Steve -
I've looked in a LOT of PTOs, and I've never seen anything but a 2N3858 in the Buffer and either a
2N3858 (early) or 2N706 (late) in the oscillator. No other changes required with either oscillator
transistor. The '3858 is just about extinct, but the 2N706 is still a common transistor.
Defective transistors have definitely been known to cause the kind of frequency changes you're
seeing. So while they may even be a 'later' modification than factory built, and may even be a
suitable substitute, they can still fail just like the originals. By the way, if you look at the
PTO schematic, the FSK 'shift' terminal is connected to the output of the oscillator stage. This
allowed you to _SHIFT_ the PTO frequency by up to 850 Hz by adding a cap from this terminal to
ground. So variations in the Buffer transistor CAN dither the frequency. And yes, it does.
I think transistors were about the third thing down on the list once you get through the
lubrication, mechanical and ground faults.
I know you said you were short on components, but '706s are cheap from Mouser, or if you can find a
couple of 2N3904 (everywhere!) transistors you could try them just to see. They may not work
perfectly, but if the PTO becomes stable you'll know. Watch the basing on whatever transistors you
use. Seems like they are all different these days!
73, Garey - K4OAH
Glen Allen, VA
Steve Wedge wrote:
Looking at the transistors in this PTO, I'm 99.9% sure someone replaced them: they are both marked
"NSRS / 2018", with the / being a line break.
I'm sort of thinking that Drake used different parts for the oscillator and buffer for a good
reason. Aside from this maddening frequency-shifting and crummy audio, the frequency calibration
is still good. What are the chances that using the "wrong" transistors could be the source of all
this grief?
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