[TMC] PAL-1K / RFD-1 tuning

[Duncan M. Brown]

I am working with a RFD-1 (p/o SBT-1KV) power amplifier that seems a little
strange on 40 & 30 M. When tuning up on 7.0 Mc, 6-8 Mc band, the Plate
Loading ends up at 00.0 (maximum capacity) with the Loading switch in
postion 3 (no extra loading capacitor) (450 ma @ 3KV; 650W out). If I
change the Loading switch to position 2 (extra 1000pf added), then the
Plate Loading capacitor ends up around 12.0 (minimum capacity - control
doesn't have much effect.) I can't find a position of the Plate loading
capacitor (with or with out extra capacitor switched in) where it tunes
well. I get this same effect on the 8-12 Mc band in the range of 8-10.2Mc.
(at 12 Mc, it is OK)
 
I have been able to repeat this effect with an MFJ 269 antenna analyzer
connected to the output and a Rp = 3.3K resistor across the PL-172 plate to
ground; tuning the Plate & Loading controls for 50 +j0. At 7 Mc., Loading
position 3, and Loading = 00.0, I get 1.1 SWR (but should be able to tune
to 50+j0). When I switch in the extra 1000pf cap (C-274), the best SWR I
can get is about 4.2 (Loading control about 14.0).
 
I have measured the individual components out of circuit and they all read
OK. But it acts like there is some extra reactance being switched in along
with C-274. (there is abouty a foot of 1/4 strap between the loading
contro; and C-274, including going through two switches)
 
Also, at 4.0 Mc, unit tunes up with Loading control at minimum C (counter
12-15) and Load switch at position 3. (best SWR measured with MFJ 269: ~1.1)
 
At 14 Mc and above, tuning seems normal.
 
This RFD-1 is sn 17529 and there are some differences between it and the
PAL-1 manual I have (IN-523, June 1962, with changes through Dec 1967).
This unit has been modified to cover down to 1.75Mc. (new label plates for
input and output bandswitches have been installed) and has different fixed
output loading capacitors (glass, instead of ceramic as shown in manual;
can see mounting holes & lables where the ceramic ones were mounted.).
These fixed output loading capacitors (C-244, C-272, C-274) are located on
a bracket above the Loading switch, not where the original ceramic ones
were. 
 
Has anyone seen this type of tuning? 
 
Thanks,
 
Duncan Brown, K2OEQ
 
AWA Electronic Communication Museum
http://www.antiquewireless.org/