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Johnson 122 VFO




 
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Author Topic: Johnson 122 VFO  (Read 9192 times)
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sndtubes
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« on: July 02, 2010, 01:38:13 PM »

I have a Johnson 122 VFO.  It seems pretty stable on 80/160, but above that it is kind of unstable.  It wobbles a bit and drifts some.  I have replaced the notorious 18 K resistor and one other that was out of tolerance.  I replaced both tubes. 

Does this sound normal for this VFO?

I am a little suspect of my line voltage.  It is kind of unstable, also.  It varies from 105 to 120 plus VAC.  Most of the time it's around 110 VAC.  When the AC in the house kicks on, it sags quite a bit. 

Should I try a Sola xfmr?  I'm not sure how well that would work.  I could also try to run the VFO off it's own regulated supply.  Maybe that would improve things a bit??

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WQ9E
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« Reply #1 on: July 02, 2010, 01:52:15 PM »

The VFO-122 drifts a little but you shouldn't be seeing the wobbling or erratic changes.  A separate variable capacitor section is used for the 40 and up section so make sure that the contacts on that are clean along with switch contacts.  A problem with one of the mica caps could also be an issue.

For operation on 40 meters, with most transmitters you can leave the VFO-122 set for 80/160 meter output and let the transmitter driver/multiplier chain take care of the additional frequency multiplication.   Doing this will also avoid having the final and VFO on the same frequency which can cause frequency shifts (this is commonly observed on Rangers and Valiants on 40 meters).  Unless you are using break-in on CW, set the VFO up so it runs continuously during transmit.  It will already operate this way on AM but if keyed on CW it is going to have some chirp.

The VR tube should take care of the plate/screen voltage but rapid drops in line voltage can cause a frequency change via changing filament voltage.  I recall years ago someone traced frequency shift in an SB-301 to shifts in the VFO filament voltage.  I would think that it would take a pretty dramatic (and rapid) drop in line voltage to create this effect.

I use 122 VFOs with Viking 1 and 2 transmitters and I have never been bothered by frequency shift.  There is going to be some drift with any vintage VFO but after 10 minutes warm up it should be fairly stable and only require correcting slightly every 10 to 15 minutes.
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« Reply #2 on: July 02, 2010, 05:21:42 PM »

I've used my stock VFO 122 with my Viking II for years on 40, 20, and 10 meters and never experienced any frequency wobble. There is some initial drift when I turn it on, but after awhile, it settles down.  Still has the original 18 K resistor too. Actually, this original "notorious" resistor is still operating in my 53 year old Ranger and my 51 year old Valiant.
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« Reply #3 on: July 02, 2010, 05:39:41 PM »

The other issue too is monolithic vibration from the wires, etc.

Or sub-sonic earth tremors.
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« Reply #4 on: July 02, 2010, 09:00:58 PM »

My 122 is used with a Viking II CDC mostly on 10-15-20M and there is no warble which sounds more like you are discussing many Hallicrafters receivers Grin

Remove the cover and lightly tap components with a screwdriver handle or other non metallic object. Look for a poor ground, switch contact, trimmer cap. Changing filament voltage would likely be a slightly delayed steady drift.  A defective VR tube could certainly cause a warble, even NOS ones are suspect, check with a DVM or scope.

Carl
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« Reply #5 on: July 03, 2010, 12:26:39 AM »

Presuming you have cleaned the tube sockets and checked the tubes, there may be a bad mica capacitor.  They are over 50 years old now and nothing like that lasts forever.

Forget about a Sola transformer.  Magnetic regulation requires a load fairly near the rating of the unit, they are not designed for this application, and the noise will drive you nuts.
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Geoff Fors
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sndtubes
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« Reply #6 on: July 03, 2010, 02:10:09 AM »

Thanks for all of the great ideas.  I'm fearing it might be one of those mica caps.  I have had problems with them in other gear in the past.  I

One thing I thought of doing was running the VFO on it's own regulated power supply.  I have a couple of regulated supplies around and may try wiring one up to run the VFO.  That way, varying line voltage should'nt bother the VFO.  If it's still unstable, then I can conclude it's probably a cap.

