Question, What issues are there with Permeability Tuned Oscillators.

[ka3zlr]

Good Morning,

 I'm looking for input regarding this, any stability issues and or harmonic problems.
 
 I'm starting a project and any input would be greatly appreciated.
 

[Jeff W9GY]

In general, a perm tuned oscillator will want some negative temperature coefficient capacitance to help improve temperature stability. 

[WA1GFZ]

Tracking depends on pitch of inductor wiring. Get yourself a T195 PTO 1.5 to 3 MHz and your done. Go crazy and solid state it if you want.

[k4kyv]

Tracking depends on pitch of inductor wiring. Get yourself a T195 PTO 1.5 to 3 MHz and your done. Go crazy and solid state it if you want.

Inside the sealed can, the T195 PTO is identical to the T368 one.  There are a couple of minor wiring differences in the sub-assembly exterior to the can.  One is that the filaments are wired in series in the 195 unit while they are wired in parallel in the 368.  Another is that the 195 has a muting circuit that shuts  down the untuned amplifier that follows the oscillator, by applying a negative bias to the suppressor grid.  And the  rf output connectors are different.

I once ordered a couple of T-195 PTO's from Fair radio when they were selling them for $8 each, to use as spares for my T-368 master oscillator unit that I converted to  station VFO.  The ones that arrived in the parcel were in mint condition, shiny new, and made by Collins.  Then I discovered that some A-hole who had removed them from the transmitters at Fair had apparently brute-forced the units out instead of going to the trouble to follow the disassembly procedure, and they each had their tuning shafts bent about 15%.  I called Fair, and they instructed me to return them and they would send replacements.  The replacements that came were made by some off-brand company and were dirty and corroded, like they had been sitting outside exposed to the elements for several months, although they did seem to work OK when I tested them.  I was pissed off to say the least.

If you use the T-195 or any other Collins type PTO, or take one out of equipment  for repair,  be very careful about turning the shaft to the end points, since the bare PTO is no longer protected by the end stops in the equipment's tuning mechanism.  If you turn the shaft far enough to feel resistance to further turning, you may have already damaged the unit.  It is best to apply power to the oscillator and use a freq counter or accurate receiver to find what frequency it is set on, and make sure you never turn it beyond its nominal  range.

If you use one of these PTO's in a homebrew project without fabricating any kind of end stop, make sure you don't allow a visitor to casually turn the knob and inadvertently ruin the PTO.

My T-368 oscillator has been running continuously 24/7 for almost 20 years now.  It has been briefly shut down a few times for maintenance and repair, or when the power has gone off.  I added enough shielding and filtering that it is inaudible in my receiver, so I don't have the annoying drift following stand-by periods.

One thing I did find was that mine was extremely sensitive to changes in filament voltage caused by line voltage fluctuations, even with regulated plate/screen voltage.  Even a change of one volt, which is less then 1%, was enough to make it noticeably  drift when I used it on 40m.  On 40m CW, that change was enough to make it drift out of the passband of my 300~ CW filter.  I solved the problem by running the oscillator tube filament off regulated DC.  Now it is rock stable, and I have never bothered to connect my DDS oscillator up to use as station VFO.  Of course, the filament voltage problem would be solved by converting it to solid state.

I noticed the same thing with my 75A-4 receiver.  I fixed that one by running the entire receiver from a small Sola constant voltage transformer I picked up at Dayton.  Unlike every other Sola transformer I have ever seen, that one is not noisy.  It was a recently constructed transformer apparently designed to run a PC in an office environment, so they must have paid attention to the acoustical noise problem usually associated with those regulators.

[WA1GFZ]

Don,
I have a clean one made by Collins. Do you use the heater?

[KD6VXI]

The 'parting out' process at Fair Radio was a painful thing to see in progress! I questioned an employee there about this practice and was told that each piece was worth twice it's normal selling price when 'parted out'. Cannibalized T-368's, BC-610's, T-195's, etc. were laying around everywhere! Likewise receivers of every type. These guys had hacked up so many old mil. pieces that there was a big cardboard box there full of nothing but those old copper bellows type of flexible shaft couplers and another full of the insulated ceramic ones!

One really big, 5-10kw all mode HF transmitter (Collins or TMC?) was sitting there about to be destroyed. It was offered to me for about $1000-$1500, 2 employees negotiated that low price in an attempt to get out of the work involved in dismantling it, but I knew my pickup wouldn't haul it. The thing looked like new inside and out, may have been a NOS mil. item. A quick call to a local rental place verified the availablility of a trailer for one way rental but the size & weight of those 3 phase transformers finally discouraged me. I still cringe at the thought of it being hacked up into pieces.

A trip to Fair Radio during the 60's-80's was well worth it, just to look at all the surplus there.      

Mac,

The west coast equivelant was Murphys Junk in San Diego (El Cajon).

At one point, he had a wharehouse of those Collins 208-10Us and a bunch of the 5kw models.  All the single phase models where sold to (I guess) amateur communities, and most of the 3phase models where parted out.

If you want to see a huge junkbox, Murphys Electronics on Johnson in El Cajon, Ca will make you cry.  Before that, it was California Electronics, just down the street.

--Shane

[WA1GFZ]

I spent an afternoon in Mike's back room a few years ago...lots of cool stuff.

[k4kyv]

Don,
I have a clean one made by Collins. Do you use the heater?

No.  According to a DA manual I have here somewhere, the heater in those PTO's is for extremely cold environments, not for precise temperature control at normal temperatures.  I'm not sure it is even connected to anything in the '195 and '368.

[ka3zlr]

Thanks Guys,


 I don't have anything here laying around this will be a complete build up from scratch with a Buffered output to feed a toob circuit i'm looking at...

 I appreciate all the input...

