37pd5200e smps

[zx81]

Re: 37pd5200e smps

Cheer Tom.

Will let you know how I get on.

[tom66]

Re: 37pd5200e smps

A short in the other boards should not matter.

I always, always replace the driver, regardless of apparent damage or not, even if it tests OK. Reason is, when the FET shorts, it basically shoves 300V DC into the 15V output of the IC... who knows what kind of damage that could do? It might not damage the output but instead some other parts of the IC, like control logic.

I also usually replace nearby SMD resistors at the same time.

[zx81]

Re: 37pd5200e smps

Yes Tom same FET. Initially I did not change or check the driver IC picture uploaded in post#19. Since the bulb across the fuse went off when I replaced The MOSFET. I thought it would be safe to fire up with the fuse back in place.

Done some very basic testing of the driver IC. No obvious shorts. Will try and reflow in the next day or so when my Atten reflow station arrives and pop it back in.

I just realised that I haven't tested Q003 and Q004 in the PFC section as suggested by Paulsef in an earlier post. Will check too.

Could a short in any of the other boards cause the issues I'm having?

[tom66]

Re: 37pd5200e smps

Sorry meant, same FET shorted? Did you replace the driver IC at same time?

[zx81]

Re: 37pd5200e smps

Yes Tom same one.

[tom66]

Re: 37pd5200e smps

Same resistors shorted?

[zx81]

Re: 37pd5200e smps

I can only get a standby green led solid on when I remove the cable that goes to to the io board.

The fireworks started as soon as I connected all the other boards to the power supply. Popped in the cable but as soon as I pressed the power button Bang

Luckily I orderd 2 set of resistors and an extra Mosfet.

[paulstef]

Re: 37pd5200e smps

You plug the TV set in and it makes BOOM right away?

Or you plug it in it goes to standby then you turn it on and then it makes BOOM?

[paulstef]

Re: 37pd5200e smps

Not sure how I would test the capacitors marked in red in the scan?

I'm also unsure how to test voltages when the set is plugged in do use Hot or cold ground ?

They are not necessarily capacitors, could be inductors too, but less likely. Truly testing them requires them to be removed and measured. They are unlikely to have lost capacitance. they could be short though.
There are diodes too (black , 2 pins) and transistors. One would need to draw the diagram to be sure what is what.

If you turn the set on and then Q201 blows up then it's because its gate is driven ON. PC201 is open as long as the secondary voltage 5V (page 96) is lower than 5V. So the IC201 does not seem to activate OUT1 which would turn off Q201. It stays on, T201 saturates and boom. Overcurrent.

You measure all voltages to their respective ground (if that's what you want to do). Everything that is between AC IN and one of the SMPS transformers is live and measured to hot ground. Everything after the SMPS transformers is measured to cold ground. Then there can be more SMPS following these, not necessarily in this TV though.

Don't get confused. There can be many more than just two grounds or references in a circuit. References can also be positive. What the reference is for a certain part of a circuit depends on the circuit.


What you need to understand and repair this is:
- draw at least part of the circuit of IC 201
- an oscilloscope
- an isolation transformer

You can also try to at least check shorts in power semiconductors to start with. But then you will need to do some serious troubleshooting.

[zx81]

Re: 37pd5200e smps

So far I've replaced R003 (5watt 10ohm) and Q201 (K2843) Mosfet. Tried the bulb across the fuse holder and the bulb went out. So I proceeded to replace the fuse and fire up the set. Unfortunately there were a few sparks followed by a bang Main fuse blown R003 open again Q201 blown again.

I've removed the daughter board IC201 which is the driver for the damaged Mosfet. Also attaching some scans of the IC components. I'm trying to check for shorts as best as I can. Not sure how I would test the capacitors marked in red in the scan?

I'm also unsure how to test voltages when the set is plugged in do use Hot or cold ground ?

Any tips on how to proceed would be appreciated.

Thanks in advance.

[zx81]

Re: 37pd5200e smps

Paul I can answer questions

4) All other PCB where connected.

