[Central-Electronics] The 458 VFO again

[David Olean]

Hi CEers

First off, a big thank you to W5AFY, Dan, for sending me a copy of his 
ten meter add on kit for the  CE 458 VFO. It is dated 1958, and has some 
nice useful information not available in earlier manuals.  I am in the 
process of reading it all and sorting things out.  Thank you so much Dan!

The schematic issued with the 1958 manual shows two tubes in the ten 
meter kit, a 12AT7 that works as an oscillator at 48.4 MHz and a mixer 
in section #2 that drives a 6AU6 amplifier at 37.4 MHz.

In my unit, there are THREE tubes: 2 X 12AT7s and a 6AU6. Apparently, 
the added 12AT7 performs as a Butler Oscillator with the crystal 
installed between the two 12AT7 cathodes.  I tried sketching out the 
circuit, but my shaky hands are so bad that my results are pretty 
illegible. I was hoping that the newer manual would show the correct 
circuit mods that I am seeing here. It does not, so I guess I need to go 
back and reverse engineer it.  My circuit does not look like a hack job. 
It looks like a factory unit. The original add on kit has the crystal 
with a coil to the right, then the 12AT7. My unit has the crystal, then 
a 12AT7, and then the osc coil.  The rest of the circuit with the 6AU6 
looks similar to the drawings and the schematic.

In working on my unit, I finally have it working properly. I was seeing 
some intermittent action that turned out to be a broken cathode pin on 
the oscillator 12AT7. It took me awhile to find that as the pin was just 
about invisible. I found it when I went to try another 12AT7 in the 
socket!  I also swapped out the selenium rectifier and that big 3300 MFD 
can. I found a 2800 MFD can to replace the old dead one and it is in 
very fine shape, so it all looks authentic. There is about 20 volts p to 
p of 37.4 MHz energy available, which is in the ballpark with the other 
frequencies. Stability is very good at 37.4 MHz. Not much drift in the 
48.4 MHz crystal, but I did note that getting the crystal to run well 
required a really good 12AT7.  The oscillator coil must be well tuned 
and there is not much travel either way for the slug for the oscillator 
to drop out. I ran the oscillator for an afternoon, and only saw about 
30-50 Hz drift at 37.4 MHz.

One error I did find in my unit was that the 50 pf air trimmer on the 
458 front panel was not connected when the VFO was switched to ten 
meters.  The 1958 revision manual shows it connected for ten meters and 
it allows for accurate calibration. Without that trimmer, the ten meter 
freq was off by about 10-15 kHz.

Now I am on to fixing up a few MM2 monitors.

Dave K1WHS