Troubleshooting Mag LT865B

[Welchs101]

Re: Troubleshooting Mag LT865B

ok. I found U7.

[Welchs101]

Re: Troubleshooting Mag LT865B

ok. found u7.

[PlainBill]

Re: Troubleshooting Mag LT865B

ok. I found U7.

Good, I was getting a little worried.

For what it's worth, U7 processes the inputs from the front panel buttons and also switches the power LED by turning the appropriate colors on or off. It is the I/O processor for U10.

PlainBill

[Welchs101]

Re: Troubleshooting Mag LT865B

Hi Bill,

Here is what i did. I removed the main board disconnecting it from the speakers and inverter board. Removed main board entirelly from unit. I connected the on/off controls to the main board. Powered up the main board with the "power adapter" cord.

Using multimeter set to dc volts:

Placed black probe at gnd. Placed red probe at pin 44. Verified 5.31 volts. Place red probe at pin 22...verified pin 22 is grnd. Then i placed red probe at pin 10 and pressed the on/off button........NO CHANGE in voltage.

David

[Welchs101]

Re: Troubleshooting Mag LT865B

ok. Just fumbling around i found something and wanted to get some input from you all.

See pic. Device circled in red. It has the following markings on the case:
First line: "WST"
2nd line: "TL 431" ........... or its ............"TL 131" (hard to tell)
3rd line: "-A238"

(i have enclosed a pic of the actual device......best i could do)


When i measure (in ohms) from pin 1 to pin 3 i get a short. Pin 2 to 3 is ~ kOhm, pin1 to 3 ~ kOhms.

Should pins 1 and 3 be shorted. If the marking TL431 is correct then this thing is a "shunt regulator" (i guess) and its hard to tell if pins 1 and 3 should be shorted.

[PlainBill]

Re: Troubleshooting Mag LT865B

Hi Bill,

Here is what i did. I removed the main board disconnecting it from the speakers and inverter board. Removed main board entirelly from unit. I connected the on/off controls to the main board. Powered up the main board with the "power adapter" cord.

Using multimeter set to dc volts:

Placed black probe at gnd. Placed red probe at pin 44. Verified 5.31 volts. Place red probe at pin 22...verified pin 22 is grnd. Then i placed red probe at pin 10 and pressed the on/off button........NO CHANGE in voltage.

David

We have a failure to communicate. Pin 10 is Reset, and should not change. It should be 0 volts (I think, sometimes these designations are wrong). Pin 2 is the input from the power button.

TD1 is a TL-431, and pins 1 and 3 are shorted together (Page 23 of the service manual, left - center of the page).

PlainBill

[Welchs101]

Re: Troubleshooting Mag LT865B

Again, poking around and i have another question. I was looking at what Bill suggested and i started tracing back in the circuit. I came to TC9 (see enclosed pic). I found TC9 on the board but i am not sure where it is.......i mean i see the letters "TC9" but i am not sure where the device is.

Is it the "circle" where a cap should be?

[Welchs101]

Re: Troubleshooting Mag LT865B

ok Bill. I see pins 1 adn 3 are shorted. Thanks.


Referring to pin10 measurement:
So from what you said ..........the measurement was to be expected that pin 10 does not change.

Ok what now?

[PlainBill]

Re: Troubleshooting Mag LT865B

ok Bill. I see pins 1 adn 3 are shorted. Thanks.


Referring to pin10 measurement:
So from what you said ..........the measurement was to be expected that pin 10 does not change.

Ok what now?

Again, we are failing to communicate. What is the (approximate) voltage at U7 pin 10? As I mentioned earlier, U7 is the control I/O processor. It 'tells' U10 when a button is pushed. U10 in turn commands U7 to turn the power LED on or off depending on the presence of video signal and the On / Off state set by the power button.

We've ad a number of monitors exhibiting the 'no power LED' symptom where the problem is actually the reset controller for the processor.

And does U7 pin 2 change when you push the power button? And what voltages do you see?

PlainBill

[Welchs101]

Re: Troubleshooting Mag LT865B

Hi Bill,

I am enclosing a pic just to make sure that what i think is U7 is indeed U7.

Based on the enclosed pic what i did was the following.

I plugged in power. I verified pin 44 was VCC and Pin 22 was gnd.........i did this mainly to make sure i had the right device.

I then placed measurement probe on pin 10. Pin 10 measured 0volts. Even when i pressed on/off it stayed the same.......0 volts (dc).

