Dynex dx-lcd32 power supply half dead

[The Performer]

Finally joined and needing some help with a board issue. I know I can buy the board online for 40-50$ but to be honest Im worried about putting the money into it since this tv was litterally found on the side of the road, haha! Whoever threw it away wasnt a jerk about it however, they stood it up and didnt put a shoe through it so I figured lets give it a shot! Plus I feel much more enthusiastic about learning the troubleshooting rather than throwing a power supply that may be just a cheap fix.

This is the service manual for the power supply I've been able to locate.


Anyway, as it stands the power supply seems to be working half way. I've been toying with it on and off for over a year now, so remembering all the parts I've changed is listed as followed,
-both big 100uf caps. (in the pictures you can see I ordered ones with different legs so I had to extend legs on them to attach to the board.
-all 7 1000uf caps on the dc side of the board
-ic902 (there was a thread on here a while back of someone describing they had a issue with this part so I changed it as well to no avail) In all honesty Im not familiar with the purpose of these IC's on the board do) My limited knowledge can only assume they oscillate the cycles for the transformer? I've noticed I get around 80v ac on all the pins of this IC, is this typically correct?

Aside from those, I still have the original 470uf on the dc side caps as those seemed to test fine.

Im getting the 5v output, and 12v output just fine but nothing at all on the 24v and 15v outputs and it's driving me crazy. If it means anything as well, while overlooking the schematic I notice there is a B+390V which I havent stumbled across while probing around on the board, but I assume it MUST be there atleast or the 12v and 5v side wouldnt be working, correct?

Any help would be greatly appreciated, I'd love to if anything learn how to troubleshoot this powersupply and acheive fixing it. If the TV works and has a good picture, hell I may use it for my computer monitor!

Thanks !

Oh and heres the pictures!




Please let me know if I need to take closer shots of certain areas!

[The Performer]

Re: Dynex dx-lcd32 power supply half dead

Update, just did a DC measure across the primary of both T902, and T901 and they measure 167vDC.

Could this be where my problem lay? Should they be the listed 390v? If so how is my 12v and 5v active?

[stj]

Re: Dynex dx-lcd32 power supply half dead

resolder the transformers for a start.

[The Performer]

Re: Dynex dx-lcd32 power supply half dead

resolder the transformers for a start.

Do you mean re flow the terminals or would it be in my best interest to remove and re install them?

And thank you for the reply!

[stj]

Re: Dynex dx-lcd32 power supply half dead

remove the old solder & put fresh - no need to actually pul them off the board.

[budm]

Re: Dynex dx-lcd32 power supply half dead

Can we get the straight shot pictures of the board, both sides? Also bottom side in small sections so I can clearly see the traces.

[selldoor]

Re: Dynex dx-lcd32 power supply half dead

Pictures should be JPG (max resolution 2000x2000 and 3MB) using the manage attachments button, which is found by clicking "go advanced" under quick reply.

Please do not post inline and offsite as they slow down the loading of pages.

[The Performer]

Re: Dynex dx-lcd32 power supply half dead

Thank you for the responses everyone! I am sorry about the picture issue, I'm used to other forums that operate differently. Was just a habit to upload from the cell to photobucket

Ok here are the updated pictures budm asked for. Please let me know if the pictures are useless so I can fix them.

Also if it makes any difference, this thread has the same powerboard with the other users pictures.
https://www.badcaps.net/forum/showth...light=dx-lcd32

One other thing to note is I have replaced d920 in the photos as I took the original off to find out it was fine, (replaced it with a legged diode) and as well c932 was substituted to see if anything changed.

Thanks again!

[The Performer]

Re: Dynex dx-lcd32 power supply half dead

Update,

Just out of curiousity I pulled C907 and tested with my fluke and it gets a reading of 1.016uf Which to me seems fine as it's supposed value is 1uf.

Also re flowed fresh solder into all the transformer legs across the board. May I ask what the thought process behind that is ?

I need to scrounge up an ESR meter if I keep this electronic tinkering up, but first I need a new scope. My dso nano just fried on me. (metal case grounded out on a circuit I was reading)

[stj]

Re: Dynex dx-lcd32 power supply half dead

transformers resonate at the running frequency,
this slowly breaks the soldering

also, anything large or bolted to a heatsink puts a lot of stress on the soldering - specially when it's bumped or moved around.

that's why bad solder joints are so common in psu's

[The Performer]

Re: Dynex dx-lcd32 power supply half dead

I gotcha, that makes sense. So in general thinking, if I can't find the 390v present on the circuit, where may my 12v and 5v line be getting their power?

[stj]

Re: Dynex dx-lcd32 power supply half dead

i see how it works - i think.
first there is some type of stepup generating B+
then there are 2 seperate psu circuits.
one generates the 5v and 12v
the other based on ic902 generates the +24v using q902 driving t902.
the 15v you can forget for now, it's generated from the 24v so no 24v means no 15v

replace c937 and meter the voltage supplying ic902.
and check if t902 or any of the related diodes are shorted.

[stj]

Re: Dynex dx-lcd32 power supply half dead

btw, i think B+ reading 167v is o.k.
i suspect your schematic is based on a European 240v input voltage.

[martino]

Re: Dynex dx-lcd32 power supply half dead

shouldn't the psu be forced on in order to get the 15 and 24 volt? As in ps-on ?

[budm]

Re: Dynex dx-lcd32 power supply half dead

You should have about 360~400V on those two main filter caps (100uF 450VDC) when TV is powered on since this TV has PFC voltage booster circuits, without that boosted voltage the 24V power supply will not work. So do you know if you have PS_ON signal from the main board?

[The Performer]

Re: Dynex dx-lcd32 power supply half dead

You should have about 360~400V on those two main filter caps (100uF 450VDC) when TV is powered on since this TV has PFC voltage booster circuits, without that boosted voltage the 24V power supply will not work. So do you know if you have PS_ON signal from the main board?

I'm getting 160v on those two caps, and yes I get 2.5v at the ps_on.

I feel like were getting close, aside from not knowing how yalls logic is running circles around my rusty cogs.

[The Performer]

Re: Dynex dx-lcd32 power supply half dead

i see how it works - i think.
first there is some type of stepup generating B+
then there are 2 seperate psu circuits.
one generates the 5v and 12v
the other based on ic902 generates the +24v using q902 driving t902.
the 15v you can forget for now, it's generated from the 24v so no 24v means no 15v

replace c937 and meter the voltage supplying ic902.
and check if t902 or any of the related diodes are shorted.

Seem to be fine, what else?

[budm]

Re: Dynex dx-lcd32 power supply half dead

So the PS_ON does go from <1VDC when TV is off and then swings up to >2V when TV powered on, then that means you need to fix the PFC Voltage booster section first.
Need to see the P/N of the PFC Controller IC next. It is IC901 FAN7529, please verify.

[The Performer]

Re: Dynex dx-lcd32 power supply half dead

So the PS_ON does go from <1VDC when TV is off and then swings up to >2V when TV powered on, then that means you need to fix the PFC Voltage booster section first.
Need to see the P/N of the PFC Controller IC next.

PS_ON is at 0 v then goes to 2.5v. but let me move on the the PFC controller...

[The Performer]

Re: Dynex dx-lcd32 power supply half dead

From what I can tell on the schematic, the pfc controller part number is "FAN7529".

Edit, just verified it on the board as well.