Modifications for the Yaesu FT-690
Extending the tuning range of the Yaesu FT690mkII
Although the FT-690mkII covers 50-54 it can be 'stretched' to cover any six
megs between 44-54Mhz. A very useful mod for listening to early warning
video signals on 48.25+-, 49.25+- and 49.75+-.
Firstly replace the crystal on the 55Mhz vcxo (part X4002) on the pll/pa
unit with a new one of the following spec.
Frequency - 27.34925Mhz, fundemental cut.
load capacitance - 28pf
series resistance - low (15-20ohm)
Holder - HC-18/T/3P, middle pin to case.
To modify the PLL divider and display, first switch off the memory back-up
battery and then install a 1N4148 diode, soldering the cathode to pin 5 of
P2501 and the anode to pin 6 of P2502 of the CPU unit.
Turning the rig back on, the VFO's will display 5.000.0 (this is 45.000.0).
The mark "HIGH" will be displayed above 0.000.0 (50Mhz). Now adjust TC4001
on the VCO unit for the desired range. Next adjust T3018 and T3021 on the
transmit path as well as T3001 to T3005 on the recieve path for optimum
flatness and sensitivity over the chosen frequency range.
In case the range 48 - 54 is required, there is no need for the above VCO
and RF circuit adjustments. Please note that the performance will decrease
approaching the lower 44Mhz limit.
For accurate frequency display, adjust T4001 on 0.000.0 against a known
50Mhz signal and VR4004 for 25Hz frequency difference between 9.998.475 and
9.998.500 using the 25Hz step mode. The step when moving from 9.998.4 to
9.998.5 is the most critical as it represents the maximum possible error of
the display reading.
This counter shows the number of hits since the
9th February 2000
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