"No transmit on UHF"
One day I realized my good old Yaesu FT-897 (not the D model) from 2013 did not transmit at 70 cm: power output was less than 1 mW and the built in meter of the radio did not indicate any forward power nor high SWR. Radio drew a high current on transmit and UHF receiving seemed to work well. VHF transmit and receive also worked – the radio uses the same final transistor for V and UHF so the finals were clearly intact.
The FT-897 service manual was super helpful since it included complate schematics and layouts of all PC boards. Pena suggested (correctly as it turned out) that the UHF-transmit chain was broken after the final transistor. In 897 the V/UHF T/R switching is accomplished using PIN diodes, the UHF TX section of which is controlled by signal U_TX5V_2. This control line was +5 V on transmit and 0 V on receive which I presumed meant that the control was working as it should. On the other hand probing the RF trace with an oscilloscope revealed a RF voltage of approximately 30 volts rms on the anode of the first PIN diode (D3015 in the service manual) in the UHF TX chain and 0 V on the cathode. So clearly at leastthe first PIN diode was broken. Afterwards other PIN diodes in the assembly were tested using an multimeter and seemed to work properly. The first broken diode did not conduct in either direction.
The broken PIN diode was removed, and since no original spare parts were readibly available the diode was replaced with one salvaged from old VHF base station. The replacement diode was in similar ceramic package. The replacement of this PIN diode seemed to solve the "missing TX" problem, and a full transmit power of 20 watts on UHF was measured after this repair. UHF RX and VHF TX/RX also still worked. The location of the defect PIN diode can be seen here. As for all electronics work ESD safe workstation and a microscope were necessary.
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