[TenTec] Omni 6 Opt 3: Problem on 15 and 10M band decoding

[Ken Brown]

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 at 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.
>
>
>