This device senses the temperature difference between its environment (ambient temperature) and any object passed in front of its lens. The focus distance of the bolometer is approximately one meter (1m).The output of the sensor  is an open-collector type and must be pulled high to 24V with a 4.7KΩ resistor to see voltage output.

  • With the help of the schematic diagram shown below I have managed to trace and repair faults on the bolometer sensor.

  • The schematic diagram shows the expected values at every test point so I had to check if the values labeled correspond on the PC board.
  • The steps below describes how  I carried the test.
  1.  I connected the bolometer to the cable plug.
  2. I turned on the power to the bolometer
  3. I measured the 24V supply voltage (the voltage should be between 23.5V and 24.5V.
  4. I measured the +175V and -175V supplies to the bolometer this was done on the test rig.
  5. With the aid of an oscilloscope I connected the bolometer and measure the output Vrms, which is expected to rise from 9V to 24V and later drop down to stabilize between +16V to 17V.
  6. If the output Vrms does not stabilize between +16V and 17V I had to adjust R8 until I get the correct results.
  7. I mount the bolometer on the function generator test rig and start the heat source and set the temperature to 50 Degrees Celsius.
  8. I measured the peak voltage and record it.
  9.  I increased the temperature by steps of 50 Degrees Celsius each step recording the results up to 250.



The AC snubbing unit is designed to drive a store Platt electromechanical brake, used for snubbing the type 63 AC point machine. It can be used for other makes of brakes, provided that the coil power requirements are the same (90V dc, 9 W).
The input to input is taken from across one phase of the motor stator. This is rectified by the diode D1 and charges capacitor C1 through resistor R1. Resistor R2 provides a discharge path for C1 should the supply be removed and the snubbing contacts stay open for any reason. At the end of the stroke, the power supply is removed and one of the snubbing contacts closes.C1 then discharges into the brake winding operating the brake for a short period.

  • I connected the variac to the AC snubbing unit and set the variac to 160V.
  • I set switch S1so that “b” section is closed.
  • I switched on the 160V supply from the variac.
  • I threw switch S1 so that the “a” section closes, and hold it for about 3 seconds and then restore it to the original position. The brake operated and stay operated for at least 1 second.
  • I repeated the same process with the supply set to 220V.

  • The only problem I came across during the test was failure to connect the capacitor polarity in the correct way during assembly.
  • I have fixed this problem by removing the capacitor and connect it in the right polarized way.



This is a thyristor snubbing unit designed to provide reliable contactless snubbing for DC point machines. It is usually suitable for the type 63 and the type B1 machines. It is intended for use with split field DC motors of up to 110 volts normal supply.

  • I insert the DC snubbing unit on the test rig.
  • I connected the multimeter, the negative lead of the meter to the snubbing and positive lead to the positive of the snubbing on one side of the snubbing.
  • I did the same thing to the other side of the snubbing.
  • I switched the 110V power supply on.
  • On the test rig I set the switch to control the first snubbing on and check the output on the meter.
  • The voltage of on meter two decreases until reaches zero.
  • I set off the first snubbing switch and set on the second snubbing switch and check the output.
  • The voltage of meter one decreases until it reaches zero after I had to reset and check if the meter reading of both snubbing is 110V.



The SAM7-200 card consists of a RS232 driver with handshaking, RS232 driver and surge protection and also 100BaseT Ethernet port.


The circuit consists out of a switch mode power supply, linear voltage regulator, RS232 diagnostics port, USB interface port, an ARM7 microcontroller, 24 configurational inputs, 9 status indication LEDs, 1 can driver, 4 general purpose outputs and 4 general purpose inputs.  The general purpose inputs are optically isolated but the general purpose outputs are not optically isolated.  The outputs are open collector outputs.

