700G no backlights

[sceva]

The backlights on the monitor will not come on. Not even flicker! I do see the image on the lcd with a flashlight, so that part seems ok.

I checked the caps in the inverter area with an esr meter, dropping some off the board just to make sure, and they all tested fine. I am getting 3v from the logic board for the ON to the inverter side. Is that high enough?

With it powered on, I checked voltage at the inverter transformers. Nothing on the secondary side of either one. On the primary side, I get 12v when I turn it on, and it immediately begins trailing down to almost 0v over about a minute or so.

I tried plugging in other ccfl's, but no change. I removed the two transformers and tested them with ring tester, and they showed ok (2 yellow lights on the Blue ring tester). I looked for a fuse but couldn't find anything on that part of the board. I am stumped!

[PlainBill]

Re: 700G no backlights

The backlights on the monitor will not come on. Not even flicker! I do see the image on the lcd with a flashlight, so that part seems ok.

I checked the caps in the inverter area with an esr meter, dropping some off the board just to make sure, and they all tested fine. I am getting 3v from the logic board for the ON to the inverter side. Is that high enough?

With it powered on, I checked voltage at the inverter transformers. Nothing on the secondary side of either one. On the primary side, I get 12v when I turn it on, and it immediately begins trailing down to almost 0v over about a minute or so.

I tried plugging in other ccfl's, but no change. I removed the two transformers and tested them with ring tester, and they showed ok (2 yellow lights on the Blue ring tester). I looked for a fuse but couldn't find anything on that part of the board. I am stumped!

Good pictures, thanks. A couple of points to check. What is the voltage across C207? Many of the pins on U301 and U302 are common. The pins are numbered counter clockwise from the lower right corner. I believe 2-4 are connected together and 5-8 are connected together. Check for shorts between 1 and 2, 1 and 8, and 2 and 8 of each IC.
Also, what is the voltage between pin 6 and pin 7?

PlainBill

[retiredcaps]

Re: 700G no backlights

I checked the caps in the inverter area with an esr meter, dropping some off the board just to make sure, and they all tested fine.

What about the little "startup cap"? It is usually 2.2uF 50V or 22uF 50V.

I am getting 3v from the logic board for the ON to the inverter side. Is that high enough?

I have only seen 3.3V, not 3V.

With it powered on, I checked voltage at the inverter transformers. Nothing on the secondary side of either one. On the primary side, I get 12v when I turn it on, and it immediately begins trailing down to almost 0v over about a minute or so.

I have a similar dropping voltage problem right now. You can follow along at

https://www.badcaps.net/forum/showthread.php?t=10451

[sceva]

Re: 700G no backlights

What is the voltage across C207?

17.08V

Many of the pins on U301 and U302 are common. The pins are numbered counter clockwise from the lower right corner. I believe 2-4 are connected together and 5-8 are connected together. Check for shorts between 1 and 2, 1 and 8, and 2 and 8 of each IC.

I get the same readings on each IC, with meter set to 200k:
Black probe to 1/Red probe to 2: 127
Red probe to 1/Black probe to 2: 136
1 and 8: discharge and then charge up to 1, and then repeat the process when I swap the black and red probes. 2 and 8 does the same.

Also, what is the voltage between pin 6 and pin 7?

Not sure what you mean, as I think pins 5-8 are soldered together - are you still referring to U301/2?
I did check voltage on U301/2 with it powered on:
1 = 0v
2 = 7.5v
3 = 17v
4 = 16.5v
5-8 = 0v

(the first time I tested the voltage it was lower: 0, 1.76v, 16.3v, 15.8v on pins 1-4. I turned it off/on and tested it several times more and got the voltages above...)

little "startup cap"...

I think you are referring to the cap at the bottom right corner of the large transformer. It is a 47u/25v. It tests .2 on the esr meter, and has 11.6v when powered up.

I have only seen 3.3V, not 3V.

Tested it again today, and it is 2.9v.

What is the name of u301? Is it a transistor of sorts? Thanks for the help.

[PlainBill]

Re: 700G no backlights

Not sure what you mean, as I think pins 5-8 are soldered together - are you still referring to U301/2?

What is the name of u301? Is it a transistor of sorts? Thanks for the help.

Sorry, my mistake. I meant the voltage between pins 6 and 7 of U303 - the inverter controller.

U301 is a power FET in an 8 pin package.

