Thanks, Garey -
I've done loads of rework over the years, so hoping I won't mess up the
basing (though I won't say that I've never messed it up). I'm ordering 3
each anyway. I used to have scores of 2N3904's but gave all my parts away
when I moved five years ago. Not a big deal - Mouser has 'em for less than
a quarter each. The shipping's going to cost me more than the parts :-) I
was just figuring I can order some of the caps while I'm paying $7 for
shipping anyway.
I have no idea what these transistors are that are in there now. Someone
must have had these around as generic NPN's and used them.
Even though that 3000 pF cap is huge and funny-looking, am I right in
assuming I could replace it with a standard SM one?
Here's hoping replacing them takes care of the problem.
73,
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" <[email protected]>
To: "Steve Wedge" <[email protected]>
Cc: <[email protected]>
Sent: Sunday, July 24, 2011 6:24 PM
Subject: Re: [Drakelist] R-4A PTO - transistors substituted?
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" <[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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