[JMS] Variarm VFO

[Tom Elmore]

I am making progress on the Variarm VFO project thanks to the help of
several of the members on this list. I am seeing about 15 volts AC leakage
between the chassis and earth ground even after disconnecting most of the
bypass caps. Keep in mind I haven't received the new electrolytic caps yet
and have the voltage doubler circuit disconnected so I have no active B+
just filaments right now. Does anyone have any thoughts on where the leakage
could be coming from like I said I disconnected one end of any bypass cap
that goes to the chassis. In this day and age is there any reason to keep
any of these caps in the circuit anyway?

Thanks 
-Tom 
KA1NVZ

-----Original Message-----
From: james_millen_society-bounces at mailman.qth.net
[mailto:james_millen_society-bounces at mailman.qth.net] On Behalf Of Tom
Elmore
Sent: Friday, December 31, 2021 12:12 AM
To: 'The James Millen Society Members Email Reflector'
Subject: [JMS] Variarm VFO

Hello, I recently picked up a Variarm VFO model 90701, serial number 431. It
still had the original resistor line cord and I considered keeping it as a
display unit had the line cord been functional but as it turns out the
resistive element was open.  I did replace the cord with a black cloth
covered type that I found at Lowes marked as a Globe Designer extension
cord. I went with this type of cord because of its appearance and that it
also has a polarized plug on one end and on the other I cut off the female
outlets. I ended up using the capacitor & resistor combination shown below
to get the proper filament voltage and the sure resistor runs quite cool to
the touch. This unit is going to need a total recapping but what I did find
out was that I have B+ voltage live on the chassis. It took a couple of
hours of troubleshooting but what I discovered  was that the V2 25L6 buffer
tube has its output coil L2/L3 one tap goes to the chassis.  Can anyone tell
me if these VFO's ever  had a SO-239 installed on the side of the chassis as
the VFO output. The one I have has a SO-239 installed and it looks like a
stock installation. According to the schematic there are actually two
separate coils in the output here marked as L2 and L3. L2 is part of the
tank circuit and L3 is the actual RF output that is air coupled to L2.  In
my VFO though L2 and L3 are wound as one continuous coil with several taps
and someone at some point grounded one the taps to the chassis (a big no no
since there is live B+ here) and then capacitor coupled one the other taps
to the center pin of the SO-239. I'm thinking I could break the electrical
connection at one of the taps to provide a true isolated output but I wanted
to see if anyone had a picture of their coil arrangement they could send me
please.  Lastly what's the deal with the fuse in the schematic it looks like
it is only providing protection for the voltage doubler circuit and not the
filaments and this is how mine is wired also. 

 

 

 

Volt Total=62.6

Current Total=.300ma

 

Capacitor 8.3uf 

Surge Resistor 33 ohm / 3 watt

 

 

Thank You 

 

Tom Elmore  KA1NVZ

Anchorage, Alaska 

 

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