diagnosing an old ATX PSU help identifying resistors (PowerComputing TCX-20D)

[CapLeaker]

Re: diagnosing an old ATX PSU help identifying resistors

Try a different DMM. I mean 480vdc from 240vac, even with a PFC you wouldn’t get there! It’s gotta be the DMM.

[tony359]

Re: diagnosing an old ATX PSU help identifying resistors

The PSU is from a Power Computing old Macintosh clone system, model TCX-20D. The model on the PCB says CY-03V0 and the little vertical board which has the 5VSB regulation and ON/OFF logic says CY-03 so I suppose it's CY-03 Rev 0.

@Capleaker
Oscilloscope also reads 240V RMS at the caps so I don't think it's the DMM.

@R_J
Thanks, I'll take a look

@STJ
That makese sense thanks. I am not familiar with voltage boosting circuits so this helps.

Still, the caps are rated 200V and I've got 240V across them. Is that expected? Not to mention the burnt bleeding resistors. Is that just bad design?

[CapLeaker]

Re: diagnosing an old ATX PSU help identifying resistors

Doesn’t make sense either. If you rectify 240VAC x 1.414 = ~340VDC. All your numbers are way off. So these 200V caps have to be in series on the 240V setting.

[tony359]

Re: diagnosing an old ATX PSU help identifying resistors

Yes but as @stj pointed out, the thing is has a voltage doubler in it. Something like the below - which should only be on when 115V is selected though?

Hence I guess the recommendation to try without the 220/115 switch I think.

[CapLeaker]

Re: diagnosing an old ATX PSU help identifying resistors

That is correct. We thought maybe that switch was sitting on the 120V position and then you feeding it 240V. Your measurements are all over the place, so I am not sure if it is your testing equipment or its the ground reference point you are using, or both?

The voltage doubler part on these old PSU are all the same. 2 caps in series for 240VAC operation. You can see, there is no chance in hell for these voltages you are getting. Unless you have not the right point for your ground reference, or faulty equipment.

[tony359]

Re: diagnosing an old ATX PSU help identifying resistors

With the Fluke DMM I am measuring across the caps and across the (-) and (+) of the bridge.

With the scope I am using a differential probe, still across the caps and the bridge. So the reference should be ok - happy to be proven wrong of course.

My DMM is reading the correct AC input and other lower DC voltages. The Scope is backing up the DMM.

I agree those voltages are weird but I would be surprised if both my DMM and my scope broke the same way. Happy to test another DMM but I doubt it would make a difference!

[stj]

Re: diagnosing an old ATX PSU help identifying resistors

i always remove the wires to the switch,
it stops other people making expensive mistakes.

last time a saw someone get the switch wrong the caps exploded and it killed the motherboard!

[tony359]

Re: diagnosing an old ATX PSU help identifying resistors

well clearly the switch was not switching properly. I removed the 115/220V switch and now I have 390V at the bridge. 194V per cap. Much better! Yes I tested it but in continuity mode, maybe I missed something? Maybe with high voltage it would leak something? I've now tested it in resistance mode and it switches perfectly so.... no idea.

Thanks for that! I suppose that's what killed things in the first place. I'm actually surprised the caps haven't exploded.

The PSU still won't start - but now I see why. The CS pin is getting to 8V and I understand that that is the overvoltage optocoupler stopping the IC. The CS pin would be normally be clamped to 3.6V (it turns on the IC at 0.24V). 8V means "stop".

So I have a lead to follow. Any ideas appreciated.

Example design from the controller IC datasheet

[CapLeaker]

Re: diagnosing an old ATX PSU help identifying resistors

390VDC is closer to to what it should be, but it still not correct. You should be having roughly 340VDC not 390VDC. You should have 390VDC IF there is a PFC circuit in there.

[tony359]

Re: diagnosing an old ATX PSU help identifying resistors

ah you're right. I remember 390V from the PFC boost.

I did leave the other black cable connected, that is connecting the common of the capacitors to one leg of the AUX Transformer - but I assume it's powering the AUX PSU itself. I am going to lose 5VSB if I remove that.

I looked expensively and did not see a PFC in there. Could that AUX PSU wiring designed to boost the voltage a bit?

[stj]

Re: diagnosing an old ATX PSU help identifying resistors

does it have a big choke bolted to the case?

[tony359]

Re: diagnosing an old ATX PSU help identifying resistors

the metal case of the PSU? No. All you see is in the pictures.

[Per Hansson]

Re: diagnosing an old ATX PSU help identifying resistors

@Capleaker
Oscilloscope also reads 240V RMS at the caps so I don't think it's the DMM

You are not supposed to measure the AC RMS voltage: since the caps will have AC ripple on them you will see an arbitrary high reading.
The meter should be in DC mode and then you should be getting the readings posted by others.
That is with 240VAC RMS mains: 240v x √2 = 339VDC
It is also possible the capacitors are dead (dried out) then you will have high frequency unfiltered DC that might get up to those levels.
So check the capacitance of both capacitors.

[tony359]

Re: diagnosing an old ATX PSU help identifying resistors

Correct me if I am mistaken, the RMS value of a DC voltage is the DC voltage!

What I mean is that the Oscilloscope measured the same 240V DC - the "measuring" was in RMS but being DC it does not matter. To be fair the scope reads 16V of "peak to peak" which is the ripple measured. But if you check the trace, it's definitely not 169.5V, would you agree?

[Per Hansson]

Re: diagnosing an old ATX PSU help identifying resistors

Correct me if I am mistaken, the RMS value of a DC voltage is the DC voltage!

Yes it is, but what I was getting at is that there in no way to get such a voltage reading, and thus the conclusion would be that there is something wrong with the measurement.

An example:
240VAC RMS mains: 240v x √2 = 339VDC
Two caps in series will see half, or 169.5VDC each.
However if you flip the voltage boost switch the voltage will double.
So the recitified DC voltage will be ~680VDC (or ~340VDC per capacitor).
Note how none of these readings are even close to what you saw so a stuck voltage boost switch does not explain the readings you reported.

The only way I see to explain it would be if the capacitors are not in circuit at all (i.e. failed) then the voltage level could be any arbitrary value between 0v and 340v depending on circuit loading.

[tony359]

Re: diagnosing an old ATX PSU help identifying resistors

I fully understand, I appreciate my reading doesn't seem possible.
The switch must have been marginal, it tests 100% in resistance mode. I guess it leaks some voltage when higher voltage is applied. That might explain the "in between" voltage of 490V I measured?

I did not test the capacitors but I know they were making sparks and lighting up my lightbulb when I went to discharge them - so they are not dead. I can test the capacity no problem.

Let me be clear, Per, I am not questioning what you say. I understand what I am reporting doesn't make sense.

[tony359]

Re: diagnosing an old ATX PSU help identifying resistors

Capacitors test fine which is a surprise as

1. there are burn marks on their sides due to the baking resistors
2. I powered them with 240V while testing (they're rated 200V)

They are 330uF caps.

Apologies for the rotated pictures, they look ok on windows, something is not taking the rotation into account.

[Per Hansson]

Re: diagnosing an old ATX PSU help identifying resistors

What is the voltage reading now on the primary side with the power supply soft off and started up?

[tony359]

Re: diagnosing an old ATX PSU help identifying resistors

without the capacitors in place?

[Per Hansson]

Re: diagnosing an old ATX PSU help identifying resistors

No the caps always need to be in place, the last reading you gave was 390v, is that still valid?