Over the years I have accumulated a lot of surplus electronics. I had a bunch of these small PCBs and I had no memory of where they came from. The only marking on the board is "AMPRIX 3-1391". A Google search for Amprix doesn't turn up much, but I did find a few useful pieces of information. First I found a surplus site selling an Amprix voltage regulator.
I also found this site which says this was a company from Texas that existed from 1981 to 1984.
https://opencorporates.com/companies/us_tx/0054892500
Finally I found this quote:
“Amprix designs and builds electronic components and products for the automobile aftermarkets . “The board only has two inputs, power and ground, and 4 LEDs, so my assumption was that this was some sort of voltage monitoring device.
Here is the schematics I traced from the PCB.
The D27 zener diode creates a reference voltage which goes through a resistor network to one input of three LM339 comparators. There is a also a variable resistor, R22, to calibrate the reference voltage. The other input of the comparators goes to the supply voltage. With this setup LED 8 and 20 will come on around 5 volts, 8 will come on alone around 9 volts, all the LEDs will go off around 11 volts and finally 14 will come on at around 13 volts. Based on the mention of automotive components and all the LEDs being off around 12 volts (auto battery voltage), this will likely a device for monitoring car batter y voltage.
The one mystery with this circuit is LED D20, I have found no scenario which turns this one on. It is hooked to the top LM339. The non-inverted input come from the supply voltage and the inverted input from ground. For LED D20 to turn on the non-inverted would have to be higher then the inverted, and I am not sure how that could ever happen.
My one theory is that the circuit if actually designed incorrectly. Sine I have a bunch of these they were likely surplus components, so maybe these were defective and sold as surplus.