Steve -
No, typically they are smaller values. The TC caps are some of the ones with asterisks (*) beside
them. Those are the selected caps, picked at final test. It's certainly possible that the 3000 or
90 pF ARE TC though. The small disc ceramics caps that are TC type have the top half 'dipped in
paint', and the color indicates TC. The tubular _glass_ caps have a complex, colored dot
pattern. The capacitance value is often "Body - End - Dot" for the capacitance value (first -
second significant figures and third multiplier) and typically the rest of the dots specify either a
third significant, or TC, or .... There is a set of tables in the back of the ARRL Handbooks that
explain a lot of the codes, or I'm sure they're on the internet.
So far though, all the PTOs that I've had to replace devices in have resulted in good linearity. I
don't think the active device has much of an effect on TC, it's mostly the relationship between the
coil's, form, core, capacitance. Don't overheat anything in there, and be sure to let it cool to
room temp before becoming concerned about TC.
WATCH the device basing........
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>
Steve Wedge wrote:
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" <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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