PlainBill

[sceva]

Re: 700G no backlights

Sorry, my mistake. I meant the voltage between pins 6 and 7 of U303 - the inverter controller.

As my partner's father used to say, "it ain't a mistake if you can work your way out of it." My other two favorites of his were: "Some jobs you just have to walk away from." and finally, "You can stay awfully busy if you work for nothing."

I get 3.4 or 3.5v on said pins.

[PlainBill]

Re: 700G no backlights

As my partner's father used to say, "it ain't a mistake if you can work your way out of it." My other two favorites of his were: "Some jobs you just have to walk away from." and finally, "You can stay awfully busy if you work for nothing."

I get 3.4 or 3.5v on said pins.

That is definitely 'On'. The threshold is 1 volt.

PlainBill

[sceva]

Re: 700G no backlights

I have just gotten back to this one. Since most of PlainBill's questions centered around the area before the mosfets I have decided to keep working in that direction. Attached is a datasheet for the inverter controller. Here are the readings from the pins on the controller:

pin# - power off - power on
1/INN - 0 - 1.28vdc
2/CMP - 0 - 4.44
3/LOAD - 0 - .7
4/CTOSC - 0 - 1.4
5/TIMER - 0 - 3.94
6/ONOFF - .11 - 3.7
7/GND - 0 - 0
8/NOUT2 - .15 - 0
9/NOUT1 - .25 - 7.56
10/VDD - .25 - 7.57
11/PWMOUT - 0 - 2.02
12/CTPWM - .01 - 1.8
13/PWMDC - .78 - 1.5
14/CLAMP - 0 - 0
15/ISEN - .04 - .4
16/MODSEL - 0 - 0

I saw another post that said if pin 15 is below 1.3Vdc an error signal is set - which it is, and if pin 5 is greater than 2.5Vdc - which it is, the outputs are shut down.

I tried testing the transistors in that area, and only 1 or 2 of them tested normal, but I did not remove any of them from the board yet. None of them seemed to be shorted completely out.

Next I tested the resistors without removing, and below are some unexpected readings:
R312 - 2703 - 90kohm (dmm set at 200k unless noted)
R306 - 104 - 87.3kohm
R310 - 564 - 171.3kohm
R334 - 474 - .391 with dmm at 2M
r325 - 683 - 50.1kohm
R324 - 164 - 88.2kohm
R316 - 683 - 52.4kohm
C321 - 164 - 73kohm (notice the marking on the board is "C"321, not "R"321)
R309 & R304 - 220 - 1.0 with dmm at 200M

I must admit I am puzzled by the C321 with a resistor in place. It does not appear to have been replaced, unless the tech did an awesome job of soldering.

I also removed the inverter transformers and tested with ring tester. The meter only showed 1 or 2 yellow lights for both primary and secondary on both transformers.

Once again I have exhausted my limited knowledge...

[PlainBill]

Re: 700G no backlights

You've done some good work, the problem is trying to figure out what is going wrong. This would take less than 15 minutes with an oscilloscope. Without one, I'm trying to figure out some practical tests.

Pins 8 and 9 are the outputs that drive U301 and U302. They are turned on alternately, with a 'dead' time between on times (so they will never be on at the same time). They will go active for about two seconds (maybe less) whenever the power light is turned on.

If I follow this correctly, the signal from pin 9 of U303 goes under R310, through JR309 to the cathode of D301, to the left end of R304, then from the right end of R304 to either pin 2 or pin 4 of U302 and U301. Please verify.

Now the drive for the other pair of pins had me scratching my head. It appears it is routed through Q303, Q304, Q305, and Q306. Check those transistors, please.

Thinking about it, we REALLY need someone to build a $50 LCD oscilloscope - battery operated and hand held. Put it in the same package as a DMM and capacitance / ESR meter and sell the lot for $150.

PlainBill

[sceva]

Re: 700G no backlights

You've done some good work, the problem is trying to figure out what is going wrong. This would take less than 15 minutes with an oscilloscope. Without one, I'm trying to figure out some practical tests.

If you google "diy oscilloscope" there are several for under $50. Pick one an I will see if I can build it!! Don't know how they work, but it seems they are useful...

If I follow this correctly, the signal from pin 9 of U303 goes under R310, through JR309 to the cathode of D301, to the left end of R304,

When you get to D301, the left side of it is connected to the left side of R304 AND the right side of D301 is connected to the right side of R304.

then from the right end of R304 to either pin 2 or pin 4 of U302 and U301.

