[AMRadio] Oscillator Filament Voltage change and frequency drift

Donald Chester k4kyv at hotmail.com
Thu May 11 19:25:47 EDT 2006

>...I later experienced band switch contact issues with that VF-1, and
>it was more like a spread spectrum exciter. Off to the dump that POS

Recently, I have seen VF-1's go for big bux amongst the collectors.  I 
wouldn't throw one away nowadays.

One problem I discovered with my VF-1 was thermal drift.  I took it apart 
and noticed the construction; the slug tuned coils are mounted on one wall 
of the oscillator subchassis, and the slugs and their bushings are mounted 
on the opposite wall.  So as the temperature varies, the whole compartment 
expands and contracts, and moves the slugs in and out of the coils.  No 
wonder the things drift!  Then I noticed the insides of a DX-100, which has 
a nearly identical VFO.  The coils and slugs are mounted on the same piece 
of chassis panel.

So I remounted the coils in mine to the same side of the subassembly.  It 
was easy - just drill a couple of  mounting holes on either side of the 
original coil slug bushings (with the slugs removed during the process).  I 
had to slightly re-route some of the wiring, but it was easy to hook 
everything back up, and it worked FB once reassembled.  It didn't stop the 
drift entirely, but reduced it by at least 90%.  It was nice and stable on 
160/80, but was still drifty on the 40m. range.

I ran the VF-1 off a CVT.  It too, drifted with slight changes in fil. 
voltage.  I think both the VF-1 and Collins PTO's use a 6AU6 as oscillator 
tube.  My "silent" Sola beats fooling around swapping tubes.  I suspect even 
a good tube would eventually become drifty with age.

Don k4kyv

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