need help with a gateway lp2407/fpd2484

[PlainBill]

Re: need help with a gateway lp2407/fpd2484

I count 5 voltage regulators on the signal card - U601 and U603 near the right hand edge of the card, U602 near the lower edge toward the left; U605 above it and slightly to the left, U604 above U605. Using the DMM measure the voltage on each pin. 3.5V is not a standard output voltage.

PlainBill

[chetreynolds]

Re: need help with a gateway lp2407/fpd2484

With the signal board connected I still have 5 and 24 volts I checked the out of the regulators on the signal board and measured 3.5 and 1.8 which is correct per the data sheets . connected the power switch strip no led .I removed the power switch to insure the ribbon connector was connected .The power switch is a strip that is glued to the front panel . It has a synapocs chip on it there is no mechanical switch .
I connected the inverter no change .

[PlainBill]

Re: need help with a gateway lp2407/fpd2484

I have 5v at the output of d855 and 24v from d853

OK, now why isn't it working??? Have you tried hooking up the signal (logic) card only to the power supply? Let's leave the LCD panel and the inverter disconnected for this test. If you still get 5V and 24V it's time to add loads (front panel to see if the LED comes on, etc.)

As an aside, the other power supply designs I have seen switch the PFC on when turning on the main supply. It appears they use a different design here. The PFC portion of the controller is set to turn on when the load increases to a certain point.

PlainBill

[chetreynolds]

Re: need help with a gateway lp2407/fpd2484

I have 5v at the output of d855 and 24v from d853

[chetreynolds]

Re: need help with a gateway lp2407/fpd2484

pin 5 is 175vdc pin 3 is a 40v 90 percent duty cycle pulse pin 6 50v simular to a sine wave 7 approx 100v wave pin 8 50v wave

[PlainBill]

Re: need help with a gateway lp2407/fpd2484

I have a 12v 2us pulse ay the gate of q850 and pin 12 of the controller nothing at pin 14
pin
1 0v 11 7.9v
2 1.74 mv 12 1v
3 1.65v 13 gnd
4 .835v 14 1v
5 0v 15 15v
6 0v 16 2.2v
7 0v 17 2v
8 5v 18 2.1v
9 1.2v 19 0v
10 1.5v 20 2.5v

Excellent!! That is the drive for the power FET. Now if I can figure out how to SAFELY test T850, the SMPS transformer. Do not probe the source lead of Q850 unless you are using a scope probe rated for at least 1000V.

Got it. Referring to the picture of the bottom side of the power supply, T850 is just to the left of center (good position, politically); Q850 is above it. There are 5 pins for T850. From left to right, they appear to be (1) Hot ground, (2) unused, (3) the tertiary winding, (4) Drive (DANGER!!), and (5) DC input.

Check the DC voltage on pin 5, and see if there are pulses on pin 3. Also, continuing clockwise the pins are numbered 6-11 right to left. Check for pulses on pins 7 and 9-11.

PlainBill

[chetreynolds]

Re: need help with a gateway lp2407/fpd2484

I have a 12v 2us pulse ay the gate of q850 and pin 12 of the controller nothing at pin 14
pin
1 0v 11 7.9v
2 1.74 mv 12 1v
3 1.65v 13 gnd
4 .835v 14 1v
5 0v 15 15v
6 0v 16 2.2v
7 0v 17 2v
8 5v 18 2.1v
9 1.2v 19 0v
10 1.5v 20 2.5v

[PlainBill]

Re: need help with a gateway lp2407/fpd2484

I wont blame you if you give up on me it is almost impossible to trouble shoot when you are getting info from an idiot . I was using my scope for the readings . I rechecked using a dvm and pin 15 was 15 v and 20 was +2.5

One of the problems with troubleshooting with a DMM is it only shows the average voltage. A scope will show activity. 15V is right at the start-up voltage for the controller.

It would be interesting to know if there is a drive signal to either of the power FETs.

PlainBill

[chetreynolds]

Re: need help with a gateway lp2407/fpd2484

I wont blame you if you give up on me it is almost impossible to trouble shoot when you are getting info from an idiot . I was using my scope for the readings . I rechecked using a dvm and pin 15 was 15 v and 20 was +2.5

[PlainBill]

Re: need help with a gateway lp2407/fpd2484

I am a little bit confused is the 10volts a problem and is the reading on pin 20 ok ?

I'm confused too, and at somewhat of a loss as to what to tell you.

Looking at the datasheet for the IC, I can identify some of the components feeding pin 20, but I'm at a loss if the voltage you are seeing is correct. My gut reaction is it is not, but that raises the question - is it the cause or a symptom? Tentatively, I'm saying it's a symptom.

The voltage at pin 15 indicates there is a problem. Ordinarily it should be in the 15 - 20 volt range. Again, I have the same question - why is it low? And are the strange readings due to your desoldering with power on, or the original cause of the problem?

