Switching power supply repair

lynx said:

measuring again the components around the IC i found that R104 was blown...it wasn't obvious until i
looked at it from the sides and i've noticed the coating from the bottom side of the resistor was missing,
from the schematic i can see it goes to the pin that says ISENSE i guess it senses current...but since i
don't know much about switching power supplies i don't know if that resistor is the cause that the
previous IC got blown and now the new one is not functioning.

it's value must be 0.47ohm (yellow, blue or purple? and silver) and now it measures 7Mohm and it is 0.5W
which i don't have because it is rare number...with what else i could substitute that resistor?

do you think i should power up the supply after i change the resistor or it might blow the IC again?

thank you guys.

Click to expand...

Hi again,

Well, the current sense resistor could have blown because of an overcurrent, but i would think it would blow OPEN not to a near SHORT (or zero). This makes it hard to say if the IC will blow again or not, but usually the way power supplies are tested is the input is raised little by little checking everything with a scope and if anything looks wrong it is fixed before continuing. Once we get up to the full input voltage with no problems then we assume it's ok. This includes checking for spikes across the main transistor(s).

But as i said before, this is a difficult thing to fix without having it right in front of me where i can examine many aspects of it, and even then it may be too hard to fix. Could there even be something wrong with the thing it is powering.
That's another reason why i strongly suggest building your own power supply. Yes you have negative voltages, so what, that's what a power supply does You probably only need one negative voltage and then regulate it down using regulators chips. The current probably isnt much but you can measure it at start up to see if it goes too high, then upgrade that line to a higher current. At least this way you have a definite way to fix it.

It's still up to you of course, but you may never get this fixed unless you get lucky.

Resistors can blow both ways. Carbon composition had a tendency to increase value. Metal films can do just about either, including pudde.
Wirewounds tend to blow open.

Metal oxides tend to blow completely open by design.

The current limiting resistor could be the cause of all of the problems and could also be a design deficiency and it's a good reason why the supply wont start. Sometimes those resistors are metal oxide so it acts as a fuse.

lynx said:

measuring again the components around the IC i found that R104 was blown...it wasn't obvious until i
looked at it from the sides and i've noticed the coating from the bottom side of the resistor was missing,
from the schematic i can see it goes to the pin that says ISENSE i guess it senses current...but since i
don't know much about switching power supplies i don't know if that resistor is the cause that the
previous IC got blown and now the new one is not functioning.

it's value must be 0.47ohm (yellow, blue or purple? and silver) and now it measures 7Mohm and it is 0.5W
which i don't have because it is rare number...with what else i could substitute that resistor?

do you think i should power up the supply after i change the resistor or it might blow the IC again?

thank you guys.

Click to expand...

Hi Guys

I do not normally stick my neck out and get involved in the middle of a discussion...but its quite and and I am totally bored.

So, here is what we know:

1. No parts are blowing so far(good).
2. The Current Sense resistor has gone from 0.47 ohms to open (MegaOhms). So it is blown.
3. I would say it blew when the original Chopper IC failed.

What I would do is the following:

1. Replace the Chopper IC
2. Replace the 0.47 ohm Current sense resistor
3. Remove and check D128 to see if it is OK (no reverse leakage). If it reads faulty, the Optocoupler is possibly damaged too.
If the above are suspect, replace them.
If the above are OK....be brave and feed the PSU Mains and see if it magically works.

It's very hard to repair things that are not in front of me.

Regards,
tvtech

tvtech said:

be brave and feed the PSU Mains and see if it magically works.

It's very hard to repair things that are not in front of me.

Regards,
tvtech

Click to expand...

it magically works! i've replaced the R104 awhile ago (a friend had that value but it was 1/4 watt so we used 3 of them)

BUT NOW I AM WHERE I STARTED! and i wrote at the first post.. "the LM317T gets quite hot (120c) and the dvd doesn't spin..."

between the D108 and the ground it's 23.20VDC while the schematic says it should be 15VDC, so i guess that's the reason the LM317T gets that hot, but what causes that?

Good

Leave it for now. Turn all off and I will try to help you Tomorrow..Deal

Regards,
tvtech

Congrats:

Take a look at the voltages on the transformer which have values. They might all be high.

The 5V source is what's regulated directly by the SMPS IC. Everything else, get's post regulated.

I can't remember if the frequency relationship is linear, but, I think, there was a 67 kHz or so version. FWIW: 22*67kHz/100kHz is about 15V.

You could resort to a transformer supply for the 12V supply.

lynx said:

i've replaced the R104 awhile ago (a friend had that value but it was 1/4 watt so we used 3 of them)

Click to expand...

All together? In parallel or series?

KeepItSimpleStupid said:

Congrats:

Take a look at the voltages on the transformer which have values. They might all be high.

