Mag LT916s Model 900p 19" LCD dropping like flies

[A Man]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

the bridge rectifier is the black "box" below the "Caution" warning.

Any ideas how to test this?

[PlainBill]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

Any ideas how to test this?

With an ohmmeter. On the lowest resistance scale, resistance between all pins should read open. You can test it in circuit.

PlainBill

[A Man]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

With an ohmmeter. On the lowest resistance scale, resistance between all pins should read open. You can test it in circuit.

PlainBill

I only have an autoranging Fluke meter. It does have a diode check function. Using the ohm meter, I get 1meg plus (it fluctuates) from the top pin to the other three and continuity between pins 2, 3 and 4. I show 0.564v in the diode check mode from pin 1 to 2, 3 & 4 with the negative lead on pin 1 and nothing when I reverse the leads.

Can you help me decipher this?

Thanks!

[PlainBill]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

I only have an autoranging Fluke meter. It does have a diode check function. Using the ohm meter, I get 1meg plus (it fluctuates) from the top pin to the other three and continuity between pins 2, 3 and 4. I show 0.564v in the diode check mode from pin 1 to 2, 3 & 4 with the negative lead on pin 1 and nothing when I reverse the leads.

Can you help me decipher this?

Thanks!

No, because I'm not sure what you are saying.

Make the following tests:

1 - 2, 1 - 3, 1 - 4

2 - 3, 2 - 4

3 - 4

All should read greater than 1K, possibly greater than 1 Meg (a reading might start low, then rise. That is OK). A steady reading of less than 100 ohms is a sign of trouble. A steady reading of less than 1 ohm is a sign of a short.

PlainBill

[Regenpak]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

Continuity between any pins on a bridge rectifier means it's blown and must be replaced with a possibly beefier one (more current and voltage). The reading of 0.564V is from the only diode that's not blown...

[A Man]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

No, because I'm not sure what you are saying.

Make the following tests:

1 - 2, 1 - 3, 1 - 4

2 - 3, 2 - 4

3 - 4

All should read greater than 1K, possibly greater than 1 Meg (a reading might start low, then rise. That is OK). A steady reading of less than 100 ohms is a sign of trouble. A steady reading of less than 1 ohm is a sign of a short.

PlainBill

Ok, it measured less than one ohm from 2-3, 2-4 and 3-4 so it's shorted. I've had some basic electronics classes ages so long ago I wasn't sure exactly what I was testing so I took four seperate diodes and made a bridge. I used the diode test function and made some tests concluding I should also have twice the voltage that I am showing further proving this rectifier is toast. Hey, it even looks like a little slice of toast now that I think about it lol

A few more questions... is this a common occurance or did this happen when I shorted the fuse against the heat sink of the transistor? Is there a good way to test the transistor? Is this defective rectifier the reason the replacement fuse blew immediately? My theory is shorting the fuse shorted the rectifier and that causes an immediate fault in a new fuse.

Again, thanks for your help and how are the eyes?

[PlainBill]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

Ok, it measured less than one ohm from 2-3, 2-4 and 3-4 so it's shorted. I've had some basic electronics classes ages so long ago I wasn't sure exactly what I was testing so I took four seperate diodes and made a bridge. I used the diode test function and made some tests concluding I should also have twice the voltage that I am showing further proving this rectifier is toast. Hey, it even looks like a little slice of toast now that I think about it lol

A few more questions... is this a common occurance or did this happen when I shorted the fuse against the heat sink of the transistor? Is there a good way to test the transistor? Is this defective rectifier the reason the replacement fuse blew immediately? My theory is shorting the fuse shorted the rectifier and that causes an immediate fault in a new fuse.

Again, thanks for your help and how are the eyes?

I would do a similar test on the power FET. Any short indicates it is bad. And yes, shorting the fuse to the heat sink could have damaged the rectifier. A shorted rectifier will blow the fuse immediately.

I came through the surgery OK. Right now I have a shield over the eye. I'll update the report after seeing the doctor tomorrow.

PlainBill

[A Man]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

Continuity between any pins on a bridge rectifier means it's blown and must be replaced with a possibly beefier one (more current and voltage). The reading of 0.564V is from the only diode that's not blown...

This rectifier is a KPB206 spec'd out at 2A and 600V. Will a 4A 400V work as a replacement?

Thank you for your help!

[EGuevarae]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

Is this defective rectifier the reason the replacement fuse blew immediately?

[EGuevarae]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

I came through the surgery OK. Right now I have a shield over the eye. I'll update the report after seeing the doctor tomorrow.

PlainBill

I was about to ask about the same. I hope all went exactly like the 1st eye, or better ....

[PlainBill]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

I was about to ask about the same. I hope all went exactly like the 1st eye, or better ....

It didn't go as well. There has been more discharge from the eye. Right now it feels like I have about 5 lbs of goop on the eyelashes of my right eye. On the other hand, vision out of that eye is already better than before the surgery. The other problem is I have NO glasses which work at computer distances, so I'm typing at arm's length. (I KNEW I should have learned to touch type).

