[Central-Electronics] The 458 VFO again

[George Maier]

Hey Dave; Nice to see you show up on the CE reflector!!!  For some reason, I have not looked at this mailbox for quite a while, and this morning, I discovered a lot of new posts, including yours.  Welcome to the group!!!!

Glad that you found what you need for info.  I've had several CE 458's with the 10M add-on, but never really pursued that band as I tend to hang out on 75M with the CE gear.  Sounds like you have made great progress with it and sharing your work with the group is certainly appreciated. 

Somewhere around here, I have a P&H 600A transmitting converter; that's the little one that upconverts 20M to 6M with about 10W output from a 6360.  It's just waiting to be used with a 10B or 20A! 

Best wishes for the new year, hope to see you again when NearFest resumes.

73 and Happy New Year to all,

George Maier - W1LSB

-----Original Message-----
From: central-electronics-bounces at mailman.qth.net [mailto:central-electronics-bounces at mailman.qth.net] On Behalf Of David Olean
Sent: Wednesday, January 13, 2021 10:09 AM
To: central-electronics at mailman.qth.net
Subject: [Central-Electronics] The 458 VFO again

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

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