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Johnson VFO 122 Doubled Frequency Output




 
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Author Topic: Johnson VFO 122 Doubled Frequency Output  (Read 877 times)
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wa2fxm
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« on: August 11, 2019, 11:30:51 AM »

How is it that a Johnson VFO 122 can double it's output frequency between the oscillator plate and the output rf coax cable? The VFO is powered by a Viking II with the VFO output coax cable disconnected. With the VFO 160/80/40 band setting I'm getting 1850 kHz on the freq counter at the VFO 6AU6 plate. On the other side of the C67 output capacitor which connects to the rf output coax I read 3700 kHz. I also read 3700 at the center tap of L51, but the tap is only wired to terminal 12 of the bandswitch and in the 160/80/40 position terminal 12 is unconnected to anything else. I've replaced a few out of spec resistors, tightening ground screws, going over solder joints and mechanically testing and poking the sloppy wiring job but I haven't found anything yet. Anyone have any ideas about what's going on?

WA2FXM Mark
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KK4YY
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« Reply #1 on: August 11, 2019, 01:56:35 PM »

Probably all the harmonics are there but you frequency counter can only display one at a time and it locks onto whatever it likes. Look at it with a spectrum analyzer or tune a receiver to the calculated harmonics. The tuned circuit that you will drive with the VFO will discriminate which harmonic to use.


Don
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« Reply #2 on: August 12, 2019, 02:15:38 AM »

How is it that a Johnson VFO 122 can double it's output frequency between the oscillator plate and the output rf coax cable? The VFO is powered by a Viking II with the VFO output coax cable disconnected. With the VFO 160/80/40 band setting I'm getting 1850 kHz on the freq counter at the VFO 6AU6 plate. On the other side of the C67 output capacitor which connects to the rf output coax I read 3700 kHz. I also read 3700 at the center tap of L51, but the tap is only wired to terminal 12 of the bandswitch and in the 160/80/40 position terminal 12 is unconnected to anything else. I've replaced a few out of spec resistors, tightening ground screws, going over solder joints and mechanically testing and poking the sloppy wiring job but I haven't found anything yet. Anyone have any ideas about what's going on?

WA2FXM Mark


What Don said about harmonics is right on.  Smiley

If you want extra stability from the VFO you can regulate the internal B+ as well. This also results in doubly regulating the screen voltage of the 6AU6 or better yet, the 6AH6. See schematic 1.

If you want the VFO to be portable schematic 2 offers an add-on power supply with the transformer and the DPST switch mounted on the back, or externally to the VFO.


Phil - AC0OB

* VFO 122 Regulator and PS.pdf (145.8 KB - downloaded 70 times.)
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wa2fxm
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« Reply #3 on: August 12, 2019, 12:21:40 PM »

Probably all the harmonics are there but you frequency counter can only display one at a time and it locks onto whatever it likes.
Thank you Don, exactly right. Tuned the VFO up and calibrated it to the R-388 and the dial is eyeball spot on.

If you want extra stability from the VFO you can regulate the internal B+ as well. This also results in doubly regulating the screen voltage of the 6AU6 or better yet, the 6AH6. See schematic 1.
Phil, thanks for this. Once I get the VFO working with this Viking I've got another Viking that has the keyer mod in it. Apparently you need to make some modifications to the VFO to have it work with the modified Viking. This regulator looks like a good add-on when I open it up again.

WA2FXM Mark
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wa2fxm
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« Reply #4 on: August 12, 2019, 08:34:18 PM »

Except where do you find 100 and 150 volt zeners in stock anywhere? Not at Mouser or Digikey.

WA2FXM Mark
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MikeKE0ZUinkcmo
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« Reply #5 on: August 13, 2019, 12:15:35 AM »

It's simpler to just daisy chain a 0C3 on top of the existing 0A2, and adjust the value of R-18.

Mount the new VR tube socket on a couple of 1/2" spacers to minimize the impact of the added tube.

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Mike KE0ZU

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Dave K6XYZ
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« Reply #6 on: August 13, 2019, 11:03:09 AM »

Except where do you find 100 and 150 volt zeners in stock anywhere? Not at Mouser or Digikey.

WA2FXM Mark

1N5383BG 150v Zener at Mouser....thousands in stock.

https://www.mouser.com/ProductDetail/ON-Semiconductor/1N5383BG?qs=sGAEpiMZZMtQ8nqTKtFS%2FJ7m6e1KBCguIz%2Fz4Rc2znI%3D
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Tom WA3KLR
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« Reply #7 on: August 13, 2019, 08:41:32 PM »

There was an article on the apparent problem with the 122 VFO in Electric Radio many years ago.  Looking at the output voltage waveform with an oscilloscope will probably reveal why there is the duality nature driving a frequency counter.  I just found the article in an index on the Electric Radio website.  I do not have the issue.  Perhaps someone else here can dig it up and scan it in, I believe it is 2 pages:

- ELECTRIC RADIO, September 1995, issue 77, page 24. "Johnson Viking VFO Model 122 And Its Little Secret", by Bob Thomas W3NE.
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MikeKE0ZUinkcmo
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« Reply #8 on: August 13, 2019, 10:23:06 PM »

Tom said;
Quote
...There was an article on the apparent problem with the 122 VFO in Electric Radio many years ago...
Actually it isn't a problem, it seems to have been designed that way.

According to the article Tom referenced, the author found that grid tank of the 122 is tuned to 1.9MC, but the plate tank is tuned to 3.8MC, as evidenced in the plate tank pic below.



This was done because the Viking II for which the VFO was apparently designed, and subsequent EFJ transmitter oscillator circuits, had band switch selected 1.9MC plate tanks.   This meant that there would be sufficient 1.9MC fundamental content in the VFO's output, that the crystal oscillator stage could clean it up and deliver the required 1.9MMC signal to the following "multiplier" stage in the transmitters.
---------------
If you want to clean up the 1.9MC you would need a total of about 500pF for the plate tank to resonate properly, but then, there would be virtually no 3.8MC content.
    
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Mike KE0ZU

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