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]> 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> > > > > 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? >>>> >>> >>> ______________________________**_________________ >>> Drakelist mailing list >>> [email protected] >>> http://mailman.zerobeat.net/**mailman/listinfo/drakelist<http://mailman.zerobeat.net/mailman/listinfo/drakelist> >>> >> >> >> ______________________________**_________________ >> Drakelist mailing list >> [email protected] >> http://mailman.zerobeat.net/**mailman/listinfo/drakelist<http://mailman.zerobeat.net/mailman/listinfo/drakelist> >> >> > ______________________________**_________________ > Drakelist mailing list > [email protected] > http://mailman.zerobeat.net/**mailman/listinfo/drakelist<http://mailman.zerobeat.net/mailman/listinfo/drakelist> >
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