N1 % RPM Indicator

This is another engine instrument which indicates the low pressure compressor speed in percentage of the nominal speed.

The Unit...

Brand: Smith Industries
PN: WL101EED3 

Some insights...

Top picture:

From right:

From bottom:

From left:

Face:

Some mechanics:

How this works...

This instrument needs only a DC power supply between 24V and 28V, and it expects pulse signal where the frequency of this signal will be proportional to the engine rotating speed.

This unit, I guess, works with a magnetic pick up sensor.

A magnetic pickup is essentially a coil wound around a permanently magnetized probe.  When discrete ferromagnetic objects (such as gear teeth, turbine rotor blades, slotted discs, or shafts with keyways) are passed through the probe's magnetic field, the flux density is modulated.  This induces AC voltages in the coil.  One complete cycle of voltage is generated for each object passed.

This is why the instrument has a differential input to measure the pulses.

When the sensor coil is "open" (this means broken) the instruments shutdown the LED display indicating a failure.

Normal operation with the N1 limit bug in manual mode:

Normal operation with N1 limit bug as an external signal (here in zero):

The dashed display indicates that the N1 limit has not been configured yet. 

During the BITE test:

Applying some pulses...

I just have a very simple pulse generator and I'm applying a range of frequencies between 90 Hz and 2.78 KHz.

 

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...

TAT / SAT / TAS Indicators

So this is a typical 737 unit that displays three important values in the same device.

TAT: Total Air Temperature
SAT: Static Air Temperature
TAS: True Airspeed

Some info about the unit...

Brad: Smith Industries
PN: 2261-02-1

How this works...

This unit needs 26VAC 400Hz power supply, and it is fully digital.

It is featured with one Arinc 429 low speed bus input (12.5 KHz), which expects three Labels.

The Labels (written in Octal) are:

TAT: 211

SAT: 213

TAS: 210

Pinout info...

This unit is easy to open and doing back engineering I got the following:

A picture of the device during the BITE test:

Hydraulic Pressure Indicators

Hey! I got two Hydraulics indicators, one is indicating the brake pressure and the second one indicates the pressure of the systems A and B.

Some info...

Type: Hyd Brake Press
Brand: U.S. Gauge
PN: SRL-0C7EM

Type: Hyd Sys Press
Brand: U.S. Gauge
PN: SRDL-0C7E

The back part of the instruments looks like this:

How they work...

These two indicators are fully analog and they are driven directly by Servo Receivers.

So I opened them and here are the schematics:

These two where the easier ones.. hehehe!!

Oil Temperature Indicator

This post is about an Engine oil temperature gauge.

Some info about the unit...

Brand: The Lewis Engrg. Co.
PN: 162BL804A

Some pictures...

Unfortunately this is another unit that it is sealed, so again I had to cat the housing to be able to get an schematic.

This is a totally analog device, so it wasn't so much effort to figure out the pinout.

The pinout...

Later I will make a curve to see the relation of the position of the pointer with the input current.

Digital Chronometer

Hey! I have a new Digital Chronometer from a Airbus A320, which is similar to the used in the Boeing 737.

The Unit...

Brand: Smith Industries
PN: 2610-13-1

Likely this unit doesn't work with AC power source, so a normal low DC power supply can be used between 24 and 28Vdc.

Lets see inside...

Here you will see the power supply of the device.

This board is providing 5V to the digital bus and aprox -24V (negative) to drive the LCD display.

And this is the processing unit.

Perpendicular to the processing unit and the power supply there is another board that has all the drivers of the LCD display (CD4056B).

This device is also providing an Arinc 429 output with all the information regarding the UTC time, Chronometer Information and Date.

Later I will evaluate the proper Arinc Labels that this unit is sending!!

Repare process...

The first time that I powered up this unit nothing was happening. I checked the power supply and was working nice and the microcontroller (MCU) was receiving the proper power feed.

After further checking I could see that the RESET signal of the MCU was active as pulses all the time, which is not ok... that means that the MCU is being reset periodically not giving it the chance to execute the code inside.

I did a schematic of the reset circuity and it was clear what was happening.

The signal /RES_IN is one pin of the TL7705 which is a Supply Voltage Supervisor used to manage the RESET condition of the MCU.

The circuit above is a very simple oscillator that generates RESET pulses.

If the MCU starts properly, the firmware is going to clear the proper pin which is going to produce the reset oscillator to stop, so then the circuit stops generating RESET pulses.

All of this was working properly, so then after this I could find the mayor issue... the main oscillator of the MCU was not working!!! so that means that the microcontroller was not able to execute any code.

A picture of the broken part:

So I couldn't find the original part, but this oscillator has a standard oscillation frequency (3.6864 MHz), thus it was easy to find a replacement.

Here you can see the board with the new part:

Due the fact that the footprint of the new oscillator was quite different I had to place it in other location... even better because it is closer to the MCU... kidding!

The Pinout...

So this is what I got after tracking the boards:

The Unit working...

First power up looking:

At this point the device is expecting that we introduce the first UTC data in order that the device can start working.

Chronometer function:

This feature starts pressing the button CHR ones.

If this button is pressed again the chronometer stops... pressing a third time will reset the counter.

Unit in normal operation during the flight:

The down-left switch is set in RUN position, that means that the Flight Timer is running, and this value is being shown in the ET display.

A quick video showing how that works :)

Enjoy!!

Oil Quantity Indicator

Ok.. I got two Engine Oil Quantity Indicators and this is the way to find them:

Brand: Simmonds Precision
PN: 10031-0000-01
Boeing PN: 10-60722-8

This how this part looks like:

This unit works with a power supply of 115V 400Hz and I believe that it is measuring the level of oil using the capacitance method, I'm really not sure at this point.

Let see inside:

As we can see it is a pure analog device.

After doing back engineering I could discover this pinout:

Again I don't have a picture of the oscilloscope when I tested this, but the pins 2 and 3 are 400Hz sinusoidal outputs.

The pin 3 provides a sine wave where the amplitude is proportional to the position of the pointer.

This signal is compared outside of the instrument with the signal provided by the pin 2, which is a fixed amplitude sine wave.

I will upload some videos of the system working.