Funny, I don't recall you working for me during college........ :-)
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
Paul Christensen wrote:
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" <[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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