[VAC] Control panel tank levels

[John Leggett <pklhead2@xxxxxxxxx>]

Hello all-
I'm attempting to rejuvinate a 1968 Control Panel to
get the tank sensors to perform, but I'm running into
a problem with the water tank level monitoring
circuit.

The water tank sensor has four leads to it - a low
voltage supply line going to the probe and three wires
corresponding to each of the green, yellow and red
indicator panel lights coming back.  I have taken the
tank sensor out of the tank, and wired it directly to
the terminal strip in the panel (to take the trailer
wiring out of the circuit). When I press the test
button all three lights light up even when the sensor
is completely high and dry.  Continuity checks of the
sensor look OK - NO shorts circuits across the wires
to each other.

There are three NPN transistors on a small circuit
board, each acting as a switch for the 3 sensor wires
to control.  Can anyone ID these transistors and a
replacement source/part number for me or am I barking
up the wrong tree?

I'm also curious why the holding tank sensor omitted
the transistors? (it works fine.)  Perhaps taking the
transistors out of the circuit would be the best
solution?

Has anyone replaced the whole setup with another
monitor that has a remote output capability so you can
drive the original panel lights?

Any and all input would be welcomed.
John Leggett
San Francisco
68 Safari