Often the opener, I never thought was followed by "that the car could go that fast as you get a ticket", or "that the gun was loaded" or "that she would end up pregnant". Well, that phrase also covers today's bit of blabber.
The set up ~ Both Collins and Drake used PTOs in their radios which were hallmarked by a linear readout and excellent stability after a warmup. Collins in the rigs prior to the KWM-380 were all vacuum tube and Drake started with tubes and then went to Solid State.
My "I never thought is followed by that they did that this way" involves a Drake PTO. While rummaging around one of my junk stashes, I found a box marked Drake PTO. I seem to recall having to repair a Drake PTO and that was by installing a replacement PTO as the rig I bought had an inoperative unit.
Given my current mode of operation of trying to fix things, it was time for this PTO. There was no info in the box about what was wrong with the PTO and likely what I did was replace the unit without trying to fix the original one.
In all honesty, likely I said to myself, self you would be treading in very deep water trying to fix this PTO and took the easiest path.
Above and below are some shots of the Drake (R4A) PTO. Somewhat aghast that Drake used cheap circuit board material and that the board was not more rigidly affixed to the assembly. Well, I guess it worked.
BUT what really jumped out was the staggered pitched winding of the inductor. Before commenting more on this, the PTO (permeability tuned oscillator) consists of a fixed inductor whose inductance value is changed by inserting a gear driven powdered iron core through the windings -- id est, no variable capacitor. Back to the physical winding, the stepped pitch makes the inductor act in a linear fashion. Were it a constant winding the change in inductance would not be linear.
All roads lead to Rome and all analysis starts with a schematic. The first problem -- there are so many schematics for the Drake R4 radios. The R4 series includes ones with 13 tubes, 11 tubes and 10 tubes. The latter rigs were a hybrid mix as more solid-state circuits were included like the PTO and Audio stages. There were even two different solid-state PTOs with one a bipolar device (2N706) followed by one a JFET (2N5990).
This specific PTO had the 2N706.
Q2 (2N706) is the Oscillator and Q3 (2N3563) is the Buffer. Notice anything of interest with Q2 having a Cap (10nF) to Ground. So, if I want to test this PTO where do I connect 12VDC?
We now have the old what I see in the schematic is not what is in the hardware. R85 is a 4.75K Ohm resistor at 5 watts and that connects to an OB2 Voltage regulator which is a gas fired 108VDC regulator tube --- if R85 fails you sure smoked a 2N706. With the R85 dropping resistor you get a regulated 12 or so volts into the PTO.
But wait in this PTO the lead going to R85 appears to be encased in shrink tubing and tucked away. What does follow is that a wire is installed at the junction of R85, R104 and R54 and the diode and connects to something in a radio.
This PTO likely came from later production models where additional solid-state devices were present and other provisions made for supplying low DC voltage. Thus, what you see in a schematic is not always in the hardware.
Hooking 12VDC to the wire produced no output. Next steps, pull the 2N706.
So why am I doing this? Well, the first part is playing detective, and the second is I might succumb to using an analog PTO in a radio. Despite its flimsy looks the Drake gear mechanism is pretty solid.
Thanks for traveling along -- tomorrow is the 4th of July and don't forget what that stands for -- fly your flag and remember the flag etiquette. Never upside-down Sam, the proper flying hours and it must never touch the ground!
73's
Pete N6QW