221 2KV high voltage cap broke down when under full load
The complaint of Singer 21 Inch CRT TV was dim display. The brightness control already adjusted to maximum and yet the display was still dim. There are few reasons why the CRT TV can get dim:
- PC Tube already weak which means the internal cathode materials already been used up. This means less electrons will be generated.
- Heater voltage of 6.3 volt from the flyback transformer is low. If there is a flyback problem, the heater voltage will be affected but this cause is very rare
- Low screen voltage (G2 voltage). If the screen voltage is low the display will become dim. And you can adjust the screen voltage by adjusting the knob of the flyback transformer.
- If the CRT Tube is using G1 negative voltage and if this G1 circuit have problem, the display also would be affected.
From my experience, I would usually check on the G2 voltage first (Number 3) to make sure it has the right voltage.The voltage range can be from 250 to 400+ DC volt depending on the type of TV you are checking. If you get very low voltage, obviously the display will become dim or totally no display at all. Below is the internal section of the CRT TV once the cover was removed.
When I check on the G2 point at the CRT Board point (picture below), I got only 96 volt in which it is not enough to brighten up the TV display.
I tried to adjust to increase the G2 voltage by turning the knob located at the back of the transformer (see the photo below) and the maximum voltage that I got was only 107 volt.
I knew something was not right base on the dc voltage that I got.
I followed the G2 wire from the flyback transformer to the CRT Board and found a 221 2KV capacitor that is connected between the G2 line and cold ground.
I took up my meter and check and the capacitance value was still acceptable (within range). But somehow I knew that this type of capacitor have a high tendency of breaking down when under full load. Normal meter can’t test it and you need to direct replace and retest it.
I directly swapped with a good one and powered up the TV and do the testing. True enough, the G2 voltage came back normally (419VDC) and the TV display appeared-see the photos below:
Conclusion- Sometimes in electronics troubleshooting, if you can’t find the cause of the problem, there is a chance that there is component breakdown when under full load in which you can’t test it. It can happen to capacitors, semiconductors and etc. In my case, you can use a special meter to test out the fault in the capacitor. You can get Jestine’s ebook on how he test capacitors with various meters plus there are many more testing tips that you can learn from him. Ok, will see you in the next repair article.
This article was prepared for you by Suranga Bandara who owns an Electronics repair shop in Anuradapura, Sri Lanka.
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