http://londonstransport.proboards.com/

Testing the wallwasher LEDs




The two photos below show the underside of the PCB (Fig WTL1) and the top of the PCB (Fig. WTL2). Refer to the circuit diagram to see how the LEDs connect.


The LEDs are the common “through-hole” type - i.e. the LED “legs” go through holes from the top of the PCB and are soldered to the underside of the PCB. The PCB is double-sided, which means that there are tracks running on both sides of the board. As a consequence, this means that the LEDs are soldered to both sides of the board, whether they actually connect to to anything on that side or not. This can make the LEDs more difficult to replace as the solder at the LED joints has to be melted on both sides of the board to remove the LED. Because the LEDS are packed closely together, it is very unlikely that you will be able to use desolder wick or a solder sucker to remove the solder from the joints on the top of the PCB and the only way may be to soften the solder at the joint on top of the board by heating the two LED joints on the underside of the board and, at the same time, gently trying to pull the LED free. The two holes can then be cleaned up.


Remember

As when removing any soldered component, be careful that you don’t damage the PCB track. When the solder is heated, it’s possible that the glue on the track might heat as well, causing the track to lift from the PCB. Normally that isn’t a problem as the it will stick back as it cools down. The damage occurs if you are pulling a component away and the solder hasn’t melted enough and is still stuck to the component. Instead of the “legs” pulling smoothly up through a molten blob of solder, you may find that you are pulling up a sticky blob of solder that is stuck to the leg and the track. The track may crack and part of it come away with the leg. You will then have the problem of trying to find a way of repairing the track (impossible in this case) or trying to find an alternative way of connecting the new LED leg to the circuit. Whilst not impossible, it may be very time consuming and is something you want to avoid.



Testing the LEDs. The wallwasher light LEDs can be tested in the same way as the sidelight LEDs, except that you will be doing the testing on the underside of the PCB rather than the top. This may make it harder to see if the other LEDs, or a particular LED, in the chain light up. The best way may be to lie the PCB face down on a mirror when you do the test. That way, you will be able to see a reflection of the light and you will have easy access to the underside of the PCB.


As always, be extra careful when testing so that you don’t short out anything with the probes. The solder pads are very close together. If using a multimeter, make sure that it’s set correctly. If the meter is set to read current and you connect the probes, this will effectively create a through path between the probes with no resistance and would be the same as if you just stuck a wire between the two points you were testing and there is a good chance that you might damage more LEDs or other components. (Been there, done it. Blew all the LEDs in a group as a consequence of not checking the meter setting!).


When testing the LEDs with a diode tester, the black COMmon probe should be connected to the LED cathode, and the Red probe should be connected to the LED anode.


see the Sidelight / Testing LEDs page for details of how to test the LEDs.

























Fig. WTL1:  The wallwasher light LED groups and their connections from the underside of the PCB























Fig. WTL2:  Identifying the wallwasher light LEDs



A better quality PDF version of this is available from the Downloads page