Samsung 940N No Backlight - BN400121A Analog Board

[C152Heavy]

Hello Samsung Fixers

Quick introduction. My training is in Mechanical Engineering not electrical so don't toss me in the deep end without a lifeline. That said I have fixed a handful of monitors, power supplies, motherboards, routers etc., mostly through recapping so I'm not too dangerous with a soldering iron

OK, on to the job at hand. Scored a Samsung 940N a few days ago and I'm trying to get the back light going again and could use a little help if you have time (Thanks!).

So what we know so far is:

When I got it, it had a slight flicker in the back light when turned on but almost instantaneous. Changed out the caps (CrapXcons) for Panasonic & Rubycons and the flicker is gone, so is anything from the backlight but the panel passes the flash light test. The 470u @25V beside the daughter board was bulged.

Took some measurements using a Fluke 77 (auto-ranges only):

Transformers:

T1, Primary= 0.2 Ohms, Secondary= 1116 Ohm
T2, Primary= 0.2 Ohms, Secondary= 1124 Ohms

Conclude, the transformers should be OK

Voltage at the white, 9 place connector to the video/logic board (connected of course):

5.1V ~ about right
13V is about 16.XX (don't remember exactly & didn't write it down)

Diodes:____FWD_____Rev.

D101______0.456____0.0
ZD101_____0.493___1.806
D102______0.491___1.809
ZD102_____0.602___1.966
ZD201_____0.330___0.560
D202______0.202___1.380
D203______0.202___1.382

The readings were taken when the values had stabilized. The rise to the indicated resistance probably means a capacitor was being charged so all but D101 might need to be pulled to check them but I suspect the problem is elsewhere.

So, tested for a bad fluorescent tube with a known good (new) spare: No joy so suspect there's a problem elsewhere and that's about where I am tonight.

I don't see anything suspicious except on the small daughter board which is conveniently soldered in on standoffs so that you can't see what's on the board. This is interesting because any of the other 940Ns I see on Badcaps has another (Power/inverter) board with two 8 pin SO FET packages on the bottom. I suspect they are on the side of the daughter board you can't really see on this one. In fact I can see that there is at least one 8 pin SO package on there in the zone of the board that is turning colour from the heat.

So, where from here?

Thanks!

D

[budm]

Re: Samsung 940N No Backlight - BN400121A Analog Board

The Power MOSFETs for driving those two Inverter transformers are that daughter board.
So right now you do not even see the backlghts come on for seconds? The red round fuse next to the cap you replaced is good?

[C152Heavy]

Re: Samsung 940N No Backlight - BN400121A Analog Board

The Power MOSFETs for driving those two Inverter transformers are that daughter board.
So right now you do not even see the backlghts come on for seconds? The red round fuse next to the cap you replaced is good?

The backlight never did come on for more then an instant and yes, both fuses are good, forgot to mention that.

The MOSFETs are starting to look like a smoking gun??

I'm not looking forward to pulling that daughter board off, at least it's (the father board) not a multi-layer board. I have a Weller solder station but it's still just an iron, any hints to do that without lifting/damaging the pads in the process?

Tks.,

D

[budm]

Re: Samsung 940N No Backlight - BN400121A Analog Board

If the backlights did flash then shutdown, it means it is getting the BL-ON signal from the logic/main board but the inverter may have sense an error so it goes into protection shutdown.

[C152Heavy]

Re: Samsung 940N No Backlight - BN400121A Analog Board

If the backlights did flash then shutdown, it means it is getting the BL-ON signal from the logic/main board but the inverter may have sense an error so it goes into protection shutdown.

OK. What's the path to determining what's causing the protection to shutdown (component fault/failure)?

D

[magistertc]

Re: Samsung 940N No Backlight - BN400121A Analog Board

I fixed a 941BW which was flat dead -- would not respond at all.
You will need to remove the daughter board to replace an SMD component.

First, look at my very rough drawing, marking the test points. Check for +15v at Vcc and signal at ENA. If those are good, check Q302D for voltage. Controller is OZ960SN, check the test point on top foil side near the connection marked 'BT.' If that is good, the problem is further along in the circuit.