Pin2: Mesaured 5.31 volts dc and did not change when i cycled on/off button.

[Welchs101]

Re: Troubleshooting Mag LT865B

NOte: For some reason in my previous pick the letters "U7" did not appear. But the device in question does have a "red" "1" to show where i think pin1 is located.

[PlainBill]

Re: Troubleshooting Mag LT865B

Hi Bill,

I am enclosing a pic just to make sure that what i think is U7 is indeed U7.

Based on the enclosed pic what i did was the following.

I plugged in power. I verified pin 44 was VCC and Pin 22 was gnd.........i did this mainly to make sure i had the right device.

I then placed measurement probe on pin 10. Pin 10 measured 0volts. Even when i pressed on/off it stayed the same.......0 volts (dc).

Pin2: Mesaured 5.31 volts dc and did not change when i cycled on/off button.

Yes, that's U7. 0 Volts on pin 10 is good; that's the reset line. It should go high for a few milliseconds when the monitor is plugged in, then go low and stay low.

Pin 2 is a problem. Locate CN4 on page 24 of the service manual. Note that the function of each line is shown. Pins 4, 5 ,6, and 7 should go low when the corresponding button is pressed.

PlainBill

[Welchs101]

Re: Troubleshooting Mag LT865B

Hi Bill,

Yes, i guess pin 2 is a problem.

I found cn4. From the schematic how did you know they should go low when buttons pressed?

What do i do now?

[PlainBill]

Re: Troubleshooting Mag LT865B

Hi Bill,

Yes, i guess pin 2 is a problem.

I found cn4. From the schematic how did you know they should go low when buttons pressed?

What do i do now?

Look at the circuit. Pins 4-7 are pulled to +5V through RP11 and RP12. Pins 2 & 3 drive the LEDs, pin 1 is not used in this design, pins 4-7 are tied to the buttons and the only other pin is pin 8. The only way for this to work is each of the four lines are pulled low when the corresponding button is pushed. OK, it is theoretically possible that the switches are normally closed, and open when pushed, but that's a more expensive design.

The next step is to measure the voltage on each of pins 4-7 while pushing every button while checking each pin. Then it's time to start measuring resistance on the cable to the front panel board. Maybe there's a break. Or you could try CAREFULLY shorting pin 7 to pin 8 and see if the board responds.

One thing to remember is to not get too attached to a particular diagnosis. Obviously everyone here thinks I know everything (except the 99% who know better). A few months ago I got a call from a guy who wanted to know what it would take to repair his Dell 19" LCD monitor. By the symptoms (have to push the power button multiple times to get it to turn on), it would have to be recapped, but I couldn't be sure until I opened it up. After discussing this, he decided he'd rather have a new wide screen monitor, and an hour later he dropped the dead one off for me.

When I got around to looking at it, I discovered it was a mechanical problem, not an electronic one. There was a tiny nipple on the power button that actually 'clicked' the switch. That had worn down to the point it no longer clicked the switch. After a LOT of work, I discovered a tiny dot of crazy glue would build it back up.

PlainBill

[Welchs101]

Re: Troubleshooting Mag LT865B

Bill,

Thanks. Very informative. I will do as you suggested. I have to go for now. But i will get back to the forum if not today then tomorrow.

thanks!!!!!!!

[Welchs101]

Re: Troubleshooting Mag LT865B

ok. Woke up this morning and started measuring. I am enclosing 3 pics to help explain what i did. I wanted to make sure what i did was what you were asking me to do.

Terminology:
Connector has 8 pins labeled C1 through C8.
Button....there are 5 push buttons labeled B1 through B5.

What i did was plug in power.
Measure C1 through C8 to see what voltages were present.
C1 had 5.3V
C2 had 0.099V
C3 had 0.097V
C4 had 5.3V
C5 had 5.3V
C6 had 5.3V
C7 had 5.3V
C8 had 0.002V

Now what i did was measure voltage on C1 and toggled/push all the buttons.
B1 did toggle voltage on C1 to 0V when B1 pressed. B1 did not effect any other C's.

Next i measured voltage on C2 and toggled/pushed all buttons.
C2 not toggled by any button.

Next i measured voltage on C3 and toggled/pushed all buttons.
C3 not toggled by any button.

Next i measured voltage on C4 and toggled/pushed all buttons.
B4 did toggle voltage on C4 to 0V when B4 pressed. B4 did not effect any other C's.

Next i measured voltage on C5 and toggled/pushed all buttons.
B5 did toggle voltage on C5 to 0V when B5 pressed. B5 did not effect any other C's.