OK, I've got a plan of action. Looking at the circuit, pin 20 should never show a negative voltage. The circuit on the main board is not quite the same as the one in the datasheet. Near the bridge rectifier are two diodes, D858, D859. They are the ultimate source of the voltage at pin 20. Since their anodes are on opposite sides of the AC line, the voltage at their cathodes should never read less than 0 volts. Note that their cathodes are tied together, and they feed resistors R871, R872, R873, and R874. Further note capacitors C857 and C876.

1. Have you replaced C857 and C876? They are part of the startup supply for the controller, and fail without visible signs. The COULD have been responsible for the original failure.

2. With power off, check diodes D858 and D859. If one is bad it could explain the voltage readings.

3. Check resistors R871, R872, R873, and R874. If they are open it could cause problems.

Now, the SG6902SZ PFC / SMPS controller uses the same design to provide start-up power as the SG6841SZ SMPS controller. The SG6841SZ controller is notorious for failing with the same symptoms you are seeing. Given that Mouser has the SG6902SZ for under $2.00 (+ shipping), be prepared to order it AFTER completing step 4.

4. I notice 4 additional diodes in this area, plus I believe there are two more on the daughter board. With power off, check these diodes. Also, visually check the resistors for any sign of damage.

PlainBill

[chetreynolds]

Re: need help with a gateway lp2407/fpd2484

I am a little bit confused is the 10volts a problem and is the reading on pin 20 ok ?

[PlainBill]

Re: need help with a gateway lp2407/fpd2484

Thanks for the heads up PlainBill. I knew something was wrong when you said "bottom of page 3". On the page 3 I was looking at is a block diagram. It turns out there are two data sheets for this chip: the original written in 2007 by System General and the Fairchild one from 2009 I was looking at. The Fairchild datasheet is just an edited version of the SG datasheet. Of course, the Fairchild version left out the "provide start-up current for VDD", but it's probably mentioned somewhere else.

Yup. It's discussed later, but when the offspring of a bachelor rewrote the datasheet he omitted that information from the description of pin 20.

PlainBill

[jetadm123]

Re: need help with a gateway lp2407/fpd2484

Note the last entry on page 3 of the datasheet. Pin 20 provides startup current.

PlainBill

Thanks for the heads up PlainBill. I knew something was wrong when you said "bottom of page 3". On the page 3 I was looking at is a block diagram. It turns out there are two data sheets for this chip: the original written in 2007 by System General and the Fairchild one from 2009 I was looking at. The Fairchild datasheet is just an edited version of the SG datasheet. Of course, the Fairchild version left out the "provide start-up current for VDD", but it's probably mentioned somewhere else.

[PlainBill]

Re: need help with a gateway lp2407/fpd2484

From the SG6902 spec sheet, it looks like once VDD (pin 15) hits 10V or less, the chip shuts down. 16V is required to turn the SG6902 on. VDD voltage is generally supplied by a tertiary winding on the main transformer, which feeds a diode and startup capacitor. Try checking the diode. If ok, then try replacing the startup cap. Spec sheet has it around 10uf, but it could be higher.

Note the last entry on page 3 of the datasheet. Pin 20 provides startup current.

PlainBill

[jetadm123]

Re: need help with a gateway lp2407/fpd2484

pin 15 is 10v pin 20 is -1.25
pin 12 is a 12v pulse with a 20 percent duty factor

From the SG6902 spec sheet, it looks like once VDD (pin 15) hits 10V or less, the chip shuts down. 16V is required to turn the SG6902 on. VDD voltage is generally supplied by a tertiary winding on the main transformer, which feeds a diode and startup capacitor. Try checking the diode. If ok, then try replacing the startup cap. Spec sheet has it around 10uf, but it could be higher.

[chetreynolds]

Re: need help with a gateway lp2407/fpd2484

pin 15 is 10v pin 20 is -1.25
pin 12 is a 12v pulse with a 20 percent duty factor

[PlainBill]

Re: need help with a gateway lp2407/fpd2484

Interesting. They combined PFC and SMPS controllers into a single package. A very busy single package.

I'd check the voltage at pins 15 and 20. Use pin 13 as the ground point.

PlainBill

[Scenic]

Re: need help with a gateway lp2407/fpd2484

http://www.alldatasheet.com/datashee.../SG6902SZ.html

EOL part. not available anywhere

http://search.digikey.com/scripts/Dk...02SZ&x=21&y=17
http://www.fairchildsemi.com/sitesea...s=fairchild_db


edit: HA! mouser still has 709pcs. of them
http://www2.mouser.com/ProductDetail...TXCa9NLw%3d%3d

[chetreynolds]

Re: need help with a gateway lp2407/fpd2484

the smps ic is an sg6902sz

[darnwalls005]

Re: need help with a gateway lp2407/fpd2484

perfect! thanks so much