[ka3zlr]

Thanks Guys,


 I don't have anything here laying around this will be a complete build up from scratch with a Buffered output to feed a Lo toob circuit i'm looking at...

 I appreciate all the input...

[w3jn]

Jack, it's a lot easier to get a relatively linear tuning with a capacitor tuned VFO than a PTO.  The wire pitch on the form is really hard to adjust so it tunes at a linear, rather than geometric, rate.  I know, I tried 

ALso difficult is making a mechanism that takes up the space as the leadscrew unscrews from the coil form.  As you unscrew it, you need some sort of spring to absorb the decreasing space between the end of the leadscrew and the shaft/knob.

I would recommend finding a good gear reduction drive for a variable capacitor.  One of the finest assemblies around is the gearset/capacitor out of an ARC-5 xmitter.  The only downside is that the capacitor will mount parallel to the front panel rather than perpendicular.

[ka3zlr]

Thank You John,

 I am much obliged, I have been looking at some different basic building blocks for Lo's especially in the variable aspect, thinking was simplicity, and i just by chance found a circuit that employs it's own buffering and from the designers spec's offer a 9VPP  50 Ohm@load, along with a High Impedance RF tunable coupler stage, that is able to be "Directly Keyed' with minimal drift...when i saw this the flags went up, subsequently i posted this thread my thoughts were on the same lines as posted here but i wanted to make sure my thinking was correct..as I questioned it as you posted..better to ask than to assume...

 So, i am going to build this, and test it, I know there are better options, but this design is so simple at component level and the stated results were interesting...


 Here check this out and tell me what you think:

http://myweb.cableone.net/adamsmed2/PTO/pto.htm

 
 Something like this has alot of potential from a builders aspect.

[w3jn]

That's a pretty poor design in several respects.

First, the leadscrew has no mechanical support.  This will cause instability.

Second, there's WAY too much coupling between the tank circuit and the transistor.

THird, the base circuit for the buffer is really strange.  The diode will cause clipping; if the oscillator had been designed properly there would be no need for it.

Fourth, looking at the wire pitch on the form, it's gonna tune at a geometric rate (very slow at one end of the band and extremely fast at the other).

I would run the oscillator and first buffer continuously, and key a subsequent buffer stage.  Or use a NAND gate at the output and use one input of the gate to key the output.

[WA1GFZ]

The best cap/ gear combination know to man is the one used in the BC 221. It is a work of art.

[k4kyv]

One of the earliest PTO's, and made by Collins, can be found in the ART-13.  I recall most of the '13's I have heard on the air have a very stable carrier, even if the audio has shortcomings.  This was the predecessor to the "modern" Collins PTO's used in the 75-A and 32-V series receivers and transmitters, and all that followed.

Unfortunately, its nominal frequency range goes down to 2 mHz.  It will overlap into the very top end of 160, however.  I wonder if the "AM calling frequency" at 1985 is a result of the extra margin in the frequency range of  numerous 2 to XX hHz surplus military and  commercial rigs.  The PTO could probably be padded down to cover the entire band, but I'm not sure  how much that would affect linearity of tuning.

One solution to the tuning linearity problem would be to use the old National HRO and BC-221 tactic of a dial with arbitrary scale (whatever you have in the junkbox that will work) and make up a calibration chart.

[ka3zlr]

Thanks guys I Value these opinions greatly, not ever building a "PTO" of any kind or even close to this, alot of the microwave projects I built used such stringent LO's and mixers when i looked at this thing I was kinda surprised...I like the simplicity of this, but tuning an oscillator with a screw..OK..Better ask the guys and be sure of where i am...

Thanks alot fellas...

[Steve - WB3HUZ]

If you put a freq counter on it, who cares if the tuning is linear.

[w3jn]

Because you might be faced with a situation where 1/4 turn tunes the VFO 100KHz right near your frequency of interest, and the slightest jar will jump the frequency several KC.  A better design would spread out the pitch of the windings so it tunes relatively slowly throughout its range.

[Steve - WB3HUZ]

I totally agree. I was only addressing the linear tuning not the rate. The design shown did look rather mechanically unstable, but not a bad concept, in general.

[w3jn]

Some keys to building a stable VFO:

1)  Mechanical rigidity and stability of ALL components.
2)  Smooth tuning mechanism.
3)  high quality capacitor, preferably with ball bearings.  All contacts MUST be clean.
4)  Use as large a wire for the inductor as you can.  Wind the inductor then put it in boiling water for a few minutes.  THis will anneal the wire.
5)  Air core inductor is best, second is powdered iron, worst is brass.  Of course using an air core inductor, it will be more difficult to adjust the VFO frequency range.
6)  Use high quality padder and trimmer capacitors.
7)  Use as light a coupling as possible to the tank consistent with reliable oscillator operation.
  Use at least one buffer stage, and that buffer should put out voltage, NOT power.  If you need power add another stage, preferably physically separate from the oscillator so as to keep heat-producing elements toa minimum close to the tank.
9)  Use good voltage regulation.
10)  Bypass the hell out of the power (and filament if using a tube).  Appropriate is 1 uF, .1 uF, and .001 uF all in parallel.
11)  Use a single point ground for the oscillator and tank.
12)  Avoid clipper diodes.  They cause thermal instability and phase noise.
13)  Use the least tuning range you can.  For example if you're building an xmitter VFO for 75, design it for 3600-4000, NOT 3000-5000.
14)  Some books say use Hi-L/Lo-C values to reduce circulating tank currents and thus heating of the tank coil.  I've been successful using Lo-L/Hi-C combinations.

I've built many VFOs (most being "false starts") and it's not hard to build a good, clean, and stable oscillator following these rules.  The builder of the circuit Jack described has violated almost all of these principles.