You would not be able to see a few tens of Hertz change on a scope.
One method of observing small changes in frequency (and phase) is to utilize
the X-Y function on a scope to display a Lissajous pattern. I have no doubt
Garey has done this in the past. For others, one simply takes a
known-stable RF generator and feed it into Channel 1 of the scope. The PTO
output is fed into Channel 2. The scope time base is set to "X-Y" mode.
Next, the stable generator amplitude can be set to approximately equal that
of the PTO. Amplitude equality does not have to be very precise. As
frequency changes, so will phase. Two in phase and on-frequency signals
will show on the scope's displays as a straight 45 degree line. As the PTO
changes frequency, the line will turn into an oblong loop and continuing
further, a perfect circle at 90 degrees of phase difference, then it will
show as a straight line again but at the 135 and 315 degree points on the
scope's display.
With the Lissajous method, even the slightest change in frequency between
two sources can be displayed on a scope.
Paul, W9AC
----- Original Message -----
From: "Garey Barrell" <[email protected]>
To: "Robert Fish" <[email protected]>
Cc: <[email protected]>
Sent: Wednesday, October 26, 2011 12:21 PM
Subject: Re: [Drakelist] T-4XC Unstable PTO (I think)
Bob -
You would not be able to see a few tens of Hertz change on a scope. The
warbling of the calibrator signal is often a power supply thing, C179 on
the Calibrator board. Since it's intermittent, it's more likely a bad
ground, I'd suggest 'exercising' the Calibrator plug by moving the board
up and down a bit. DeoxIT on the connector pins wouldn't hurt.
There are two primary causes for PTO instability. One has been going on
for years, which is the intermittent grounding of the PTO slug and drive
screw. The grounding path(s) are through the drive follower riding on the
chrome rider bar at the top of the gearbox, and the ball bearings in the
front plate. Hardened grease forms little 'chunks' that break the path
through the ball bearings, causing frequency jitter when tuned. Usually
not a problem when tuning is stopped. Drake offered a 'fix' of a small
piece of wire braid soldered from the drive follower to the side wall of
the gearbox. The field change is on the CD in the MISC directory.
The second cause seems to be happening more often lately, and is the
breakdown of one or more of the capacitors in the frequency determining
circuit of the PTO. There are a couple of 'common' culprits, depending
upon which of the 17 versions of PTO that you have. You should be able to
see the jitter on a counter, the changes are tens of Hz if you can hear
them. Freeze spray can help to isolate the defective cap(s), but not
always. In the C-Line PTO C157 and/or C203, which may be either two 45 pF
NP0 tubular ceramic caps or a single 90 pF is the most common
troublemaker. It's best to measure the value of the existing cap and then
choose a replacement as close as possible. C158 and C159 are selected at
test to compensate for variations of C157 for linearity, and if you select
C157 accurately linearity doesn't seem to be impacted.
Less likely causes are a bad FET transistor Q8, or a bad Zener diode CR6.
I have seen one case of a leaky C151.
The trials and tribulations of PTO repair are in the archives about a
month ago on this list and on DrakeRadio.
--
73, Garey - K4OAH
St Charles, IL
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
Robert Fish wrote:
Hi Guys,
I have been ignoring an issue with my T-4XC until recently. When I use
the R-4C PTO to control the receiver it seems to work fine.
When I use the T-4XC PTO to control the receiver I get sort of a chirpy,
jumping around of the cw signal I am trying to copy. Both units
work fine in transceive using the R-4C PTO, which is the way I have been
using them. But with the recent DX expeditions (T32C, TX7M)
running split, I have been having to use the C-line as separates to work
them. I have had some success working them, but heaven knows what my
signal sounds like.
I have been putting off fixing this problem as I felt it was probably the
PTO and I haven't been in the mood for performing major surgery lately.
I decided to get off my arse and do something about it yesterday. So, I
got out the scope and took a look at the T-4XC PTO output while it was
controlling the R-4C.
It seems stable. So I looked at the Plate of V-8 with the scope, I have a
composite signal (PTO, Xtal Osc.) it looks solid as a rock. So then I
looked at the injection signal
right at the jack on the back of the T-4XC and once again, I can hear a
cw signal I tuned in on the R-4C jumping all over the place, while I'm
looking at a rock solid injection sig
leaving the T-4XC. I switch back to the R-4C PTO and the jumping around
stops, the receiver copies the same cw sig with stability.
What am I missing here? It is hard to fix something when it seems like
the only evidence of a problem is coming out of the speaker. If I could
find some evidence of the problem
I could follow it to it's source. This could be operator error (probably
is) I only put about an hour in to this last night as I got called to
dinner and a man has to have priorities.
I am going to dig in to it this morning, I will eventually find the
problem, but maybe you guys can save me some time.
Oh........another clue: When I use the spot function, in CW mode, I get a
warbling sound in the R-4C instead of a solid cw tone when tuning in the
T-4XC sig with the receiver. It seems
sort of intermittant (it switches to a cw tone sometimes and then goes
back to warbling like you get in SSB mode with the spot function.)
Thanks in advance, I hope this long winded post made sense.
Bob K6GGO
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