[Johnson] Invader VFO Modification

[David C. Hallam]

Jim,

I have looked that the differential capacitor but never tried any 
adjustment.  After I finish the SX-100 project, I will be starting on 
restoration of an Invader 200.  The SX-100 has come to a temporary halt 
because my HP 606B signal generator developed a serious problem.  Some, 
probably all, of the power supply capacitors need to be replaced, lots 
of ripple on the B+ with resultant frequency instability.  Naturally, 
these are all high capacitance, multi section 475V can capacitors.  I 
have contacted most of the usual sources and replacements are going to 
be a challenge.

I have a list of Invader modifications, some field and some applied to 
later production runs, put out by Johnson sometime in the 1962/1963 time 
frame.  One of them deals with the VFO.  Basically, the ceramic and mica 
capacitors associated with the differential capacitor are moved from the 
area below the chassis and put in the box with the variable capacitor. 
It is a real PITA job as you are working in a very tight space and some 
rewiring is necessary.  This box is heated by a 2K resistor connected 
across the 120VAC line and is always on.  Also a new always on filament 
transformer is installed for the VFO tube.  After completing the 
rewiring, the differential capacitor is to be reset to be fully meshed 
rather than 1/2 meshed as set at the factory.

This modification without the new filament transformer has been done on 
my 2000 which is my main station rig.  I have heard disparaging comments 
about drift in the Invader VFO, but no one has ever complained about mine.

After I finish the 200 refurbishment with VFO as it left the factory, I 
will measure the VFO drift and decide what to do; leave it as is, adjust 
the differential capacitor, do the Johnson modification, or install the 
X-Lock.  It will be sometime in the future before this happens though.

73
David
KW4DH

On 8/5/2013 1:13 PM, james.liles at comcast.net wrote:
> Good morning David:
>
> If drift is a problem, the Invader VFO has a unique drift adjustment.  I
> don’t believe it is addressed in the alignment procedure.  The only
> other VFO that I have seen the same implementation is the Hallicrafters
> SX-115.  The Hal designers do not want you to touch them.  It is
> actually an easy but time consuming adjustment.
>
> C55 is a differential cap that uses C53, a 22pf NPO and C54 a 22pf N750
> cap to align the minimum drift to band center.  The drift will increase
> toward the band edges because of the positive drift attribute of the
> tuning cap.
>
> Measure the drift at 2, 15, and 75 minutes at the low, middle and high
> frequency settings --- would be interesting to hear what they are.
>
> Kindest regards Jim K9AXN
>
l

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