Tek 494AP VR board mystery


Sergey Kubushyn
 

Can somebody come out with any reason for the CR3068 diode on VR 2nd Filter
Select board (left bottom corner on the schematic on page 185 of 494A/AP
Vol.2 Service Manual)?

There are 5 filters there, each having same exact switch on input and output
made from couple of diodes and a 2N2907 switch applying either +15V for ON
or -15V for OFF via 64uH inductor thus forward/reverse biasing those diodes.
Two switches per filter, on input and output so all inactive filters are
disconnected from both ends.

The switches are driven from an open collector 74LS145. Same filter is
inserted for 1MHz and 3MHz (it is a pass-through with only gain adjustment
potentiometer -- it is SECOND stage filters so actual filter is implemented
in the first stage). To make it happen they tied 'LS145 pins 2 and 9
together so either one would pull the 2N2907 switch bases low thus inserting
that bypass "filter" for corresponding BCD code.

The 100kHz/10kHz/1kHz filters are all separate so 'LS145 3/4/5 pins each
control their own pair of switches.

Then, 10Hz/100Hz share the same filter, exactly like 1MHz/3MHz, but this
time it is an actual filter. That means 'LS145 pins 6 and 7 should be tied
together to implement wired OR exactly like it is done with pins 2 and 9 for
the 1MHz/3MHz one.

Tying OC outputs into wired OR is perfectly fine when active level is LOW,
it is done all the time and this is what 1MHz/3MHz have. However for
10Hz/100Hz they put a DIODE between corresponding pins that is equivalent to
a short pulling the Q2020 and Q8035 bases to ground when either pin 6 or 7
is low.

But that is not all -- all switches are identical with 3.3K resistor to -15V
EXCEPT the one implemented with Q2020 that has 2.2K instead of 3.3K with
everything else EXACTLY identical. That also doesn't change anything as all
those switches are DC-blocked by 10nF capacitors on both sides of each
diodes/transistor switches. Diodes are also identical so there is absolutely
no reason for 10Hz/100Hz part to be any different from 1MHz/3MHz one.

Am I missing something?

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