On Sun, 2009-07-12 at 09:26 -0400, Mike Bryce wrote:
> the counter-on-a-chip is a MK50398N
>
> the multiplex chips are 75491APC and 75492apc
>
> there's two 491's they are the segment multiplexer and the 492 is the
> display multiplexer
>
Actually both are simply drivers, one open collector, the other open
emitter. One for driving LED cathodes, the other driving LED anodes.
> ( I guess technically, the 50398 does the multiplexing, and the three
> other chips are used to decode and drive the LEDS)
Only drive.
>
> both the 75491APC and 75492apc are not stocked anymore by ten tec.
> These chips are really hard to come by, but again, there's the part
> miners out there that have them listed, but with $250 min to buy.
>
> I did a quick look at NTE and they seem to have a replacement for the
> 491, but have not looked deeper into the project.
Jameco has 75492. Mouser has NTE75491, under $5 each.
The MK50398N is more elusive. I found a decent data sheet. Its not a
speedy part, limited to 1.5 MHz input, but its complex, 6 decades
putting out 7 segment and digit drive signals which are amplified by the
75491 and 75492 on the multiplexed display. And it uses all 28 pins of
the package. The MK50399N is similar, just BCD output instead of 7
segment so it would need a BCD to 7 segment driver which used to be a
common part.
If there was enough demand, it would be practical, I think without doing
it, to work up a circuit using a PIC chip with many I/O pins
(PIC24FJ48GA004-I/PT has 35 I/O pins) to emulate it. It likely would
require an adapter board holding the PIC chip and its auxiliaries, like
its own clock oscillator. Since the 50398 doesn't set the gate interval,
there has to be more logic driving from some frequency reference that
sets the gate period to make it work. Then a PIC with an internal
reference could work from that gate control signal, but loss of that
gate signal to the 50398 would keep it from working. Is there a display
diagram available on-line? And there has to be a mixer to mix the PTO
frequency down to the counter's range. Failure in any of these circuits
can prevent the counter from working.
>
> there's three possibilities here with a dead Ten Tec Omni LED display
>
> 1. Get another junker for parts
> 2. Use a external freq counter
The band switch selected preset (which is a capability of the 50398)
complicates using the general purpose frequency counter.
> 3. The dead one becomes the part donor
>
> and...
>
> there's a few places that sell "digital dials" that maybe used,but
> none that I have seen drive LEDs, they are usually for LCD displays.
>
>
> Mike, WB8VGE
73, Jerry, K0CQ
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