In my case I believe that Q302 was not adequate for the surge current. Feeding about 12V directly to that point got the CCFLs to turn on.....finding a suitable replacement was not easy for me
Tom

[C152Heavy]

Re: Samsung 940N No Backlight - BN400121A Analog Board

The job just got a little bit harder.

I was setting up to take some more measurements and in a moment of brain fade I set the board down on something conductive and shorted(?) the common point of R105, C102 and D101 to "some other" point on the board and did some damage. Drew enough current to weld the protruding end of R105 to the piece of metal I set it on (the back of the LCD panel......). It seems to have affected at least D202, 203, ZD102, ZD101, D102, ZD201 and F101 or at least the measurements appear to have changed. No visible sign of damage and no indication of what the other "return" path was.

Unless anyone has any better advice, the plan is to isolate these (and other??) components for proper measurements and try to determine how far the damage spread before the fuse blew. Any advice is most welcome.

BTW, I have a service manual but given that there are at least 2 power conversion boards that turn up with this monitor I have doubts whether it has the right schematic. For example, I could not find reference to some of the components in the parts list. I'm thinking I may be able to trace some of the circuit to scope damage and see if it looks like the schematic.

This always was a "technical" challenge to see if I (with the help of you kind folks) could bring the monitor back to life, so it's not a matter of whether it's worth it for the value of the monitor. I just like to fix things and if/when it's fixed it's likely I will offer to donate it to a local not for profit club to replace a CRT.

And the odyssey continues......

D

[C152Heavy]

Re: Samsung 940N No Backlight - BN400121A Analog Board

The job just got a little bit harder.

I was setting up to take some more measurements and in a moment of brain fade I set the board down on something conductive and shorted(?) the common point of R105, C102 and D101 to "some other" point on the board and did some damage. Drew enough current to weld the protruding end of R105 to the piece of metal I set it on (the back of the LCD panel......). It seems to have affected at least D202, 203, ZD102, ZD101, D102, ZD201 and F101 or at least the measurements appear to have changed. No visible sign of damage and no indication of what the other "return" path was.

Unless anyone has any better advice, the plan is to isolate these (and other??) components for proper measurements and try to determine how far the damage spread before the fuse blew. Any advice is most welcome.

BTW, I have a service manual but given that there are at least 2 power conversion boards that turn up with this monitor I have doubts whether it has the right schematic. For example, I could not find reference to some of the components in the parts list. I'm thinking I may be able to trace some of the circuit to scope damage and see if it looks like the schematic.

This always was a "technical" challenge to see if I (with the help of you kind folks) could bring the monitor back to life, so it's not a matter of whether it's worth it for the value of the monitor. I just like to fix things and if/when it's fixed it's likely I will offer to donate it to a local not for profit club to replace a CRT.

And the odyssey continues......

D

While I'm at it, if anyone has a schematic/parts list for the analog/power board I'd appreciate that, the P/N is BN44-00121A

Tks,

D

[C152Heavy]

Re: Samsung 940N No Backlight - BN400121A Analog Board

Update:

While I'm working through the other problem, I have removed the daughter board and checked components, one of the 4502C FETs shows physical evidence of damage so it's unsurprising that measurements of the two differ.

U301: (showing damage)

S1 G1__~300 - 400 Ohm, variable
S1D1___~300 Ohm - variable
S2G2___160 Ohm
S2D2___0.4 Ohm

U302:

S1 G1__2.48 mOhm
S1D1___1.96 mOhm
S2G2___20k Ohm
S2D2___1.97 mOhm

Now, for a couple of questions:

1) Where to find replacements - are there suitable cross references for the Anachip 4502C's available from Mouser where I have found all of the other components I am about to order (Digikey does not stock all). I'm not keen on eBay, particularly Paypal so I'd prefer an industry source like Mouser/Digikey.

2) I will presume to replace both MOSFET packages but do the measurements indicate that U302 is normal?

Any other thoughts while we're at it?

Thanks,

D

[budm]

Re: Samsung 940N No Backlight - BN400121A Analog Board

S2D2___0.4 Ohm = shorted MOSFET.
I am not sure how close the diagram is.

[C152Heavy]

Re: Samsung 940N No Backlight - BN400121A Analog Board

OK, I’m getting back to this thing.

