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| Photo from radiomuseum,org |
I couldn't resist posting another great message from Rick Campbell, KK7B. This one was sent by Rick to the r2pro mailing list. Rick mentions the Collins Permeability Tuned Oscillator. More info on this magnificent bit of old tech can be found here:
Rick writes:
Great stuff here on different approaches to designing-building-experimenting with stable VFOs. Some of it is a mature art, and some really is a lost art--but that's good. There's nothing quite so gratifying as recovering a lost art while taking advantage of modern tools and techniques, and perhaps exchanging ideas on some new-fangled communications medium.
If you ever want to see folks having a blast recovering a lost art, head to a wooden boat festival and watch the kids learning from old codgers who can do magic with a little bit of centuries old technology. Just don't mistake it for reenactment--some of the new boats these kids are designing, rowing and sailing may include some venerable tools and techniques and give an aesthetic nod to classics, but they are lighter, easier on the environment, easier to transport, and much much faster than historic craft.
We can do the same with our radios. Spend as much time designing and building your VFO as I spent designing the small sailing rig for my little boat--or convert an old Collins PTO to solid state. For highest performance use a mix of old and new--I use hand stitched 1968 sails from the Schatteaur Loft in Seattle on my bigger old boat. Old radios are a treasure trove of parts and technology, and since 1959 well-designed HF gear has been stable enough for SSB.
I measured one of my converted old Collins 2.5 to 3.5 MHz PTOs at 18 Hz per hour drift. It was originally built in the mid 1950s. Some problems were solved long ago, and the techniques and solutions hold up well over time.
As with the kids studying old wooden boat techniques and then taking their new creations out on the Salish sea, it is well worth our time and effort to spend some time studying how our forbears constructed a good VFO a half century ago. It's not just the schematic: the construction, materials, temperature coefficients...all are important, and the quickest way to come up to speed is to study old stuff that works well.
Good stuff, and have fun with the experiments.
Best Regards,
There were construction articles in the sixties for PTOs, in "QSR" and "73". They are mechanically intensive, but in this age of frequency readouts, a lot less fuss needs to be made about linearity. It gives multiple turns, and no variable capacitor.
ReplyDeleteIn 1974, there was an article in "Ham Radio" about locking a 50MHz VCO to a broadcast band variable oscillator. In this case, he used the PTO mechanism from a car radio, an eat way to take care of the mechanics. At the time, the local surplus place was selling the mechanisms for about a dollar. Of course, now the car radios tuned that way are well in the past, digital tuning taking over, so no longer easy to find those mechanisms.
And yes, it's easy to put an FET in those Collins PTOs. About 1974 I got a few 1-1.5MHz Collins PTOs for cheap at a hamfest. I soldered an FET to the tube socket, maybe changed the plate resistor to a lower value, and ran it off a much lower voltage. All was fine, the oscillator was simple, it was the mechanism that was complicated.
Michael