----- 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,
Steve, W1ES
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.
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 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.
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.
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.
Now, I am thinking out loud so take that into account.
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.
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.
I just looked at the Garey's Drake service manual but
it does not show the PTO so I have no memory refresher.
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 its obvious. Sometimes
another person will spot it right away.
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.
Lastly, this is just a dumb machine and its outsmarting
a bunch of fairly smart people, we should be able to beat
it!
--
Richard Knoppow
Los Angeles
WB6KBL
dickb...@ix.netcom.com
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