IMAC 24 PSU Bad Caps replacement

[bottleneck]

Re: IMAC 24 PSU Bad Caps replacement

R83 is toast.

Check D9 near the cap that keeps blowing.

What is the device in the gray goop next to that cap? 3 legged, TO-92 package.

Identify (case markings) M6, M9, M10?

T

I checked D9, it looks like the following(four lines on top) of A6

=
=
A6

It measures (in circuit) .555 v one direction and OL the other using the diode test with my dmm. Doesn't that mean it should be functioning correctly?


The grey goop guy is:

TL431AC: datasheet included.


The others are as follows:

M6: fc762
m9: fc762
m10: Lm393

I have included couple of the datasheets. not sure on the fc762? Please advise of any further tests or ideas you might have.

[Francisco.s]

Re: IMAC 24 PSU Bad Caps replacement

Where is the NCP1381?

I attached the datasheet.

T

Here you have a NCP1381 schematic application, but it differs in many aspects from the circuit I'm looking for.
Is there someone who knows de value of r83?
Thanks in advance.

[Toasty]

Re: IMAC 24 PSU Bad Caps replacement

@ Francisco.s - *WHERE* is the NCP1381? I do not see it.

@ bottleneck - D9 readings look fine. It is C53 that keeps blowing out, correct?

T

[Francisco.s]

Re: IMAC 24 PSU Bad Caps replacement

@ Francisco.s - *WHERE* is the NCP1381? I do not see it.

@ bottleneck - D9 readings look fine. It is C53 that keeps blowing out, correct?

T

Hi Toasty, I can`t understand your questión, the NCP1381 is in the pcb of the psu I have.
Could be posible that we are talking about different psu`s?
I will take some photograps and uploads it.

[bottleneck]

Re: IMAC 24 PSU Bad Caps replacement

@ Francisco.s - *WHERE* is the NCP1381? I do not see it.

Toasty... ncp1381 is located under what I'm guessing is supposed to be a heatsink. The Copper thing in the center of the board. I could retake a picture with it lifted up if it will help.

@ bottleneck - D9 readings look fine. It is C53 that keeps blowing out, correct?

Yes C53 is the correct cap in question. I noticed there is something under some silicone as well VR1. would this be a regulator? It doesn't look like anything I have seen before, its completely metal. Rather than march ahead, I was hoping that you might have an idea?

I've been looking for a value for r83, no luck yet.

Thanks!

[Toasty]

Re: IMAC 24 PSU Bad Caps replacement

@ bottleneck - Thank you. That's what I wanted to know. Since there are no pictures of the NCP1381. Can someone please take some? Also, I believe the copper is a shield, rather than a heatsink.

re: C53 - okay, just want to be sure we weren't chasing the wrong cap. Again, no one has given a cap layout of this PSU.

VR1 is the adjustment for the TL431AC device. Do not try to move it. It is preset and the silicone locks it from moving.

I really need to find one of these for CHEAP.

Check the cap attached to one end of R83 to ground for short. If, as Franciso.s says, R83 is attached to pin 3 (DMG), it's feed is directly off the transformer. Depending on which schematic you follow, there are a few scenarios where R83 could get damaged.

Looking at the schematics and coming from Pin 3 (DMG), which position is R83 in? :

Is it R12 or R13 in Francisco.s' schematic?

-or-

In the datasheet, the resistor above the NCP1381 or the one next to the transformer?


T

[Francisco.s]

Re: IMAC 24 PSU Bad Caps replacement

Hi Toasty, I have taken some photos to clarify the position of some componentes, I hope so.

Also, I believe the copper is a shield, rather than a heatsink.

You are wrigtht, it is a shield.

re: C53 - okay, just want to be sure we weren't chasing the wrong cap

. I can read the value of that capacitor, and is an electrolitic one, value: 22uF 50v.

Check the cap attached to one end of R83 to ground for short

there is no short in that capacitor (c54) in my psu, and it has other resistance (r67) conected to r74 and this one, to pin3 of NCP1381. There are other components in this series of resistances, going to PFC circuit.

Regards.

[Francisco.s]

Re: IMAC 24 PSU Bad Caps replacement

In order to not doing a big post, I decided to cut in two post.

Looking at the schematics and coming from Pin 3 (DMG), which position is R83 in?

R83 would be R12 in NCP1381ADAPEVB_SCHEMATIC, I can`t find the equivalent of R13, but as I said before, there are other resistances going to PFC circuit.
Finally, I`ve found D9, it is a smd diode that feeds c53, and if this capacitor is damaged, probably the optocoupler (m7), the ic TL431, or an open resistance in that circuit can be the bad part. Also in the primary side of the transformer, there are one diode (D11) and some electrolitic capacitors that can do a bad job.
I would like to take some captures with the oscilloscope, that will help to find the problems, but I can´t get my PSU work continuously, and it is because the driver circuit, of the power mosfet has damaged componets that I can't identify. (I get some pulses in the mosfet drain, but then, nothing)
There is a smd transistor (Q5)that amplifies current to drive the mosfet, and it is imposible to read the name. (it puts 35 and something more)
I'm doing some research to identify that transistor, and repair this PSU that is a big headache.
Thanks and good luck.

[Toasty]

Re: IMAC 24 PSU Bad Caps replacement

Using R12 as R83 - DMG (3) monitors the voltage from the trafo. If R83 failed, then either the cap to ground shorted, or the IC failed / shorted.

