Fuel Flow/Used Indicator

I have another interesting engine instrument from the 737-300. This is a Flow meter indicator that provides two indications... by default the fuel flow rate and as a second option the consumed fuel.
So basically this instrument integrates the fuel flow rate and stores the consumed fuel by the engine.

Info about this gauge...

Brand: Smith Industries
PN: WL203EED4

Lets take a look inside...

This picture shows very clear the DC motor plus the encoder and the signal conditioning board.

This is a closer look of the mechanics... as it can be seen it is pretty simple!

This picture shows 4 electronic boards.

From top to bottom, the first one is the signal conditioning board; the second one is the processing unit; the third one is the digital display driver and control unit; and the last one is the power supply and DC motor driver.

Some repair...

I currently got two of this units, but it turned out that one of them (the one in the picture) was not working at all, so I had to start looking for the issues.

I must say that it took me a lot of time to got it to work because there were 6 issues in this unit !!!!

I will explain them, just in case you have some similar issues.

First step that I did was to disassemble the back board which implements the input power protection and power lines filtering.

On that board we can see a circuitry like this: 

This is a simple implementation of a regulator plus some line filtering.

So the first issue that I found was that the power transistor (2N5497) was burned out... so I bought a replacement named as BD247C.

But if this input protection is broken the power supply should be checked just in case that it is burned out as well... so that is what I did.

This is the power supply assembly:

After analyzing the complete board I could identify the following components:

So these are the main components on this board that should be verified.

I found out that all of them were in bad condition, for example:

The power MOSFET was presenting a short circuit, so I put this component aside and I applied power again.

Then the power supply controller (SG1524J) was not oscillating and was not providing the proper 5V voltage reference, so that means that this IC was burned out as well.

So I bought a new set of MOSFET plus the controller (Very difficult to find!!!) and still was not working !!

Then I could verify that the voltage monitor integrated circuit (LM324) was not performing properly so I had to replace it.

I also found out that the zener diode that feeds the voltage monitor IC presented short circuit situation, so I had to replace it as well.

After this the device was starting properly, but the display was not working !! but I could see some blinking in the moment of power down the instrument.

So the device has a light senor just right up the number 1 (you will see it in the first picture) that is regulating the intensity of the LED display.

Observed schematic:

Doing back engineering of the signal conditioning board I found out that one resistor was "open", it is marked in the following picture:

Hopefully the colors on the old package still were visible to I could identify that it was an 100K normal resistor.... so I replaced this componente and WORKING AGAIN!!!!

Pinout and other schematics...

Unit powered up...