PlainBill

[A Man]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

This rectifier is a KPB206 spec'd out at 2A and 600V. Will a 4A 400V work as a replacement?

Thank you for your help!

Anyone know if this will work properly or not? I went ahead and replaced this after testing the FET (no shorts and 500 ohms). It no longer blows the fuse. I have 160VDC (is this enough/too much for this application?) leaving the rectifier but no outputs on the 5 or 12VDC.

Where do I go next from here?

[PlainBill]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

Anyone know if this will work properly or not? I went ahead and replaced this after testing the FET (no shorts and 500 ohms). It no longer blows the fuse. I have 160VDC (is this enough/too much for this application?) leaving the rectifier but no outputs on the 5 or 12VDC.

Where do I go next from here?

The rectifier will work fine on 120VAC, but is marginal for 240VAC. It's not the cause of the lack of output voltage.

Double check the orientation of the caps, especially the 10uF and 100µf. Then check the voltage across the large capacitor (it should be 1.4 x input voltage), and the voltage on the power input pin of the SMPS controller. If you need help, post a picture of the bottom side of the board and the part number of the SMPS controller.

PlainBill

[A Man]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

The rectifier will work fine on 120VAC, but is marginal for 240VAC. It's not the cause of the lack of output voltage.

I am powering with 120.

Double check the orientation of the caps, especially the 10uF and 100µf. Then check the voltage across the large capacitor (it should be 1.4 x input voltage),

I have not replaced either of these caps on the input side as they were not bulging, only the output filters. However, I have 160V which is roughly 1.4x 120VAC on the output of the bridge as well as across the 100uF.

and the voltage on the power input pin of the SMPS controller. If you need help, post a picture of the bottom side of the board and the part number of the SMPS controller.

PlainBill

I'm not sure about this... I posted pics of both side of the board earlier though.

The eye any better?

[PlainBill]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

I am powering with 120.

I have not replaced either of these caps on the input side as they were not bulging, only the output filters. However, I have 160V which is roughly 1.4x 120VAC on the output of the bridge as well as across the 100uF.



I'm not sure about this... I posted pics of both side of the board earlier though.

The eye any better?

The eye is fine.

This line of monitors is notorious for failing to power up when the two small caps on the primary side of the power supply fail. Caps in the 10 - 100 uF, 50 Volt range won't bulge when they fail. And what is the part number of the SMPS controller - the 8 pin SMT IC on the bottom of the board in the power supply error?

PlainBill

[A Man]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

This line of monitors is notorious for failing to power up when the two small caps on the primary side of the power supply fail. Caps in the 10 - 100 uF, 50 Volt range won't bulge when they fail. And what is the part number of the SMPS controller - the 8 pin SMT IC on the bottom of the board in the power supply error?

PlainBill

My mistake... I actually did replace the two small 10uF 50V and the two 100uF 25V electrolytics. They are oriented correctly. I did not replace the large 100uF 400V nor the small 47uF 50V caps just to the right of the large one.

The 8 pin IC is a 20306.

[Scenic]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

the large primary cap (100uF 400V) usually doesn't fail, but the 47uF 50V is most likely one of those plainbill was talking about.
when those dry up, the powersupply (usually) just won't start up. even if everything else is OK.

[A Man]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

The 8 pin IC is a 20306.

My mistake... AGAIN!

Upon a closer inspection, it is a 203D6 PQGE

I just replaced the 47μF 50V cap and it's still dead.

[PlainBill]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

My mistake... AGAIN!

Upon a closer inspection, it is a 203D6 PQGE

I just replaced the 47μF 50V cap and it's still dead.

That would appear to be a NCP1203D60R2. Points to check. Pin 4 should go the the - lead of the large cap. Pin 8 should go to the + lead of the large cap - possibly through a resistor. Pin 8 should go to the gate of the power FET. Verify those, please.

PlainBill

[A Man]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

That would appear to be a NCP1203D60R2. Points to check. Pin 4 should go the the - lead of the large cap. Pin 8 should go to the + lead of the large cap - possibly through a resistor. Pin 8 should go to the gate of the power FET. Verify those, please.

PlainBill

Yes, pin 4 is on the ground side of the cap and shows continuity. Pin 8 does go to + of the cap through two 23K SMT resistors. When I connect power, I have 16V at this pin. However, I believe pin 5 (not 8) is the driver output for the FET according to the datasheet schematic I attached. But hey, I''ve never done any of this kind of troubleshooting! ... I work on natural gas compressors lol. Anyway, looking at the datasheet for the 7N60B FET, I believe the gate is the pin I circled in blue. I'm having a hard time physically seeing a connection to pin 5 through all of the SMDs but ohming to pin 5 of the SMT IC, I get 10.2 ohms.

We are making progress, right?