V2 and 122 vfo

Discussion in 'Amplitude Modulation - AM Fans' started by N8ZL, Jan 12, 2019.

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  1. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Unless there is something wrong with the 75A-4 PTO, the stability is excellent.

    Also, the difference in internal capacitance will have an effect on the calibration. How much, I really don't know.

    In the VF-1, replacing the 6AU6 with a 6AH6 will cause the calibration to shift slightly. However, most people do not worry about this change. But, a slight "touch up" of the trimmer capacitors will get the calibration back to where it was before. The variable inductors should not be adjusted if the calibration was fine before the tube swap.

    Glen, K9STH
     
    N2EY likes this.
  2. K4KYV

    K4KYV Subscriber QRZ Page

    My working spare 75A-4 has excellent stability. The other one, the one I use most of the time, has PTO issues. From a cold start, it drifts about 1 kc, but after 30 minutes or so it settles down and stays put. While warming up it occasionally jumps 100~ or so in frequency. The little slug-tuned coil that adjusts frequency range, so that it covers exactly 1 mc/s in 10 revolutions, has run out of adjustment and there is still about 2 kc/s too much bandspread over its full range.

    The standard fix for the end-point adjustment is to take the PTO apart, remove the little trimmer coil, take off a couple of turns and re-assemble the unit. The problem is caused by ageing of the main tuning slug, something that is both inevitable and irreversible (the manufacturer probably never dreamt that these receivers would still be in use 60 years later). The purpose of the smaller coil is to compensate for this permeability change by reducing the inductance which is in series with the main coil, but when the slug is all the way out the only way to further reduce the inductance is to take turns off the coil. I haven't ventured to try that just yet. The warm-up drift may be due to moisture accumulation inside the sealed unit, so swapping out the oscillator tube probably wouldn't help much. While apart, the whole insides should be gently baked a few hours in an oven, maybe at 150° F or so, to drive out the moisture. Others have reported that this helps any drift problem. While apart it would need cleaning and lubricating, and maybe adjusting the leaves on the cam that regulates linearity. More to it than a simple one-evening project.

    One should use only an electric oven to bake out electronic components, never a gas oven, since a product of the combustion process is water, adding humidity to the hot air in a gas oven, which is what you are trying to eliminate in the first place.

    I use the receiver with the problem PTO as my main receiver because of a set of modifications that I consider essential but don't particularly want to replicate in my spare one. I have a third A-4 that I picked up cheap at a hamfest. It wasn't working when I got it but it is operational now. It is cosmetically in better shape than my main rcvr, but it still has some some major issues that will take time and patience to resolve. I have considered simply swapping PTOs in the two receivers, since that one has good stability, linearity and proper end-point frequency range with still some adjustment left to the trimmer coil.
     
  3. N8ZL

    N8ZL Ham Member QRZ Page

    Tnx Glen
     
  4. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    KYV:

    Probably, one of the temperature compensating capacitors has gone bad. The truth be known, the sealing, of the PTO, probably didn't last for even 24-hours if even that long. There are just too many places that are going to leak even when the unit was brand new.

    Glen, K9STH
     
  5. K4KYV

    K4KYV Subscriber QRZ Page

    I don't doubt that. After running long enough to reach a stable temperature the positive air pressure inside the sealed unit would force out a few molecules of air , then upon cooling, there would be a slight vacuum, which would suck in moisture-laden outside air to replace the dry air that was forced out. Over decades, the interior humidity would build up. Maybe that's why the S-line PTOs are not sealed at all.

    OTOH, the R-390 series PTOs are sealed and pressurised with dry nitrogen. I have taken apart scrapped PTOs that had been tossed out after sitting on the shelf for decades, and upon releasing pressure with the little valve on the sealed enclosure, I heard a hissing sound from gas escaping so the unit must have remained inflated for that long period. As far as I see, the physical construction of the A-4 PTO and the core of the 390 series PTO are practically identical, except no little gas valve on the A-4 unit. Collins could have pressurised the A-series PTOs as well, but maybe that would have driven up the cost too much; the 390 PTO must have a tighter seal for the positive gas pressure to last that long, inevitably adding costly steps to the manufacturing process.
     
  6. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Also, the R-390 and R-390A were built for the military and the military probably wanted the PTOs specially sealed, etc.

    Glen, K9STH
     
  7. AB2RA

    AB2RA Ham Member QRZ Page

    See R55 in series with C66 on grid of 6AU6. This is very obscure, but I found a problem with VHF parasitics when I used the 11 meter position for 5 MHz output to get on 60 and 30 meters. Check that it is in place to at least eliminate that as a source of the problem. Use a carbon old fashioned composition resistor, value not critical. I used 120 ohms. Or new OX or OY series non inductive resistors from Mouser.
    http://bama.edebris.com/download/johnson/vfo122/vik-vfo.pdf
    This manual is correct. The other manual on BAMA does not have R55. Some 122s do not have it, and can act squirrely on the 40 M range with certain tubes. Which is why EFJ added it later.

    One other troubleshooting technique would be to disconnect the VFO from the V2. This eliminates the TX as a source of interference. You will be able to hear a 2nd harmonic on the 40 M range without the TX attached. If it is clean without the TX, the TX is generating the spur. Also try a different RX. I found false spurs on one project DX-20 with a modern solid state transceiver, but it was clean in an old single conversion RX.

    Be sure to attenuate the signal to the receiver, because they will generate spurs internally from overload.
    Also weird grounding issues can cause cross modulation problems; connect a short wire from the RX to the VFO ground cabinets. No explanation, but the false spur may disappear.

    The V2 is known to have interactions with the xtals plugged into the sockets in positions 1 thru 3. Also, I have seen some xtal oscillators to generate spurs about 50 KC away from fundamental, but you are running the VFO here. Try removing all the xtals and see if it disappears.

    Also, there are a lot of factory mods in the BAMA manuals. Be sure to check that all the mods have been installed, including the 56 ohm resistor instead of the inductor at the screen of the 6146s and the resistors at the 807 plate and grids. Not all V2s are created equal. Early ones do not have the clamp tube.

    This is one reason I have a spectrum analyzer on my Christmas List.
     
  8. KE5YTV

    KE5YTV Ham Member QRZ Page

    Glen,
    Are those tubes drop in replacements in the Johnson VFO?
     
  9. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    YTV:

    Yes, the 6AH6 and 0B2 require no circuit changes.

    The 6AH6 has slightly different inter-electrode capacitances and there might be a need to slightly adjust the calibration capacitors (NOT the inductors) to maintain the calibration. However, these adjustments will be slight and many people don't even bother!

    The input voltage for the Johnson VFO and the Heath VF-1 really needs to be from a 150-volt regulated supply. I suggest using the 0A2 removed from the VFO to regulate the input voltage. Regulating the input voltage also helps with stabilizing the frequency.

    Glen, K9STH
     
  10. KE5YTV

    KE5YTV Ham Member QRZ Page

    Glen,
    Thanks for the info. I just picked up a Johnson VFO to use with my Viking 1.
    73,
    Mike KE5YTV
     

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