Understanding the x-ray protection circuit in Television

High Voltage shutdown commonly referred to as X-Ray protection circuit shut down the horizontal deflection in the event of a detected faults if that faults causes CRT radiations to exceed acceptable limits.

It is important to note here that HV can increase even if the power supply is okay; a good example is if the safety capacitor (snubber) opens or looses some of its original capacitance.

If any of the above faults occurs in Television, It will cause the Fly back to produce undesirably high secondary voltages. The good news is that as the HV increases all other output from the fly back also increases and therefore this circuit of X-ray protection is not attached to the HV output which could have caused more challenges to the designers but another output of low voltage is monitored.

Usually a voltage reference is taken from the FBT and rectified by a diode to get a DC voltage, this DC voltage is applied to a precision resistor divider, the values of these resistors are chosen to produce correct X-Ray protection trip threshold for each CRT.

Below you can see a simple X-ray protection circuit usually found in CRT Televisions.

If the voltage increases above normal (say 13 Volts) the Zener diode which in our case is a 12 Volts Zener will conduct and voltage will appear at the X-ray in pin. The X-ray protection pin is usually zero volts when things are normal and if this pin get voltage, the I.C internal shutdown circuit will trigger and shut the horizontal out signal and hence shutdown the Television.

This will then prevent dangerous X-ray being emitted which could have otherwise caused health hazard to the user or even break the CRT tube.

Conclusion

I think now you can see why you should never replace the safety capacitor with one with lower capacitance value because this will cause the HV to increase a lot and hence emit this dangerous X-ray radiation.

One sign that the snubber capacitor has lost some of its original capacitance is that the screen usually become narrow (E/W) and sometimes you can even hear cracking sound caused by this excess HV. When you get these symptoms this is an indicator that this capacitor has lost some of its original capacitance and replacing it usually cures the problem.

Wishing you all the best

Humphrey Kimathi

Author CRT Television Repair course

Homemade ESR meter assembled n working

Roger:  what is the purpose of 100k potentiometer in your ESR diagram? what will be the settings, for now I started to assemble the ESR  tester… it’s almost done…the IC TL062 or TL072 is lacking,,,  it’s hard to find the ic…but I’ll try other electronics stores …

Humphrey: The 100K pot is like the zero adjusts of your analogue meter. So it will be used to adjust to zero when using the esr meter attached to your analogue meter. Please try and get any of these two ic...they are very cheap in the market.

Roger:  sir, good day…I finish assembling the esr…but when I switch ON, only little flick…when I short the two leads at the esr… no movement…I used ceramic disc caps,,102m,,,104m,,, is this right sir? I check the vcc pin of ic TL062,pin 8 there is 5 volt there, but I wondering why that no movement or not functioning…I check also the ic pins 5-6-7-8,,jumper,,pls,advise me sir...thanks

Humphrey: That little flicker is ok...now try to turn the P.O.T knob all the way with the  two probes shorted together and see if it goes to zero and come back to me. The 102 and 104 are ok..best of luck.

Roger:  Sir, I try your advise no effect even though I adjust the pot 100k ohms…

Humphrey: Let me have the picture of your assembled circuit both bottom and components side (main esr circuit only)

Humphrey: your pictures are not clear but I am able to pick some few things...

1. Did you cut the 4 track on the underside of the ic 4 pins...i can’t see them.

If they are not that means the ic is doing nothing because it is just shorted on the 4 pins...

2. I can’t see the jumper wire from pin 8 of the ic to 100 K resistor.

3. I see jumpers on the underside what are they doing there.

Finally let me say this…This circuit which I reproduced here is the one I used to make mine and I have not added or subtracted anything and so it should work.

Please note my template is the road map. You should follow it the way it is...


Please if you think you have not followed the template to the letter you can start again because i see the ic is on socket and therefore you can always re-use it...

Again after assembly you will get something like the one I have done because I used the same template to do mine.

Roger:  Good news sir, the esr is working now…I analyzed what you have advised…I think I mislead the jumper at the bottom of ic…, when I looked at the diagram…so I immediately cut the jumper at pins 5,6,7,8,,,is shorted….after that i test its viola sir….thanks for your support sir, I really appreciate…God bless you…,now I can practice testing bad caps……

Compiled  by  Humphrey Kimathi

Author CRT Television repair course

GLD Television with a single horizontal line

This Television belong to a Colleague of mine, She has a phone which she wanted me to check for her which was having some problems and in the course of our chat she told me that she also has a TV which is lying in her house and if I know a good technician who can look at it.

