Darryl -
That's what I get for being in a hurry. Seems to be happening more
often these days! :-)
You are correct, you can only do one filter at a time. You can use the
dial skirt as a reference, i.e., turn on the CAL, and tune to zero
beat. Set the dial skirt to "0". THEN tune through the calibrator in
either filter position and you can see the approximately 2-1/2 kHz marks
on either side where the S-Meter reads near maximum and with a drop to
noise on either side. Then turn OFF the calibrator, and with the dial
set to "0", switch between USB and LSB, listening for 'similar' noise
pitch in either position. If they are not the same, then tweak C130
slightly one way or the other until they are. Again, they won't sound
identical, but not 'bassy' or 'hissy' on either.
Yes, the drop to the 'valley' between the two filters will only be about
20-30 dB due to the skirt slope of the filters. You can see this in the
diagram of the filter response mentioned earlier. What we are
attempting to do here is get the CO at the right place to be between the
passbands, NOT necessarily in that maximum notch position. The Balanced
Modulator will take care of the additional attenuation of the carrier.
So, sorry for the confusion on my part. Hopefully the above will clear
it up and 'un-traumatize' you!!! :-)
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
kb8bku wrote:
Garey,
Thanks for the explaination! I have never heard this double hump when tuning
the calibrator signal in any of my Drakes (TR-3, TR-4, 4A-line twins). I
assume the null that you describe tuning through is the BFO frequency, between
the two filter passbands. But it would seem to me that you would only have one
passband filter active at a time (depending on the selected sideband), and so
no null in the middle(between the two), but rather the signal would just fall
off on either side of the single (LSB or USB) passband. This is more what I
think I hear from my radios. I must be missing something or perhaps I am
misunderstanding your explaination. Or perhaps I have some work to do! I can
switch sidebands while tuning and get close to what you describe, but my dip in
the middle is only about 20dB, not all the way to noise.
KB8BKU, Darryl in Dayton
On Jan 25, 2011, at 11:12 PM, Garey Barrell wrote:
Kevin -
A scope can be used, although I find it easier to just listen to the receiver.
Think of the two filter passbands as the letter "M" as depicted on Page 29 of
the TR-4 Manual. The goal is to adjust C130 so that it sits in the valley between the
two filters. If you turn on the CALibrator and tune through it in either direction in
receive mode, you'll see the S-Meter go from noise level up to 10 over S-9 or so for a
couple of kHz, then down to noise again, back up to 10 over S-9 for a couple of kHz, and
then back down to noise level. If you get this pattern, then C130 is close. Fine
adjustment is to listen to the receiver noise (no antenna, high band, no signals) and
switch back and forth between USB and LSB. The 'pitch' of sound should be similar in
both positions. If one sounds low or 'bassy' and the other side sounds high and 'hissy',
then adjust C130 slightly until they sound about the same. They will NOT sound EXACTLY
the same due to anomalies in the filter passbands, but they should sound similar.
Just in case you are 'really' asking about nulling the Carrier in the Balanced Modulator,
in this case you can clip the ground lead of the scope to the chassis and then just lay
the probe tip near the Driver tube socket. Set the SIDEBAND switch to "X", key
the transmitter in X-CW with the GAIN at full CCW, and adjust C127 and R85 for MINIMUM
signal. Again, it's easier to listen on an external receiver if you have one available,
and do the above, adjusting for minimum signal in the receiver.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
Kevin Elliott wrote:
I have been reading all of the posts in the new Drake archive and the old Drake
archive trying to learn as much as I can about my TR4. To you old pros this
will be a rookie question, but I am of the belief that if you don't ask you
don't learn. Okay I have read where it has been said that you can use a couple
of different ways to set C130 correctly. One being use a digital receiver and
listening for the matching null and the one that I want to ask about is using a
scope to adjust them. Where will the scope probe be connected to read this
information or is it indirectly connected with a loop of wire around the
oscillator tube to see the nulls.
I got the Great service CD and I love it and will be going through it and
digesting all I can.
Thanks
Kevin _ KG0MN
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