[Johnson] Re: Valiant chirp - fixed (long)

[Al Parker]

Hi agn folks,
    Well, I think I've made the cure, thanks to all.  I did get 3 responses 
from the BA list, and 2 from the Johnson list.  4, 2 from each list, said it 
was 40m RF getting back into the VFO, which was operating on the output 
freq.
    3 suggested fiddling the mechanical switch that changes the VFO freq. 
from 1.8mc to 7mc on 40m & higher freqs.  That would keep the VFO from 
operating on the output freq. exc. on 160m, where it doesn't seem to be a 
problem for anyone.
    Tom, W8JI, correctly identified the actual cause of the problem by 
sniffing with a probe.  The final tuning capacitor, C8, is isolated from 
ground for some reason.  Usually a capacitor rotor and shaft is grounded. 
In this case the shaft, which is about 8" long, and goes thru the front 
panel to it's knob, picks up RF from the final tank coil which is 
conveniently close coupled to the capacitor rotor and shaft.  I quote Tom's 
analysis here:
-----------
"I got out my H field probe and started sniffing around
inside the cabinet and found all of the problematic current
was coming from the shaft that runs back to the plate tuning
capacitor in the PA tank. I measured almost 1/2 amp of RF
current on that shaft on 40 meters in mine!! It was making
the VFO shaft hot with RF."
------------
    I made myself a crude but sensitive probe and also found a large amount 
of RF on the shaft.  I did as he did, with a long insulated screwdriver 
grounded the shaft - the frequency shifted slightly, and the chirp was gone 
(maybe).  I say maybe because I my case once I had slid the chassis about 
10" forward out of the cabinet, there was no chirp to be found.  Ain't these 
things fun?  Anyway, to lengthen the story, I really didn't want to horse 
the thing all the way out of the cabinet and remount the capacitor to ground 
it, as Tom had done.  If I had a strip of phosphor bronze I would have made 
a grounding strip for the shaft to rub on, and mount it using existing 
hardware that would't require getting underneath.  I almost started making a 
spring strip from a hacksaw blade, to mount a graphite motor brush on to rub 
on the shaft & ground it.  But the motor brush pair nad a nice 3" long 
stranded copper wire, probably a bronze alloy.  So I cut it off, twisted it 
arouond the 1/4" shaft 4 times, crimped teh ends in a spade terminal and put 
teh spade under a VFO shield screw, with some "preload" to keep the coils 
tight on the shaft.  A cupla drops of DeOxit ProGold, and it checked out 
fine business with the RF probe, and even after going back in the cabinet. 
It may not be a long term permanent fix, but I think it'll do the job for 
many months.  I could have remounted the capacitor in less time, but just 
don't feel strong today.  I've horsed it around enough in the last month.
    Thanks to all who helped.
73,
Al, W8UT
New Bern, NC
www.boatanchors.org
www.hammarlund.info