Dell 1800FP no 5v standby.

[triplefour]

got another monitor on the workbench here...this time its completely missing standby power. so we automatically know theres something wrong with the PSU, right?

i measured 120v across the fuse
and 68v on the big filter cap.

theres a small 4 legged umm... IC? mosfet? chip? sorry i dont really know what to call them...it says L0306 on it...anyway 2 of the legs read 68v but the other 2 are zero. does that mean anything or are they just zero cause theyre both grounds?


heres some pics.

[triplefour]

Re: Dell 1800FP no 5v standby.

hmm according to this ebay item, its usually the 8 legged chip that fails. its the inverter chip apparently. http://www.ebay.com/itm/DELL-1800FP-...item2574ad02a0
i know i could save myself some time by just buying this but i'd really like to scavenge the parts for free. where else might i find such a chip? oh and how should i test to make sure it really is this chip that is wrong instead of taking some ebay guys word for it?

[budm]

Re: Dell 1800FP no 5v standby.

"i measured 120v across the fuse and 68v on the big filter cap"
You are using the correct ground ref point, when you work in the primary side, you use the Negative leg of the main filter cap as the Ground ref point for the primary DC voltage testing.
So check the DC voltage right at the two legs of the cap.

[budm]

Re: Dell 1800FP no 5v standby.

BTW, this power supply is an always on power supply so the 5V and the 15~18Vdc are always present. You are correct that the SMPS IC is the commonly failed IC. If you check the resistance of the VCC pin and the Ground pin, you will find that it has very low resistance, the later version has added Zener diode so the startup voltage cannot exceed the VCC spec of the IC.

[triplefour]

Re: Dell 1800FP no 5v standby.

not sure if i understand how to test this IC... if i get low resistance between the VCC pin (whichever one that is) and the ground pin, does it mean the IC is bad?

[triplefour]

Re: Dell 1800FP no 5v standby.

also why do i have to use the negative leg of the main filter cap? thats the sketchiest place to take a ground measurement i can think of...

[budm]

Re: Dell 1800FP no 5v standby.

First, you are working the primary side of the circuit, the chassis/safety ground is not the ground ref point for the meter to test the DC voltage in the primary side. The secondary side of the circuit, the negative legs of those output filter caps are the ground ref point of the secondary side and 99% of the time are tied to the chassis.
The primary side ground ref is the negative leg of the main filter cap and it is not tied to the chassis, it is isolated from the secondary side for safety. You need to look at the typical power supply circuit to understand what the ground ref points are and how they are used. If you do not use the correct ground point, your voltage readings will be no use to us to tell if you are getting the correct voltages or not. I hope you understand the difference between the primary and the secondary side of the circuit since primary circuit is the DEADLY side and you must be careful when making voltage measurement in that section.
I am not sure how much electronics training you have, but it may be a good idea to read up on the basic electronics books first. I do not want you to make the mistake that can kill you.

[budm]

Re: Dell 1800FP no 5v standby.

not sure if i understand how to test this IC... if i get low resistance between the VCC pin (whichever one that is) and the ground pin, does it mean the IC is bad?

Yes it can be the IC or the components that are connected the VCC pin of the IC, first you need to get the spec sheet of the IC so you will know what the pin out of the IC are.

[triplefour]

Re: Dell 1800FP no 5v standby.

ok i understand what you say about having the primary and secondary circuits isolated from each other. makes sense. but how do i tell from looking at the power supply where the primary side ends and where the secondary side begins?
i guess to be safer, i could solder a little wire to the negative terminal of the main filter cap and take my readings from there. its just too easy for a probe to slip while measuring from the leg! but yeah i guess i dont know how to really tell which part is primary and which part is secondary. i mean i assume that from where the plug comes into the unit, and from there to the main filter cap is all primary but after that im not sure where it ends?

so i guess ill have to locate the datasheet for that IC and do some further testing.

[budm]

Re: Dell 1800FP no 5v standby.

Most of the time, the PC designer will put the thick lines to show the separation between the Primary and the secondary side, or if you work on stuff long enough then you can tell.
For example:
https://www.badcaps.net/forum/showth...752#post453752

[budm]

Re: Dell 1800FP no 5v standby.

BTW, here is the spec sheet of the ICE2AS01

[triplefour]

Re: Dell 1800FP no 5v standby.

ahh yes i see the black line on some power supplies and it says hot on one side and cold on the other..cold is secondary and hot is primary, right?
and thanks for finding that spec sheet. ill try make sense of that later today and post my findings

[triplefour]

Re: Dell 1800FP no 5v standby.

thank you for your concern about my life, dont worry i will be very careful. is wearing rubber gloves a good idea when working on these things?

[triplefour]

Re: Dell 1800FP no 5v standby.

ok i get 1.3~ ohms resistance between pin 7 and pin 8 (VCC and Ground) ...
is that "very low" ? does this mean the IC has failed?

[triplefour]

Re: Dell 1800FP no 5v standby.

what exactly is going on here.. what role does the resistance between these two pins play in the operation of the IC?

[triplefour]

Re: Dell 1800FP no 5v standby.

man this datasheet is intense.. i hope one day i can understand all of this!

[R_J]

Re: Dell 1800FP no 5v standby.

Do you mean a short between pin 6 and 7 ? pin 8 is not connected (N.C.) If you have 1.3 ohms between pins 6 (vcc) and 7 (gnd) the ic is most likley shorted.

[triplefour]

Re: Dell 1800FP no 5v standby.

oh yeah i meant to say pins 6 and 7... how did i mess that up? yes pin 6 and 7 resistance is 1.3ohms ...what is it supposed to be?

[triplefour]

Re: Dell 1800FP no 5v standby.

i ordered one from ebay... coming from china so who knows..could be a few days..could be a month...

[triplefour]

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...