Monitor Repair - Advice Needed

[MaxRambone]

I'm trying to repair an old monitor for my uncle. I realize monitors are cheap these days and this one is so old that it doesn't even have HDMI, but I love trying to repair things like this as often as I can to expand my knowledge and skill.

So, when you first plug into mains the light will come on and sometimes the screen even comes to life, but it will quickly turn itself back off. From my somewhat limited experience, I assume immediately that this is a PSU problem.

I did the usual visual check and all the caps look fine, so I decided to start with checking the DC voltage out of the bridge rectifier. It's putting out a tad over 100 volts DC and I do not think that is what it should be putting out. I looked up the number on the BR (KBP06) and this was the closest thing I could find:

https://media.digikey.com/pdf/Data%20Sheets/Vishay%20Semiconductors/2KBP005M-10M,3N253-259.pdf

Although, these have the letter "M" at the end and mine does not.  I assume that the voltage I'm interested in is the RMS voltage.

This one says 420 volts. My concern then is that the main cap is rated at 400V, which is probably within tolerances.

Does anyone have any advice? Do you think that this datasheet is accurate to my component? Is the 420 volts just like a maximum that it might only rarely hit? Is that why the big capacitor is only rated for 400v? Could another faulty component cause the output voltage to drop?

In any case, even if I can fix this, I will probably give him one of my old monitors since he will have it hooked up all the time and I only use them for repairing other people's computers. I would hate to think that something I did caused a fire or worse.

Thanks for reading.

[helius]

1. Poking around inside a live power supply unit without knowledge of all the dangers is a great way to get yourself killed.

I did the usual visual check and all the caps look fine, so I decided to start with checking the DC voltage out of the bridge rectifier. It's putting out a tad over 100 volts DC and I do not think that is what it should be putting out.

2. The voltage from the output pins of a bridge rectifier is not constant DC. However, using a typical (non-isolated) oscilloscope to measure this circuit is dangerous and can destroy the equipment, or you.
3. The first stage of many "universal" PSU circuits acts as a voltage doubler so that the 120 VAC line voltage is converted to around 340 VDC. This is a potentially lethal shock if you touch any live circuit on the primary side of the transformer, and is even dangerous after the PSU is unplugged: the primary-side capacitor can hold a lot of charge.

I do not suggest learning electronics by poking in mains power supplies.

[MaxRambone]

Thank you for your concern, but I'm well aware of all the dangers.

I use a current limiter, I check caps before touching the PCB, and I have large resistors to discharge caps when needed.

This isn't my first rodeo. I've fixed quite a few electronic devices. I have many tools and some knowledge. I'm just not the best at diagnosing what the problem is.

it's easy when it's a swollen/burst cap or some fried component on the PCB, but when something just fails with no physical indication, I'm kind of lost.

BTW, I'm 47 years old and been tinkering with this stuff for at least half of that.

[inse]

You can easily check the bridge rectifier with a multimeter for an open diode, but usually they fail as 'short'.
I would suggest the good old hairdryer method on the PSU: heat it up to see whether it starts more reliable.
If so, most probably electrolytics are at fault.
The shutdown of the PSU can also be caused by overcurrent on the secondary, which needs more investigation.

[MaxRambone]

I did the multimeter test and it tested as expected, but it only outputs about 105VDC which seems very low to me. I even removed it and checked the voltage out of circuit.

[inse]

Switch your multimeter to AC and check for ripple voltage, maybe the DClink capacitor has failed.

[tunk]

If it has a bridge rectifier, filtering and a smoothing capacitor, then
you should read 150-160Vdc (110Vac * 2^0.5). As inse suggests,
the capacitor could be bad, or maybe it has a bad soldering joint.

[madires]

CRT or TFT with CCFL backlight?

[helius]

Thank you for your concern, but I'm well aware of all the dangers.

Your post doesn't give me that impression at all, particularly with all the questions about very basic things.
A bridge rectifier is simply a network of 4 diodes. Since diodes do not have any gain, they cannot "put out" a peak voltage higher than what is "put in" to them. Being familiar with how each component is built and its possible failure modes is part of safety awareness, and also part of understanding how to test, measure, and repair electronic devices.
Since the bridge rectifier is just 4 diodes, the proper way to test it out of circuit is to measure the voltage drop on each leg using a DMM's diode mode. Each silicon diode has a drop of around 0.450 V at the DMM's test current. When the direction of current is reversed, each diode should read OL. The possible failures are one or more diodes shorted (0 V) or open (OL in both directions). This is really all you need to do to verify a bridge rectifier is working as designed.

[strawberry]

''turns on then off'' aged CCFL tube
retrofit with LED

[MaxRambone]

Your post doesn't give me that impression at all, particularly with all the questions about very basic things.

I'm sorry. I was under the impression this was a forum that people could seek help with repairs.

Looks like I'm in the wrong place.

Sorry to waste your time.

[jdragoset]

Not in Wrong Place
Wait for additional responses
True that bridge rectified mains need safety precautions (isolated DVM/Oscilloscope  or analog volt meter) to measure output, but we do that out of habit without question.

[james_s]

when something just fails with no physical indication, I'm kind of lost.

That's how most stuff fails, it's a luxury when you can find parts that are visibly damaged. You need a basic understanding of how a SMPS works, and you'll need an ESR meter unless you want to just shotgun replace all the electrolytic capacitors, and a multimeter with a diode test function.

Are you sure you're feeding it a valid signal? Some monitors will shut down if they're driven out of range or have no signal. Also check the cable.

[fzabkar]

If the mains supply is 120VAC, and if there is a single 400V capacitor, then this would suggest that the capacitor is part of the APFC circuitry.

Does this monitor have a model number?

[inse]

It also might be a wide range power supply covering both worlds.
Regarding PFC: in EU, PFC is mandatory for appliances >75W, I don't know whats the regulations in other regions.

[MaxRambone]

The monitor is working now. I replaced the main cap.

Thanks to all of you who were actually trying to be helpful.

[inse]

Congratulations!