Samsung 930B re-capped, no backlight

[leperinc]

A while ago I was given a dead Samsung 930B and made an attempt at fixing it by replacing the obviously damaged electrolytic caps. On my first attempt i replaced:

cap# orig uf/v ---> new uf/v
-------------------------------------------------
301 820/25 820/25 nichicon pw
111 820/25 820/25 nichicon pw
112 820/25 820/25 nichicon pw
109 1000/10 1000/25 nichicon pw
110 1000/10 1000/25 nichicon pw

When I pressed the power button the green LED stayed on followed by a high pitched screeching noise coming from everywhere on the board. I replaced the rest of the electrolytics inluding the large one. Here's the rest of the list showing what I replaced:

cap# orig uf/v ---> new uf/v
-------------------------------------------------
113 330/10 330/16 nichicon pw
123 10/50 10/50 nichicon pw
107 47/50 47/50 nichicon pw
105 100/450 100/450 chemicon kxg
345 33/25 33/35 nichicon pw

Now the screeching is gone, it's all nice and quiet. When pressing the power button the LED is green and steady. It appears fine but there's no backlight whatsoever. I can faintly see a 'no input' message moving around the screen when I shine a light at it.

Attached are some pics of the board. After reading other threads in here I've checked my work again and i can't find anything wrong. I've checked the high voltage transformers both primaries read 0K ohm (my $cheap meter only has a R * 1K setting so i can't tell something like 10 ohms from a true short) and the secondaries read 1.3K ohms. There is no continuity between the primaries and secondaries.

I am unable to find any fuses (mentionned in another thread for a 930b) apart from F101 which is good. U202 and U203 look rather suspicious though. I can't read all the numbers on them and am not sure how to test them. Anyone?

[jetadm123]

Re: Samsung 930B re-capped, no backlight

A while ago I was given a dead Samsung 930B and made an attempt at fixing it by replacing the obviously damaged electrolytic caps. On my first attempt i replaced:

cap# orig uf/v ---> new uf/v
-------------------------------------------------
301 820/25 820/25 nichicon pw
111 820/25 820/25 nichicon pw
112 820/25 820/25 nichicon pw
109 1000/10 1000/25 nichicon pw
110 1000/10 1000/25 nichicon pw

When I pressed the power button the green LED stayed on followed by a high pitched screeching noise coming from everywhere on the board. I replaced the rest of the electrolytics inluding the large one. Here's the rest of the list showing what I replaced:

cap# orig uf/v ---> new uf/v
-------------------------------------------------
113 330/10 330/16 nichicon pw
123 10/50 10/50 nichicon pw
107 47/50 47/50 nichicon pw
105 100/450 100/450 chemicon kxg
345 33/25 33/35 nichicon pw

Now the screeching is gone, it's all nice and quiet. When pressing the power button the LED is green and steady. It appears fine but there's no backlight whatsoever. I can faintly see a 'no input' message moving around the screen when I shine a light at it.

Attached are some pics of the board. After reading other threads in here I've checked my work again and i can't find anything wrong. I've checked the high voltage transformers both primaries read 0K ohm (my $cheap meter only has a R * 1K setting so i can't tell something like 10 ohms from a true short) and the secondaries read 1.3K ohms. There is no continuity between the primaries and secondaries.

I am unable to find any fuses (mentionned in another thread for a 930b) apart from F101 which is good. U202 and U203 look rather suspicious though. I can't read all the numbers on them and am not sure how to test them. Anyone?

U202 and U203 look in pretty bad shape and are probably burned out. They look to be AF4502C dual mosfets. Datasheet below:


http://pdf1.alldatasheet.com/datashe...P/AF4502C.html


With you meter set on resistance:

Measure across the following for shorts:

S1-G1
G1-D1
S1-D1

Do the same test S2-G2, etc

[jetadm123]

Re: Samsung 930B re-capped, no backlight

I also forgot to mention the inverter fuse might be between the large cluster of caps and the heatsink. It might be underneath the white adhesive. It might be green and stands upright.

[leperinc]

Re: Samsung 930B re-capped, no backlight

U202 and U203 look in pretty bad shape and are probably burned out. They look to be AF4502C dual mosfets.

[...]

With you meter set on resistance:

Measure across the following for shorts:

S1-G1
G1-D1
S1-D1

Do the same test S2-G2, etc

Thanks for the reply.


I put the black lead on the first point mentionned and the red lead on the second i.e. black to S1 and red to G1, etc. It's hard to see so I've included a pic pointing to what I think is pin 1. Hupefully I've gotten it correctly

I've checked both U202 and U203 and they read identical.

S1 --> G1 = 2.5K ohms
G1 --> D1 = 30K ohms
S1 --> D1 = 700 ohms
S2 --> G2 = 400 ohms
G2 --> D2 = 100 ohms
S2 --> D2 = 350 ohms

No shorts or opens.

I also forgot to mention the inverter fuse might be between the large cluster of caps and the heatsink. It might be underneath the white adhesive. It might be green and stands upright.

I still can't find it. I've been looking where you said from before but this time I cleaned off the component and it appears to be a 1 watt(?) resistor. it's light pink. There's noting green in there. I've attached updated pics of the area.

thanks!

[retiredcaps]

Re: Samsung 930B re-capped, no backlight

See if U301 is shorted. You can measure it in circuit. Report readings.

edit: Are you seeing 13V DC on the connector CN2?

[jetadm123]

Re: Samsung 930B re-capped, no backlight

Hmmm, you did pick the correct location for pin 1. No shorts indicated from your readings. I'm wondering if what's on U202 is leftover residue from the soldering process. Try scratching the top of U202 with your fingernail to see if it will flake off. As for the fuse, Samsung usually puts a fuse at the 13V output, just before the inverter section. And I don't see a fuse either?! As a sanity check, try measuring for the 13V at the middle pin of heatsinked D105.

