Dennis -
And watch those reverse breakdowns too!! :-)
The 2N2222 is the 6L6 of the transistor world. No matter how much you abuse them, they just keep
going....
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>
Dennis Monticelli wrote:
For the oscillating device in the LO I would add "noise" to the list of what to look for in a
transistor. This varies a lot from type to type and even from maker to maker. For example, the
2N2222 is not exactly the world's quietest transistor. To just check to see if that osc
transistor is the root of the problem, then free subsitution is fine. Otherwise stick to a device
that was designed for RF service, which is different than a device designed for general saturated
switching. I know this from experience; my company was a transistor manufacturer in the past.
Dennis AE6C
On Sun, Jul 24, 2011 at 2:20 PM, Garey Barrell <[email protected]
<mailto:[email protected]>> wrote:
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 <http://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]
<mailto:[email protected]>>
To: "Steve Wedge" <[email protected]
<mailto:[email protected]>>
Cc: <[email protected] <mailto:[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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