  • The diagnostic menu is available via the USB connector J4 on the card, so I used the hyper terminal to link the card with the windows machine (the machine should be XP). The hyper terminal also supplied power to the card to power up the microcontroller.
  • I connected the hyper terminal to the laptop and wait for about 30 second for the PC to install the drivers to ensure proper connection.
  • I ensured that the hyper terminal is setup up for buadrate of 11500 kbps and there is no handshaking.
  • I used the computer program called SAM-BA 2.9 via JTAG cable to program or load a bin file to ARM7 microcontroller. Bin file is known as a binary file with an extension (.bin). After I opened the SAM-BA 2.9 program I got pop up window shown below
  • Here I had to select the connection type from the list which had JLINK and also the board type which in my case was SAM7.
  • After I have selected the connect option shown on the above window, I browsed for the bin file and send it to the microcontroller via JLINK connection as shown in the above window
  •  Noticing that the connection was successful I removed the hyper terminal to recycle power. I used the comment at@2 and achieved the window below
  • Achieving this window simply tells me that the program is loaded safely without any error. I followed all commands shown above to test the SAM7-200 card.
  • To fault find on this card I had to know the function of each section as highlighted on the card. I have used alphabets to show each section which are interpreted as follows:

·         A –JTAG interface                             B – ARM7 microprocessor
·         C – USB connection                           D – 100BaseT Ethernet PHY
·         E – General purpose inputs                 F – Diagnostics RS232
·         G – Open collector outputs                 H – CAN driver
·         I – RS422 driver                                 J – Configurational inputs
·         K – RS232 driver                                L – Switch mode PSU
Ø In most cases I was failing to load the program to the microcontroller this was due to power failure.
Ø  So I had to check the orientation of the microcontroller IC before applying power.
Ø  The possible common problem that I experienced was the dry joints of the pins or short circuits as the pins are very close to each other.



  • The first thing I did was to connect the B24 power leads of the test rig to one of the dual power supply channels and set to 24V.
  • I connected B24 (TJ) to the other channel of the power supply and set it to 24V.
  •  I plugged the relay interface card to the test rig as shown above.
  •  These relay interface card consists of various stages that performs each function therefore I will break down each stage to explain how I went through with the testing
I carried out the test using the diagram attached at the back of this page

 Point detection relay
  • I had to make sure the switch CR is set off.
 Point detection related to plus detection
  •  I had to take note that when switch SO_T1 is on, LED si_b_CHR will automatically be switched on which is covered in the fourth step (crank handle)
  • Switch KR_P will also affect LED po_E_P and po_E_F which is not specified it was covered earlier.
 Point detection related to minus detection

Switch KR_M will automatically switch on LED po_E_M and changing state of LED po_E_F which is not specified because it was carried previously. 

Point call sequence and lockout switch
  • Before I started the test I had to make sure switch SO_T2 is on which automatically switch on LED si_b_KR_M.
Status lamp indication driver
  •  I connected the B24 leads to one channel and B24 (TJ) to the other channel of the dual power supply.
  •  I set both channels to 24V.
  •  On the test rig, I set on both switch B24 and B24 (TJ).
  •  I set on switch so_SK on the test rig and LED D14 on the card automatically switched on.
  •  LED po_SK switched on same time as LED D1
 VSD voltage detector stage
  • I had to use both channels of the power supply but now the grounds of the two channels had to be linked together.
  • The channel that I connected B24 (TJ) leads was set to 10V and the other channel of B24 remained at 24V.
  • I put the link on the analog side of the jumper (JP1).
  • On the test rig I set on switch vi_VSD_ON and LED si_VSD_OFF switched off.
  • I tuned down the voltage of B24 TJ from 10V to 0V. The LED si_VSD_OFF automatically switched on when the voltage from power supply was about 5V to 0V.
  •  From the test rig I set off switch B24 (TJ) and vi_VSD_ON, LED si_VSD_OFF remained on.
  • I put the link on the digital side of the jumper, LED si_VSD_OFF automatically switched on.

  • The most common problems I experienced during test were on Points call sequence and lockout switch and VSD voltage detector stage.
  • To trace fault on Points call sequence I used the schematic diagram and followed these steps :
  1. The first thing I did was to check if the correct relays were used with the correct terminals making contact.
  2. I set on switch CR and check if the relay marked CR on the PC Board picks up.
  3. With relay CR picked up I set on switch WCR_P and check if the relay marked WCR_P on the card pick up
  4. Again with the relay CR picked up I set on switch WCR_P and check if the relay marked WCR_M on the card pick up.
  5. I set off switch CR and switched on Lockout and checked if the relay marked Lock_out on the card pick up.
  • I used the schematic diagram of VSD voltage detector circuit as shown on the right corner of the diagram above and carried the task in the following order :
  1. I checked if LED si_VSD_OFF does not switch on and off as mentioned above. I used a multimeter to check IC B1 pin if the is any changes in voltage when tuning B24 (TJ) channel from 10V to 0V on the power supply.
  2.  I checked for possible dry joints.
  3. I checked each component individually for functionality using the multimeter on the VSD voltage detector circuit.

