The band select lines in the Omni VI versions 1-3 take a rather
complicated route through the radio, existing in several forms. In some
places they are twelve parallel lines, one for each band including the
WARC bands and four separate segments of ten meters. In some places they
have been combined down to fewer parallel lines and when they get to the
25 pin connector on the back of the rig there are only six.
Those six lines are driven by an integrated circuit and one transistor
on the XTAL OSC LO Mixer Board, where they are nine parallel lines,
later to be combined down to only six on the Front End Mixer Board. The
parts that drive those lines are U2 a UDN2985 IC and Q5 a 2N5807
transistor. It appears they used an eight line parallel driver chip and
one transistor as the ninth line driver.
You could probably fix your problem by using some transistors to drive
your small relays, thus drawing less current from that IC and
transistor. Or you might use six lines of another eight line parallel
driver chip. You may want to look up the specs on the UDN2985, and then
when you design your driver circuit put some series current limiting
resistors between the Omni VI band select output lines and your circuit.
Use resistor values that would limit the current to well below the
UDN2985 capabilities even if they went straight to ground. Remember it
is providing current to a bunch of stuff inside the Omni VI, so the
additional current available to the back panel connector is way below
the fully capability of that IC.
Ken N6KB
Dr. Gerald N. Johnson wrote:
> On Sun, 2006-06-18 at 18:47 -0400, PaulKB8N@aol.com wrote:
>
>> This has been an ongoing problem. For the first hour or so, I get voltage
>> off the DB-25 for every band I select. I use this to drive some small
>> relays
>> which, in turn, select the appropriate LC combination on my bandswitching
>> antenna tuners. After about an hour, the 15M voltage drops off and maybe
>> another half hour the 10M voltage drops off. I can sometimes recover the
>> 10M
>> voltage by switching to 12M first, then to 10M. I have this identical
>> problem
>> with both radios, but I don't believe it is my relays that are causing the
>> problem.
>>
>> Paul, K5AF
>>
>
> Probably you are drawing too much current from those ports causing the
> driving device to overheat. Its probably an IC, but I don't know that
> since I don't have the schematic of the Omni 6 Opt 3.
>
>
>
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