iMac G5 AcBel 17" API3PC94-290 no voltages

[torgeson5kqs]

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

I think I was using the open fusable resistor, I had a couple of them and I think I got them switched. Once I relized that I narrowed it down to the C28.

[torgeson5kqs]

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

Earlier in this post you mentioned hooking up a DC supply along with a scope. I have another power supply with this same problem but can't isolate the short. Could you explain how to hook both those up to run diagnostics?

[Toasty]

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

The scope allows you to see if the PWM is working by seeing the signal in the circuit. You don't even have to connect the scope, just hold the probe near the transformers and observe the pattern.

The bench supply allows you to inject a voltage before and/or after a regulator device or supply section to see if it is functioning or is shorted on input/output.

What is the issue with this supply and what have you done? Is 5v standby working?

Look to the "common" failures in these for the solution before going after the wacky ones. Q1, ZD19, M24, bad start/filter caps, etc.

T

[torgeson5kqs]

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

I know of the Q1 and it's fine (not shorted) but where are ZD19 and M24? Topside, bottomside? what are they near?

[Toasty]

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

I probably got the ID numbers wrong. Scratch the ZD19, these don't have that one.
M24 is correctly M6, the KA78M24, 24v regulator.

Again, "Is 5v standby working?"

T

[torgeson5kqs]

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

M6 was replaced and is showing about 3.9 volts and the 5v rail bus is showing only about .6 volts. Readings taken while unjumpered.

[Toasty]

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

Q1 voltages?

TOP?

3845B?

[torgeson5kqs]

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

Q1:
Voltage across C & E jumps between 1 and 2 volts
Voltage across C & B jumps between .5 and 2 volts
Voltage across B & E jumps between .2 and .4 volts

3845B:
voltage accross ground and Vcc are between 0 & .4 volts
voltage accross ground and output are between 0 & .2 volts
all others pin outs are 0 volts

TOP244YN:
input is 366 volts
output at S Pin appears to be 366 volts as well (not sure if I'm reading this right) this is also the same going into the medium sized transformer (T4) got this from data sheet

pg 4

[Toasty]

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

Q1 - From circuit ground you should have:

B = 3v, C = 16v, E = 2v

Circuit ground is the (-) of C37, 100uF cap (adjacent to the Collector of Q7). You should have 16v across that cap, Vcc for the circuit.

366v is correct. Source (4) to Drain (7).

On the other side of the standby transformer (T2), measure from the two pins that are tied together to the cathodes of the diodes for 5v (D20) and 24v (D23). You should have 5v and ~31v.

T

[torgeson5kqs]

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

voltage across C37 jumps from .2 - 1.6 volts

using circuit ground
B = .3-.8v, C = 1.3-2.6v, E = .7-2.4v (all these voltages jump around and are NOT stable)

0v for both D23 & D20 when measured from the cathode to the tied end of the T2 transformer as you mentioned in the above post.

[Toasty]

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

Short somewhere on output side. Lift one diode cathode at a time and see if other voltage comes up.

[torgeson5kqs]

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

Even after lifting one cathode at a time and measuring voltage across tied end of T2 the voltages are still zero. Is it possible T4 is shorted at the output?

[Toasty]

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

T4?

Measure from heatsink of TOP to cathode D18.

[torgeson5kqs]

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

reads 1.3v - 2.5v (jumps, not steady)

[Toasty]

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

Lift D18 cathode, does 5v and 24v come back?

[torgeson5kqs]

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

no still the same as before

[Toasty]

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

Scope the transformer input and outputs. You should have very clean sweeps as in the lower part of the attached image.

Remember to use a 10X probe on the primary. Your peaks can be 350-400v coming off that TOP.

You are using the isolation transformer, yes?

T

[torgeson5kqs]

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

here is a couple of pics of the setup and what I get on the scope

[Toasty]

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

You have both probes attached. That's not correct.

Remove the left probe and attach the ground clip (supplied with probes) to the right probe at the exposed silver barrel section. Where you have the right hand probe is okay. You want the ground clip to the pin just below it.

The Upper Left 2 pins are the TOP output and the primary of T2.

This transformer has a primary and 3 secondaries. The HOT side secondary is 16v and comes from the 2 pins on the Lower Left.

The isolated secondary is the 2 on the Upper Right and the 2 on the Lower Right. Their common is the 2 tied together in the middle. Upper Right is 5v, Lower Right is 30v.

You should have 400v+ P-P on the Upper Left pins.

Corresponding lower readings on each of the other 3 pairs.

T

[torgeson5kqs]

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

I both pics should show what your looking for however the Pk - Pk tops out at about 370 volts on the two upper left pins when.