dell 172fpt 23v but no 5v

[alexanna]

I have a dell 172fpt.Initaly there was no sign of life.D815 a transient voltage suppressor was in a real bad way, It has been replaced

.I have 165v across the big cap c823.I also am reading 23v on the 35v 470uf caps in the c831 area.

I am calling the 5v rail 0v although sometimes I will see about a voltage of 3.5 v to 4v that is very unsteady. Most of the time its 0v

.On the back of the power supply there is a 14 pin ic #i801 Three numbers are on it #1 Lta201p---#2 4e843343 #3—h603421.

I have been probing around checking voltages. And one thing I have noticed on pin #4 of L817 optocoupler I see 12v most of the time but sometimes the voltage goes down to 5v and at this time the voltage somewhat comes back to the 5v rail but it is low and unsteady.

Direction on this would be welcomed.
Thanks Al.

[PlainBill]

Re: dell 172fpt 23v but no 5v

I have a dell 172fpt.Initaly there was no sign of life.D815 a transient voltage suppressor was in a real bad way, It has been replaced

.I have 165v across the big cap c823.I also am reading 23v on the 35v 470uf caps in the c831 area.

I am calling the 5v rail 0v although sometimes I will see about a voltage of 3.5 v to 4v that is very unsteady. Most of the time its 0v

.On the back of the power supply there is a 14 pin ic #i801 Three numbers are on it #1 Lta201p---#2 4e843343 #3—h603421.

I have been probing around checking voltages. And one thing I have noticed on pin #4 of L817 optocoupler I see 12v most of the time but sometimes the voltage goes down to 5v and at this time the voltage somewhat comes back to the 5v rail but it is low and unsteady.

Direction on this would be welcomed.
Thanks Al.

Replace C834, C835, and C837.

PlainBill

[alexanna]

Re: dell 172fpt 23v but no 5v

Would Nichicon KT series be acceptable? I have some of those.
Al

[PlainBill]

Re: dell 172fpt 23v but no 5v

Would Nichicon KT series be acceptable? I have some of those.
Al

They might work for a test, but I wouldn't trust them for long term use.

PlainBill

[alexanna]

Re: dell 172fpt 23v but no 5v

Well The test did not work, I tried them.Also this time with power on my 23v rail was down to 20v powering down and hooking up logic board 23v came back.Was the test with the kt valid?I will order some fm series those are just some I had on hand?

[alexanna]

Re: dell 172fpt 23v but no 5v

Well I tried some thing I hooked up a 1157 automotive bulb to the 23v rail and it will not light. testing the bulb and socket with a 12v power supply it dose light.Should the 23v rail have enough current to light the bulb?
Al.

[PlainBill]

Re: dell 172fpt 23v but no 5v

Well I tried some thing I hooked up a 1157 automotive bulb to the 23v rail and it will not light. testing the bulb and socket with a 12v power supply it dose light.Should the 23v rail have enough current to light the bulb?
Al.

Probably not, it depends on how you hooked it up. An 1157 bulb is a dual filament bulb rated at .6A and 2.1A. If you hooked the two in parallel that would be a total of 2.7A at 12v. If you use only the smaller filament, the power supply should have the capacity of lighting it - at least for a few seconds before it burns out. I would suggest hooking the smaller filament to the 5V output - that's the one that is regulated.

You mentioned replacing D815. Obviously something BAD happened in that area. R852 also looks like it may have overheated, even though there is no sign of damage to the PC board.

PlainBill

[alexanna]

Re: dell 172fpt 23v but no 5v

I just used one filament to hook up to the 23v It would have been the one that would be brighter, drawing more amprage.I thought possibly since I was also seeing the 23v line go low with no load possibly a power supply was not flowing enough current.I will check R852 and get back to you .
thank you Al.

[alexanna]

Re: dell 172fpt 23v but no 5v

R852 has About 156v on both sides.It is red-purple-silver-gold .27ohms?And its reading so close to 0 with my meter.
Al.

[PlainBill]

Re: dell 172fpt 23v but no 5v

R852 has About 156v on both sides.It is red-purple-silver-gold .27ohms?And its reading so close to 0 with my meter.
Al.

Probably a load sensing resistor.

PlainBill

[alexanna]

Re: dell 172fpt 23v but no 5v

I would like to learn what is going on here,I have the larger size fm series caps I could install if need be.I guess what I am wondering if the reading on the volt meter is fooling me.Is the voltage there but not enough amprage? or what would be a good way to diagnose this?
Al.

[alexanna]

Re: dell 172fpt 23v but no 5v

Just for infomation I powerd the lamp with a 12v battery charger The amprage of the bulb was 1.7amps larger filament.And 500ma on the smaller filament. The 23v rail will not light up the smaller filiment.I hooked it up ground to low volt board ground and positive lead to center lead of the schottky diode.I would not even light it much less burn it out. I agree.something bad must have happend to the board.Is there a way to test these schottys in circut?Or am I even thinking right?
Al.

[PlainBill]

Re: dell 172fpt 23v but no 5v

Just for infomation I powerd the lamp with a 12v battery charger The amprage of the bulb was 1.7amps larger filament.And 500ma on the smaller filament. The 23v rail will not light up the smaller filiment.I hooked it up ground to low volt board ground and positive lead to center lead of the schottky diode.I would not even light it much less burn it out. I agree.something bad must have happend to the board.Is there a way to test these schottys in circut?Or am I even thinking right?
Al.

The best way to test the diodes is out of circuit, or better yet by replacement.

One point - are you doing this with a known load on the 5 volt supply? The design of the SMPS is very straightforward. It regulates the 5Volt supply; the other supply (usually 12 volts) will vary depending on the load on it and on the 5V supply.

