another imac psu ADP-200DFB

[rookieme]

hello, the power supply is in a working - not working state
if switched off the 12V are dropping to 3V before going to 12V again - when you have 12V you can hear a electronic noise and when the voltage drops the noise stops
the cycle repeats every 4-5 seconds

well if you press the power button at the time you have the 12V system can boot and work fine so with load the PSU can work

so far I have tested the capacitors at the 12V line
any suggestions?

[sam_sam_sam]

Re: another imac psu ADP-200DFB

Check all capacitors that are less than 100uf with an ESR meter if you do not just results

[rookieme]

Re: another imac psu ADP-200DFB

thank you for the reply,
by saying all capacitors you mean the electrolytic only or the polymer capacitors too?

[sam_sam_sam]

Re: another imac psu ADP-200DFB

I was referring to electrolytic ones

I will check capacitors with my ESR meter but if I do not like the results that I am getting I do this test if I do not have this value on hand

I will check the capacitor at 100*F then a room temperature then at 50*F if the results are more than 10% of a new capacitor between the the 100*F and the 50*F temperatures I consider them to be bad but this just me personally

Here is what I do when I polymer capacitors and there are several of them with the same UF value and voltage rating if you have ESR reading all over the place more than 0.50 I would compare them to a new one and just replace them for good measure but that just me personally

I use this rule when I repair Servo drive and switching power supply for these machines and have been able to get them back functioning correctly again but this is my personal experience with this type of testing capacitors for these devices

[rookieme]

Re: another imac psu ADP-200DFB

So far there is only one at 33uF.
I have access only at capacitance meter so no way to measure the ESR.
I have replaced it as it was metering at 30uF with no luck.
Which is your ESR meter? Maybe I should consider buying one.

[sam_sam_sam]

Re: another imac psu ADP-200DFB

My is Anatek Blue ESR Meter from EBay there are two versions one is a kit and the other one is assembled which cost a little bit more and if I remember correctly this seller has both versions

https://www.ebay.com/itm/18423015559...53.m2749.l2649

Yea it is a little bit expensive but it is a good one and it has help me troubleshooting Servo drives and VFD drives and Switching power supply with capacitor issues

A little while back I bought another one so I would have a back up tester for testing capacitors at a customer place of business because the one that I am using is a little rough around the edges from a lot of use

If after you replace the capacitors in this switching power supply and you still have this issue then check the power supply rails for short circuit conditions to make sure that this not an issue

[rookieme]

Re: another imac psu ADP-200DFB

I have bought some years before this one but I'm not sure if it is trustworthy
https://www.ebay.com/itm/28475083100...0AAOSwlb5iTpf7

[rookieme]

Re: another imac psu ADP-200DFB

Thank you for your assistance I managed to repair it.
There was a cap 100uf under a heatsink that was measured 73uf and 6,5 Ohm ESR!
I should start using my meter more often too.