Hi Steve,
Of course, take heed to initially eliminate "...the usual suspects" just
as
Garey suggests in finding the cause of your receiver PTO's raspiness &
instability...but sooner or later, you just *may* have to bite the
proverbial bullet & get inside of that PTO shield can to see exactly
"...what's what" therein...
Now, if anyone on the Reflector starting to read this has no issue(s)
whatsoever with the PTO(s) in their rigs, consider yourself blessed!---and
feel free to delete this mesage before proceeding any further (it IS
rather
long).
My experiences with the PTO of my T-4X are pretty much well-documented in
previous posts...but to summarize briefly, the signal from my transmitter
PTO would intermittently become raspy and off-frequency, as well. After
bravely(!) removing the PTO cover---and the entire PTO circuit board
itself!---I first cleaned everything that I could see with isopropyl
rubbing
alcohol...no change.
I then replaced the zener diode therein...no change.
I next added the Drake-prescribed after-market grounding strap inside the
PTO...no change.
Finally, I did a visual inspection of the solder traces on the board of
the
PTO, & discovered that one of the joints of the 3000-pfd. silver mica
capacitor there appeared to be "crystalized" (as in "cold") under a
magnifying glass. I ultimately removed this capacitor outright, & replaced
it with a pair of 1500-pfd. s.m.'s in parallel.
Did that solve the issue...? YES, and NO. As long as the rig is allowed to
thoroughly warm-up for about 1 to 2 hours, all seems well and
good---however, even after this "heat soak", there continue to be very
infrequent (but still there!) minor frequency "creeps" up in frequency
that
I can detect, of a few tenths of hertz (but no raspiness).
Now, I should point out that my Drake Twins are located in my basement,
where the average ambient room temperature hovers at 50F (+/-)---and I
cool
the 6JB6A finals of the T-4X by fanning air OUT of the enclosure. My
suspicion initially at this point was that any of the temperature
compensating capacitors (themselves almost a half century old now!) inside
that PTO shield never really have much of a chance to really & truly
warm-up, as such, and were acting "flakey" in getting up to speed---hence
the continued display of off & on drift. And so I pondered the possibility
of installing a small 12-volt pilot lamp "heater" inside the can, to speed
things up to a constant level of warmth...
This past week I proceeded to look into that notion, and started by
removing
the PTO cover. HELLO!!! the locking tabs on one side were NOT FULLY
SECURED
into the mounting holes! As well, I concluded that ANY "heater bulb" entry
into the tight quarters of PTO shield would necessitate that wires be in
close proximity to the coil field---and I had no idea what the
consequences
that might have upon the dial calibration...and I really did NOT want to
find out, either!
So I simply replaced the can/cover, being extra careful this time that the
locking tabs were engaged.
Now, fast forward to yesterday, when I stumbled upon this nugget in the
matter of ferrite-rod oscillator tank coils (as in Drake PTOs), which I
quote now from page 85 of Harry D. Hooton's (W6TYH) 1968 book entitled,
"SINGLE SIDE-BAND: THEORY AND PRACTICE"...
To whit: "IN ORDER TO MAINTAIN ITS HIGH-Q THE FERRITE-ROD COIL SHOULD BE
KEPT AWAY FROM THE METAL SHIELDING, BECAUSE IT IS QUITE SENSITIVE TO ANY
EXPANSION IN THE SHIELDING DUE TO HEAT. THE SHIELD BOX SHOULD BE
CONSTRUCTED
OF FAIRLY HEAVY-GAUGE ALUMINUM. UNLESS THE BOX SEAMS AND THE TOP AND
BOTTOM
PLATES MAINTAIN A GOOD ELECTRICAL CONNECTION DURING EXPANSION OF THE
METAL,
THE COIL FIELD WILL BE DISTURBED AND CAUSE THE OSCILLATOR FREQUENCY
SUDDENLY
TO INCREASE FROM TIME TO TIME."
Hmmm...
If it takes that little to adversely affect oscillator frequency, the
detrimental effect upon stability of one of the "ears" of one of the
locking
tabs being loose now makes sense to me---just as the fact that continual
removal & replacement of the can probably has a bearing upon shielding
integrity, that heretofore was not compromised in any way since the rig
left
Miamisburg in 1966!
I'm next going to try some sort of "RFI gasketing" scheme at the base of
the
PTO shield can, to ensure that there are no "cracks" in the shielding that
could be affected by normal heating of the rig...
Steve, hopefully your problem might be traced to a simple need for a
clean-up of dried-up grease/dirt in the PTO mechanism. While my own
personal
"...journey of discovery" here with my PTO has been enlightening, I
wouldn't
particularly wish the trip I've been taking upon anyone else...! Hi Hi
~73~ de Eddy VE3CUI - VE3XZ
*****************************************
----- Original Message -----
From: "Steve Wedge" <w1es1...@earthlink.net>
To: "Diane and Edward Swynar" <deswy...@xplornet.ca>
Sent: Saturday, January 08, 2011 8:37 PM
Subject: Re: [Drakelist] R-4A PTO Woes
I tried to take it off when I cleaned the receiver, Eddy. Those
spring-loaded catches didn't want to cooperate, so I stopped. The PTO
was
removed in its entirety prior to my dunking and drying the rig (which
came
out pretty nice, otherwise after being pretty filthy).
Steve Wedge, W1ES/4
To be is to do - Socrates
To do is to be - Plato
Do be do be do. - Sinatra
All my computers have my signature with various pearls of wisdom appended
thereto.
--------------------------------------------------
From: "Diane and Edward Swynar" <deswy...@xplornet.ca>
Sent: Saturday, January 08, 2011 6:17 PM
To: "Steve Wedge" <w1es1...@earthlink.net>; "Drake List"
<drakelist@zerobeat.net>
Subject: Re: [Drakelist] R-4A PTO Woes
> Hi Steve,
>
> Have you---or any previous owner(s) that you may be aware of---had
> occasion
> to take the PTO shield can/cover off more than once from that receiver,
> for
> any reason...?
>
> There is method to my madness in asking this---I'll explain more when I
> get
> your answer...
>
> ~73~ de Eddy VE3CUI - VE3XZ
>
>
> *****************************************************************
>
>
> ----- Original Message -----
> From: Steve Wedge
> To: Drake List
> Sent: Saturday, January 08, 2011 5:52 PM
> Subject: [Drakelist] R-4A PTO Woes
>
>
> I've got my twins warmed up and continue to have problems with the PTO
in
> the R-4A. When controlling the set from the receiver, the CW tones
sound
> raspy and the frequency tends to jump around. If I touch the shield, I
> can
> make the pitch change. Pressing on the tuning knob also seems more
> sensitive.
>
> When I control the frequency from the T-4X, the notes sound beautiful
and
> don't vary. It seems less sensitive to mechanical contact with the
> knob
> as
> well.
>
> Is there a way of tightening up the PTO? Why does everything sound
> T-7-ish
> when using the receiver's PTO and not the transmitter's?
>
> TNX es 73,
>
> Steve Wedge, W1ES/4
>
> To be is to do - Socrates
> To do is to be - Plato
> Do be do be do. - Sinatra
>
> All my computers have my signature with various pearls of wisdom
appended
> thereto.
>
>
>
>
> _______________________________________________
> Drakelist mailing list
> Drakelist@zerobeat.net
> http://mailman.zerobeat.net/mailman/listinfo/drakelist
>