Next i measured voltage on C6 and toggled/pushed all buttons.
B3 did toggle voltage on C6 to 0V when B3 pressed. B3 did not effect any other C's.

Next i measured voltage on C7 and toggled/pushed all buttons.
B2 did toggle voltage on C7 to 0V when B2 pressed. B2 did not effect any other C's.

Next i measured voltage on C8 and toggled/pushed all buttons.
C8 not toggled by any button.


Also, i was just curious how you knew that CN4 was the on/off buttons.

Did i answer all your questions?

[PlainBill]

Re: Troubleshooting Mag LT865B

ok. Woke up this morning and started measuring. I am enclosing 3 pics to help explain what i did. I wanted to make sure what i did was what you were asking me to do.

Terminology:
Connector has 8 pins labeled C1 through C8.
Button....there are 5 push buttons labeled B1 through B5.

What i did was plug in power.
Measure C1 through C8 to see what voltages were present.
C1 had 5.3V
C2 had 0.099V
C3 had 0.097V
C4 had 5.3V
C5 had 5.3V
C6 had 5.3V
C7 had 5.3V
C8 had 0.002V

Now what i did was measure voltage on C1 and toggled/push all the buttons.
B1 did toggle voltage on C1 to 0V when B1 pressed. B1 did not effect any other C's.

Next i measured voltage on C2 and toggled/pushed all buttons.
C2 not toggled by any button.

Next i measured voltage on C3 and toggled/pushed all buttons.
C3 not toggled by any button.

Next i measured voltage on C4 and toggled/pushed all buttons.
B4 did toggle voltage on C4 to 0V when B4 pressed. B4 did not effect any other C's.

Next i measured voltage on C5 and toggled/pushed all buttons.
B5 did toggle voltage on C5 to 0V when B5 pressed. B5 did not effect any other C's.

Next i measured voltage on C6 and toggled/pushed all buttons.
B3 did toggle voltage on C6 to 0V when B3 pressed. B3 did not effect any other C's.

Next i measured voltage on C7 and toggled/pushed all buttons.
B2 did toggle voltage on C7 to 0V when B2 pressed. B2 did not effect any other C's.

Next i measured voltage on C8 and toggled/pushed all buttons.
C8 not toggled by any button.


Also, i was just curious how you knew that CN4 was the on/off buttons.

Did i answer all your questions?

Excellent information. Now we have to figure out why the button presses didn't make it through to the MCU.

As far as figuring out CN4 was for the front panel.... Well, promise me you won't hurt yourself after reading this.

I'm sitting in front of a monitor with buttons labeled Up, Down, Power, Auto, and Menu. There is also an LED over the power button that can be blue, amber, or dark. If you notice the labels on the lines connected to CN4, it's pretty easy to make the correlation.

Also, the front panel information has to be passed to the MCU somehow, it's a matter of tracing the lines.

The datasheet for U7 is available here. You've got the PLCC package, as you surmised, pin 1 is indicated by the dot, then the pins are numbered counterclockwise. Please verify the power and reset signals (pins 22, 44, and 10 I had requested earlier. Also, check that the button presses do make it through RP2.

PlainBill

[Welchs101]

Re: Troubleshooting Mag LT865B

ok.

With board powered up.

Pin44: Is at 5.31V and i pressed each button and there was no change in pin44 voltage.

Pin 10 and Pin 22: Both were at 0.002V and i pressed each button and there was no change in their respective values.

I had a question: What is RP2?

[Welchs101]

Re: Troubleshooting Mag LT865B

ok. Found RP2 (see enclosed pics). Also found RP11 and RP12 (see enclosed)

I am not 100% how to make sure signals are getting through. I assume i measure on both sides of RP2. First measure on one side see if buttons do anything and then measure on the other side and see if the buttons do anything................is this right?

should i also do the same for RP11 and RP12?

[Welchs101]

Re: Troubleshooting Mag LT865B

ok.

I measured voltages on the side of RP2 shown in the enclosed picture. I have labeled the 4 pins coming out of RP2 as 1,2,3,4 (see pic).

1: Is at 5.31V. When i press B2 it goes to 0V.
2: Is at 5.31V. When i press B3 it goes to 0V.
3: Is at 5.31V. When i press B5 it goes to 0V.
4: Is at 5.31V. When i press B4 it goes to 0V.

So does this mean voltages coming into the U7 device are ok and that the U7 device is the problem?