5) Line voltage is 240v the bulb is also 240v

I will test and get back to you re 1,2 and 3

I purchased this set faulty from EBay specifically to help me learn about fault finding and learning more about SMPS and TV's in general. I'm in no real rush to fix or sell it. I sometimes have long gaps before I can carry out tests due to work. So please bear with me. It's just a hobby but I'm very eager to learn as much as I can. SMPS seem very complicated to me as a beginner but If I can learn how these things work and how to identify and diagnose faults it should be a very valuable experience.

Paul your helpful guidance is really appreciated.

zx81

[paulstef]

Re: 37pd5200e smps

A 240V/150W lamp has about 380 Ohm at 150W. (cold it's much lower). That means you have 380 Ohm in series with the rest of the circuit. Something in the circuit has a much lower equivalent resistance than 380Ohm.

1. Which relay clicks?

2. Can you measure the coil voltage of RL002 (DC) to see if it ever gets a signal (initially)? For R003 to burn (without the light bulb) the relay RL002 must be open.

3. What's the output voltage at the bridge rectifier when the light bulb is fully on?

4. Is the rest of the TV PCBs connected or are you working with just the PSU?

5. What is the AC line voltage? What is the voltage on the light bulb?

[zx81]

Re: 37pd5200e smps

A quick update.

I have now replaced R003 which was open circuit and a K2843 MOSFET which was shorted.

When I fired up the set with a 150watt bulb in place of the main fuse the bulb goes from off to very bright within half a second or so the relays click and then after a short time this whole process cycles again and again.

I'm not sure what to make of this result does this mean I still have a short somewhere else in the primary side?

The good thing is that non of the replaced or other components blew using the light bulb.

Any guidance on how to proceed and what to check next would be very much appreciated.

[zx81]

Re: 37pd5200e smps

Thanks for your replies guys.

I feel a little more confident now and will try and replace the the faulty components.

[paulstef]

Re: 37pd5200e smps

Nothing looks damaged on IC201. There are not a lot of components on this board. I would at least diode check the diodes/transistors but that's about it.

A light bulb instead of the fuse would limit the current in case of a short, yes. If it stays lit once the main caps are charged and there is no load on the PS output (standby) there is a problem.

[multimeter]

Re: 37pd5200e smps

I now have the parts and will be looking to replace them this weekend.

Could any of you guys please suggest a way to determining if the driver IC (pics attached) is not shorted before I go ahead and replace the K2843 mosfet and 5watt 10ohm resistor? I'd like to avoid damaging the new parts if at all possible.

Would the 100watt bulb across the main fuse help in this instance?

driver ic's that i have seen always go short VCC to ground pin,check the datasheet to determine the pins.

yes, use the light bulb across the fuse holder,it will limit the current and avoid more damage,a dim lightbulb is a good sign,if it stays bright,there is still a short in the primary section

[zx81]

Re: 37pd5200e smps

Any help or advice would be much appreciated.

[zx81]

Re: 37pd5200e smps

I now have the parts and will be looking to replace them this weekend.

Could any of you guys please suggest a way to determining if the driver IC (pics attached) is not shorted before I go ahead and replace the K2843 mosfet and 5watt 10ohm resistor? I'd like to avoid damaging the new parts if at all possible.

Would the 100watt bulb across the main fuse help in this instance?

[zx81]

Re: 37pd5200e smps

Not managed to find Q003 or Q004 I think they may be hiding under one of the heatsinks. They should be in the vicinity of IC001 and the bridge rectifier. I'll take another look for them.

This board has documented issues with the Mosfets Q220 Q201 both K2843's but I can't find those parts in stock locally. I think they've been discontinued so may need to source an alternative also r003 and r005 are quite commonly faulty from what I've read.

Will start taking bits out and testing. Will revert back when I've done more tests.

Cheers,

zx81

[paulstef]

Re: 37pd5200e smps

You don't need to understand in detail the whole circuit right away.

What's with Q003 Q004?