[Johnson] Rebuilding Johnson Navigator ???

[Glen Zook]

A while back several people commented either here, or
on one of the boat anchor reflectors, about removing
this resistor from the VFO compartment.  It seems that
removing the resistor generally resulted in poorer
stability than improving the stability.  The consensus
of opinion was that the compensating capacitors in the
VFO circuit were selected with the heat from this
resistor taken into consideration.  Relocating the
resistor actually resulted in a much longer time for
the VFO to stabilize (if at all).  Now increasing the
wattage of the resistor is definitely required!

One thing that improves the stability of the Heath
VF-1 VFO and the VFO in the DX-100 and DX-100B is to
replace the 6AU6 with a 6AH6.  This is a direct
replacement.  The only thing is that the 6AH6 has a
slightly different interelectrode capacitance and the
trimmer capacitor for each frequency range has to be
adjusted slightly (the inductor should not require any
adjustment) to maintain calibration.  Heath changed
the 6AU6 VFO tube in the early TX-1 Apache
transmitters (and added the separate filament
transformer to keep it "lit") to a 6AH6 early in the
production which helped the stability considerably.  I
suspect that making this change should also improve
the stability of the Johnson VFOs.

I wonder about adding a small 6.3 volt filament
transformer to the Johnson transmitters that has the
primary wired before the off-on switch to power just
the heater ("filament") of the VFO oscillator tube. 
The fact that this definitely helps in the Heath
equipment leads me to believe that it should help with
the stability of the Johnson equipment as well.

Glen, K9STH


--- C E <catman351 at yahoo.com> wrote:

The VFO is identical to the other famous EFJ xmtrs
from the Ranger, 500, Valiant, et al. As you can
probably guess, that vaunted 18K resistor has been a
source of trouble (like others have said in this
thread). My solution would be to get that resistor and
clip it to expose just the leads. Get a pair of wires
and hook it up the leads as well as bringing it down
through the VFO and out to the bottom of the chassis.
Substitute (like what some had said) with a higher
power version of the 18k resistor. In my case, I
mounted it to a terminal strip and used a 20k 5 watt
unit (which worked fine). I bolted the terminal strip
to the bottom so it had an anchoring point on an
exisiting piece of hardware in the Ranger. Draws the
heat out of the VFO. Also, change out or check the
6Au6 and OA2 tubes while you're at it. Inspect the
insulated shaft coupler to see if there are any
cracks. This is another thing  you have to watch out
in these VFO's of this nature.









Glen, K9STH

Website:  http://k9sth.com


 
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