  • This card consists of various sections mostly controlled by different ICs with different purposes I have managed to test and repair faults.
  • However the test was carried in the order below:
  1. I set up the digital storage oscilloscope to measure the microprocessor oscillator frequency. These was done on IC U4 on pin 19 with the expected output frequency of 11.0592MHz ± 0.1%
  2. With the oscilloscope set up to measure frequency I measured real time clock oscillator frequency, on IC U3 on pin 3 the output frequency was expected to be 4.1943MHz ± 0.1%.
  3. I also used the oscilloscope to measure the discrete oscillator frequency, amplitude and duty cycle. The measurements were taken at IC U17 on pin 6 with the frequency of 5.12MHz ± 0.1%, amplitude of 5V peak to peak and 50% duty cycle
  4.  I measured the frequency of the analogue to digital converter oscillator on IC U13 at pin 19 with expected outcome of ± 404 KHz.
  5. I used digital multimeter to measure analogue to digital converter reference voltage which is 2.5V on IC U15 on pin 2 (if I don’t get 2.5V I adjust R6 until the reading is obtained).
  6. The multimeter was also used to measure the two RS232 converters namely U7 and U21 ICs. For both circuits the measurements were carried on pin 2 and 6. Pin 2 must have positive voltage greater than +8V and pin 6 must have negative voltage greater than -8V respectively.
  7. I connected the two inputs of the DIN 41612C connectors (U22) together. Input 1 is on C6 and input 2 on C22.The LED D1 on the card switched off. I measured the output voltage on A5 and got 0V. I removed the link between two inputs and LED DI turned on and the measured output voltage was 5V.

  • The common problems I came across during the test were with the RS232 converter. I was failing to get the required voltages on MAX232 IC (U7 and U21) pin 2 and 6.
  • I have used the multimeter to check if the capacitor values are still in good conditions, the capacitors were likely to get damaged if not the IC itself.
  • To fix this problem I had to replace the capacitors or IC with new ones.


  • The digital input card (S2/DIP22) is used to interface the signals from 32 separate make-to-operate input devices and the S2 system.
  • Each input is filtered and buffered by logic gates with a LED to indicate the state of the input.
  • The scanner module (S2/SCN11) reads this card in group of bits (1 byte).
  • The card is designed for used in a dual highway system.

  • The digital output card (S2/DOP22) provides an output interface between an S2 system and the associated external equipment.
  •  It is capable of storing 4 bytes of data (32 data bits).
  • The card is designed for used in a single highway system.
  •  It recognises its address (determined by backplane strapping) when the correct address code appears on the address highway and the highway Output Control (HOC) line is low.
  • The data present on the data highway is strobed into one of the four sets of latches when the write line goes hThe S2/DOP22 card has a unique card identification built in.
  • Prior to finally strobing the data into the card, the S2 scanner module checks that the card responding is an S2/DOP22 and not some other type of cards by outputting the card address together with the card Identification. 


Now I was dealing with SMT components I followed the guidelines below to achieve the output as shown on the PC Board above.


 I used 60W desk lamp on my work bench and adjusted the light low to the bench to give maximum intensity. The idea was to illuminate the work area adequately and not produce shadow because most faults are visible with the lighting.

Hand tools

  • I did not use sharp ones because they may damage delicate SMT transistors instead I have used flat ended type.
Magnifying glass
  • After populating each PCB I have used the magnifying glass to check if the is no track broken during soldering
  • I also used this tool to check if pins of the microcontroller are not soldered together by mistake since there too close to each other.
Soldering technique

Soldering a two padded SMT component
  • It was always important to check the orientation of the component especially diodes because the polarity matters most.
  • I placed small amount of solder on the two pads. Too much solder can end up damaging the component
  • I slight one side of the component with the tweezer, while on the other hand I apply soldering iron to the pad.
  • Sometimes the alignment of component to the pad was difficult so I had to remove the solder from the pad to avoid causing damage.


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