Click to expand...

thanks i will...although at some point in the past (before the IC blows up) that issue
with the hot regulator was fixed by itself for awhile and everything! was working pefect...
i told you about it in past pages...so now i don't know why it returned to tha state.

atferrari said:

All together? In parallel or series?

Click to expand...

in parallel, we tried to match the 0.5 watts we used three just in case 0.5w is little, but probably i'll replace it again as soon as i find a single resistor because i don't like it the way it looks on the pcb, it might be funny what i said..but yes when i fix something i like it to be as it was, tight and clean.

in parallel

Click to expand...

That means the current limit for the FETs inside the IC will be ~ 3 times the designed value. Your IC probably won't last long

alec_t said:

That means the current limit for the FETs inside the IC will be ~ 3 times the designed value. Your IC probably won't last long

Click to expand...

i wanted to say... a friend had that value but it was 1/4 watt so we used 3 resistors to make that value

KeepItSimpleStupid said:

Congrats:

Take a look at the voltages on the transformer which have values. They might all be high.

Click to expand...

all the voltages around the board seem to me they are fine! other than the voltage that goes to the lm317t which is 23.20V and makes it hot and maybe the 32v output which when i measure it's 30v and if i keep the multimeter on the rail any longer it falls up to 28V the transistor the schematic shows at that point it's a C337 maybe it supplies too little current at that voltage and even a multimeter it's a load for it??

Hi again,

Wow that's great news. So the only problem now is the 15v rail is too high in voltage?

MrAl said:

Hi again,

Wow that's great news. So the only problem now is the 15v rail is too high in voltage?

Click to expand...

yes it's too high... only way for the 317t to remain cool is if i don't connect the dvd power connector.

lynx said:

thanks i will...although at some point in the past (before the IC blows up) that issue
with the hot regulator was fixed by itself for awhile and everything! was working pefect...
i told you about it in past pages...so now i don't know why it returned to that state

Click to expand...

Hi again

The only thing that makes sense now is a "dry joint" or a hairline crack on the PCB that is causing havoc.

I would do this:

1. Unplug the PSU from Mains.
2. Take a 560 Ohm to 1K Ohm resistor (5W) and hold it across the main smoothing cap to discharge the circuit completely.
3. Closely examine the PCB for any signs of bad (dry) joints. If in doubt, resolder it/them.
4. Next, take your Meter and thoroughly check that there is continuity on all tracks that are connected together.

Regards,
tvtech

tvtech said:

2. Take a 560 Ohm to 1K Ohm resistor (5W) and hold it across the main smoothing cap to discharge the circuit completely.

Click to expand...

that's what i'm doing all the time when i take off the board... also most of the joints have been resoldered...but i'll check again
since you say it doesn't make sense.

something irrelevant i want to ask, do you think it could be powered from a square wave inverter?
and if not what issues it could cause to the specific PSU?

What I'd like to see is the transformer pins:
pin, should be value
(18) 23
(15) -22
(12) +8
(11) +8

I found both ends of FL1, but not FL2 on the schematic.

Normally SMPS do not have issues running from Inverters. Be that pure Sine Wave or modified Sine Wave......why do you ask??

OK..here's what! the lm317 input goes to 23.20v WHEN i have connected the large connector (the one most of the voltages go to)
when it is disconnected then the input of the lm317 is 13.55v exactly and steady!

tvtech said:

Normally SMPS do not have issues running from Inverters. Be that pure Sine Wave or modified Sine Wave......why do you ask??

Click to expand...

because i have an inverter but i'm only using it for resistive loads...and now viewing the
input of this device i thought if it could be possible to damage the smps ic if i use it.

lynx said:

OK..here's what! the lm317 input goes to 23.20v WHEN i have connected the large connector (the one most of the voltages go to)
when it is disconnected then the input of the lm317 is 13.55v exactly and steady!

Click to expand...

Seeing this thing has been plugged in and out so many times...is there at all a possibility that maybe a wire broke off somewhere and you did not solder it back in the right place.....

Trying to reason logically here and not complicate things

Regards,
tvtech

tvtech said:

Seeing this thing has been plugged in and out so many times...is there at all a possibility that maybe a wire broke off somewhere and you did not solder it back in the right place.....

Trying to reason logically here and not complicate things

Click to expand...

no wires seem broken, everything it's in place... having the PSU outside the dvd-r unit if i connect it to the mains without any connectors on it the voltage at the input of the lm317 it's 13.55v, now if i connect the large connector where all the output voltages go to the processing board
then the input of the lm317 becomes 23.20v and if i try to connect the 12v/5V connector of the dvd to the PSU, so it can spin then the lm317 becomes extremely hot and no motor spins.