Thanks, Garey -
Yeah - I've seen that they're only about a buck-seventy-five on Mouser. I
think that this is where I'm going next.
Just in case this doesn't work out, which capacitors are the ones with the
negative temp coefficients? I suspect the 3000 pF would be one of them.
Steve Wedge, W1ES/4
"I can't complain, but sometimes I still do."
- Joe Walsh
If the above message appears, it came from Steve's Son of Laptop!
----- Original Message -----
From: "Garey Barrell" <[email protected]>
To: "Steve Wedge" <[email protected]>
Cc: <[email protected]>
Sent: Sunday, July 24, 2011 2:45 PM
Subject: Re: [Drakelist] R-4A PTO - transistors substituted?
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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