[Central-Electronics] Lake Shore Band Hopper VFO update

[Jim Candela]

Hi Group,

   I put my lakeshore Band Hopper VFO on the operating table once again.
It appears to be much improved. That said, it will take a while to be
sure.

What I found:

The 6CB6 oscillator grid leak resistor was bad. It was supposed to be
51K, 1/2 watt, CC. The color code was correct, yet it read 91K. A slight
warmup with the soldering iron, and it rapidly increased, 91K, 92K, 93K,
94K, etc. I replaced it with a metal film 52K (combination of 1% metal
films).

I also replaced a silver mica that was ~ 50pf and across the oscillator
tank. On the schematic it is labeled with a * which I presume is to set
the tuning range for a given unit. I replaced it with a COG 50pf ceramic
1 kv disk. The COG designation is a more modern term for NPO.

The oscillator cathode is 'RF Hot' since the circuit is a Hartley
oscillator. The filament power is common to all tubes which includes the
high voltage rectifier, the 6X4. The filament circuit floats with
respect to ground. I measured the filament potential with a DC meter
referenced to ground, and the potential was about +100 volts. I figured
the 6X4 was showing a little heater cathode leakage and that is why the
filaments assumed a common mode potential. I took that same 91K resistor
to load down the filament potential to ground. The +100 volts dropped to
about 5 volts. When doing this the oscillator frequency shifted a few
hundred hertz. It appeared that the 6X4 was pumping about 50ua from the
cathode to heater. I decided to replace the 6X4 with two silicon diodes,
and the leakage was gone since the common mode filament potential was
now zero. 

Adding the silicon diodes instead of the vacuum tube 6X4 rectifier
caused another issue. Now the B+ was too high (over 325v when in standby
and the filter capacitor rating was only 300 volts). I needed to replace
the capacitor anyway since it was the original, so I changed the power
supply to choke input filtering instead of capacitor input filtering,
and used a 47uf @ 450 v filter capacitor. Now the B+ was 250v in standby
and 200 volts when in operation. The Manual says that the nominal B+ is
225v when in operation. I'd rather be a little low than on the high
side. 

So how does it work? Well the 150 hz ( ~ +/- 75 hz) rambling around is
now gone. I have not yet experienced the sudden 200-300 hz shifts like
before either. The cold start to 1 hour later drift was about +800Hz
before, and now it is reduced 4x to about +190 hz. When I started
writing this email (70 minutes from a cold start) I had set the 20A
frequency to 3880.000 Khz. Now 30 minutes later it is at 3880.004 Khz. 

My fingers are crossed; maybe I've licked this one...

Regards,
Jim Candela
WD5JKO