Under/Over Voltage Sensor
 

My model 1968 C Skymaster has a Under/Over Voltage Sensor that is located behind the throttle quadrant. This unit is suppose to sense High Volts and Batt Discharge situations and then lights up a light above the Master Switch. Mine hasn't worked for a long time and I was trying to get it back on line. I have two other units that are "as removed" from Skymasters but can not get either of those to work either.

Has anyone had any experience with this unit? There is a board with resistors and transistors also a capacitor. I tried my board with another capacitor and nope.

See the attachments for info

I'm guessing you did an ohm check for all the resistors? Could very well be a friend resistor that opened the circuit.

Any chance there is no power to the circuit, like a broken power or ground wire somewhere? Would be consistent with no unit working.

There is power to the unit as the light does glow dim. There are pots that can be adjusted for calibration but no luck trying to use those.

I will check with Craig to see if he ohmed the resistors

Thanks guys

Herb:

The most common component in that schematic to fail is the capacitor (C1). These are inexpensive foil caps that tend to leak.

Dave

Tried a different capacitor off one of the "as removed" units on my unit and it did the same thing as my unit was doing. Any ideas where a person can get the capacitor?

Does the test function work?

Dave

Something like this?: http://www.newark.com/vishay/tva1315...itor/dp/18F975

It doesn't look the same but not sure it won't work.........

It sounds like you have a few of these and they are all not working. That suggests its not the units but the wiring in the plane. If you could get a bench power supply that will produce up to 30-32V you could try just connecting it on the bench to the supply and see if the circuit functions as expected. You will have to wire a couple of 24V bulbs onto the Hi and Lo outputs. As the voltage is raised from 0 to 32V I would expect to see first the low voltage light come on, then go out, then the high voltage light come on. The pots are to set the low turn off and high turn on voltage threshold values, but the circuit should function even if they are out of adjustment. If it works on the bench you can start looking at the plane. If it doesn't I may be able to suggest a few things to isolate the problem on the board.