[AMRadio] Broadcast transmitter rescue


D. Chester k4kyv at charter.net
Sat Feb 14 15:09:24 EST 2009


>> So my question to the group. Who is the Collins 20v-2 guru and how much 
>> will
>> be involved to move it from 610khz to 1885khz.
>>
>> 73, Al  K3TKJ

Why be content to operate on just ONE frequency?

It is very easy to convert those xtal oscillator stages to function as 
buffer stages for an external VFO, and to make rf stages the rig tuneable 
across the band so that the rig will function with VFO or multiple xtals.

If you have a VFO with low impedance output, and capable of at least 0.25 
watts, the best solution is to ground the grid of the xtal oscillator stage, 
disconnect any feedback circuitry going to the cathode, add an rf choke from 
cathode to ground if necessary, and inject the rf signal from the VFO 
directly to the cathode through a coupling capacitor.  That is what I do 
with my Gates BC1-T and homebrew HF-300 transmitter.  With the other 
homebrew rig presently tuned to 40m, I inject the VFO signal directly to the 
control grid of the 6AG7 1st buffer/amplifier stage.

I converted my BC1-T to external VFO, which is a modified T-368 m.o. unit, 
and it will tune from 1800 to 2000 with all the transmitter tuned circuits 
accessible from the front panel, and the neutralisation holds perfectly from 
one end of the band to the other without readjustment.

One of the faults I find with much of the present-day AM operation on 160 
and 75 is the same as with the SSB dead-air groups: insisting on operating 
on one and only one frequency, with daily 8 to 10 member roundtables amidst 
QRM from adjacent QSO's less than 5 kHz away in both directions, while there 
may be tens of kHz of vacant space in other parts of the band.  We need to 
spread out a little and maintain more widespread AM presence throughout the 
bands, with reasonable sized QSO's and roundtable groups.

Don k4kyv




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