What's wrong with my power supply?

[sbyseven]

Hi folks, I have a power supply from an Apple Cinema Display 27 that is not giving me dc output correctly.

Things that I have done already:

1. Change all the Taicon Caps but nothing has improved.
2. Change a Zener diode rectifier, but nothing has changed.

What I have observed is that Rubycon caps 420 68v are not passing more than 10v and fuses are not gone.

Where can I begin diagnosing a power supply?
Any help is welcome.

[momaka]

Re: What's wrong with my power supply?

Are you measuring the voltage on the 420V caps directly across their leads? I mention this, because they are on the primary side, so you CANNOT use secondary side ground.
If you're measuring them correctly, check the two green fusible resistors on the upper-right side of this picture:
https://www.badcaps.net/forum/attach...0&d=1676727450
^ These might be sense resistors for the APFC boost circuit. They should read very low resistance. If open-circuited, that could be the reason why you don't see any voltage on the 420V caps.

Last but not least, I hope you haven't discarded the Taicon caps yet. While they may not have a stellar reliability track record, they should be many classes better than Jackcon, which will likely fail in a year or two if the PSU is used normally.

[sbyseven]

Re: What's wrong with my power supply?

Are you measuring the voltage on the 420V caps directly across their leads? I mention this because they are on the primary side, so you CANNOT use secondary side ground.
If you're measuring them correctly, check the two green fusible resistors on the upper-right side of this picture:
https://www.badcaps.net/forum/attach...0&d=1676727450
^ These might be sense resistors for the APFC boost circuit. They should read very low resistance. If open-circuited, that could be the reason why you don't see any voltage on the 420V caps.

Last but not least, I hope you haven't discarded the Taicon caps yet. While they may not have a stellar reliability track record, they should be many classes better than Jackcon, which will likely fail in a year or two if the PSU is used normally.

First, thanks for the prompt reply.

1. Are you measuring the voltage on the 420V caps directly across their leads?
- Yes, I'm measuring them as shown in the picture.

2. These might be sense resistors for the APFC boost circuit. They should read very low resistance. If open-circuited, that could be the reason why you don't see any voltage on the 420V caps.
- Ive measured them and they gave me 0.004 Ω each one. Do I have to change them?

3. Last but not least, I hope you haven't discarded the Taicon caps yet. While they may not have a stellar reliability track record, they should be many classes better than Jackcon, which will likely fail in a year or two if the PSU is used normally
- Yes, I didn't discard them.

What I have seen is that the diode on the left side of the sense resistors is giving me continuity on the circuit, like being shorted, but if I detach it is working fine.

[R_J]

Re: What's wrong with my power supply?

Check the voltage across the outside leads of the bridge rectifier, it should be around 325vdc, if it is there, then follow the trace between the BR and the main filter capacitors?

[Devil_rider]

Re: What's wrong with my power supply?

This psu is a 2 stage psu as you can see there is a switching supply completely located on the hot side of the board and that increases voltage to 400Vdc . Second stage is a psu that makes the low voltages for the system and charging port(magsafe) . You need to check them as 2 different power supplies and recommend to use a series light bulb to limit the current when you start repair/ replace parts . Warning 400v !!!!!!!!
When you say 'DC output correctly ' you mean no output or fluctuating? Fluctuating output or clicking noise is common in this machine and if you Google sysmpton there is a way to bypass the protection circuit of the controller that causes this problem....

[sbyseven]

Re: What's wrong with my power supply?

Check the voltage across the outside leads of the bridge rectifier, it should be around 325vdc, if it is there, then follow the trace between the BR and the main filter capacitors?

Hi thanks for the reply. I'm a newbie how can I check that without danger? Is there any passive way to check that? I mean without connect it to the socket

[sbyseven]

Re: What's wrong with my power supply?

This psu is a 2 stage psu as you can see there is a switching supply completely located on the hot side of the board and that increases voltage to 400Vdc . Second stage is a psu that makes the low voltages for the system and charging port(magsafe) . You need to check them as 2 different power supplies and recommend to use a series light bulb to limit the current when you start repair/ replace parts . Warning 400v !!!!!!!!
When you say 'DC output correctly ' you mean no output or fluctuating? Fluctuating output or clicking noise is common in this machine and if you Google sysmpton there is a way to bypass the protection circuit of the controller that causes this problem....

It is fluctuating sometimes I get 4v sometimes less from each pin but I can't get the 24v to the motherboard

[Devil_rider]

Re: What's wrong with my power supply?

Look at this link. Like I said seems to be common problem and it is described nicely in this article.... Easier than me explaining step by step. https://www.instructables.com/Fixing...le-27-Display/

[sbyseven]

Re: What's wrong with my power supply?

Look at this link. Like I said seems to be common problem and it is described nicely in this article.... Easier than me explaining step by step. https://www.instructables.com/Fixing...le-27-Display/

thanks, but that's a completely different problem that I have

[Devil_rider]

Re: What's wrong with my power supply?

Thee are more articles around the web and I just picked this because it explains the circuit a little.

[sbyseven]

Re: What's wrong with my power supply?

Thee are more articles around the web and I just picked this because it explains the circuit a little.

If Im asking here is because I have checked all those articles, and I didn't have an answer to my problem. But as I said thanks anyway

[Gentian]

Re: What's wrong with my power supply?

Hi Sbyseven!

I think you have to check first the diode bridge in dhe AC side just before the 420V 68uF capacitores. Maybe it is faulty open circuit in some of the 4 diodes. Second, remove glues at AC side near the 420 V capacitors (especially the black glue which cover the smd resistors). Some times it become conductive and low the voltage. Third remove these 420V capacitors and measure resistance (should be very high (Mega Ohm range)) and then the capacitance and compare it with 68uF for each. If you don't have capacitance meter you can try soldering only one of them at a time and check if voltage restore to 320V DC (i think they are connected in parallel).

Be careful every time you check don't touch capacitor legs but discharge before them with 2kOm 5W resistor, measure voltage then and if it is small value you can do another test.

Good luck!