[sdiy] Question for MOTM300 UltraVCO owners...

[Jean-Pierre Desrochers]

I never blow like this on sensitive boards like VCO's..
I was just checking the 'sensitiveness' of that particular
3.3M HF tracking resistor..
So any advices about this adjustment on MOTM300's ?

JP


 On Tue 13/03/12 12:56 , blacet at blacet.com sent:
> 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
> thePCB 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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