CRT MONITOR 21 inch repair

Rather a strange idea replacing the LOPTX with no indication it might have been at fault?, particularly as it almost certainly wouldn't cause your fault anyway.

1: Check for HT on the collector of the LOPT transistor - usually something like 100-150V.

2: If that's OK, then check for line drive on the line output transistor.

3: If no line drive, check that any safety circuits haven't tripped - there's almost always an over-voltage protection circuit, called x-ray protection in Japanese manuals, and this removes the line drive if triggered.

I presume you have a service manual?.

Hello friend, first of all thanks for your response.
I have spare parts for this monitor so, first of all i assumed that the LOPT transformer was bad so i replaced it. After replacing it i realise that even the heater for the electron gun is off so that means something is cutting down those power lines. Next i replaced the TDA4854ic because it is the main chip for the deflection and it controls the horizontal output transistor. And same story, the monitor powers on, degauss works perfectly, relay works, green LED, but no screen and obviously no EHT and heater not glowing. I measured the B+ voltage and it is perfect, 146v power line good, 81v power line good, 15v good, 12v good. And i am now stuck i do not know what to do.
Now, you wrote me. I checked the HT as you say on collector of LOPT transistor. This is not very clear to me what did you mean but i assume that LOPT transistor is the horizontal output transistor. The collector on horizontal output transistor is 0 volts.

I do not know what is line drive on line output transistor so i am pleasing you to explain me.
i will now send you the service manual for this monitor and i am hoping that you will help me to repair it.

Thanks a lot, and sorry for my a bit late response.

Service Manual:

CRT_Enthusiast said:

Hello friend, first of all thanks for your response.
I have spare parts for this monitor so, first of all i assumed that the LOPT transformer was bad so i replaced it. After replacing it i realise that even the heater for the electron gun is off so that means something is cutting down those power lines. Next i replaced the TDA4854ic because it is the main chip for the deflection and it controls the horizontal output transistor. And same story, the monitor powers on, degauss works perfectly, relay works, green LED

Click to expand...

Randomly changing parts isn't the way to repair anything - you need to fault find, locate what's wrong, and replace it.

So far you've changed two parts, neither of which had much probability of being responsible.

The CRT heater is usually fed from the LOPTX, so if the line isn't running, the heater won't be on.

Do you have a multimeter?

Do you have an oscilloscope?

Do you have the service manual for the set?.

Quick edit:

IC's 7601 and 7602 are involved with the over current and over voltage protection, check the voltages round those to see if either has shut it down.

Sorry i have not finished my message and accidentally pressed Post reply, but you saw my edit. Anyway i apologize for that.
I have multimeter but i do not have oscilloscope.
I will check the voltages arround those 2 chips and will reply back. Thanks a lot.

So according to the manual, the ic 7601(uc3842) is the chip for xray protection so i measured the voltages of that chip.

Pin 1 - Compensation - 0.15v
Pin 2 - voltage feedback - 0v
Pin 3 - Current sense - 0.46v
Pin 4 - Rt Ct - i assume this is measured with oscilloscope but i do not have one so i measure the voltage - 0.15v
Pin 5 - Ground
Pin 6 - Output - 0.1v(as soon as i turn off the monitor it jumps to 3.24v and the discharges to 0v)
Pin 7 - Vcc - 12.53v
Pin 8 - Vref - 0.73v
All voltages measured with respect to ground(pin 5)

I suppose this chip is not outputing the signal to drive the mosfet for some reason or maybe i am wrong, this is just my opinion. What shall i do next ?


I must mention that i have another monitor of the same type and i know that on that monitor the xray protection kicks in because on startup i can hear the high voltage cracking sound for about 300-400ms and then xray kicks in and shuts down the EHT. I am not bothering with that monitor at the moment, i wanted to tell that on the monitor that i am trying to fix i cannot hear the EHT at all so i assume that the xray protection is not kicking in and something else is the problem.

However, i am voting for your suggestion what shall i do next. Thanks.

Pin 1 appears to receive the input from the OVP circuit, and should be 3.2V according to the manual, but is pulled low if the OVP is triggered.

Try the voltages round IC 7602.

Hello

voltages around IC 7602(LM358) are:

Pin1 - Output A - fluctuating from 4.4v to 4.6v
Pin2 - Input A - fluctuating from 4.4v to 4.6v
Pin3 - Input A - 4.7v
Pin4 - Ground
Pin5 - Input B - fluctuating from 0.3v to 1.5v
Pin6 - Input B - fluctuating from 6v to 6.5v
Pin7 - Output B - fluctuating from 6.5v to 7v
Pin8 - VCC - 11v
All voltages are measured with respect to ground(pin4)

I am waiting for your response. Thanks in advance.

Well I'm puzzled how pin 5 can be low when pins 6/7 are high, as it's a non-inverting buffer.

However, I would suggest trying temporarily lifting one end of the two diodes 6606 and 6607 out of the board, this should remove the protection.

I pulled out diodes 6606 and 6607. Nothing changed, still no high voltage and heater not glowing. I hoped that by disabling the protection the monitor should give picture, but still nothing has changed, same problem. I do not know what to do else. What is your next suggestion ? Thanks in advance.

Did you start by following power through and out the power supply?

Can you be more specific on what you mean by that ?

CRT_Enthusiast said:

I pulled out diodes 6606 and 6607. Nothing changed, still no high voltage and heater not glowing. I hoped that by disabling the protection the monitor should give picture, but still nothing has changed, same problem. I do not know what to do else. What is your next suggestion ? Thanks in advance.

Click to expand...

Buy an oscilloscope

Yes, i know that the oscilloscope is needed for this kind of job but i cannot afford it. Anything else to try ?

CRT_Enthusiast said:

Yes, i know that the oscilloscope is needed for this kind of job but i cannot afford it. Anything else to try ?

Click to expand...

No, I'd look to see what signals are around the uc3842 with a scope. Really a scope is essential for electronic repairs.

I have one last suggestion if you are willing to help me i will be grateful.
Let's assume that the signals are bad. I have spare parts for this monitor. Can you tell me what components should i replace for signals to become good, no matter if i need to replace 50 parts, i just need to get it working.
I do not understand well this kind of electronics with Frequency inputs and stuff like that because i am only 24 years old. I can repair all kind of stuff with dc low voltage circuits but this is totally whole new world to me. I do not know even to use scope.
Again if you are willing to tell me everything i should replace i will do it.
If not,again, thanks for this help i respect it.

Sorry, I can't just randomly pick components without any information - been 24 years old isn't a problem, I started repairing electronics professionally at 16 years old, which was probably the usual age.

To repair anything, you have to understand how it works, otherwise you're just randomly swapping things, and likely to cause more problems than you fix.

I know that very well, but in my place, there is not anybody that knows something and if it knows it won't tell anything and i want to learn. However, thank you a lot.

Have you been studying the PDF you posted?