Right end of R304 goes to pin 2 of U301 and U302.

Now the drive for the other pair of pins had me scratching my head. It appears it is routed through Q303, Q304, Q305, and Q306. Check those transistors, please.

Just wait until you see the attached drawing - your head will explode! The dashed lines are showing the jumper connections when they pass under other parts. This is the first time I have attempted to map something like this out and when I run across something I don't understand (like both ends of D302 and R309 being connected) it makes my brain think I made a mistake. But I recheck and things are the same... But if you see something that looks totally off let me know and I will triple check.

I checked each of the transistors still on the board with DMM on diode mode. The readings below are based on the assumed pins are as shown in the transistor image attached:

Pins - Q301 - Q302 - Q303 - Q305 - Q306
black probe on B:
BE --- open - open - open - .65 --- .66
BC --- open - open - open - .6 ----open
black probe on E:
EB --- open - open - .74 -- .72 ----1.5
EC --- open - 1.74 --open - .87 ---open
black probe on C:
CB --- open - open - .73 -- open --1.65
CE --- open - open - open -open ---.6

I don't know for sure that the BCE are the correct pins. The part numbers on the transistors are:
Q301 - RKM/18
Q302 - 26/15
Q303 - W04/57
Q305 - W06/57
Q306 - RKM/18

Time for a break!

[jetadm123]

Re: 700G no backlights

i don't know for sure that the bce are the correct pins. The part numbers on the transistors are:
Q301 - rkm/18
q302 - 26/15
q303 - w04/57
q305 - w06/57
q306 - rkm/18

time for a break!

rkm = rk7002
w04 = pmbs3904
w06 = pmbs3906

[PlainBill]

Re: 700G no backlights

Good work. I suspect the purpose of that circuit is level shifting of the gate signals to the mosfets. Fortunately, I just realized a functional test is easy.

You will be working on your choice of U301 or U302. The test is simple. With the monitor plugged in press the power button until the LED turns off. Measure the voltage from pin 2 of U301 or U302 to ground. Press the power button. The power LED should turn green. Does the voltage on pin 2 change? Repeat the test for pin 4 of U301 or U301. Now place the probe on pin 5 of U301 or U302 and repeat the test. Does the voltage change?

I'm not sure if you want an explanation, but you are going to get one. In standby one output pin of U303 will be at 7.5 volts, the other at 0 volts When you press to power button the outputs of IC301 switch fro. When the inverter controller is active the output pins will be switching from 0 volts to about 7.5 volts. This will be reflected on the output pins of U301 and U302.

PlainBill

[sceva]

Re: 700G no backlights

Didn't see a scope you liked?

Pin 2 is .2v with power off, 7.5v with power on.
Pin 4 is 14v with power off, climbs to 16.7v with power on.
Pin 5 is 0v on and off.

I also checked pins 8 & 9 on U303:
Pin 8 is .2v with power off, goes to 0v with power on.
Pin 9 is .1v with power off, 7.5 with power on.

[PlainBill]

Re: 700G no backlights

Didn't see a scope you liked?

Pin 2 is .2v with power off, 7.5v with power on.
Pin 4 is 14v with power off, climbs to 16.7v with power on.
Pin 5 is 0v on and off.

I also checked pins 8 & 9 on U303:
Pin 8 is .2v with power off, goes to 0v with power on.
Pin 9 is .1v with power off, 7.5 with power on.

That would indicate both U301 and U302 are bad.

PlainBill

[sceva]

Re: 700G no backlights

I removed them both from the board and tested them on diode setting:
On each of them All pin combinations were open except:
Red probe on Pin 1 to Pin 7 and/or 8 - .5.
Red probe on Pin 5 and/or 6 to Pin 3 - .5.
And pins 5&6 and 7&8 are closed circuit.

I noticed after removing them that the numbers on them are slightly different:
Both have 4511GM on the first line. Then one has 531026, the other 531016. I found a datasheet for 4511GM and it shows one side is N and the other P channel, so that explains why they are reversed, but not why one is ..16 and the other ..26...

So do you think they still test bad?

[PlainBill]

Re: 700G no backlights

I removed them both from the board and tested them on diode setting:
On each of them All pin combinations were open except:
Red probe on Pin 1 to Pin 7 and/or 8 - .5.
Red probe on Pin 5 and/or 6 to Pin 3 - .5.
And pins 5&6 and 7&8 are closed circuit.

