Paul Christensen wrote:
> Can you elaborate on your BFO mods and what values/part types you used to
> improve temperature stability? At one time I had considered replacing the
> majority of BFO trimmer caps with subminiature ceramic NP0 types.
> -Paul, W9AC
I replaced the BFO Osc, Buffer, and Agc xstrs with the metal can (4 pin)
2N918 type. These are supposedly very slow heating. Collins Radio (who I work
for) always seemed to use these types in xtal oscillators in the HF and slightly
above area (I am referring to older designs). They have even published svc
bulletins on various equipments changing 2N5179 to 2N918 because of noise. The
Osc base coupling cap was changed from a .01 discer to a .022 monolythic cer.
This was a SWAG because .022 is closer to 9mc for self resonance. The
variables, except for FSK , were changed to ceramic piston type and the fixed
ones were changed to ceramic COG temp spec. I have silver micas in for the two
470pf, although I do not understand these high vaules in this colpitts ckt.
Realistically they should be about half that value for 9mc. I am sure TT has a
reason. I do plan to play around with this some more as the SM caps are unkowns
as far as temp variations are concerned. I would like to change the switching
diodes, 1N4148 type, to PIN diodes but I am not sure about putting 20-30ma
through them and screwing up the xtals.
This is all fun for me, no matter how much the TT Omni cost.... hi hi
73 Bill Cook N4WC
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