Dell 1800FP no 5v standby.

[bammbammfran]

Re: Dell 1800FP no 5v standby.

did you also change the main power mosfet (The 3 pin diode on the center most heatsink)
you should usually do that when you do that ICE

check your 2 rectifier diodes on the other heatsinks to make sure that they did not short out. if one is shorted it will usually shut down the system

[triplefour]

Re: Dell 1800FP no 5v standby.

nope didnt think to test those...live and learn! will get to that

[triplefour]

Re: Dell 1800FP no 5v standby.

how would i test the main power mosfet? and im getting confused cause i thought diodes only had 2 "pins" and a white line on one side of the diode to show which way current can flow. this 3 pin power mosfet is also a diode?

[bammbammfran]

Re: Dell 1800FP no 5v standby.

the two on the same heatsink with the 3 pins are rectifier diodes
the two outer pins are on the same circuit coming from the tranformer, so basically it is like a two pin .
The center pin is your DC output

Depending on the manufacturer or the diodes- If you look closely at the black body, you might see 2 arrows pointing to the center.

these look similar to some power mosfets, but they are not the same testing wise

just test in circuit with your ohm meter from the center pin to one of the outer pins to make sure that they are not showing Zero or really low ohms

[bammbammfran]

Re: Dell 1800FP no 5v standby.

rectifier diode locations from bottom

[triplefour]

Re: Dell 1800FP no 5v standby.

thank you for the explanation.
ok so testing the 2 rectifier diodes using the black probe in the center pin and the red probe on the outer pins, i get 100~200 ohms resistance on all of them.
weird thing happens when i use the red probe on the center pin and the black one to test the other 2...the resistance just keeps climbing. whats going on there?
also, how do i test the main power mosfet?

[bammbammfran]

Re: Dell 1800FP no 5v standby.

Hi
Yep the ohm meter will usually keep climbing for a while in one scenario of probe locations, then if you reverse the tester probes after it builds up, it will probably drop back down and then up again
The resistance usually eventually slows down and stops.
Caps are affecting this
If you take them out, your readings will differ.

So those should be ok, or should I say good enough to not cause power on issues. If you ever find one that is shorted. It will stop all power, so this is always a good starting place

you can only do limited testing on the power mosfet with a standard tester.
to see if it may be shorted while it is on the board, you can do a similar test with your ohm meter
between the center pin and each side pin and then side pin to side pin

[jetadm123]

Re: Dell 1800FP no 5v standby.

:/ the IC came in the mail today and i switched it out...still no 5v standby... and i dont get any voltage to any of the pins on that IC (i am using negative leg of main filter cap as ground) come to think of it..i never checked the voltage on those pins so i dont know what they were, but they are all zero now...

If you're getting zero voltage at every pin, then check the resistance of the ground and Vcc pins of the ICE2AS01 again to see if there's a short. If no short,then power up the board and measure the DC voltage across the leads of the main 450V cap. You should see around 165VDC.

[bammbammfran]

Re: Dell 1800FP no 5v standby.

can you post a clearer picture of the bottom of the power board targeting the center section ??
the other picture gets too fuzzy when I try to get in close on it.

[triplefour]

Re: Dell 1800FP no 5v standby.

ok heres a better picture of the back of the board.
also i guess i should mention that while changing the IC, one of the pads got a little damaged... it still looks like it would be making a connection...but is this really bad?

[triplefour]

Re: Dell 1800FP no 5v standby.

also, one of the pins was never soldered in and does not even have a pad...

[bammbammfran]

Re: Dell 1800FP no 5v standby.

also, one of the pins was never soldered in and does not even have a pad...

actually both pins on the one end do not get used on this chip.

so you have no voltage between pin 6 and pin 7 ??
If not, check to see if pin 6 is grounded with an ohm meter
then check the diode at D902 to see if it shows zero resistance in one direction
then check resistor 907 to make sure it is not open (no resistance)

[triplefour]

Re: Dell 1800FP no 5v standby.

just to be sure, to check if pin 6 is grounded i put my meter to ohms setting and put black probe on chassis ground and red probe on pin6?

[triplefour]

Re: Dell 1800FP no 5v standby.

pin 6 is VCC and pin 7 is ground... also pin 6 is the one the pad got damaged on.. how likely is that to cause a problem?

[triplefour]

Re: Dell 1800FP no 5v standby.

oh wait this is in the primary side of the psu so i have to use negative leg of main filter cap as ground?

[triplefour]

Re: Dell 1800FP no 5v standby.

ok using the negative side of the main filter cap as ground ref, i get 0.3 ohms resistance on pin 7 (GND) and 3.1 ohms on pin 6 (VCC)

if pin 6 were grounded would that mean it should be 0.3 like the ground pin? and that would be bad, right?

[triplefour]

Re: Dell 1800FP no 5v standby.

on d902 i get 13 ohms one way and infinite ohms the other way...

[triplefour]

Re: Dell 1800FP no 5v standby.

10 ohms on r907....

[triplefour]

Re: Dell 1800FP no 5v standby.

in my experience, diodes always have some resistance even in the direction they are allowing current...what would zero resistance mean there?

[bammbammfran]

Re: Dell 1800FP no 5v standby.

ok using the negative side of the main filter cap as ground ref, i get 0.3 ohms resistance on pin 7 (GND) and 3.1 ohms on pin 6 (VCC)

if pin 6 were grounded would that mean it should be 0.3 like the ground pin? and that would be bad, right?

0.3 would mean it is totally shorted
but 3.1 does not appear to be anywhere near where it should be
based on one I just checked in a different model board, it should be well over 100K

pins 2 and 3 should also be showing decent resistance to the ground pin

what is the power mosfet model number