I’ve replaced suspect components from my little episode from a few weeks ago and am back where I started or even a little ahead since I have replaced both MOSFETS on the daughter board. One was definitely damaged, presumably because of the leaking cap and I reasoned that while I was at it the other might as well be replaced too.

They were Analog Power AM4502C and are now Fairchild FDS8858CZ which appears to be a suitable replacement for the obsolete AP device. The attached photo is prior to the MOSFET replacement, you can see a slight bump on the left MOSFET (U301) caused by internal damage.

Needless to say the monitor is not back to life so I’m hoping for some more advice in troubleshooting & repairing.

So, a bit of housekeeping;

budm, the schematic is close but with differences. I also found another (attached) which is again close but still not the same, especially the daughter board. My monitor is an October 2006 date of manufacture with just a VGA input, no DVI so I expect it is something like Rev 0 or A of the boards.

Magistertc, the daughter board looks like yours and I have done some measurement to understand where I may be.


So, moving ahead:

Q302 (and Q1), marked RA4, appear to be to an AUK SRA2204S which appears to change up to a Toshiba RN2404 (Digikey RN2404TE85LFCT-ND). The link is perhaps tenuous but the RC4 marking pointed to the AUK and since that’s the same maker as some of the discrete components so it seems reasonable.

While we’re at it, Q301 (RC4) would appear to be an AUK SRC1204S which appears to change up to an NXP PDTC144ET,215 (Digikey 568-6438-2-ND).

Finally, Q2, marked as 2F appears to be an AUK SBT2907A = Fairchild KST2907A (Digikey KST2907AMTFCT-ND ).


OK, I have some updated measurements & a few questions they raise:

At the 9 place connector on the main power board (lower right in my photo in post #1), with power on and off respectively (as indicated by the power button illuminating), I measure relative to GND:

PIN----------ON------------OFF
BDM------- 0.049V------0.038V
13V---------16.25V------14.91V
5.1V--------5.12V--------5.12V
A-Dim------0.023V------0.01V
O/F---------2.347V------0.011V


At the contacts on the back of the daughter board:

PIN---------ON----------OFF
Vcc--------16.25V----14.82V
On/Off---2.347V----0.011V
AG--------0.025V-----0.01V
BT---------0.052V----0.038V


At the MOSFETs (power off of course since the daughter board is now removed) I measure:
(+ probe always contacting PIN SX)

PIN---------U301------U302
S1G1------11.82k-----2.55M
S1D1------2.61 M-----2.53M
S2G2------20.04k-----20.11k
S2G2-------INF---------INF

Of the above measurements, there’s one which does not make sense to me, there are others I do not have enough knowledge at this point to judge. The one that does not make sense is the difference between the S1G1 measurements at the MOSFETS.

U301 was the one that had obviously (visibly) failed, I’m wondering whether related components have failed and are producing the anomaly. Following this up I measured:


On the board, Q301 (RC4, SRC1204S) measures:

PIN-------------PIN-----------------Resistance
1 (Base)-------3(Collector)------3.63M
3----------------1--------------------Infinity(Inf)
1----------------2 (Emitter)-------83.2k
2----------------1--------------------83.1k
2----------------3--------------------3.59M
3----------------2--------------------Inf



On the board, Q302 (RA4, SRA2204S) measures:

PIN-------------PIN-----------------Resistance
1 (Base)-------3(Collector)------ Inf
3----------------1--------------------3.54M
1----------------2 (Emitter)-------92.7k
2----------------1--------------------92.6k
2----------------3-------------------- Inf
3----------------2--------------------3.62M


On the board, Q1 (RA4, SRA2204S) measures:

PIN-------------PIN-----------------Resistance
1 (Base)-------3(Collector)------ 115.4k
3----------------1--------------------115.5k
1----------------2 (Emitter)-------92.2
2----------------1--------------------92.6k
2----------------3-------------------- 207.6k
3----------------2--------------------207.5k


On the board, Q2 (2F, AUK SBT2907A) measures:

PIN-------------PIN-----------------Resistance
1 (Base)-------3(Collector)------ 46.7k
3----------------1--------------------47k
1----------------2 (Emitter)-------47.5k
2----------------1--------------------47.5k
2----------------3-------------------- 590 Ohm
3----------------2--------------------582 Ohm


To my untrained eye, Q1 & Q2 are suspicious because of the base - collector and base - emitter measurements.

