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Monday, August 26, 2013

BITX Build Update #3

This weekend I built the VFO.   I used Doug DeMaw's FET Hartley, from his "QRP Notebook," page 50.

Getting a VFO into the desired frequency range always seems to involve a bit of "cut and try."  Not only do you have to get in the correct freq range, but you also have to try to get the tuning range of your variable cap (or coil) to match the band you want to cover.  In my case, thinking of 20 meter SSB, I need about 200 kHz of tuning range.  And sticking with my 9 MHz (for 20) and 13 MHz (for 17) plug-in filter plan, I need the VFO to be in the 5 MHz range. 

That main tuning cap you see above has more capacitance than necessary (19-148 pf).  One option would be to pluck out some of the rotor plates (been there, done that).  But that seems a bit barbaric, so instead I just reduced the overall capacitance by putting a smaller fixed capacitor (about 20 pf)  in series.      

Yesterday I had it percolating nicely.  Waveform looks beautiful on the Tek 465.   But it was drifting too much.  I had a cheap plastic trimmer cap in there.  This morning I replaced it with a more substantial ceramic trimmer.  This seems to have improved stability quite a bit.

I need to put some stabilizing substance on that toroid.   DeMaw prescribes Q-dope.  I'm all out.  What is the field expedient substitute?   Was it clear nail polish? 

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  1. disolve pieces of a clear plastic box in acetone till you get the right sticky consistancey you need. Instant Q dope Hope this helps Bob NG4R

  2. I use clear nail polish for this all the time

  3. I use hot glue on larger coils to keep them from shifting around, don't see why it wouldn't work on toroids, but I haven't actually done it.


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