It does sound as if there is some problem.  I think it's stable enough for AM and works fine on 80/160, so I'll focus on the 40 M and higher bands. 

Stay tuned....(don't drift too much).
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w3jn
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« Reply #7 on: July 03, 2010, 04:45:24 AM »

Wet a q-tip with a bit of DeOxit.  Clean the variable capacitor bearings and any wipers.  Do the same with the bandswitch.  Then, get one of those little plastic oilers with the teflon grease in it from Radio Shack and shoot a dot of lubricant on the cap bearings.  This will settle down many jumpy oscillators.

DO NOT succumb to the temptation to douche the thing down with DeOxit.  That's a quick way to create many more problems.
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« Reply #8 on: July 03, 2010, 12:36:39 PM »

Lots of good advice here already, but I'll add my own redundant information. After 15 minutes or so, both of my 122's settle down nicely on the upper bands. Wobbling or slight jumping around is not normal. As others have said, dirty tube sockets, dirty variable capacitor wipers, degrading solder connections, weak tubes or tubes with degrading internal structures that are shifting, or old carbon resistors could be causing issues. Consider shotgunning all the resistors inside there since there are not many to deal with. Don't overlook issues inside the transmitter the VFO is being used with, since that is the power source for the 122. Anything in the path of the voltage source for the VFO could be suspect.
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« Reply #9 on: July 04, 2010, 12:54:41 AM »

In my HB xmtr I had some issues with drifting.  One thing I had to do is to start the oscillator at half the operating freq.  My xmtr only operates on 75M and 40M.  I had some FMing on 75M but none on 40M.  Freq multiply up to 40M but ran straight thru on 75M.  Funning thing is I remember knowing not to start a VFO on the output freq from my early days of ham radio.  Somehow I forgot all about this issue when I designed this xmtr.  Completely solved the FMing by starting the VFO on 160M.

Changing filament voltage will also cause a drift.  I run the VFO filaments on a separate 6.0VDC supply.  I use a simple 50W 6.0V zener diode to keep the voltage steady.  This solved the drift due to changing line voltage.

The B+ voltage is regulated with a 150V VR tube.  These tubes will work fine BUT you must leave them on for them to be stable.  IMO switching the B+ on and off to a VR tube will cause some issues.

In the final design the VFO runs from a completely separate supply for the B+ and filaments.  The VFO is on at all times.  The VFO output is a very low level signal, maybe 20 - 30 millivolts.  It can be heard in the nearby receiver but the signal is only about S-2 or S-3 so it really isn't a problem.

Fred,  KA2DZT
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WA1GFZ
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« Reply #10 on: July 05, 2010, 07:07:02 PM »

Check the power connector socket on the back of the rig for loose contacts. bend them tighter if there is poor contact. Also check the SO 239 on the back of the rig to make sure it is also tight. The last 122 I bought was stable as a rock and it was missing 1/2 the hardware. Check the tube socket pins as well. Deoxit is a good idea but don't go crazy with the spray.
My CDC had both SO239 connectors with very loose contacts when I got it.
Carefull bend them inward with a dental pick or straight hat pin.
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« Reply #11 on: July 06, 2010, 01:08:31 PM »

In my HB xmtr I had some issues with drifting.  One thing I had to do is to start the oscillator at half the operating freq.  My xmtr only operates on 75M and 40M.  I had some FMing on 75M but none on 40M.  Freq multiply up to 40M but ran straight thru on 75M.  Funning thing is I remember knowing not to start a VFO on the output freq from my early days of ham radio.  Somehow I forgot all about this issue when I designed this xmtr.  Completely solved the FMing by starting the VFO on 160M.

Changing filament voltage will also cause a drift.  I run the VFO filaments on a separate 6.0VDC supply.  I use a simple 50W 6.0V zener diode to keep the voltage steady.  This solved the drift due to changing line voltage.

The B+ voltage is regulated with a 150V VR tube.  These tubes will work fine BUT you must leave them on for them to be stable.  IMO switching the B+ on and off to a VR tube will cause some issues.

In the final design the VFO runs from a completely separate supply for the B+ and filaments.  The VFO is on at all times.  The VFO output is a very low level signal, maybe 20 - 30 millivolts.  It can be heard in the nearby receiver but the signal is only about S-2 or S-3 so it really isn't a problem.

