Modifications for the Yaesu FT-690

Modifications for the Yaesu FT-690

19-07-1998 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.

Modification as supplied by SV1DH and printed in Six News, Issue 41. All modification done at owners risk.