The meters on my Yaesu FC-902 antenna tuner have stopped functioning,
but the antenna tuner functions normally. I used an external Power/SWR meter to perform my matching.
This tuner has a small number of parts, and I noticed in the schematic diagram
that the meters use a couple of small PC boards with a few components.
Any thoughts on what to look at? I see a couple of small signal diodes on one of the boards. Thanks
The CM Coupler in the Yaesu FC-902 Antenna Tuner uses 2 diodes, D101 and D102.
1SS16, Schottky barrier diode, Yaesu part number: G2090038http://www.foxtango.kc9foz.com/FT101ZD-901-902/fc902.pdf
Yaesu used this same diode in other Yaesu equipment, such as the mixer unit of the
FT-7 and FT-9xx transceivers.
DO NOT subtitute a generic silicon diode in this CM coupler unit.
A Silicon diode has a voltage drop between 0.6–1.7 volts.
A Schottky diode has a voltage drop between approximately 0.15–0.45 volts.
A Germanium diode has a voltage drop of approximately 0.30 volts.
The lower voltage drop of the Schottky provides higher switching speed and efficiency.
Couplers for amateur radio equipment (Drake, Heath, etc) that used meters (SWR, etc.) often used germanium diodes (1N34A, 1N60, 1N270).http://www.angelfire.com/electronic2/index1/1N270.pdf
Substitution of these germanium diodes with modern Schottky diodes is common.
I use the Central Semiconductor CDSH270, Schottky replacement for 1N270.
This part is readily available from Mouser.http://www.centralsemi.com/PDFs/products/CDSH270.PDF
Small signal Schottky diodes, like the 1N5711, 1N6263, 1SS106, 1SS108 or the BAT41–43, 45–49
can also be substituted for the 1SS16 in your FC-902