[leperinc]

Re: Samsung 930B re-capped, no backlight

See if U301 is shorted. You can measure it in circuit. Report readings.

edit: Are you seeing 13V DC on the connector CN2?

I don't see a U301, I'll assume U201. I tried looking up the datasheet on it and can only find the BIT3193. Is this close enough, and what pins should I be paying attention to for measuring?

I see 13Volts on CN2, yes.

Hmmm, you did pick the correct location for pin 1. No shorts indicated from your readings. I'm wondering if what's on U202 is leftover residue from the soldering process. Try scratching the top of U202 with your fingernail to see if it will flake off. As for the fuse, Samsung usually puts a fuse at the 13V output, just before the inverter section. And I don't see a fuse either?! As a sanity check, try measuring for the 13V at the middle pin of heatsinked D105.

There is also 13volts on the center of D105. I've tried to clean the chips, first with my fingernail, followed by a white eraser and rubbing alcohol but it didn't change anything. I took a webcam which can be made to focus really close up and wetted the tops of the chips with alcohol. The look burned alright. Also, the plastic sheet has marks where the chips are.

[leperinc]

Re: Samsung 930B re-capped, no backlight

I figured I'd kill some time and poke around on U201 and measure everything from anywhere to anywhere in both directions. There appears to be one short between pin 7 and 16. I'm not sure if this is normal or not. On the similar model (3193) sheet pin 7 is GND and pin 16 is MODSEL. C345 (33uF/35v, replaced) is connected by traces across pin 7 (GND) and 10 (VCC)

[retiredcaps]

Re: Samsung 930B re-capped, no backlight

I don't see a U301

The look burned alright. Also, the plastic sheet has marks where the chips are.

U301 is on the front side.

Even though the mosfets are not shorted, the one chip looks really bad and could be malfunctioning.

[jetadm123]

Re: Samsung 930B re-capped, no backlight

I agree with retiredcaps about the mosfets. Anything that heated up like that should be suspect. Also, they should always be replaced in pairs.

As for pin 16 connected to pin 7, since MODSEL sets the output polarity of the PWM controller, I'm assuming that connecting it to ground specifies a negative polarity.

Also, when you first power up the monitor, do you get any type of quick flash from the backlights?

[leperinc]

Re: Samsung 930B re-capped, no backlight

U301 is on the front side.

i'm going crosseyed. I can't find it. Can you point it out to me?

[jetadm123]

Re: Samsung 930B re-capped, no backlight

i'm going crosseyed. I can't find it. Can you point it out to me?

U301 is next to transformer T301. What's the part number?

[leperinc]

Re: Samsung 930B re-capped, no backlight

U301 is next to transformer T301. What's the part number?

Sorry about that, there are no opens or shorts no matter what pins/directions I measure.

here's the pic:

[jetadm123]

Re: Samsung 930B re-capped, no backlight

If you're not even getting a flash from the backlights when you push the power button, I would suggest you try replacing the two mosfets in question before going any further.

[leperinc]

Re: Samsung 930B re-capped, no backlight

No problem, I appreciate your help with this. I'd like to hedge my bets on this one and ask what other components might be needed in case U202/3 are fine. I'd rather spend $5 in extra parts now as insurance rather than having to spend yet another $8 on shipping later.

Thanks!

[retiredcaps]

Re: Samsung 930B re-capped, no backlight

I'd rather spend $5 in extra parts now as insurance rather than having to spend yet another $8 on shipping later.

On the front side between the two transformers are a whole bunch of resistors and diodes. You may want to test them all "in circuit" to see if you have a bad component.

As a fellow Canadian, I can relate with the $8 shipping cost.

[leperinc]

Re: Samsung 930B re-capped, no backlight

On the front side between the two transformers are a whole bunch of resistors and diodes. You may want to test them all "in circuit" to see if you have a bad component.

They all look fine. No opens or shorts.

If you're not even getting a flash from the backlights when you push the power button, I would suggest you try replacing the two mosfets in question before going any further.

I wasn't able to find the original part at any of the usual places. Elsewhere in the forum here i saw a couple of people claiming success with a MMDF2C03HD from another manufacturer. A quick search shows Mouser is currently the only supplier with stock in individual chips at $1.47 each.

However, at $20 shipping, this last item is too expensive for me to continue. Getting this far has cost me $31. I'm currently not working and getting the parts from Mouser will push the total cost of this "free" monitor to just under $60. I saw the $8 USPS option but when you let them collect fees upon receipt it usually means an additional $15-20. The term "egregious" comes to mind. Well, other terms come to mind as well but...

Big thanks to both of you for helping me with this but unless I can find cheaper shipping I think I'm gonna have to call it a day

[retiredcaps]

Re: Samsung 930B re-capped, no backlight

but unless I can find cheaper shipping I think I'm gonna have to call it a day

ebay has 10 (ten) for $7 CDN + $4 CDN for shipping. Just search for AF4502C.

[jetadm123]

Re: Samsung 930B re-capped, no backlight

Fairchild shows a FDS8928A as a possible replacement. It handles less current than the AF4502C, but might be more readily available. Since they're so cheap, it would be good to order a couple extra just in case.

[leperinc]

Re: Samsung 930B re-capped, no backlight

ebay has 10 (ten) for $7 CDN + $4 CDN for shipping. Just search for AF4502C.

<smacks head> Thanks for pointing that out! I've placed the order, so now we wait...