Or even a 2N2222. Between the 2N3858, 2N3904, and 2N2222, the fT / GBP,
hFE, and C in/out parameters are reasonably identical. Probably the biggest
variant will be the hfe value across samples, but I agree with Garey to give
it a shot. The PTO is only running at 5 MHz.
During college, I worked for an engineer who's philosophy was to replace
with "2NAnyThing" that worked. He certainly knew the widely different
transistor parameters, but his point was that in many general purpose
switching, amplification and oscillating circuits, "2NAnyThing" is often an
adequate substitute, taking into account the need to watch for NPN, PNP,
FET, etc. configurations.
Paul, W9AC
----- Original Message -----
From: "Garey Barrell" <k4...@mindspring.com>
To: "Steve Wedge" <w1es1...@earthlink.net>
Cc: <drakelist@zerobeat.net>
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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