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Re: PTC thermistor question

At 02:17 AM 12/31/00 -0000, you wrote:
>Where is the mod published for the osc for the old DEM converter?
>Thanks... Fred W0FMS

Hi Fred,
I haven't seen one published (so I SHOULD have...) 

When I built mine, which was quite a while ago, I noticed that 
the oscillater would quit when it was set to the exact frequency. 
The closest I could get was about 300 Hz off. So, I took a closer 
look at the schematic and I noticed that the circuit didn't include 
compensation for the XTAL holder capacitance. 

The fix is trivial, you need to add a small coil across the Xtal to 
tune out the capacitive reactance. The coil needs to have the same 
reactance as the xtal holder's capacitive reactance. It is not very
critical, it just needs to be close. Alas, I don't remember the details 
but I do recall the coil was just a few turns around a 1/8" - 1/4" drill 
bit as a form. I don't have ready access to my unit or I would just 
look at it to see what I did.


Tony AA2TX@amsat.org
North Andover, MA

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