Thursday, June 23, 2016

Attitude Directional Indicator: First steps...

Hey! This time I desired to investigate an analog ADI unit used in a former 737. (See the video at the end !!)

The Unit...

Brand: Collins
PN: 787-6892-003

The device is a mixture of digital and analog electronics, so tat means that expects synchro transmitters signals and simple analog signals.

The insights....

Here we can see an overview of the mechanics parts:

This is the electronics section that is composed by 5 controller boards and 2 power supply boards attached in the back cover.

Mechanics from the top:

A closeup of the flight directo gears:

Some more pictures:

The test bench...

For this first test related with the Gyro inputs, I'm just powering up the unit with 26Vac 400Hz and connecting two synchros to emulate the Pitch and Roll signals.

A video about how this is working:

Sunday, June 19, 2016

EGT Indicator

Hello! I got a engine EGT indicator from a former 737-300.
This unit is very simple and uses thermocouples as sensors.

The unit...

Brand: Smith Industries

The electronics analysis...

This is what I got after analyse the boards inside:

Some testing...

Normal operation:

During BITE test:

N2 % RPM Indicator

This time I'm analyzing a N2 RPM indicator from a former 737-300.
This unit is working in a similar way than the N1 indicator that was reviewed in a previous post.

The Unit...

Brand: Smith Industries

The pinout...

After analyzing the important boards inside, I could take this notes:

Pretty simple to wire it, right?

Test of the unit...

In normal operation should look like this:

During the BITE test:

Applying pulses with a frequency range between 90 Hz and 2.78 KHz, produces this result:

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:


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.


Saturday, June 18, 2016

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

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: