Dynex dx-lcd32 power supply half dead

[The Performer]

Re: Dynex dx-lcd32 power supply half dead

So I cant get any voltage readings on DC anywhere on ic901, but in AC I get 12v around all the pins, is this right?

On another not, If I trace back a random line at D932 I get a voltage drop from 72v to 14v, (again this is with meter set on AC. Any thoughts?

[budm]

Re: Dynex dx-lcd32 power supply half dead

What do you use for ground for your probe? You are measuring DC voltages in the primary side of the circuit, you have to use the negative leg of the main filter cap as the ground ref point for your meter. Did you verify that IC 901 is FAN7529?

[The Performer]

Re: Dynex dx-lcd32 power supply half dead

What do you use for ground for your probe? You are measuring DC voltages in the primary side of the circuit, you have to use the negative leg of the main filter cap as the ground ref point for your meter. Did you verify that IC 901 is FAN7529?

Yessir, the Part number is correct. FAN7529

And on the note of ground point, I've been using the chassis as my ground... And now I feel like a dumbass...
But seriously, was this taught to do so somewhere that I missed? Im not sure I follow which cap is the main filter cap, do you mean C901 as in the first cap in parralell after power goes into the board?

[budm]

Re: Dynex dx-lcd32 power supply half dead

There are two on them, black laying side way and has 100uF 450VDC printing on it, so now I know that you did not use the correct ground point for the primary side of the circuits, please measure the DC voltage again RIGHT AT THE TWO LEGS of either one of the two main filter caps when TV is ON. Do not let the probes slip on you and do not touch anything on the board.

[The Performer]

Re: Dynex dx-lcd32 power supply half dead

There are two on them, black laying side way and has 100uF 450VDC printing on it, so now I know that you did not use the correct ground point for the primary side of the circuits, please measure the DC voltage again RIGHT AT THE TWO LEGS of either one of the two main filter caps when TV is ON. Do not let the probes slip on you and do not touch anything on the board.

165v DC at standby and drops to 160 DC at power on.

Ok so now answer me this, the whole time I'm doing my testing I have the board unscrewed from the chassis, but I ran a jumper from the circuit board mounting tab to the chassis to recreate the original ground, is this wrong?

[budm]

Re: Dynex dx-lcd32 power supply half dead

That is fine, the secondary (cold side) ground is connected to the chassis.
So we need to check the resistance of that PFC IC 901 between the ground pin 6 of IC 901 against all the other pin, no power to the board of course.
Make sure to report if the reading is in Ohm, K Ohm, or M Ohm.

[The Performer]

Re: Dynex dx-lcd32 power supply half dead

Ok so resistance in relation to pin 6 is as followed,

6-1 _ 66 ohms
6-2 _ 53.8 ohms
6-3 _ 86.5 ohms
6-4 _ 307.8 ohms
6-5 _ 15.03k ohms
6-7 _ ---- nothing seems to be connected by looking at the board, and no reading from meter (weird because the schematic shows it goes to ... and what do you know R 965 is showing open...
6-8 _ 41.9 ohms

[budm]

Re: Dynex dx-lcd32 power supply half dead

Your IC is damaged due to shorted out Gate to Drain that is why the Gate drive resistor is blown open, you need to replace the MOSFET Q901 and the IC at the same time. You should also test the R921 (Source resistor) to see if it still shows <1 Ohm.
You also need to check the Gate discharge R/D network R932 (10 Ohms), and Diode D919 (1N4148) to see if they are damaged also.
You should also test the resistance again on the Gnd pad against all other pads when the IC is removed.

[The Performer]

Re: Dynex dx-lcd32 power supply half dead

Your IC is damaged due to shorted out Gate to Drain that is why the Gate drive resistor is blown open, you need to replace the MOSFET Q901 and the IC at the same time. You should also test the R921 (Source resistor) to see if it still shows <1 Ohm.
You also need to check the Gate discharge R/D network R932 (10 Ohms), and Diode D919 (1N4148) to see if they are damaged also.
You should also test the resistance again on the Gnd pad against all other pads when the IC is removed.

Sweet action!

yup, after I investigated the r965 I found that r932 was reading wrong too. Well that being said, looks like I have some parts to order!

Now if you dont mind, how do you know from my readings that the IC is bad. I looked over the datasheet but it seemed kinda confusing.

And thank you everyone for everything so far,

It really shows genuine kindness when people are so nice to help all for the purpose of helping someone else learn.

[budm]

Re: Dynex dx-lcd32 power supply half dead

Let me know the resistance reading of the Ground pin pad against all other pads of that IC after the IC is removed.
"Now if you dont mind, how do you know from my readings that the IC is bad" You look at the function blocks inside the IC then you know what you should expect what the reistance should be, yours are all very low, and the blown resistor also made it clear that lots of current has flown through the circuit.

[The Performer]

Re: Dynex dx-lcd32 power supply half dead

Let me know the resistance reading of the Ground pin pad against all other pads of that IC after the IC is removed.
"Now if you dont mind, how do you know from my readings that the IC is bad" You look at the function blocks inside the IC then you know what you should expect what the reistance should be, yours are all very low, and the blown resistor also made it clear that lots of current has flown through the circuit.

So now come to think about it, I do remember about a year ago finding that mosfet shorted and I replaced it, so it's a brand new one in there right now. I didnt know to check any of the components feeding it for issues.

So on terms of you knowing what the resistance should be, does that just come from years of crouched over these devices to know such things?

Also Bud, are most of you wizards electronic engineers? I'm hoping to get to go to college for this. Started college in 08 and didnt do so well, and just now getting to give it a shot again. Electronics is just one of those things I dont lose as much attention with in comparison to other subjects.

[budm]

Re: Dynex dx-lcd32 power supply half dead

I would replace the MOSFET and the IC at the same time. If you look at the diagram inside the IC, you will see for example the on VCC pin8 which has Clamping Zener diode inside, it will not have 40 Ohms reading, it should be in 1000's of Ohm range.
Pin 1 (use for feed back to maintain also show low resistance, this is an input pin to the comparator and op amp input circuit, so to have 66 Ohms, that is bad, etc.

[The Performer]

Re: Dynex dx-lcd32 power supply half dead

Ok so update, got the IC 901, swapped it out, with the new one in place I did a check on q901 and it appears to be fine. Like I said, I found it shorted through my first tear down and replaced it then.

All in all, still no 24v and Im at more of a loss. Any ideas what to check next?