Re: Tek 494AP VR board mystery


Sergey Kubushyn
 

On Thu, 15 Jul 2021, Ed Breya via groups.io wrote:

Eh, there is NO logic needed. Wired OR is done without any diodes using
OC/OD outputs. And it is done like that at the very bottom of the schematic.
On the same 'LS145, just different 2 outputs.

There is no different biasing -- 2N2907 emitter is connected to +15V, 10K
from emitter to base, 10K to 'LS145 OPEN COLLECTOR output. When that output
is high, 2N2907 base is connected to its emitter and that 10K to 'LS145 OPEN
COLLECTOR output is just hanging in the air. Transistor is closed, no
collector current. When 'LS145 output is active (low), that 10K is grounded
and there is base current that makes transistor open fully, to saturation.
2N2907A has beta at least a hundred, so that base current is more than
sufficient for full saturation. Emitter is at +15V wrt ground so a diode
drop makes no difference.

The schematic is classical RF diode switch. No level shifting with diodes.
No special biasing -- it is either ~+15V or ~-15V. Plus/minus couple of
volts makes absolutely no difference.

Please, don't play a guessing game with trivial advices. Just look at the
schematic.

I'm 99.9999999% sure it is just a snafu caused by multiple people and even
departments working on that schematic before it reached manufacturing. There
is no sacred knowledge or black magic there, the thing is trivial.

If it is purely a (TTL) logic section, it could be that a little
diode-logic was added to save a gate package or such, and required a
little different biasing to assure proper levels. Often, a Ge or Schottky
diode could be used with a saturated transistor output to get a valid
TTL-low when isolation is needed, or a regular Si diode could work with
some biasing help.
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