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Clicking sound in CRT TV, any speculations of what's the problem?

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eagle1109

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Hello,

I'm working on CRT TVs repair course.

The one I worked on yesterday has very clicking sound; like 60Hz clicks near the SMPS unit.

The screen charges, and won't start because of this sound.

What could it be?
 

Switching supplies in TV's often go into a hick-up mode for protection if there is an excessive load due to a fault somewhere.
 

Does the screen have pronounced trapezoidal shaped jagged edges if visible? If it's fairly loud 60hz buzzing then could be open mains filter capacitor usually large black one like 200mfd 250V
 
E-design said:
Switching supplies in TV's often go into a hick-up mode for protection if there is an excessive load due to a fault somewhere.
Click to expand...

I don't think there's an excessive load. There's might be a short in the LV area.
If there's a short at the HV area, I would get explosions.

BradtheRad said:
In line with post #2, hiccup mode is liable to drop the frequency down into a slower range, which could explain audible noises coming from the inductor.
Click to expand...

That's most what I think!

I have another working TV, now it has a problem because I switched the boards with the monitors.

chuckey said:
Is the earth on the CRT, is could be a corona discharge due to leakge somewhere.
Frank
Click to expand...

What is the corona?
Do you mean the degaussing coil? I disconnect this cable when troubleshooting, the TV still works without it.


retrogear said:
Does the screen have pronounced trapezoidal shaped jagged edges if visible? If it's fairly loud 60hz buzzing then could be open mains filter capacitor usually large black one like 200mfd 250V
Click to expand...

The HV area works fine.




IMG_20160418_191844324.jpg



>>>>>>>> current situation.

The working TV now has a problem, when I de-solder D838, most voltages go OK except vertical transistor doesn't work. It still receives the 112V, and it doesn't have the switching signal which is coming from the MCU!

When I connect the diode, the LV area goes very low. Which means there is a short CKT I guess.
 

Make sure the zener diode is not faulty. A shorted or very leaky diode will tend to shut down the regulator or keep the output voltage very low.

The 33 k resistor value seems a bit low for me. Without seeing the complete circuit, it will keep the opto on. I would have expected more like a 330 k value for it to have some regulating action.
 
The 16V zener D838 is over voltage protection. If the regulated 112V rises to break over that zener, it turns on Q832 which turns opto isolater diode fully on reducing the power supply output. I agree the zener must be leaky
 
I checked the zener diode, and replaced it multiple times.

I want to know about the other diode which I circled around it and wrote my comment about what's the function of it.

These resistors are the original TV parts, the R837 is 33k on the schematic, and in real time it's 15k!

I don't care much because they are the original parts, so they should work if I solved the problem.
 

I mean D839 which I replaced multiple times.

What should be the voltage on the anode of D838?

So, yes you are right, when I connect this diode, the SMPS 3 voltages on the high frequency transformer rectification diodes go very low!

So, that's because of this diode to be faulty.

I should try to check its resistance with another one.

Also, I should disconnect the opto-isolator and connecting the diode to see what's the action.
 

Is R837 color coded or labeled as 15K ? That value is critical. The schematic is faulty. D833 is drawn reversed. That is a negative supply. Disconnecting the opto-isolator removes any regulation. Removing that could be catastrophic !! The important voltages are the cathode of D840 is a 6V reference. The base of Q831 is a sample of the 112V for comparison. As the 112V rises, Q831 conducts more, lighting the opto-diode which turns on the opto-transistor junction which ends up increasing the off time of the output FET to lower the current being supplied, dropping the output back to 112V
 
Connecting all parts gets all the output voltages to go very low.

D830 gets 1.8V instead of 12V
D831 gets 1.5V instead of 8V
D833 gets 10V instead of 112V

With very low noise at the switcher IC.

When I disconnect D838, most voltages get to the normal values. Except the flyback transformer with the vertical deflection transistor doesn't work as it has the 112V on the collector.

CKT.jpg

FLYBACK.jpg

MCU&AUDIO.jpg

SMPS.jpg


I have another question:
I want to buy replacement parts from Aliexpress.

I'm looking for the RT801 in the models I have in the workshop, they are MZ73 20RM, where there's only MZ73 18RM.

So, are they similar? Is it OK to buy the 18RM?
 

I mean D839 which I replaced multiple times.
Click to expand...

No doubt it gets overloaded due to some other component being bad. You must track down that bad component, or else it will do no good to replace D839 repeatedly.
 
Yes, I think so.

As I mentioned in the previous post that the output voltages go very low when I connect D838.

And get to normal values when I disconnect this diode. Except flyback vertical transistor doesn't work.
 

If D839 shorts out repeatedly, Q830 is bad. Leakage current from collector to base of Q830 will short it out. Also check R832. The 8V reference off Q830 has to be there or 112V supply will lower it's voltage greatly. The 10V you are reading on the 112V supply should be sufficient to produce the 8V out of Q830
 
What about this question I asked ealier:

I have another question:
I want to buy replacement parts from Aliexpress.

I'm looking for the RT801 in the models I have in the workshop, they are MZ73 20RM, where there's only MZ73 18RM.

So, are they similar? Is it OK to buy the 18RM?
 

I would think MZ73 18RM would be ok. The only difference I see is 18 ohm vs 20 ohm for the MZ73 20RM.
 
I fixed one this morning :)

WOW .. feels so awesome!

I couldn't presue more troubleshooting :)

Most the failures I found are semiconductor parts, optocoupler, transistor, power switcher and diodes.

Thank you all for support.

Now, I have to go to the next level, which is to fix more monitors :)

Regards,
 

That's great. If you need help to fix another and can find a schematic, send me a message so I get email notification. I don't check here regularly.
I did CRT monitor service for over 20 years.

Larry G
 
Yes, I have to fix other CRT monitors of the same model.

Thank you very much for offering more support, it feels awesome :)

Regards,
 

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