So, questions:

1. Are the cross ref parts I have selected suitable? Data Sheets attached.
2. Do the measurements at the MOSFETS and transistors indicate what I think, that Q1 & Q2 are failed?
3. Are there any other things I need to check, replace or that appear suspicious and bear further investigation?

Thanks!!!!!

D

[magistertc]

Re: Samsung 940N No Backlight - BN400121A Analog Board

My daughter board is marked 'PowerNet PWI1704'

I researched RA4/RC4 a bit, decided they might be MOSFETS (I might be wrong!) and found some possible substitutes:

Q301, RC4, N-chan?: Si2304 or BSS138

Q302, RA4, P-chan?: Si2303 or BSS84

If you have diode test option on your multimeter, you can do a basic mosfet test, negative lead to source, touch positive lead to gate, then switch to drain to check whether it is switched 'on'

Touch finger to source and gate simultaneously, reading should rise dramatically. For P-chan, reverse polarity of leads.

Q302 gate did need some pulldown, I connected a one meg SMD resistor from the gate to ground to hold Q302 gate low when ENA signal was off. As always, the soldering was very finicky work....

Addendum: I did unsolder the parts for measurement, Q1 appeared ok, Q302 did not and was replaced.

[C152Heavy]

Re: Samsung 940N No Backlight - BN400121A Analog Board

@magistertc

Can I respectfully say I'm skeptical about the FET angle?

With the AUK or equivalent data sheets I see that the RA4 & RC4 are both pre-biased with 47k resistors and that's what I think the measurements I have made (post #11) would seem to indicate.

In the case of the 2F, the Fairchild part has the same markings and comparable specs to the AUK part.

Finally, there are other parts (discretes on the motherboard) which are AUK manufacture so it wouldn't surprise me that they would use a common supplier, AUK and Powernet are Korean and the data sheets are dated from the era of manufacture of the monitor.

OK, that said, have I made a valid observation that Q1 & Q2 are suspicious base on the measurements I have made?

Any other measurements to make?

I welcome any input forum members may have.

Thanks,

D

[magistertc]

Re: Samsung 940N No Backlight - BN400121A Analog Board

@C152Heavy
I'm inclined to agree with you. I did not find those AUK datasheets, which do match the Q301/302 (RA4/RC4) markings. Clearly, a SMD transistor with sufficient mA capacity would work in that circuit. Replacing Q302 with a MOSFET out of my junk box solved my problem, so I stopped searching at that point.

Tom

[C152Heavy]

Re: Samsung 940N No Backlight - BN400121A Analog Board

OK, I'm inclined to put in another order to Digikey for replacements for Q1 & Q2 but before I go there, are there any other measurements which should be done or do any of the measurements I have posted have any indications of things to check out first?

I'm wondering about the measured voltages at the 9 pin connector I mentioned in post #11 and those at the pins between the mother and daughter boards.

Thanks,

D

[C152Heavy]

Re: Samsung 940N No Backlight - BN400121A Analog Board

OK, I'm inclined to put in another order to Digikey for replacements for Q1 & Q2 but before I go there, are there any other measurements which should be done or do any of the measurements I have posted have any indications of things to check out first?

I'm wondering about the measured voltages at the 9 pin connector I mentioned in post #11 and those at the pins between the mother and daughter boards.

Thanks,

D

All quiet out there?

No-one out there with any more advice?

I've been away but am planning to put in an order for parts so any advice on the measurements or observations would be welcomed.

Tks,

D

[C152Heavy]

Re: Samsung 940N No Backlight - BN400121A Analog Board

OK, I'll follow up my last posting.

I received the parts from Digikey to replace the components suspected on the daughter board and.....no change. From what I can tell from the measurements I made (not being trained in the art of electronics), I replaced good components.......

So, to summarize, to date I have replaced all of the semiconductors on the main board (in part due to my finger trouble, see above) and the FETs and several transistors on the daughter board with no success in bringing the monitor back light to life.

