[AMRadio] Broadcast transmitter rescue

D. Chester k4kyv at charter.net
Sat Feb 14 21:52:57 EST 2009

> Don, I'd be curious to hear your thoughts on maximum connection
> distance electrically for a VFO. Barry used to run a VFO on the 300G
> and included a short cable with it for that purpose. I know someone
> (maybe Paul, VJB?) actually removed the lower window in the door and
> stuck the same T-368 unit inside their 300G. I'm hoping to have it
> back on the air by fall and don't mind starting out crystal-controlled
> to get things sorted out, but will want the flexibility to move around
> for the exact reasons you mentioned. I'd like to have it on the bench
> next to the transmitter, but that could end up being 8-10 feet going
> through the transmitter's access holes.
> ~ Todd,  KA1KAQ/4

If it is a high-Z output directly from the plate of the tube through a 
parallel tuned circuit, like the stock T-368 unit or commercial ham VFO's 
like the VF-1 or Johnson 122, I would say maybe 5-6 feet at most, because 
the capacitance of the coax running to the transmitter begins to swamp out 
the tuning capacitance in the VFO output circuit.

If it uses link coupling or other system of low-Z output, you can probably 
use a long a run as you need without any problems.

I modified my T-368 unit to link coupled output..  I salvaged a bandswitch 
wafer from a junker parts unit, disassembled the bandswitch in the good unit 
and added the new wafer.  I cut the metal sleeves and shortened one section 
by the thickness of the wafer, and reassembled the switch with the 
additional wafer.  I then removed each of the slug tuned output coils and 
added a few turns of wire at the cold end.  One end of the coupling coil was 
grounded and the other end went out through a new connection terminal.  I 
don't recall whether there was a spare terminal or if I had to fabricate an 
additional one.  The new bandswitch wafer is used to select the appropriate 
output coil link and take the output to a BNC connector I installed at the 
rear of the unit.  I  removed the stock BNC near the output tube and 
disconnected the wire lead going to it.  After the mod, I can get 200-250 mw 
of power into a 50-ohm  load.  I replaced the 6000 tube with a 6AG7, which 
required rewiring the octal socket.  I run only about 200 volts on the 6AG7 
and the rest of the tubes in the unit.

The unit wouldn't track perfectly across the entire range as it did before 
the mod, but it does track across each of the ham bands with negligible 
error.  Since I don't have any use for frequency coverage outside the 
amateur bands, that's no problem.  I might have a slight problem to work out 
if I ever try to cover both 40m and 30m using the 6-12 mHz range; I have 
never tried it to see  how much output I get on 30m.  I use RG-59 to couple 
the VFO to each of the transmitters.  The longest run goes to the Gates on 
160m, about 15 ft.

I get more than adequate drive for any of the rigs.

Don k4kyv


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