Passband Tuning NotesSubmitted by "Jim Shorney" jshorney@inebraska.com - nu0c@amsat.org
Passband Tuning Notes:
Fixed Passband Pots
Following Information from Bill Frost at R.L. Drake
"The fixed passband controls are located on the power supply board of the TR-7. Remove the wrap-a-round cover of the TR-7, the power supply board is located in the right front corner. You will see five small "blue" controls along the top of this board. They are in the same left to right sequence as the "mode" switch.
The left most control (you are facing the TR-7) is the AM control, next
(going to the right) is the RTTY control, then the CW control, then the
LSB control, and last the USB control. The control to the far right
(sometimes a "black" control) is the 10 volt adjust control
DO NOT BUMP OR ADJUST THIS CONTROL ! !
Once you have become familiar with the controls and their location, proceed as follows:
- Select the desired mode to be aligned.
- Ensure that the PBT button is released.
- Tune the receiver to the exact frequency which corresponds with the
desired mode.
- AM = 13695.0
- RTTY = 13697.5
- CW = 13694.2
- LSB = 13696.4
- USB = 13693.6
- As the receiver approches the desired frequency, a beat note can be heard. The beat note should be zero when the receiver is on frequency. It maybe necessary to increase the audio gain since zero beat will be on the slope of the filter and weak.
- If the beat note is not zero, adjust the corresponding PC control for zero beat.
- If additional modes are to be inspected, repeat the above procedure."
Alternative Filter Adjustment Method
Submitted by "Jim Shorney" jshorney@inebraska.com - nu0c@amsat.org
A good way that I have found to set filters when no service info is available (or the filter isn't quite at spec) is to use an audio oscillator connected to the mic input to sweep the filter passband on transmit. Use a dummy load, VSWR meter in 'calibrate' mode, and low power; sweep the oscillator to find the point of peak RF output.
Set the VSWR meter sensitivity control for an exactly full-scale indication on the meter, then set the audio oscillator to 300 Hz and adjust the carrier oscillator frequency control for half-scale on the meter.
Recheck the peak and half-scale points a couple of times, as they do interact. This gives us a -3 Db point on the filter slope at 300 Hz on the low end of the audio passband, and should be pretty darn close. You could, of course, go for 200 Hz, 250 Hz, or whatever else suits your fancy...
FWIW, this procedure comes from a Hy-Gain model 623 CB rig service manual...