Richard Knoppow wrote:
----- Original Message ----- From: "Steve Wedge" <w1es1...@earthlink.net>
To: <drakelist@zerobeat.net>
Sent: Friday, July 29, 2011 11:54 AM
Subject: Re: [Drakelist] R-4A PTO: It wasn't the transistors
You know, Garey, I've been thinking about the xtal oscillator. It would have to be part that
doesn't get bandswitched - which would include THAT transistor.
When the T-4X controls the frequency, it sounds great and never jumps frequency. In looking at
the schematic, I have been aware that the two solid-state oscillators (PTO and LO for the band)
get mixed in V8 (IIRC) and that this combination gets overridden by the output of the T-4X's
premixer.
Looks like I am going to need a counter to find out. I am loathe to dig into the T-4X because
it's working so well and the PTO is much more difficult to remove from it due to the volume of
wires in the area.
73,
I am now a bit confused. Is the PTO in question in the TX or RX? If the RX I can see some
possibilities from the circuit diagram. These are perhaps far fetched but, since you have checked
all the obvious stuff they may have some merit.
We're working on an R-4A. Bipolar oscillator and buffer. I 'thought' we had replaced the Zener
early on, since it's one of the most likely culprits.
1, There is a connection for an FSK adaptor. Its a sort of solder tab going through a hole in the
shield can. This goes directly to the tap on the PTO coil that couples the output to the buffer,
Q3. Perhaps something is affecting it.
The output is pretty insensitive to load variations. Faulty buffer transistors have been known to
vary the oscillator load and shift the frequency..
The output of the PTO goes to the PTO-Crystal switch. I am not sure how much influence on
frequency this point can have since it is on the output side of the buffer but worth checking that
there isn't something flaky about the switch.
Same reasoning, pretty well isolated from the oscillator.
C-90 is the main frequency determining capacitor. I think this is the "dogbone" cap which has
been discussed. I am curious as to what kind of cap this really is. Most silvered mica caps I've
seen were either the old molded bakelite case kind or the more modern type in dipped epoxy cases.
I think the migration problem, also called scintillation, is mostly in the molded case kind. I am
not sure if this is due to some moisture being able to get into the molded cases or to a change in
manufacturing technique but it does not seem to happen, at least not as frequently in the dipped
case kind. All the "dogbone" style caps I've seen were ceramic. I don't know if ceramic caps can
develop this or a similar problem. The diagram shows two seleced value caps in parallel with the
main cap. I presume these are temperature compensating caps, which means they are ceramic types.
Temperature compensating caps are always ceramics, they can be in at least three varieties of
casings, molded bakelite, ceramic shells sealed at the ends, and the dogbone type. There may also
be disc types.
The 'dogbone' cap IS the 3000 pF silver mica dipped epoxy type. I refer to them as Dur-Mica which I
think was a brand name, made by CDE. They do develop the silver migration problem, albeit at a MUCH
lower rate than the older molded 'postage stamp' type.
I don't think a counter will be of much help because they respond too slowly. To isolate the
variation I think you need a separate receiver capable of hearing the PTO frequency and also the
other oscillators. You should be able to get an isolated signal from any of them with the aid of a
one-turn loop on the end of a coax. Set the external receiver up with its BFO adjusted for a
convenient beat note and just let it go. You might also have a signal tuned in on the Drake so you
can tell if one jumps when the other doesn't. Obviously, if the PTO is jumping both will jump but
if the Drake jumps but the external RX indicates the PTO is stable you have to look elsewhere. The
fact that the RX is stable when driven by the TX PTO seems to eliminate other oscillators in the RX.
Yes, listening, even on a less than good receiver is the best test to find the small variation we
are looking at. The PreMIxer signal (BAND minus PTO) goes to the 1st Mixer, and is the signal that
goes back and forth on the INJ cable. A DC control voltage is superimposed to switch (bias) the
'other' PreMixer signal off.
Now, I am thinking out loud so take that into account.
I thought that's how it was done!!! :-)
I think you said you had opened the PTO and poked a the circuitry. If not try it. I find
those big wooden toothpicks that are used to hold sandwiches together handy for this. Some of the
component positions in the PTO are evidently critical so don't move thing much but wiggle all the
leads and also check carefully for a cracked trace on the circuit board. They can be quite
invisible and intermittant in effect. I am guessing at a couple of things because I've had only
one Drake PTO open and that was partly curiousity and partly because I managed to slip a gear in
the drive and wanted to make sure it was back right.
Yes, mechanical problems are often the case. The firmness of the shield can spring clip / gearbox
connection is critical, as is the spring loaded yoke / drive screw / rider bar interface. Bad
solder, particularly at the two coil tap connections, is also not uncommon.
I don't remember if the core itself is exposed all, I think it is. If so poke at it. Its
highly unlikely but possible for it to have developed a crack. If that's so I am not sure what to
do. I think cracked cores can be repaired with cement but getting it out to work on it may be
difficult. I think this is _very_ unlikely, but we are looking for unlikely things.
The core is visible as you go lower in frequency, or it is possible to remove the circuit board as a
unit and expose the entire core. I have seen _ONE_ cracked core, after looking at LOT of PTOs,
and a drop of Super Glue took care of that one. Drag on the rear support pin for the core is also
important, from a loose phenolic washer, to one that has been re-glued 'incorrectly', i.e., as a
'square' rather than a 'diamond' orientation, to grease and/or nicotine causing it to 'stick'.
I just looked at the Garey's Drake service manual but it does not show the PTO so I have no
memory refresher.
My thinking at the time was that anyone who could not trace out the simple circuit of the PTO
probably had no business being in there! I may need to do an 'addendum' for the PTO.
Also, if you have a friend who has some electronic skill get him or her to look also. Its very
easy to overlook something over and over, even when it's obvious. Sometimes another person will
spot it right away.
Absolutely. That's been an 'open secret' all the way back to the early
Heathkit manuals!! :-)
Since so many Drake PTO's seem to work fine with no additions or modifications I would be very
leary of adding anything. This problem exists because some component has gone wrong and not
because the _design_ is wrong.
Certainly. Drake built over 100,000 of them, essentially unchanged, so probably had the kinks
worked out pretty well.
Lastly, this is just a dumb machine and its outsmarting a bunch of fairly smart people, we
should be able to beat it!
Well, that's half-rught anyway.... :-)
73, Garey - K4OAH
Glen Allen, VA
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