Failing to find R13, then by the NCP1381 datasheet schematic (pg. 2), it would be the resistor above the chip. Again, the same failure scenario.

T

T

[Francisco.s]

Re: IMAC 24 PSU Bad Caps replacement

[/QUOTE]Hi bottleneck,

ncp1381 is located under what I'm guessing is supposed to be a heatsink.

Could you take a photo of the ncp1381 near the smd transistor q5?

I've been looking for a value for r83, no luck yet

In my psu the value is 15 Kohm, it is a value more realistic than 470 Kohm. If it regulates de output power, it can be the problem of your psu.
Thanks in advance for the photo.

[Francisco.s]

Re: IMAC 24 PSU Bad Caps replacement

Using R12 as R83 - DMG (3) monitors the voltage from the trafo. If R83 failed, then either the cap to ground shorted, or the IC failed / shorted.

Failing to find R13, then by the NCP1381 datasheet schematic (pg. 2), it would be the resistor above the chip. Again, the same failure scenario.

T

T

Hi Toasty, here is the Bill of Materials for the NCP1381 Evaluation Board. Surprisingly the value of R12 is 100 kΩ. (but again, as we don´t know the values of the IMAC PSU, then....)
Regards.

[Francisco.s]

Re: IMAC 24 PSU Bad Caps replacement

Hi, I have found more information abut this psu, it´s about chip NCP1351, that seems to work like NCP1381, and what is more important for me: the output driver for The mosfet incorporates a pnp transistor.
Regardas.

[bottleneck]

Re: IMAC 24 PSU Bad Caps replacement

Could you take a photo of the ncp1381 near the smd transistor q5?

I apologize for the delay on the picture, i havent had time to get back here. I have included it now. Let me know if you need more.

In my psu the value is 15 Kohm, it is a value more realistic than 470 Kohm. If it regulates de output power, it can be the problem of your psu.
Thanks in advance for the photo.

Can either one of you clarify a little on what i should check next and or how. You both obviously are more advanced than i am and i want to make sure I am not chasing my tail.

Thank you

[Francisco.s]

Re: IMAC 24 PSU Bad Caps replacement

Thanks for the photo, now I can see more clear the name of Q5. I will try to find a replacemet, and see if the psu starts to work. As we said, it's very likely, your problem is the value of r83.
I will confirm you that, as soon as possible.
Regards.

[Francisco.s]

Re: IMAC 24 PSU Bad Caps replacement

I apologize for the delay on the picture, i havent had time to get back here. I have included it now. Let me know if you need more.


Can either one of you clarify a little on what i should check next and or how. You both obviously are more advanced than i am and i want to make sure I am not chasing my tail.

Thank you

Hi bottleneck, not luck for me, I finally put an equivalent transistor, but the psu is in the same condition. I have taken a close look to your photo, and I can see many differences. The code of my psu is api5st06-290g, and i suppose your psu code will be different.
I am afraid that without schematic it is very difficult to repair this kind of breakdown. I don´t know if in your psu the value of r83 is correct or not, but if you change r83 to a value of 15 kΩ, perhaps your psu start to work properly.
But again, if we haven´t the schematic, then it´s a matter of luck.
In my case the NCP1351, starts sending pulses to the mosfet, but with a few pulses the voltage at pin 10 (vcc) drops to 6 volts, and according to data-sheet,it is due to short-circuit protection, or over-voltage protection.
Good luck.

[bottleneck]

Re: IMAC 24 PSU Bad Caps replacement

Hi bottleneck, not luck for me, I finally put an equivalent transistor, but the psu is in the same condition. I have taken a close look to your photo, and I can see many differences. The code of my psu is api5st06-290g, and i suppose your psu code will be different.

Francisco that is correct. the code of my psu is API5ST06-291G It sounds as though there is a difference after all. Did you test all caps on the secondary side?

Toasty... Do you have any idea where to proceed? you are the veteren here after all. and i am definatly the young... ok, not so young grasshopper.

Thanks for any help!

[ben7]

Re: IMAC 24 PSU Bad Caps replacement

Bottleneck, check R35, it should be 15 ohms (based on it's markings). Check ZD2 to see if it is shorted, and then R35 will likely read open.

[Toasty]

Re: IMAC 24 PSU Bad Caps replacement

Sorry bottleneck. Without one to hand I really can't help much at this point. Too many things I can not see and too many questions that are not answered.

T

[Francisco.s]

Re: IMAC 24 PSU Bad Caps replacement

Francisco that is correct. the code of my psu is API5ST06-291G It sounds as though there is a difference after all. Did you test all caps on the secondary side?

Toasty... Do you have any idea where to proceed? you are the veteren here after all. and i am definatly the young... ok, not so young grasshopper.

Thanks for any help!

I am working already on this psu, all caps on the secondary side are tested and also diodes. I have modified the control of the optocoupler and the psu works, but the power mosfet gets very hot, so I have to study the circuit closely to see what's wrong.
Regards.

[ben7]

Re: IMAC 24 PSU Bad Caps replacement

I am working already on this psu, all caps on the secondary side are tested and also diodes. I have modified the control of the optocoupler and the psu works, but the power mosfet gets very hot, so I have to study the circuit closely to see what's wrong.
Regards.

What voltage is the MOSFET driver supplied with? Is this the original MOSFET too?

What have you done to the optocoupler control? That may be the problem.