Mangala Wijetilake from Sri Lanka
On these days I am reading your E-book on Troubleshooting of TVs. It is very interesting and several questions have arisen in my mind when reading.
It is very much appreciated if you could reply to following questions.
1. There were some tests which have been carried out by dummy load (250 V bulb). How do i decide the wattage of the bulb. Does it depend on the wattage of the TV set?
No really Mangala, you can even use 40 watts, usually it is not wise to dis-connect the B+ line and test it without the load but if you don’t have the load in place then you can test briefly to check the presence of voltage once the B+ load is isolated.
If you isolate the load and apply power and find the B+ has voltage and stable then you can conclude that the primary side is okay and no need of checking it.
Suffice to say here that this is done when you tested the B+ voltage and find it missing and when you dis-connect it you find the B+ voltage now re-appears.
2. What are the test points on PC board of a TV set to carry out light bulb test excluding HOT and main fuse?
This question is not clear, but hope will be explained while answering other questions.
3. Let say light bulb test has been carried out on HOT collector lead.
Please note that there is the dummy load and the series bulb current limiter, the dummy load is just as the name implies- A dummy load is a device used to simulate an electrical load, usually for testing purposes and the series bulb current limiter on the other hand is again as the As the word implies the main purpose of this gadget is to limit current to a circuit.
From basic electronics we know that current in a “series” circuit is same at all point but voltage is dropped across each components.
Because of this characteristics of voltage being dropped across each component is actually the reason why your electricity supply in your house is never wired in series because only one bulb is enough to drop all the voltage (240Vac or 110vac), if not so all bulb will be dim and other equipment will not even be able to power up.
This disadvantage of series arrangement is also blessing in disguise for electronics technician where it is applied to make the famous series bulb current limiter.
In series arrangement, components in a circuit are connected in successive order with the end of each component connected to the other end of the next component.
With that in mind the best place to get this arrangement in a television set is to remove the fuse (main) and attach a light bulb (60-100 watts) across the fuse holders.
(i). If bulb was dimmed what I can guess?
In that arrangement if the bulb is dim, this means there is no short on the supply and therefore you can even re-install the fuse and continue troubleshooting without fear of having other parts shorting, so dim means okay.
(ii). If bulb was lighted up brightly what I can guess?
If the bulb is bright and does not go dim even after one minute means there is still a shorted component on the supply and therefore you should not dare replace the fuse but continue troubleshooting looking for shorted component(s) you might have missed.
4. Most technicians removed PC boards & bring them to their work benches for troubleshooting. What precautions should be followed by us to avoid from hazards when carrying out troubleshooting in such situation? Because all the terminals like anode, cathode terminals, other leads exposure to air. (Also from X ray protection).
Usually if the Television was dead you can safely get the mother board out of tube and work on it on your work bench without worrying about the tube.
Safety precaution is a must when the board is separated from the tube, same to when attached to the tube…the most dangerous area of the CRT TV is the primary side of the power supply called the HOT area and it is usually marked.
For the anode cap again you don’t have to fear since the board is already dead, but since you aim is to bring it back life you can dis-connect the B+ line so that even if you fix the problem the tv will not fire the high voltage. This is usually what I do but when I am happy the B+ voltage is restored I normally connect it back and power the tv on briefly and if I hear the sound from the anode cap high voltage I know the tv is good to go(okay).
This I do very briefly though
5. Does there any possibility to burst the light bulb?
Oh no, just dis-connect the B+ and attach the dummy load there, this means also if you do something and the B+ is restored you can know if the dummy load bulb lights.
Note: From my point of view the most dangerous area in CRT Television is the primary side(hot area), though this place has 300vdc or 155 vdc at the main capacitor pins, the current is also quite high and this is the killer whereas on the anode cap the voltage is over 25,000 volts dc but the current is very low and therefore what you get here is a nasty shock which can make you fly a bit and still survive. Remember the high voltage electric fence is not meant to kill the animals but to scare them away escaping from the park but better be careful than sorry.