At this point I'm at the point of taking the monitor to the recyclers unless someone can point me in the direction of some more troubleshooting and testing, any takers?

I'd hate to throw this thing out, our landfills are too full already of electronics that are old before their time.

FWIW, I have attached the Service Manual but unfortunately the inverter board appears to be different from what is shown.

Anyhow, any further help woud be appreciated.

Cheers,

C152Heavy

[C152Heavy]

Re: Samsung 940N No Backlight - BN400121A Analog Board

OK, I'll follow up my last posting.

I received the parts from Digikey to replace the components suspected on the daughter board and.....no change. From what I can tell from the measurements I made (not being trained in the art of electronics), I replaced good components.......

So, to summarize, to date I have replaced all of the semiconductors on the main board (in part due to my finger trouble, see above) and the FETs and several transistors on the daughter board with no success in bringing the monitor back light to life.

At this point I'm at the point of taking the monitor to the recyclers unless someone can point me in the direction of some more troubleshooting and testing, any takers?

I'd hate to throw this thing out, our landfills are too full already of electronics that are old before their time.

FWIW, I have attached the Service Manual but unfortunately the inverter board appears to be different from what is shown.

Anyhow, any further help woud be appreciated.

Cheers,

C152Heavy

It's awfully quiet out there, did I offend someone?



D

[magistertc]

Re: Samsung 940N No Backlight - BN400121A Analog Board

Nope -- just that this model is probably 10 years old and by now there are not too many in use. The one I worked on went back to the owner so I do not have access to it.

[C152Heavy]

Re: Samsung 940N No Backlight - BN400121A Analog Board

Nope -- just that this model is probably 10 years old and by now there are not too many in use. The one I worked on went back to the owner so I do not have access to it.

OK.

That is about the age of all the monitors I have re-capped etc. so far though.

Being hard headed I guess I'll just have to attack this a little differently. I wonder if there's somewhere I can read up on troubleshooting inverters and work from the ground up.



I'm guessing it would start with trying to figure out if the signal which tells the inverter control chip to drive the MOSFETS is being given (and what that signal looks like, whether it is a quick transient or an on and hold voltage level) and working back or forward from there.

Maybe there's a way to trick the chip (or the various protection) to drive long enough to blink the back light or take some measurements without risking hardware damage??

BTW, I have replaced the the daughter board stand-offs with board to board connectors so I can now quickly (and safely) remove it for inspection.


I already know the back light tubes are good and the logic/video board works because I drove the back lights (only) with an inverter board from another monitor. I got the "no input, check monitor cable" message using the video/logic board turned on and a blank white screen when the logic/video board was turned off.

Conclusion: The fault must be on the inverter (and power supply) board and is either:

a) In the fault detection & shut down circuit
b) In the inverter control chip (what are the chances of damage to that one??)
c) In the (already replaced) MOSFETS
d) In another component on the small PCB that mounts the inverter control chip, O2 Micro; OZ960, the MOSFETs, a few transistors (replaced, as per above) some dual diode chips (BAV99, appear to measure OK in Diode mode) in the feedback circuit or another TBD component. Enable (ENA appears to be 1.5VDC.....)

The one thing that I find curious is the difference in the S1G1 measurements between the two MOSFETs, see below:

At the MOSFETs (power off of course since the daughter board is now removed) I measure:
(Ohms setting and + probe always contacting PIN SX)

PIN---------U301------U302
S1G1------11.82k-----2.55M
S1D1------2.61 M-----2.53M
S2G2------20.04k-----20.11k
S2G2-------INF---------INF

Of the above measurements, there's one which does not make sense to me, there are others I do not have enough knowledge at this point to judge. The one that does not make sense is the difference between the S1G1 measurements at the MOSFETS.

Using the diode test function of the multimeter I get an odd measurement as well:

PIN---------U301------U302
S1G1------1.82V------0.585V
G1S1------1.82V------0.66V

The U301 measurements are to me suspicious but I have already replaced the MOSFETS although before I replaced some of the other transistors. Not sure what to think there.

Anyone knowledgeable on MOSFETS?

And the odyssey continues......

D