I asked her what does she mean by a good technician and told me she no longer have trust with technicians because of previous experiences with them…I did not like to go into details of her past experiences with techs and therefore I offered myself as a possible option if she wouldn’t mind.

She agreed but the condition was that I must repair it on site, I requested her, the possibility of me taking the motherboard alone with me for troubleshooting and repair in my workshop and come back to her to re-assemble the set in her house.

That option also didn’t work and therefore having no other option left for me, I decided to do it on-site (while – She - wait). Besides from the description she gave, I knew it is not a major assignment as you know with single horizontal line on the screen that directly point to a problem on the vertical circuit.

So I gave her an appointment for the following day after work, in preparation for the day I took a small digital meter(the eyes of the technician), a screw driver, half feet long solder wire, solder gun, solder sucker and a side cutter and placed them in my on site traveling bag.

Also because from my experience with these China made TVs I carried with me some two Capacitor rated 1000uF/35v and 470uF/35v. Both capacitors filter the vertical supply power and they have a very high failure rate.

Come the appointment day and I went to her place ready for the task ahead. The television was brought on the dining table which for now was to serve as my repair work bench.

As usual I had first to power the TV on to confirm what she told me and also to get any other clue from the screen itself. After powering on, the TV came up and indeed there was a horizontal line across the TV screen.

Looking at the line keenly I could see the line was not smooth but had some waves like on the sides. This could points to dry joints or loose connections around the vertical circuit. I tapped the Television softly with my bare hands and the line (horizontal kept on changing) which further indicated the possibility of dry joints.

I opened the set and did some scanning on the secondary side of the power supply on the component side and I didn’t see any suspicious component, then I turned the board to the bottom side and went straight to the vertical ic (LA 78040)

 

Without magnifying glass I could see two big dry joints (I mean like pot holes on pin 2 and 6 of this I.C LA78040) below is the datasheet of this ic.

Looking at the datasheet above you can see pin 2 and 6 supply this ic with the much needed 27 Volts supply from the secondary of the power supply. 

The 27 volts goes through a diode and it is split into two, one line goes straight to pin 2 and the other goes to pin 6 always through a diode and therefore expect a small drop across this diode of about 0.7 volts (so the voltage at these two pins will be almost equal (+ or –) 0.7 Volts of each other.

After re soldering all suspect areas it now time to power up…usually after repair I like powering the set via series limiter bulb but because I never carried it to the site I had to do it directly.

I pressed the power ON button and nothing happened. Even the horizontal line is not there…DEAD. I tried to listen to the signs of life from the TV and I could hear some sound from the secondary side of the television.

What could have happened now? I figured out that because this TV was having a horizontal line before and now it’s not powering on it could mean that there is something I have done wrongly in the course of this repair…and the most probable thing could be while I was Re-soldering the dry joints I could have shorted a components pins or path because the light was not enough to see the solder are perfect.

So without the magnifying glass I used my meter set to continuity range to try to locate the shorting path/component pins. There was a connector I had re soldered and I went straight there to confirm if it was ok. After testing the two pins I could hear the meter beep (the connector had three pins and all the three had beep)

After further investigation if noticed this is connector for 180 volts RGB supply, heater voltage and heater ground. The heater pin and ground should have a beep but not for the 180 volts. So did some cutting between the heater supply pin and the 180 volts supply until I heard no more beep.

Again I powered the TV and this time the TV didn’t disappoint me…it came up nicely.

A word of advice to new technicians.

I would not advice you to repair equipment at the owner place unless you are very confident with yourself. 

You can easily forget and discharge one of those BIG electrolytic capacitor with a screw driver and with the sounds it makes and the sparks generated you might find everybody running out of the house with the fire alarm already activated for fire fighters to come in to put the fire out which they perceive you have caused.

Wishing you all happy repairing.

Humphrey Kimathi

Author CRT Television repair course

Customer support

 Here is a small conversation between me and a new customer Mr Roger from Philippines

Roger : sir, thanks for the reply, I really  amazed by your eBook, it’s very nice as a beginner in this fields of electronics, sir, can i ask you about my 14inch. TV polytron, no audio and no picture, when I switch on only horizontal lines appear from top to bottom ,pls. advise me sir..Thanks...God bless....