PlainBill

[alexanna]

Re: dell 172fpt 23v but no 5v

I some times have a hard time explaining my self.All I have done is use the bulb like a simple automotive test lamp.The 5volt is not working so i could not test it.The 23volts was there so with the board powerd up ,one lead connected to the low voltage board ground and the other lead touched to center leg of the yg902c2 diode,The voltmeter reads 23v, but it will not light the smaller filiment of the bulb. I do not know if this is even a valid test.
Al.

[PlainBill]

Re: dell 172fpt 23v but no 5v

I some times have a hard time explaining my self.All I have done is use the bulb like a simple automotive test lamp.The 5volt is not working so i could not test it.The 23volts was there so with the board powerd up ,one lead connected to the low voltage board ground and the other lead touched to center leg of the yg902c2 diode,The voltmeter reads 23v, but it will not light the smaller filiment of the bulb. I do not know if this is even a valid test.
Al.

It isn't a valid test. Please answer a simple question - do you have any load (the logic card, a 10 ohm or smaller resistor, or a light bulb) hooked to the 5 volt supply?

This is a schematic of a Dell (Benq) monitor. Scroll down to the point where a line that begins i believe that the following is the schematics is at the top of your screen.

This is a poorly drawn schematic, but it illustrates the basics of what happens with your power supply.

T601 is the SMPS transformer, the winding betwwen pins 7 and 9 is the primary while Q601 is the power FET and IC601 is the SMPS controller.

At start-up IC601 gets it's power through Q602 and the associated resistors. When the voltage across C606 hits the startup threshold, IC601 turns on Q601, sending a pulse of electricity into the primary of T601. That produces proportional pulses in the secondaries 2-3; 6-5, and 12-11. Those pulses are rectified by by diodes D7021, D701, and D604, and the DC voltage stored by C707 & C708( 5V); C703 & C704 (inverter power); and C606 Vcc for the SMPS controller.

The next thing of interest is R616. It reduces the drive pulse if the current through T601 and Q601 goes too high.

Now look at IC702 and IC602 (the optocoupler). When the 5V output reaches 5 volts the LED in IC602 begins to emit, causing the SMPS controller to reduce the duration of the output pulse.

Something that is important to remember. The inverter output is not regulated. The more energy the SMPS controller has to dump into the transformer to maintain the 5V output, the higher the inverter voltage will go. And I am suspicious; the inverter voltage you measured seems very high.

Back to your monitor - remember, the function is identical, the part numbers are different. Have you checked the output diode of the 5V supply? Have you checked the capacitors? The condition of the board around D615 etc really bothers me. Are you sure you replaced D616? with the correct part?

PlainBill

[alexanna]

Re: dell 172fpt 23v but no 5v

The logic card is hooked up to the power supply with A d sub cable and a powerd up computer.
The capacitors for the 5v have been changed with panasonic fm series.
I know your not going to belieave this,After I read your last post I went back to verify voltages,I was reading 11.83v at d820.Unpluging power cord and pluging back in the 23v came back, This is not B.S. it is realy changing.
I will stand down at this point and follow your directions, I would realy like to understand how the smps works.
I will check the of the points of interst in the afternoon
Thanks Al.

[PlainBill]

Re: dell 172fpt 23v but no 5v

The logic card is hooked up to the power supply with A d sub cable and a powerd up computer.
The capacitors for the 5v have been changed with panasonic fm series.
I know your not going to belieave this,After I read your last post I went back to verify voltages,I was reading 11.83v at d820.Unpluging power cord and pluging back in the 23v came back, This is not B.S. it is realy changing.
I will stand down at this point and follow your directions, I would realy like to understand how the smps works.
I will check the of the points of interst in the afternoon
Thanks Al.

That kind of change indicates a bad solder joint. I would suggest 'touching up' all the solder joints in the area of the SMPS and the secondary. I've outlined the general area, and numbered some that I consider suspect.

I've been unable to locate a datasheet for the SMPS controller. Have you been able to find one?

PlainBill

[alexanna]

Re: dell 172fpt 23v but no 5v

Something realy strange is going on here.I have double checked all voltages.Today I am reading 2.5 vto 3.2very unsready on the center leg of d813.And 23v on the center leg of d820.
I am doing my best to ask the correct questions so thanks for staying with me.
You say check the 5v output,Is that going to be on the hot side of the board.
I do not know If it is imporentent or not but on pin 4 of the octocoupler I am reading 10v this is on the hot side of board.
I will touch up solder joints as recomended.
You did ask if I replaced the correct diode that was damaged,I beleave so It was d815 and the markings on it were p6ke130a, and it was replaced with mouser part#
576-p6ke130a.I am not good at adding links yet.
D816 has been removed from board and tests ok.
C829 the brown cap has been replaced with on from annother board
Thank you Al.

[alexanna]

Re: dell 172fpt 23v but no 5v

That kind of change indicates a bad solder joint. I would suggest 'touching up' all the solder joints in the area of the SMPS and the secondary. I've outlined the general area, and numbered some that I consider suspect.

I've been unable to locate a datasheet for the SMPS controller. Have you been able to find one?

PlainBill

No on the datasheet.
Well Is any body else having trouble opening the attachement?
Or did i do some thing wrong?

[alexanna]

Re: dell 172fpt 23v but no 5v

That kind of change indicates a bad solder joint. I would suggest 'touching up' all the solder joints in the area of the SMPS and the secondary. I've outlined the general area, and numbered some that I consider suspect.

I've been unable to locate a datasheet for the SMPS controller. Have you been able to find one?

PlainBill

I have done a bunch of resoldering.But I am unable to open any atatchments to see your suspect ones.May be tommrow?
Thank you Al.