Floyd Sense <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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I raised a question 2-3 weeks back regarding some severe key clicks on
my TR7, and have finally arrived at a solution that's quite acceptable
to me. Some of the communications on the subject occurred off-list and
I thought it might be useful to summarize the problem and solutions
here. If you never operate your TR7 on CW, or know for sure that you
don't have a key click problem, now's the time to bail out!
First, I want to say that I wasn't picking nits regarding the key clicks
- they were severe and could be heard many KHz away from my carrier
frequency. Second, while the serial number of my TR7 is 8913, I
acquired it from an ebay seller (who claimed it was perfect) and think
it's possible that all the boards, PA brick, etc. could have come from a
number of sources. That may account for why my TR7 showed the problem
that most folks haven't seen (at least not for a long time).
The cause of the clicks was traced to two separate problems and Garey,
K4OAH, provided substantial technical guidance as I stumbled my way
through. Here's a link to a page where you can view some oscilloscope
photos of the CW waveforms along the way.
http://home.earthlink.net/~fsense/TR7/
I stumbled across the first problem as I was running down some things
Garey suggested. Transformer T1101 on the 2nd IF/Audio board was a bit
out of adjustment and was responsible for the overshoot seen on the
leading edge as the waveform rose to full power. Apparently there was a
"ringing" effect by T1101 and proper adjustment eliminated that problem.
That left me with a very sharp risetime of .5 to .6 milliseconds. Other
TR7 owners have reported 2.5 millisecond rise and fall times, and ARRL
documentation says something around 5 or 5.5 ms for both is ideal. At
this point, I substituted a different 2nd IF/Audio board obtained from
Evan and while the risetime was certainly different and to some degree
better, it didn't go far enough in eliminating the clicks. Keith,
G3VKW, told me he remembered a mod in the keying area from many years
ago, done by G3HCT (now VK4OQ) and successfully tracked down VK4OQ who
was able to retrieve his old notes on the subject from a friend in
G-land. I installed John's modification to the 2nd IF/Audio board and,
as you can see from the last two photos on the web page, the waveform is
now just about perfect. My clicks are gone and the signal has very
nearly the same bandwidth as my Icom 781 on CW.
I'm afraid that I have no idea why my TR7 with a fairly late serial had
this problem while others don't show the problem at all. I agree with
Garey that the persistence of the problem through a card swap seems to
point elsewhere, but the mod to the 2nd IF/Audio board seems to have
completely fixed it (or perhaps just masked the original problem).
The fix from VK4OQ via G3VKW is as follows:
"On the 2nd/IF Audio Board the xtal oscillator Q1110 and its associated
buffer Q1109 are keyed by Q1103. The modification is done to Q1103.
Add on the back of the board a 15k 1/8 watt resistor between the emitter
and base. Add on the back of the board a .1µfd Disc capacitor BETWEEN
COLLECTOR AND BASE. That is all there is to it but the shape will then
be " Text Book" !!
I found it necessary to increase the value of the capacitor to .22 mfd
for the best envelope shape.
Thanks to all who provided assistance in getting this resolved. I can't
recall ever having gotten this kind of support in the past from other
groups and really appreciate it.
73, Floyd - K8AC
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