iMac G5 AcBel 17" API3PC94-290 no voltages

[torgeson5kqs]

I have a couple of these units of the same model and both appear to have the same problem, which is I get no voltages after plugging them in. All caps have been replaced, fuse is good, rectifier is good. I get about 168 volts on both the large caps. Varistors are good on both. I compared my readings with a known good power supply of the same model and the voltages read a little different on the TOP244 at M1. So I swapped it out with a TOP245 (thats all I have currently) still nothing. The Diodes at D20 and D10 appear to be good (The D10 never really gives a reading even on good ones). So I'm at a loss for the moment and welcome any suggestions.

[Toasty]

Re: iMac G5 AcBel 17" API3PC94-290 no voltages

No standby 5 & 20~24v? Need to find out what's killing that first.

[torgeson5kqs]

Re: iMac G5 AcBel 17" API3PC94-290 no voltages

that is correct no voltages at any of the pins. I thought it might be the TOP244 at M1 but I believe the voltages before that point are off such as at one or two diodes but they check out fine. So that is where I am at this point. Any suggestions?

[Toasty]

Re: iMac G5 AcBel 17" API3PC94-290 no voltages

Where do you lose voltage after the TOP? What do you read across D20 & D23?

[torgeson5kqs]

Re: iMac G5 AcBel 17" API3PC94-290 no voltages

D20 and D23 both read 0. The voltages of the new TOP245 (original was top244) are around 2.4 and .08 on a couple of the pins. On another working supply of the same type they read about 5v and 24v I think?? So I suspect the voltages that are incorrect are before the TOP245.

[Toasty]

Re: iMac G5 AcBel 17" API3PC94-290 no voltages

What's the input to the TOP?

[torgeson5kqs]

Re: iMac G5 AcBel 17" API3PC94-290 no voltages

appears to be about 193v but just shows on the DMM for a second. Am I reading this correctly?

[Toasty]

Re: iMac G5 AcBel 17" API3PC94-290 no voltages

No. Should be constant. Reading from where to where?

EDIT: Across the 2 outside legs of the TOP (1 & 7) you should see ~325 volts DC. Remember this is the HOT side of the power.

.

[torgeson5kqs]

Re: iMac G5 AcBel 17" API3PC94-290 no voltages

Ok, it reads about 333v across the two outside legs which is constant, I had the meter set to a max of 200v thats why it wasn't showing correctly....

[Toasty]

Re: iMac G5 AcBel 17" API3PC94-290 no voltages

From the mounting tab/heatsink to either of the 2 transformer (T2) legs closest to the edge (right next door) you should see the same voltage.

If you do, then the problem is on the output side of the trafo.

[torgeson5kqs]

Re: iMac G5 AcBel 17" API3PC94-290 no voltages

Do you mean on T4? This is a 17" model PSU, on the 20" ones T2 is the name. In any case the voltage on one of the legs closest to the edge reads about 333v and the other reads 0v.

[Toasty]

Re: iMac G5 AcBel 17" API3PC94-290 no voltages

3PC94 has 3 transformers. T1, T2, T3.

T1 (LARGE) is the main transformer for the 3.3/5/12v
T2 (MEDIUM) is the standby supply transformer connected to the TOP for the 5/24vsb.
T3 (SMALL) is the driver for the main switcher FETs

[torgeson5kqs]

Re: iMac G5 AcBel 17" API3PC94-290 no voltages

Measurements I took were off the medium transformer which show same voltage on one of the pins (inside pin closest to TOP245 and closest to edge of PCB) which was about 333v (same as voltage accross outside pins on TOP245). So since D20 and D23 show no voltage could this mean that the transformer might be bad?

[Toasty]

Re: iMac G5 AcBel 17" API3PC94-290 no voltages

T2 is a dual coil input transformer. 4 separate input leads.

On the input side, measuring from the 2 closest to the middle of the board (3 & 4) to the 2 closest to the edge (1 & 2), do you get ~325v?

It's 4 readings. 3-1, 3-2, 4-1, 4-2

1 2   3 4
o o   o o

If yes, then:

Remove D20 & D23.
- Test for continuity between center (tied together via circuit foil) and the outside legs.
- Test the diodes now out of circuit.
- Apply power and test the output of the transformer on AC scale.

If the transformer is bad, it would be a first.

.

[torgeson5kqs]

Re: iMac G5 AcBel 17" API3PC94-290 no voltages

I did get the 4 readings at 333v volts on the input side of transformer as you mentioned. I've removed the diodes as well and also got continuity. However, when I test for AC on the ouput side it jumps all over the place. When I tested on a good one I got a constant 120v or 100v depending on which output leg I set the probes to. Have I done this correctly?

[Toasty]

Re: iMac G5 AcBel 17" API3PC94-290 no voltages

>>When I tested on a good one I got a constant 120v or 100v depending on which output leg I set the probes to.<<

With the diodes out or not?

Diodes test okay?

.

[torgeson5kqs]

Re: iMac G5 AcBel 17" API3PC94-290 no voltages

I'm sorry on the good one I had the diodes in. Once I took out the diodes on the bad power supply I tested them with a transistor/diode tester and the larger one D20 was bad so I replaced it, but that doesn't matter at this point because power isn't even getting to the diodes.

[torgeson5kqs]

Re: iMac G5 AcBel 17" API3PC94-290 no voltages

I've attached a picture of the transformer pins and removed diodes from a working power supply and also have an identical one next to it with the diodes removed as well. On the good one I also get erratic AC readings (not sure if i'm doing this correctly) when I test the output side of the transformer. All the legs on the right no matter what comination I attach the probes give me an erratic reading. I'm obviously doing something wroong here. Testing with DC gets 0v.

The picture needs to be oriented 90 degrees clockwise.

[Toasty]

Re: iMac G5 AcBel 17" API3PC94-290 no voltages

Nope. Nothing you're doing wrong. The TOP is not getting any feedback so it's jumping around trying to do something. Without the feedback, it will "bounce", trying to start.

Since both supplies provide the same results, the problem is -not- in the transformer.

Since those diodes are out of circuit, try some ohmmeter (diode test too) readings with from their cathode foil to ground. Remember you're on the COLD side now.

Try with one diode back in, then the other. Does anything come back?

Check the 3 caps at that edge you replaced. Make sure to reflow their connections as I've seen these grab the foil side of the via, but not the component side. Try to wiggle the caps a bit as you reflow. Flux on the topside too.

Check the 24v regulator. Make sure it's not shorted.

A scope would be real handy right about now.

.

[torgeson5kqs]

Re: iMac G5 AcBel 17" API3PC94-290 no voltages

cathode to foild on d20 reads about 750ohms and D23 reads about 120ohms on ground to cathode foil. On the working supply the D23 reads bout 650ohms. I checked the smalls caps that I replaced near the optos and they have a healthy amount of solder, very clean, no shorts (checked with loupe) The 24v regulator tests exactly the same as the good one I have in the working supply. I do have a scope by the way a techtronix TDS-210 I picked up along with those multimeters I sent you. The scope was an easy fix so I kept it. I don't have any BNC probes though, any suggestions for quality probes with the little quick clips that you would recommend for this particular scope? I might be able to borrow some tomorrow, let me know what I should test?