across the
PCB are pretty close to each other.


>
> 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
>
>
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