17PW20.1 blows fuse right away

[wylcot]

Hi guys,

So... I usually fix CPU boards on pinball machines, so this here is totally unknown territory to me...

Basically, a friend of mine came up to me with her TV saying that she heard like an explosion, and then the tv was off.
I've probably bragged a bit about fixing TVs, but until now, I'd only had to change caps.

Here, it's a bit different, but i thought that'd be the occasion to learn a bit more.

So the main fuse of that power supply board blows as soon as the TV is plugged in.
I've noticed that at least 5 caps are bulged, but that can't be that, right?

Any lead can help !





Also, I'd really love to understand the basics of how PS boards work, so if you can advise on some good video or tutorial that does that, that'd be awesome !

Thanks a bunch !

J

[petehall347]

Re: 17PW20.1 blows fuse right away

basically the AC comes in to the hot side then is chopped up to a high frequency so its switching on and off very fast .then it goes through a transformer and rectified to DC on the cold side to supply the voltages to run the circuits .
obviously more stuff goes on but thats the basic idea .
first check is the switching fet on the heat-sink then other things that could be blown because of the short .

[neilc6]

Re: 17PW20.1 blows fuse right away

Main fuses blowing are caused by shorted components on the board. Usual suspect are FETs or diodes that can fail short. Caps typically don't short when they fail. You should be able to measure a short on one side of the fuse to ground. Without the fuse blowing, your circuit breaker in your house would have tripped since the AC power is being shorted.

[budm]

Re: 17PW20.1 blows fuse right away

I also see at least one cap with bloated top.

[wylcot]

Re: 17PW20.1 blows fuse right away

Thanks a lot, guys.
I will start my researches for a faulty FET ou diode.
And of course, if I manage to find it, I won't mount it back without replacing the caps.

I'll let you know.

[yokoono]

Re: 17PW20.1 blows fuse right away

And pwm controller, optocoupler, resistors, diodes etc. Not only the mosfet transistor is burned.

[wylcot]

Re: 17PW20.1 blows fuse right away

And pwm controller, optocoupler, resistors, diodes etc. Not only the mosfet transistor is burned.

Huh !
Not that easy, then !....

[dick_barton]

Re: 17PW20.1 blows fuse right away

If the fuse blows immediately then there is a short circuit on the board. Use your meter set to ohms (with set off) and I would initially check all the mosfets and diodes.


Do a search for the schematic using the search function on the top of the page.

Also if you search for 17PW20 you will find more threads covering repairs to this power supply.

Your friend said "like an explosion" so I would also be looking for any components that look damaged on the top and bottom of the circuit board.

All the bowed top capacitors need to be changed.

[wylcot]

Re: 17PW20.1 blows fuse right away

Well, I think I found a faulty FET.
LM317T.
Need to wait for the local component shop to open, now...

[diif]

Re: 17PW20.1 blows fuse right away

Well, I think I found a faulty FET.
LM317T.
Need to wait for the local component shop to open, now...

It's a voltage regulator not a mosfet.

[dick_barton]

Re: 17PW20.1 blows fuse right away

Have you checked the mosfets Q803, Q813 & Q814 for short circuit using your meter set to ohms or diode mode and with the set powered OFF

[wylcot]

Re: 17PW20.1 blows fuse right away

Have you checked the mosfets Q803, Q813 & Q814 for short circuit using your meter set to ohms or diode mode and with the set powered OFF

Yes, I have.
But it seems to be Q828 that's faulty...

[wylcot]

Re: 17PW20.1 blows fuse right away

Ok. So I was completely wrong.

So i'm back on this, and i'll try to test these Q803, Q813 and Q814.
These are mtp6N60E...

I'll unsolder them from the board, but can I test these TMOS like normal FET ?

[dick_barton]

Re: 17PW20.1 blows fuse right away

Drain/Source should test as diode with meter set to diode mode.
Generally when these devices go faulty they go short circuit or the diode becomes open circuit.

[wylcot]

Re: 17PW20.1 blows fuse right away

Alright then !
So I should get a reading in one way, and no reading the other way round...

Let's get onto it !

[wylcot]

Re: 17PW20.1 blows fuse right away

Well unfortunately, these 3 look ok to me...

[dick_barton]

Re: 17PW20.1 blows fuse right away

In your first post you said:-

So the main fuse of that power supply board blows as soon as the TV is plugged in.
I've noticed that at least 5 caps are bulged, but that can't be that, right?

Have you found the reason why the fuse was blowing.?
Do you still have a short circuit or low resistance from the fuse holder to hot ground. Hot ground being the negative leg of the large capacitor on its side with the white label on it.

[wylcot]

Re: 17PW20.1 blows fuse right away


This one, you mean ?

[dick_barton]

Re: 17PW20.1 blows fuse right away

That's the one. That capacitor has been known to go faulty. I'm sure Reeceyburger has a video on this on Youtube

[wylcot]

Re: 17PW20.1 blows fuse right away

So I find a .733 ohms on the 2k position...