I Had my LVPS out while working on some fan mods i wanted to do with the Belly fans. I was probing the output rails on the back looking for a 24 vdc rail and accidentally touched the probe from the 300 something plus rail to the body and blew the F2 fuse along with the fuse in my panel box and the Ground default fuse on the plug itself! So i put a new fuse in the F2 spot and plug it in and hear a buzz then the fuse pops again. What or where should i look for the short if that is what it is?


Athanasios

Ok I pulled out my other LVPS and started measuring and checking continuity of components that are connected to the fuse that keeps blowing.

1) the big resistors R9-12 are .27 ohm but read .13 but so do the ones on then good LVPS

2) R7 is 100k ohm but both good and bad supply read near 50k and it takes awhile to get to a steady reading on the 100k Resistor is it a special kind or becaus its conected the the 470uf 450v cap?

3) R3 & R5 are 10 ohm and now read 5 ohm

4) And all the ic's/transistors? on the big heat sink are shot they all have continuity between the outer pins and center also the readings are not correct.this continuity is what causes the fuse to blow correct?
CR3 and CR5 should read 53 ohms between each outer pin and the center ground they now read 0 ohm.
Q1-3 should read near 58 ohms and now Q1 is 1 ohm, Q2 is 1.5 ohm and Q3 is 15 ohms.

What else should i check and can these parts be replaced easily with same parts ?
And the .27 ohm and 100k ohm resistors, what kind are they? ceramic? carbon? metal?


thanks for any help.

Athansios

It sounds like you are measuring in circuit. If two resistors of the same value are in parallel, the resistance you measure would be one half that value. Your meter is charging the cap and that is why the resistor looks like it is changing in value.

Ok I pulled out my other LVPS and started measuring and checking continuity of components that are connected to the fuse that keeps blowing.

1) the big resistors R9-12 are .27 ohm but read .13 but so do the ones on then good LVPS

They are all in parallel so should be about 0.7 ohms. Meter with leads might not read good with ohms that low. They are probably good, though, if the same as the good supply.

2) R7 is 100k ohm but both good and bad supply read near 50k and it takes awhile to get to a steady reading on the 100k Resistor is it a special kind or becaus its conected the the 470uf 450v cap?

Your meter is charging the big caps, that effects the reading.

3) R3 & R5 are 10 ohm and now read 5 ohm

Might be caused by blown Q1-Q3, the big mosfets.

4) And all the ic's/transistors? on the big heat sink are shot they all have continuity between the outer pins and center also the readings are not correct.this continuity is what causes the fuse to blow correct?

Pretty good chance.

CR3 and CR5 should read 53 ohms between each outer pin and the center ground they now read 0 ohm.
Q1-3 should read near 58 ohms and now Q1 is 1 ohm, Q2 is 1.5 ohm and Q3 is 15 ohms.

What else should i check and can these parts be replaced easily with same parts ?

Yes, check R6 and R38. U9 is suspect, too.

And the .27 ohm and 100k ohm resistors, what kind are they? ceramic? carbon? metal?

Metal film. R9-12 are 3W. R3-5 are 1/2W.


thanks for any help.

Athansios

The bad thing about working on this type of circuit, other than it is hot to ground and outputs 385VDC, is that unless you change EVERY bad part it is apt to blow up again as soon as you plug it in. With an isolated variac and ampmeter you can try bringing it up from 0V input and tell by the ampmeter if it is coming up ok.

BE CAREFUL. HIGH VOLTAGE! VERY DANGEROUS.

Scott

You'll be much better off just getting another supply. I've got a bunch here, there's lots out there for cheap. These are a bitch to work on, I've reparied a couple, Mike PArker is really good at them though.

I''m still going to give it a shot. Some of the parts I'm getting as sample for free so its not to bad. but what type of resistors are R9 through R12 and also R7. I'm going to replace all the parts that read different than my good supply and also the ones that i think might have been affected. If I can get it going I'll just get another when i need it. this is my spare i was just using to test fan speed and such. I though there was a 24 volt output but it must switch to it when the projector get turned on and since its being used out of the projector that wont happen. But thanks for your info guys!! I'll let ya know if i get it working again.

Athanasios

You could kill yourself doing this also.

It sounds like you are really a beginner at this. Do you have even a basic understanding of circuitry, resistor and cap calculations.? Do you even have a clue as to variac and amp meter power up. As a friend, I would say buy another power supply. They are cheap. What you are trying to do at this point in your life and knowledge is stupid.

I''m still going to give it a shot

:confused:

Mike. I pray he doesn't end up giving himself a shot.

Well now i have to think about this. Thanks guys! I sent curt a PM for a new one. and maybe I'll send the power supply to Mike or a local repair shop i know that does this type work.

Awww you guys care ;-)


Athanasios

I wouldn't even try to get it fixed, the LVPS is such a low cost replacement item that it would be easy to spend more than a new one would cost to get yours fixed

Erik

well after removing the suspect parts and testing it was two mosfets that blew and i thought also the U9 chip because there was continuity from the ground pin 3 and pin 7 the output pin. but when removed it there wasnt continuity between those pins. I didn't think anything about it but should have, to that later. so i put that chip back and the other single mosfet that was still good. put it all back and thought that i could run for a short time with one mosfett. from what scott told me it would run and i wouldnt ruin any other new parts by first testing it this way. Here is where i should have re-tested the UP chip before putting it back together and trying it out. pluged her in and poof once again. this time the mosfet had a small hole in it!! Also the U9 chip was grounding out at pin 7 again. so i removed it and thought about it and relized i should have tested the pin holes from pin 3 to pin 7. well guess what it was shorting! the board itself went bad and after further inspection i saw little copper drops comming up through the pcb board were the U9 chip was hiding it. I guess i didn't notice it before with the chip in place. so yepp its shot !

I gave it shot and i didn't give a "shot" to myself ! :P :D

it was a spare power supply so i'm good for now, i just wanted to hone my diagnastic skills and learn a little more.

Thanks for your concern guys and now i know what to look for if this happens again.

Thanks Scott for your advice.

curt you have a spare of that half of the power supply lying around? ;-)


Athanasios