Mag LT916s Model 900p 19" LCD dropping like flies

[PlainBill]

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

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?

Yes, we're making progress. First of all, while I RARELY admit to making mistakes, I will admit to typos. And you are right, pin 5 is the drive output. That 8 I posted must have been a typo.

What is the voltage on pin 6 (Vcc)?

PlainBill

[A Man]

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

What is the voltage on pin 6 (Vcc)?

PlainBill

0.426VDC to pin 4

[PlainBill]

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

0.426VDC to pin 4

That leads me to believe the controller is defective. There should be approximately 10 volts on that pin. Check to verify the power FET is not shorted, and none of the output diodes are shorted. The part is inexpensive - only $1.15 from Mouser. Unfortunately, it's on backorder.

PlainBill

[A Man]

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

That leads me to believe the controller is defective. There should be approximately 10 volts on that pin. Check to verify the power FET is not shorted, and none of the output diodes are shorted. The part is inexpensive - only $1.15 from Mouser. Unfortunately, it's on backorder.

PlainBill

I'm not sure about the IGBT 7N60B... Using the diode check in circuit, it shows 0.795V from gate to emitter and 1.326V from gate to collector w positive lead on gate. If I put the negative lead on the gate, it measures 0.645V from gate to emitter and open from gate to collector. There are two other diodes that check ok.

Any other tests or suggestions? What exactly is happening (or not!) in this circuit? This is my first SMPS to ever work with. From my understanding the controller pulses the FET which causes the transformer/coil to "fire".

[PlainBill]

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

I'm not sure about the IGBT 7N60B... Using the diode check in circuit, it shows 0.795V from gate to emitter and 1.326V from gate to collector w positive lead on gate. If I put the negative lead on the gate, it measures 0.645V from gate to emitter and open from gate to collector. There are two other diodes that check ok.

Any other tests or suggestions? What exactly is happening (or not!) in this circuit? This is my first SMPS to ever work with. From my understanding the controller pulses the FET which causes the transformer/coil to "fire".

That's not the terms I would use to describe it's operation, but you are essentially correct.

Rectified line voltage is applied to pin 8. The pair of resistors are optional. A constant current source charges the cap connected to pin 6 until it reaches 12.8 volts. At that time it turns on the FET, feeding energy into the transformer primary. The transformer secondaries are rectified, and one of them is used to control an optoisolator which feeds the voltage sense input on the controller. A separate input is used to limit the current through the FET. A tertiary winding on the transformer is rectified and used to charge the cap connected to pin 6.

One characteristic of this controller is the start-up cycle. The constant current source charges the cap until it reaches 12.8 volts. It then turns off. In normal operation the tertiary winding will provide power to operate the SMPS controller. If that fails to happen (bad FET, shorted diode, etc), the constant current source will kick in again when the voltage across the cap drops down to about 5 volts, and again charge the cap up to 12.8 volts.

PlainBill

[Curious]

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

I apologize if this question has been answered or perhaps I needed to create a new thread. I am having the same issues has described in this thread, how exactly does one get into/take apart/open(whatever the term is)this LCD? Opening it has been the hardest part thus far.

Thank you

[retiredcaps]

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

how exactly does one get into/take apart/open(whatever the term is)this LCD?

Careful with the connectors, but something like

http://www.youtube.com/watch?v=ZvQdY...eature=related

[Curious]

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

Careful with the connectors, but something like

http://www.youtube.com/watch?v=ZvQdY...eature=related

Thank you!!

[Shkoda]

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

I hope it will help You...

[keithf2]

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

I have a Monitor MA-982KC Model 900P. This monitor comes on for about 2 to 3 seconds then the screen goes black, no sign of a picture can be seen, even in a dark room or with a torch. With the monitor connected to a computer a full picture can be seen until the screen goes black. Voltages on the pins of the plug 5v OK, 12v OK Bri reads 2.8v even when the screen black. Blon reads 3.96v, when the screen goes black this drops to zero. When the screen goes black the power LED turns orange. Have changed the 6 x 470 micro fahad capacitors, 1 x 220 micro fahads, 2 x 100 micro fahads and 2 x 10 micro fahads.
and the problems remains the same.
Any comments would be appreciated.

[retiredcaps]

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

I have a Monitor MA-982KC Model 900P.

Post clear focused pictures after reading

https://www.badcaps.net/forum/showthread.php?t=1868

You can also try some of the suggestions at

https://www.badcaps.net/forum/showthread.php?t=10419

[PlainBill]

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

I have a Monitor MA-982KC Model 900P. This monitor comes on for about 2 to 3 seconds then the screen goes black, no sign of a picture can be seen, even in a dark room or with a torch. With the monitor connected to a computer a full picture can be seen until the screen goes black. Voltages on the pins of the plug 5v OK, 12v OK Bri reads 2.8v even when the screen black. Blon reads 3.96v, when the screen goes black this drops to zero. When the screen goes black the power LED turns orange. Have changed the 6 x 470 micro fahad capacitors, 1 x 220 micro fahads, 2 x 100 micro fahads and 2 x 10 micro fahads.
and the problems remains the same.
Any comments would be appreciated.

This problem is on the logic card, the fact that the LED goes yellow with signal present demonstrates that. Changing the caps on the power supply may eliminate that; what brand and series did you use?

PlainBill

[keithf2]

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

This problem is on the logic card, the fact that the LED goes yellow with signal present demonstrates that. Changing the caps on the power supply may eliminate that; what brand and series did you use?

PlainBill

Capacitors are marked NRSA, CE 85 degree, (M), T0258.
Keith

[PlainBill]

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

Capacitors are marked NRSA, CE 85 degree, (M), T0258.
Keith

Well, that's part of the problem. The replacement caps aren't close to what the power supply needs to work properly. This threadhttps://www.badcaps.net/forum/showthread.php?t=2280 has information on brands and series of caps that will work. If you live in the US, I recommend buying Panasonic FM caps from Digikey. If you choose USPS First Class Mail shipping the total cost would be about $10.

PlainBill

[keithf2]

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

I tried to send you the circuit diagrams of my 900p monitor, but got a message I did not attache a security tag, how do you do this,
keith

[Shkoda]

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

emilije's request:

[tollfree]

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

Hi guys. My experiments have lead to the worse: I've completely broken the 32 pin board-to-cable connector, the one in the middle of the circuit board on the TFT panel's back, which is HSD190ME12-A00. Any experience would be extremely welcome: it doesn't look like I could solder those pins back together. Shall I find and replace the entire board - what is its make, find a connector and re-solder it - is it possible to do a 32 pin one with a regular iron, and where to look for such a connector, or maybe its better to just forget about it. Thank you for any input!

[Wrog]

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

Hi guys. My experiments have lead to the worse: I've completely broken the 32 pin board-to-cable connector, the one in the middle of the circuit board on the TFT panel's back, which is HSD190ME12-A00. Any experience would be extremely welcome: it doesn't look like I could solder those pins back together. Shall I find and replace the entire board - what is its make, find a connector and re-solder it - is it possible to do a 32 pin one with a regular iron, and where to look for such a connector, or maybe its better to just forget about it. Thank you for any input!

Pics of the cables (plus macro shots of the connectors) would be helpful here.

[tollfree]

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

Pictures are terrible, sorry. The cable is male, and the on board connector is female -- in pic 6 this broken piece is diagonal above the soldered pins.

[tollfree]

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

The picture I've mentioned where you can see the broken piece and soldered to the board pins (kind of see, since pictures are really bad) is "connector 048", and it is # 6 on the list in the previous message. I fill totally lost about that stupid brake and await on help from the experienced people. Be grateful for any input. Thanks.