Apple iMac Power Supply Acbel API4PC47-290 Higher than normal voltage

[torgeson5kqs]

I have said power supply that has had all bad caps replaced and or checked with ESR meter to ltec specs. The power supply powers on fully both on standby and when jumpered. However, when jumpered the readings on the pinout that should be 5v or 12v read 5.5 and 13volts. I have 3 power supplies all identical that have this same problem. I've checked these values with a calibrated meter as well so I know it's not my equipment. Any ideas as to what would cause an increase in these voltages?

[PeteS in CA]

Re: Apple iMac Power Supply Acbel API4PC47-290 Higher than normal voltage

Is the power supply connected to any kind of load? If it isn't, the regulator may not be able to control the voltages. If not, you can probably load the main O/Ps - traditionally, +5V, +3.3V and +12V - to 5%-10% of the ratings and see whether you really have a problem. Getting the right value and power rating resistors could be interesting, of course.

[Toasty]

Re: Apple iMac Power Supply Acbel API4PC47-290 Higher than normal voltage

5vsb & 24vsb correct?

Does this have the 4863G chip? PFC?

Photos, por favor.

[torgeson5kqs]

Re: Apple iMac Power Supply Acbel API4PC47-290 Higher than normal voltage

I've worked on several of these and never have the voltages been this high when jumpered without a load on them, which is why I thought this was odd. 5 and 24 Voltages show on the correct pin outs other than the fact that they are closer to 5.5-5.6v and 13v. This does have a the 4863G chip and TOP245YN. Let me know if you need different vantage points on the power supply.

[Toasty]

Re: Apple iMac Power Supply Acbel API4PC47-290 Higher than normal voltage

>>5 and 24 Voltages show on the correct pin outs other than the fact that they are closer to 5.5-5.6v and 13v. <<

Huh? Come again?

Original symptoms?

Where is the 4863? Out of frame?

[torgeson5kqs]

Re: Apple iMac Power Supply Acbel API4PC47-290 Higher than normal voltage

Here is a layout of the voltages I get when jumpered. Let me know if this helps.

Voltages measured from ground are as follows:

3, 5, 7, 11, 14, 16, 17, 18 - Ground (-)

1, 2, 12 - 3.3v

4, 6, 20, 21 - 5.6v

8 - (Power Good) - 5.5v

9* - (Standby) - 5.0v

10, 13, 19 - 13.1v

15 - On/Off - jumper to ground (14 or 16) to run supply

22* - 23.9v (may read about 21v until switched on)

* Always present when unit is plugged in but not switched on.

[Toasty]

Re: Apple iMac Power Supply Acbel API4PC47-290 Higher than normal voltage

Put some load on it as PeteS recommended. Doesn't need to be anything much, just something for the supply to "work" against. Some automotive 1156 types are fine.

Connect a jumper wire between the 2 mounting holes. Where the screws go that hold it in the metal case. I think what you are seeing may be induced (radiated) voltages in the meter leads.

This has the WT7515 chip, yes? If so, and the supply is working, then everything is okay. The 7515 would not allow it to go out of spec like that. Try measuring on the board, directly at the junction points for the output leads, instead of at the connector.

Toast

[Toasty]

Re: Apple iMac Power Supply Acbel API4PC47-290 Higher than normal voltage

To figure out the resistors and wattage needed to load the supply, just follow Ohm's law.

Do the following math for 100% loading:

Rated Voltage (E) ÷ Rated Current (I) = Resistor value (R) in Ohms (Ω)

3.3v ÷ 3.1A = 1.06Ω

To determine the Wattage needed for the resistor, do this:

Rated Voltage (E) x Rated Current (I) = Power in Watts (P)

3.3V X 3.1A = 10.23W

Do that for each output. Then you can determine the load you'd like to apply by as a percentage of those values.

Let's say a 10% load for starters.

In the example:

You need a 1.06Ω resistor capable of 10.23W to load it at 100%.
That resistor is going to be -screaming- hot, so I try to double the wattage if not triple. Also, fan cooling is a must. Or a bucket of water...

For a 10% load you'd need 10 times that value in resistance, but only 1/10 the wattage, or 10.6Ω @ 1 watt. Closest choice I would use would be a 5 watt wirewound ceramic "cube".

Figuring out the others can lead to some pricey resistors. This is where using automotive headlamps becomes economical. You don't need the cooling fan or bucket of water. Remember that an incandescent lamp's resistance -rises- as it heats up. What starts out cold as a 4A load may turn out to be only 1.5A when fully on.

Toast

FWIW: Loading these above 50% is not recommended. Instantaneous surges are one thing, and they handle them well. Continuous loading is quite another. You could easily cook the supply and the caps right out of it.

.

[torgeson5kqs]

Re: Apple iMac Power Supply Acbel API4PC47-290 Higher than normal voltage

I finally got them all working. I setup the connector so I could measure the voltage on the iMac motherboard in real time while it was in use. The voltages were all correct once their was a load on the power supply.