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Author Topic: Yaesu FC-902 : Meters stopped working  (Read 2361 times)
W9GB
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Posts: 2600




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« on: April 06, 2013, 07:14:57 AM »

Quote
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: G2090038
http://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
--
w9gb
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N4ATS
Member

Posts: 800




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« Reply #1 on: April 07, 2013, 05:00:41 AM »

Very easy fix , replace them with these , this is what I use...

http://www.angelfire.com/electronic2/index1/In34a.html

I replace them quite frequent in meters
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W9GB
Member

Posts: 2600




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« Reply #2 on: April 07, 2013, 06:03:03 AM »

The original 1SS16 Schottky barrier diode is 5 Volts, 30 mA rated to 900 MHz,
with a forward breakdown voltage of 0.50 Volts.
==
The 1N34A and 1N60 are common Germanium diodes, for lower voltage circuits.
Voltage and current ratings for Germanium Diodes still found today.

1N34A : 65 Volts, 50 mA
1N60 : 45 Volts, 50 mA
1N270 : 100 Volts, 40 mA
1N277 : 110 Volts, 75 mA

===
Schottky Barrier Diode, BAT-46
Lower forward break down voltage than a Germanium diode at less than 10 mA.
Forward breakdown voltage is 0.25 , while Germanium diodes are 0.35 volt.
« Last Edit: April 07, 2013, 06:20:16 AM by W9GB » Logged
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