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

[Dr. Gerald N. Johnson]

On Sun, 2006-06-18 at 20:47 -1000, Ken Brown wrote:
> 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
> 
Better than a transistor for each line, you might use a logic level
power MOSFET to keep the steady current zero.
-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer