[Johnson] Grounding exciter / plate tune shafts on Johnson Valiant II

[Artmouton]

This was from a prior set of postings to this list.
I can send the photo directly if you wish, just email me.
I have not, yet, tried it on my Valiant.
Art K5FNQ

VALIANT CHIRP FIX IT FROM LIST SERV

 I know Valiants & Rangers chirp at least sometimes.  Mine only chirps on
40m.  I know the dropping resistor in the VFO VR tube line, 18K I 
think?, is
common trouble, I've done that on a cupl, and in this one, tho' I used a 5w
22k, mounted outside the box.  I've adjusted the keying  pot, even tried it
at both ends, but it's sitll close to where a prev. owner had marked it to
be when adj. properly.  I've got some ferrite beads on the leads into the
VFO.
    This thing only chirps on 40m.  I've tried all other bands, tried
changing the exciter tuning.  I fussed with this rig for about 2 months 
b4 I
decided it was OK to put it back in the cabinet last week.  Worked the 160
contest this weekend, of course you're not going to get anything other than
599 in a test, but it sure sounds pretty on the rcvr everywhere except on
40m  It does flatten out when I cut the final plate current down below 50ma
by dropping the drive & letting the clamper do it's job.  So, it's 
either RF
somehow doing it, or voltage drop, or ???  If that 22k has too much drop,
why only on 40M?
    Looking for suggestions b4 I haul it out of the cabinet, not a fun job.
thanks, 73,


Hello Al,

I have experienced this with my Ranger too and think it must be fairly
common.  The problem is that the vfo is running at the fundamental
frequency on 40m.  What I have done with mine is to adjust the vfo switch
linkage so that it uses the 160m tank for 40m, with the change over to
using the 40m tank on 20m and above.  It's as clean as a whistle when you
do this and there's still plenty of drive available, and it's a non
permanent modification, easily changed back again so worth a try.

73 de Adrian / ZS1TTZ

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

Hi agn,
    After all that, I forgot to say I've posted a pic of the fix:
http://www.boatanchors.org/valiant40mchirp.htm
click on the thumbnail to get a full sized one.
73,
al, w8ut

 

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


Dave Maples wrote:
> All: I have after about 3 years started working over my Valiant II.  I
> thought I had read somewhere that a source of chirp (as well as heat) was
> the ungrounded capacitor shafts on the exciter and plate tuning capacitors
> (the capacitors go to ground through the shafts and the bushings on the
> front panel).  My understanding was that grounding those shafts back at the
> capacitors themselves would result in much better performance.  Anyone here
> comment on that?
>
> Thanks,
> Dave WB4FUR
>
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