Humphrey: For the TV with retrace lines...First adjust the screen voltage on the fly back transformer bottom knob (refer to chap 11 of your eBook and see if they disappear) also check the RGB supply voltage (see details on chap 13 of your book). Please relax and read the book slowly and carefully...don't rush... for the other TV...try another connector for the DVD but make sure yellow connector on TV go to yellow connector on DVD. Yellow is always for video..... Also be sure the TV is set to A/V mode.

Roger :sir, good day, I check already, this polytron 14 inch.tv,,the supply coming from the flyback,,,when I do voltage testing got only 156v, supposed to be 185v am I right? sir?, may I share to you the history when first check this TV, I re- soldered dry joints at the fly back...and i adjust the screen knob, and the TV was ok, but after 5days the fault is back to his former fault, as of now I test also all secondary output voltages but I notice some readings low...I try your advice to check the voltages at the RGB at CRT neck board if i have time......as of now i try to adjust the screen knob, but only the brightness going up and down but no display appear read your book as of now, it’s very nice book sir,,,

Humphrey: Now we are getting somewhere...the 156  volts for RGB supply voltage  is definitely low...this could point to a filter capacitor for that line that has developed high ESR (dry up) please replace the filter capacitor for that line(usually rated 250 volts in your case 4.7uF/250V and change that directly...also if you disconnect one of its leg from the circuit and the voltage goes up means indeed it is faulty...please be sure you have come back to me with the progress. Again remember you adjusted the screen voltage upwards and therefore you will need to bring it down so that the line should fade away.

Attached please see the diagram for more info.

Best of luck Roger

Roger : ok sir, ill check the caps,,,,thanks

Humphrey: This TV should be working by now....see the pic for more details...

Roger : sir, thanks, very nice support, sir I check my TV ,i check the supply coming from the fly back, I trace the e-caps rated 2.2uf/250v,,the track is broken, intermittent, so I fix that line, but accidentally when i switch ON there is smoke little, coming from the e-caps rated 470uf/25v...because my jumper line to the broken track accidentally touch to the 2.2 uf/250v e-caps..I immediately fix the line, but the 470uf/25v is already gone, so I will purchase today to fix again this TV, hehhehe,,i think this is my lesson,,,  but sir, I didn’t see any e-caps rated 4.7uf/250v...in the tv board  ,,,, i see 10uf/250v at the crt neck board only...but i really appreciate your effort for me sir, but I’ll try myself, as of now i continue read your book, it’s very detailed book..if i fix my TV, i inform you immediately.....big thanks...sir.God bless,,,,,.

Humphrey: I can see you are progressing well; sure the caps are usually rated between 2.2 uF/250V to 10uF/250v.... the working voltage is usually 250v...so please change the two caps with the rating of 250V
Good luck ……Still waiting for your success report.

Roger : sir, good day, good news, the tv is working now sir, but before that i replace the caps,,the voltages is 156 only at the crt neck board,,,the output of fbt is only 113v...after the diode and the caps,,it read 156v,,so the tv has re trace lines again after replacements of caps, so I conduct my analysis, cos,i already finish read your book, i check also the voltage at the B positive is 112v,,,but when i check the e-caps of that line 220uf/160v,,its 113v only,,,so i replace also that e-caps,,,and i double check the terminal of fbt...i resolder also that...so i sw. on the tv,,,i adjusted the focus to low,,and turn the screen also slowly to low,the picture appear, now i test run the tv for 4-5 hours is ok,,,,heres the photo sir, thank you sir for the support , now i really amazed the crt book...especially the smps function,,,bcos,,of your book sir,,its very nice information,,,,,,

Humphrey: Thanks for that good reports...I can see you are really getting on pretty well. usually in electronics it is important sometimes to think outside the box...i have noted you are doing exactly that...whenever you notice a lower voltage than expected always suspect a capacitor which has its esr gone up...To test such capacitors you will need an esr meter.

This i highly recommend it to you because it will make your troubleshooting quite easy...if you don’t have one please note that you can make one easily from the last month article from Jestine yong on how to make your own esr meter.

if you find the problem with that tv has come back again..Still it could point to a bad capacitor still not picked up.

all the best