I've had several requests to provide additional information on the Microsemi
UM2110 PIN diodes. Here's the URL pointing to the UM-2110 data sheet:
http://www.microsemi.com/corporate/catalog/partoverview.asp?PARTID=16983
The UM-2110 is an excellent replacement for the three 1N4007 T/R switching
diodes on the Omni Six. While the 1N4007 only offers a quasi-PIN
structure, the UM-2110 is a real man's PIN diode having been specifically
developed for 1KW+ H.F. frequency range, R.F. switching applications. Don't
confuse the spec sheet's diode *dissipation* rating with its true power
handling ability.
The mod consists of replacing diodes D6, D7, and D8 on the Low Pass Filter
Board . Diodes D4 and D5 (MPN3700 ) are true PIN diodes types. These are
fine for the Aux receive antenna switching circuit and need not be changed.
Again, be forewarned that working on this circuit board is no fun; it can be
very difficult to remove. A few years ago, I had the dexterity to make the
change by loosening the circuit board's hardware and extracting the old
diodes with the board still in place. I don't recommend this, however.
It was late last night when I made my initial post and I failed to mention
that some pre-1993 Omni Sixes may be more susceptible to T/R diode failure.
The early generation Omnis used a small signal PIN diode for D6-D8 (perhaps
the MPN3700) and the factory received several reports of failure similar to
what Clark had described. The 1N4007 diodes were a compromise between good
Main-to-Aux antenna port isolation and reliability. I believe the 1N4007 is
the only diode in the 1N400X family that incorporates a PIN substrate.
A vendor/distributor source for the UM-2110 diodes should be available on
the aforementioned web page.
-Paul, W9AC
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