Randy -
OK, I'll give it a shot! :-)
The Transceive Alignment is to get the two oscillators, (Carrier
Oscillator in the TX, and 2nd Mixer LO in the Rx,) on the SAME
frequency. ANY difference between the two will result in different Tx
and Rx frequencies.
Consider the circuit. In the transmitter, the PTO is mixed with the
BAND oscillator in the PreMixer. So for 40M, with the PTO at 000, the
PTO is at 5.455 MHz, the BAND oscillator is at 18.1 MHz. The difference
frequency is the one that is selected at the PreMixer output, 12.645
MHz. This is then mixed with the CO at 5.645, resulting in the
difference frequency at the output of the 1st Mixer is 7.000 MHz.
In the receiver, the PTO set to 000 is again mixed with the BAND
oscillator in the PreMixer, and the difference frequency of 12.645 MHz
is selected at the output of the PreMixer. This signal is injected as
the LO for the 1st Mixer, resulting in an IF output of 5.645 MHz to the
2nd Mixer where it is mixed with the 5595 MHz LO resulting at an output
of 50.000 kHz. This is mixed in the Product Detector with the BFO at 50
kHz, resulting in a zero beat output from the speaker.
So, if there is a difference of say 50 Hz between the Tx CO and Rx 2nd
LO, then there will be a 50 Hz output of the speaker! The Transceive
Align procedure allows you to beat the two oscillators against each
other by tuning the "front end" of the Tx and Rx to 5.645 MHz so the CO
comes out of the Tx and is received by the Rx, allowing you to adjust
C61. Since the exact frequency of the Tx is the most critical, the Rx
2nd LO is adjusted to it.
Now, the kicker in all this is that crystals drift at different rates
and different amounts over temperature. Since we're talking SSB, a
difference of 20-30 Hz is noticeable to the receiving operator, and
crystals at these frequencies WILL drift that much over room temperature
variations. Drake's answer was to select CO and LO crystals for
"similar" drift characteristics, and code them into three or four
groups. The crystals were marked with a stripe of paint, Red, Blue,
Green or Yellow, and the "color" crystal installed was indicated by
adding a letter to the end of the unit's serial number. Best tracking
over temperature is if the Tx and Rx have the same "color" crystals
installed. The letter after the serial number is NOT a foolproof
indication of what crystal is actually installed, as some were changed
by owners who happened to end up with different, or NO "color" crystals
in their "pair". So look at the crystals and determine what color
stripe of paint is on them.
By the way, this was resolved in the C-Line by adding one more cable
between the two units, CAR OSC, that carried the Tx CO signal along with
a DC voltage to disable the 2nd IF oscillator in the receiver.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line& TR-4/C Service Supplement CDs
<www.k4oah.com>
Randy Bell wrote:
Garey,
Thanks for the pdf of the B-line alignment. I went ahead and d/l the entire
T-4XB manual from BAMA, just for reference.
Since in SPOT mode, the receiver VFO controls the received frequency and the
transmitter CO controls the transmit frequency, I have to confess, I don't get
the point of the alignment procedure.
I think I need to chew on the theory a bit before doing the alignment. My comments are based on the premise that I'm going to operate in one or another of the two different transceive modes, ie. let the CO control both the transmitter and the receiver (XMTR), or let the receiver VFO control both (RCVR).
Let's for the sake of discussion, consider XMTR mode. Assume that I have tuned
the transmitter to exactly 3900KHz as measured with an accurate frequency
counter. If I then set my receiver to CAL, I should be right on frequency,
since the transmitter is controlling the receive frequency.
Another way I look at this is that in XMTR mode, suppose I'm going to check
into one of my usual nets on 3921kHz. I tune the transmitter CO until the dial
is at or near 3921, then listening in, I find a station and tune it for a nice
sounding LSB signal. At that point, if I transmit, I would like to be assured
that the transmit frequency is right on 3921 LSB. Since in the SPOT mode, both
VFO's function independently, I don't see how the test works.
Forgive my thick head on this, but do you have an explanation that even a dummy
like me could understand?
73 de K8HRO
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