[Central-Electronics] Lake Shore Band Hopper update

[Jim Candela]

Hi Group,

   You may recall that a few months ago I posted about obtaining a Band
Hopper VFO. After my first round of cleanup and repairs I put it into
use with my Central Electronics 20a. At that time I was very happy with
it, and it was by far the most stable VFO I had tried with my 20a to
date. That was then, and over time it started miss-behaving.

The troubles I see ranged from:

* normal warm up drift, about 700hz upward drift from cold start

* short term narrow band drift where the frequency would shift around
50-150  hz over several minutes and yet not go any farther than that

* sudden jump of 200-300 hz from where it was set and then remain as
stable  as with short term narrow band drift

* I also noticed that when listening to the beat note on a receiver and
tuning the VFO, the frequency change was erratic.


So far I have found and corrected four things:

The OB2 regulator tube for the 6CB6 VFO was extinguished, and then it
would flicker outside the anode sleeve. When this would happen the VR
voltage would vary from 105 to 117 volts DC and the VFO large 200-300 HZ
frequency shifts would occur when I saw the VR tube flicker. Replacing
the OB2 seemed to fix this as the VR voltage was a steady 105.3 volts.

The oscillator tuning capacitor ROTOR to ground connection is done with
a forked wiper that is screwed to the side of the condenser frame. I
removed the forked wiper, cleaned it, and then adding a little silicon
grease lube. On the Band Hopper you can easily remove the side cover to
get access to the mounting screw. On the CE BC-458 VFO you can't do
that, but maybe you could drill a small 3/16" hole to get access with a
small screw driver. Anyway cleaning the forked wiper eliminated the
erratic tuning when trying to zero beat.

I then put the VFO on Variac power and when varying the AC line voltage
from 110-125 volts the frequency would slowly shift 250 hz. I tried
another 6CB6 and the amount was 200 HZ. I again tried another 6CB6 and
the shift was reduced to 50 HZ.  OK, good enough. I reduced my drift
from 17 HZ per volt to less than 4 HZ per volt. I have found similar
variation with tube to tube swaps with the CE BC-458 oscillator tube,
the 1626. 

I noticed that if I wiggled the 6CB6 the frequency would move around and
jump a little. I used a scribe and 'tightened' the tube socket female
pins, and scuffed up the 6CB6 tube pins with steel wool. After doing
this work and then moving the tube resulted in less frequency shift, and
the erratic jumps were gone.

At this point I tested the VFO (no case) for stability. The dang thing
after about 10 minutes stayed nearly dead zero beat to my ICOM R-75
receiver. I was tuned for about a 5 HZ beat note. Over 30 minutes later,
no change. This was great I was thinking..

So I put it in the case, hooked up the CE 20A, and let it warm up. The
zero beating was so much nicer now.  Then I noticed the frequency vary
around like before in the 150 HZ window. About 30 minutes later it
shifted up 300 HZ suddenly and then was very stable until I turned it
off.

I need to go through this again. Does anyone have any ideas other than
replacing components? My next move was going to replace the 6CB6 grid
leak resistor and make dam sure the OB2 VR tube is still behaving.

73,
Jim Candela
WD5JKO