Re: Tek 494AP VR board mystery

Sergey Kubushyn

On Thu, 15 Jul 2021, Ed Breya via wrote:

It is pure logic, ON/OFF. 'LS145 pin active (low), +15V forward bias, pass
diodes in full conductance, those shorting to ground reverse biased i.e.
OFF. 'LS145 pin inactive, -15V reverse bias, pass diodes OFF, those shorting
to ground forward biased i.e. input (after pass diode) shorted to ground,
output (before pass diode) also shorted. Both sides of switches DC blocked
by 10nF ceramics.

There is nothing fancy there. Those are just switches that are either ON or
OFF connecting one of 5 filters between IN and OUT. No fancy p-i-n diodes,
no active parts besides those switches. All diodes are just dumb rectifiers,
not even fast ones (think 1N4001). All filters are totally passive, either
LC or crystal. 10/100Hz has +15V power for thermal stabilization (oven). 4
out of 5 have a trivial gain control (trimpot as variable divider). The
fifth one doesn't have even that, its output level is used as reference to
set other filters' gain against.

There is a dual stage gain block after that (post-VR amplifier) on a
separate board but there is also nothing fancy there -- fixed gain medium
power output stage with 8th order LC filter after it preceeded by a single
variable gain stage. Both are regular common emitter amplifiers with
variable stage using a trimpot in parallel with emitter degeneration
resistor to set required gain.

There are fancy p-i-n diodes and whole lot of other interesting stuff in
494AP but not on that board.

I'm almost 100% sure it is just a snafu that happens all the time and was
quite widespread back then when CAD didn't exist yet but leave 0.00001%
doubt of missing something.

It is probably like e.g. very good vintage Kenwood KA-9100 amplifier where a
small droplet of ink made its way to one channel 10K resistor on the
schematic and made it 1.0K. This went all the way to production and all
those KA-9100 have 10K resistor on one channel final power amplifier board
and 1K on other channel.

BTW, it is not that I need this as a repair hint; it is just out of

There is another fancy thing -- IN has a ground bias return make of series
connected 64uH inductor and TWO 330 Ohm resistors while OUT has a single 680
Ohm resistor instead of those two 330 Ohm ones. Once again there is no
particular reason for that. There is no high voltage there (-2dBm level AC
signal, DC blocked by a 10nF ceramic capacitor), no high power, no
especially high frequencies (it is 10 MHz IF) and those resistors are crappy
1/4W 5% carbon film ones.

I don't know the circuit here, but you may want to look at the logic and
PIN control circuits and circuit descriptions in more detail. Sometimes a
couple extra dB attenuation or passing, depending on the situation, makes
things work much better. It may boil down to turning on a PIN harder, to
short signals to ground, or to get a little more through signal, in
certain spots. Also note that in many setups, the Q (hence BW) of the
crystal RBW IF filters is set by variable PIN RF resistance (DC control
current), so that aspect has to be handled in the overall biasing scheme.
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