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

[Dave Maples]

Al: I have been chasing what I think is the same problem--VFO pulling--for
years!  I don't mind pulling the transmitter and remounting the capacitor,
but did W8JI indicate that anything about doing it was special?  If not,
that sounds like the ticket.  While I'm in there I'm going to ground the
exciter cap shaft as well...mine has always had an "issue", and now that I
know how important that can be, I'll definitely give it the treatment.

Dave WB4FUR

-----Original Message-----
From: johnson-bounces at mailman.qth.net
[mailto:johnson-bounces at mailman.qth.net]On Behalf Of Al Parker
Sent: Wednesday, December 05, 2007 4:58 PM
To: Old Tube Radios; Johnson list
Subject: [Johnson] Re: Valiant chirp - fixed (long)


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





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