Fred,  KA2DZT

   Fred,  Your VFO experience closely mimics mine. Just looking at the filament voltage issue (long time constant as Karl said), and trying several different tubes of the same type, the drift will be highly variable where some tubes have more consistent emission than others. This is good for weeding out tubes where the cathode oxide is near end of life.

   I had a VFO inside an ARC-5 transmitter that went from very stable to sounding like RTTY! Turned out to be a galvanic corrosion issue between the VFO tank coil and the aluminum chassis.

  I have brought up the Huff & Puff topology before for VFO's that run continuously like yours. I usually run into extreme resistance from folks that say their VFO is 'stable as a rock', so why bother. Well yes, good point, but many crystals drift hundreds of hertz when inside  BA during the warmup period. And yes, and H & P circuit can only benefit a VFO that is in good shape where it does nothing erratic except for a slow warmup drift.

  I have only owned one 122 VFO. I cleaned it up, and replaced several parts. It worked fine, but I'd have to say it needed a warm-up time until it settled out. These things do drift, but I'm sure if you had several, some would be more stable than others.
 
Jim
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KA2DZT
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« Reply #12 on: July 07, 2010, 04:39:48 AM »

In my HB xmtr I had some issues with drifting.  One thing I had to do is to start the oscillator at half the operating freq.  My xmtr only operates on 75M and 40M.  I had some FMing on 75M but none on 40M.  Freq multiply up to 40M but ran straight thru on 75M.  Funning thing is I remember knowing not to start a VFO on the output freq from my early days of ham radio.  Somehow I forgot all about this issue when I designed this xmtr.  Completely solved the FMing by starting the VFO on 160M.

Changing filament voltage will also cause a drift.  I run the VFO filaments on a separate 6.0VDC supply.  I use a simple 50W 6.0V zener diode to keep the voltage steady.  This solved the drift due to changing line voltage.

The B+ voltage is regulated with a 150V VR tube.  These tubes will work fine BUT you must leave them on for them to be stable.  IMO switching the B+ on and off to a VR tube will cause some issues.

In the final design the VFO runs from a completely separate supply for the B+ and filaments.  The VFO is on at all times.  The VFO output is a very low level signal, maybe 20 - 30 millivolts.  It can be heard in the nearby receiver but the signal is only about S-2 or S-3 so it really isn't a problem.

Fred,  KA2DZT

   Fred,  Your VFO experience closely mimics mine. Just looking at the filament voltage issue (long time constant as Karl said), and trying several different tubes of the same type, the drift will be highly variable where some tubes have more consistent emission than others. This is good for weeding out tubes where the cathode oxide is near end of life.

   I had a VFO inside an ARC-5 transmitter that went from very stable to sounding like RTTY! Turned out to be a galvanic corrosion issue between the VFO tank coil and the aluminum chassis.

  I have brought up the Huff & Puff topology before for VFO's that run continuously like yours. I usually run into extreme resistance from folks that say their VFO is 'stable as a rock', so why bother. Well yes, good point, but many crystals drift hundreds of hertz when inside  BA during the warmup period. And yes, and H & P circuit can only benefit a VFO that is in good shape where it does nothing erratic except for a slow warmup drift.

  I have only owned one 122 VFO. I cleaned it up, and replaced several parts. It worked fine, but I'd have to say it needed a warm-up time until it settled out. These things do drift, but I'm sure if you had several, some would be more stable than others.
 
Jim
WD5JKO

Jim,

I never owned a Johnson 122 VFO.  The VFO in my xmtr is HB, it is part of the main chassis built in a separate box.  It can be removed to work on but it takes about 4 or 5 hours to get it out.  It is stable after the normal warm up time.  About 3 years ago I removed it for mods and heat testing.  Tried different silver mica caps and other negative temperature compensating caps.  Two weeks later I reinstalled it back into the xmtr, ran it for week and then pulled it back out again for another two weeks of heat testing.  At this point, it works well and I'm happy with it.  I keep a HP freq counter on it.  Hope I never have to remove it again, it's a real pain.

Fred, KA2DZT
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