I noticed after removing them that the numbers on them are slightly different:
Both have 4511GM on the first line. Then one has 531026, the other 531016. I found a datasheet for 4511GM and it shows one side is N and the other P channel, so that explains why they are reversed, but not why one is ..16 and the other ..26...

So do you think they still test bad?

Here's the problem in a nutshell. These two guys alternately pull a line down to Gnd, then up to 17 volts. That line is connected to one end of the primary windings of the inverter transformers. The other end of the primary windings are tied to ground.

The inverter controller APPEARS to be putting out a drive signal. That signal appears to be reaching the gates on these FETs. There is no action on the output line. Now, if the drive waveform did not go high enough to turn on either the P or N channel, that is exactly what we would see. With an oscilloscope I could see the waveform. Without it, I'm pretty much blind.

Oh, good grief!!! Sometimes I'm so blind I wonder I can find my ass with both hands!!!

This test will require a small ceramic cap - anywhere from .001 to 1.0uF. Something from a junk box or Radio Shack will be fine. Hook one lead of the cap to the + lead of your DMM. The other lead will be the probe. We want to eliminate DC bias. Hook the - lead to ground. Set the DMM to the 20VAC range. It is NOT necessary to reinstall those two ICs.

Plug in the monitor. Now probe the pad for pin 2 of either U301 or U302 while cycling the power button. Repeat for pin 4. What voltages did you see?

PlainBill

[sceva]

Re: 700G no backlights

This test will require a small ceramic cap - anywhere from .001 to 1.0uF. Something from a junk box or Radio Shack will be fine. Hook one lead of the cap to the + lead of your DMM. The other lead will be the probe. We want to eliminate DC bias. Hook the - lead to ground. Set the DMM to the 20VAC range. It is NOT necessary to reinstall those two ICs.

Plug in the monitor. Now probe the pad for pin 2 of either U301 or U302 while cycling the power button. Repeat for pin 4. What voltages did you see?

I used a .007 cap, soldered it to the probe. Then soldered the other leg of the cap to pin 2. Set DMM to 20VAC and with power off it read .05v. When I turn on the power, nothing happens for about 4-5 seconds, and then it jumps up to about 1.5v - hard to tell as the dmm did not catch it every time - and then it drops back down to .05 after another second or so.
On Pin 4 it reads .3 when off, jumps up just a bit when turn power on, maybe .32, again it is hard to tell with dmm, and my analog meter is not sensitive enough to see any movement. Then a second or so later it drops back lower than .30, maybe .28, and then settles back to .3 when goes to standby.

As for the explanations, keep them coming. If you don't explain I have to go into detective mode and try and figure out what clues I gave or you saw in the pics that lead you to your suggestion.

[PlainBill]

Re: 700G no backlights

I used a .007 cap, soldered it to the probe. Then soldered the other leg of the cap to pin 2. Set DMM to 20VAC and with power off it read .05v. When I turn on the power, nothing happens for about 4-5 seconds, and then it jumps up to about 1.5v - hard to tell as the dmm did not catch it every time - and then it drops back down to .05 after another second or so.
On Pin 4 it reads .3 when off, jumps up just a bit when turn power on, maybe .32, again it is hard to tell with dmm, and my analog meter is not sensitive enough to see any movement. Then a second or so later it drops back lower than .30, maybe .28, and then settles back to .3 when goes to standby.

As for the explanations, keep them coming. If you don't explain I have to go into detective mode and try and figure out what clues I gave or you saw in the pics that lead you to your suggestion.

Excellent!!! That shouldn't be enough voltage swing to trigger the FETs. Now check pins 8 and 9 of the inverter controller.

PlainBill

[smason]

Re: 700G no backlights

I'm not sure if you want an explanation, but you are going to get one.

Thanks Bill!
Though there have been a few "just throw me the damn fish!" folks blow through here, many of us are in the "learning to fish" camp.
We definitely appreciate the gory details that you're so good at providing.

Keep up the excellent work!

[sceva]

Re: 700G no backlights

Excellent!!! That shouldn't be enough voltage swing to trigger the FETs. Now check pins 8 and 9 of the inverter controller.

PlainBill

You are easily excited...

Pin 8 reads .33 off, about 4 seconds after turning on it drops down to around .002 and then goes back up to .33 and remains when lcd goes into standby.
Pin 9 reads .03 off, about 4 seconds after turning on it jumps up to as high as 1.74 (did it several times) then drops back down to .03 and remains there.