Gateway black screen

[davg]

This unite has black screen can see desktop with bright light but back lights are not starting. The ccfl are tested ok with another board. Caps have been replaced. Voltage checks reveal 165 dcv across the large cap o volts at all points to the logic board. 56 volts dc at both ends of the start resistor? I read a post somewhere by PlainBill that the voltage at the start resistor should read 170volts one end and 10-25 volts at the other end so is this a indication that this resistor is bad and can someone tell me the value of this resistor? This one is covered with heat shrink and I can't read it without taking it out.
Sorry this unite does not power on, no led. My bad but was thinking of another unite however all other info is as posted

[jetadm123]

Re: Gateway black screen

What is the voltage across the 47uf startup cap next to D105? Located within the heatsink with the single transistor mounted on it.

Oops! Mis-read your first sentence. Disregard what I said and proceed to retiredcaps suggestion.

[retiredcaps]

Re: Gateway black screen

1) If you can see the desktop image and the power LED is green, then your power supply is working fine. If the power LED is not on, then we need pictures of the main board.

2) I would check U301 and U302 on the backside to see if they are shorted or not. Instructions are in my 2 seconds guide under the mosfets section.

3) On the 10 pin connector to the mainboard, there is one pin labeled BL. This should be > 3V in order for the backlights (BL) to come on.

4) Great clear focused pics.

[jetadm123]

Re: Gateway black screen

Also check the black square 3.15A fuse in the lower left of your board. Next to jumper wire JP202.

[budm]

Re: Gateway black screen

Please check the voltages as shown.

[davg]

Re: Gateway black screen

1) If you can see the desktop image and the power LED is green, then your power supply is working fine. If the power LED is not on, then we need pictures of the main board.

2) I would check U301 and U302 on the backside to see if they are shorted or not. Instructions are in my 2 seconds guide under the mosfets section.

3) On the 10 pin connector to the mainboard, there is one pin labeled BL. This should be > 3V in order for the backlights (BL) to come on.

4) Great clear focused pics.

Sorry but this unite do not come on no led I was thinking of another unite and since I had this one dimantled I didn't check before posting. I do know that the unite will work with another board because I tried one. So my heading should have read Unite will not power on. Should I make a new post?

[budm]

Re: Gateway black screen

That is a big information.

[Rtech]

Re: Gateway black screen

So what tests have you done to this one with No Led etc ?? Voltage across large Capacitor ??,fuse checking ??

[davg]

Re: Gateway black screen

So what tests have you done to this one with No Led etc ?? Voltage across large Capacitor ??,fuse checking ??

As stated in my first post the voltages across the large cap is 165 volts the fuses are ok no voltage to the logis board and 56 volts at both sides of the start resistor voltage across the run cap is fluxating can't get a reading jumping from 5 to 11 volts

[budm]

Re: Gateway black screen

Do you hear ticking from the transformer? Next thing to check will be the 2 8-pin MOSFETs in the inverter section to see if they are shorted or not, P/N?

[retiredcaps]

Re: Gateway black screen

can't get a reading jumping from 5 to 11 volts

Possible problems with a fluctuating voltage on the startup cap.

1) the startup cap is bad
2) the SMPS IC is bad
3) something is shorted on the output side like a diode

Checking for shorted diodes is the easiest followed up changing the startup cap.

[retiredcaps]

Re: Gateway black screen

Also check the black square 3.15A fuse in the lower left of your board. Next to jumper wire JP202.

And course, I always miss the fuse on the boards when it is not in front of me.

[davg]

Re: Gateway black screen

Do you hear ticking from the transformer? Next thing to check will be the 2 8-pin MOSFETs in the inverter section to see if they are shorted or not, P/N?

no ticking from the transformer where can one find the specs on these 8 pin mosfets?

[budm]

Re: Gateway black screen

OK, first check the black rectangular fuse which is for the main board, if it checks OK, then you can do one of two things:
1) Disconnect the two ends of the jumper wires which feed the B+ to the Inverter circuits and see if the voltage becomes steady and the power LED will now come on. If it does not, then see step 2. If it does, then you may have shorted MOSFETS.

2) Try new start-up cap to see if the the voltage will be steady.

I will you how to check and see if the MOSFET's are shorted out or not later.

See attached pictures.

[davg]

Re: Gateway black screen

OK, first check the black rectangular fuse which is for the main board, if it checks OK, then you can do one of two things:
1) Disconnect the two ends of the jumper wires which feed the B+ to the Inverter circuits and see if the voltage becomes steady and the power LED will now come on. If it does not, then see step 2. If it does, then you may have shorted MOSFETS.

2) Try new start-up cap to see if the the voltage will be steady.

I will you how to check and see if the MOSFET's are shorted out or not later.

See attached pictures.

Thanks budm will give it a try in the morning and post results. The start-up cap is 47uf@35 volts I don't have one I do have a 47uf@50 volts. What would be a viable substitute?

[budm]

Re: Gateway black screen

That will be fine for now.

[davg]

Re: Gateway black screen

OK, first check the black rectangular fuse which is for the main board, if it checks OK, then you can do one of two things:
1) Disconnect the two ends of the jumper wires which feed the B+ to the Inverter circuits and see if the voltage becomes steady and the power LED will now come on. If it does not, then see step 2. If it does, then you may have shorted MOSFETS.

2) Try new start-up cap to see if the the voltage will be steady.

I will you how to check and see if the MOSFET's are shorted out or not later.

See attached pictures.

The fuse checks OK .01 ohms. removed the two jumper ends voltage still not stable and still no led. replaced cap with 47uf @ 25volts still no change. What about that start resistor? How can I check it?

[davg]

Re: Gateway black screen

I measured the resistance on the start up resistor in circuit with power off and I am getting .01 ohms. The voltage at either end is 56 volts dc with power on from what I read this is not normal. Am I looking at the wrong thing? Any comments welcome.

[budm]

Re: Gateway black screen

I believe you are reading 56V is because you are using the chassis ground as a ground point. When you work on the Primary (HOT SIDE) you have to use the negative leg of the LARGE DC filter cap in the PRIMARY side as the circuit ground point.

[davg]

Re: Gateway black screen

I believe you are reading 56V is because you are using the chassis ground as a ground point. When you work on the Primary (HOT SIDE) you have to use the negative leg of the LARGE DC filter cap in the PRIMARY side as the circuit ground point.

Thanks budm you are right on my ground point. I did not know that. The reading using the negative leg (the grey side) of the large cap is zero both ends of the resistor. With one leg on the positive side of the large cap I am reading 165 volts on both ends of the resistor?