Help me troubleshoot this power supply. FSP043

[JonathanAnon]

Re: Help me troubleshoot this power supply. FSP043

Hey Ben... All the 12v rails are 11.8v, including the supply to the inverter. 5v rails seems fine.... I'm checking all the components between the filter cap and the 12v part of the SMPS transformer.

[ben7]

Re: Help me troubleshoot this power supply. FSP043

Hey Ben... All the 12v rails are 11.8v, including the supply to the inverter. 5v rails seems fine.... I'm checking all the components between the filter cap and the 12v part of the SMPS transformer.

Yes, check the rectifier diode on the heatsink, and just to be sure, check the esr of the capacitors.

Try measuring the 12v rail with your DMM set to read AC, the rail should have no greater than 100mV of AC on it.

[JonathanAnon]

Re: Help me troubleshoot this power supply. FSP043

I hooked up the multimeter to the 12v rail that feeds the video board and as usual the DC voltage measured 11.8V.. I think then set it up to measure AC voltage and turned everything on and it measured 25 on the 200~ setting ... Now I've looked up the MM manual and it doesnt say anywhere that it is measuring in the mV range...

But just as a control, I hooked up a 9V DC battery and the ripple on that measured as 17 on the same setting, so I'm assuming for whatever reason the MM is measuring the mV range..

I'm putting together a chart of all the voltages across the chart and marking which components I've tested. It's good practice for me anyway.

[budm]

Re: Help me troubleshoot this power supply. FSP043

Your DMM AC Volt frequency response may be too low to read high frequency AC ripple ridng on the DC of the switching power supply, it will be in 60KHz~100KHz range. If you look up the spec of your DMM, it will tell what the frequency bandwidth in AC Volt mode will be, most low cost meters are made to measure 50 or 60 Hz AC, and 1KHz is the typical limit. The reading will also be affected if the meter is a True R.M.S. or average conversion meter.

[JonathanAnon]

Re: Help me troubleshoot this power supply. FSP043

okay my multimeter, which is a cheap one, Sealey mm20, has the following written under the AC settings.

AC VOLTAGE
Range Accuracy Resolution
200V ±1.2% of rdg ± 10 digits 100mV
600V ±1.2% of rdg ± 10 digits 1V

Overload protection: 600V dc or rms. ac for all
ranges.
Frequency range: 40Hz - 400Hz
Response: Average calibrated in rms sine wave.

[budm]

Re: Help me troubleshoot this power supply. FSP043

Wow, only 400Hz on the high side, pretty low, it is an average cal also, not True R.M.S.
So when you try to measure that high frequency AC ripple on that filter caps, it will more likely to be just noise not the real AC ripple you are seeing.

[JonathanAnon]

Re: Help me troubleshoot this power supply. FSP043

Okay I'll disregard those results so. I'm planning on get this multimeter soon anyway, as this one doesnt do capacitance or frequency.. (and I've broken the transistor tester on this one as well).

Where do you guys stand on the assertion from my book that the voltage between the filter cap and the MOSFET VCC pin is too low? To reiterate my book says the voltage should be between 16v and 20v... and I'm measuring 10.6V

I'll try to get my hands on that similar monitor that is working and hopefully get a few comparisons.

[ben7]

Re: Help me troubleshoot this power supply. FSP043

Oh LOL, I got a Fluke 87 which measures over 200khz!

Although its very beaten up (was a hand-me-down from an uncle who is now deceased), it still works great!

About the 10.6v ... I think that your cheap multimeter may be to blame for that reading, as the mosfet drain pin is switching on and off at a high frequency.

Test the rectifier for the 12v rail, it should read about 0.4v - on diode check mode that is!

[JonathanAnon]

Re: Help me troubleshoot this power supply. FSP043

Hi lads I'm back on this one again I'm eventually gonna just borrow the other working monitor from the guy that owns it and measure the voltages across the above circuit on his WORKING power board.



But having looked over the schematic again... Surely if I stick the multimeter across the two zener diodes ZD120 and ZD121, I should be able to verify that the voltage is correct at these points ... as the voltage should match the Zener's reverse breakdown voltage rating.. Is this a good approach to take...

[ben7]

Re: Help me troubleshoot this power supply. FSP043

Hi lads I'm back on this one again I'm eventually gonna just borrow the other working monitor from the guy that owns it and measure the voltages across the above circuit on his WORKING power board.



But having looked over the schematic again... Surely if I stick the multimeter across the two zener diodes ZD120 and ZD121, I should be able to verify that the voltage is correct at these points ... as the voltage should match the Zener's reverse breakdown voltage rating.. Is this a good approach to take...

You can try that if you want. Maybe, if the zener is shorted - then it could be pulling the 12v rail down by feeding extra current to the 5v rail. (yeah, the zener goes from 12v to 5v! ) That might explain the darkened board by the zener and resistor.

[JonathanAnon]

Re: Help me troubleshoot this power supply. FSP043

You can try that if you want. Maybe, if the zener is shorted - then it could be pulling the 12v rail down by feeding extra current to the 5v rail.

That zener with the burn marks around it is actually Z260, which is on the 12V rail output.. And there is no drop in voltage at this point... the voltage coming out of the dual Schottky diodes is 11.78 and on the rail is 11.78.. Oh hang on, that Zener is connected in reverse bias across the output of the Schottky diode, maybe it's dragging it down.. I've already pulled it out, and checked it for shorts, but never for the voltage.. Let me try that.

[ben7]

Re: Help me troubleshoot this power supply. FSP043

That zener with the burn marks around it is actually Z260, which is on the 12V rail output.. And there is no drop in voltage at this point... the voltage coming out of the dual Schottky diodes is 11.78 and on the rail is 11.78.. Oh hang on, that Zener is connected in reverse bias across the output of the Schottky diode, maybe it's dragging it down.. I've already pulled it out, and checked it for shorts, but never for the voltage.. Let me try that.

Did the voltage change when you pulled it out?

BTW, zeners are supposed to be connected in reverse bias

[JonathanAnon]

Re: Help me troubleshoot this power supply. FSP043

Did the voltage change when you pulled it out?
BTW, zeners are supposed to be connected in reverse bias

Havent tried it without the zener diode in the circuit.. I took out he zener diode and connected up a little circuit here with a couple of batteries.. When I put 13.2V across it, the zener diode reads 9.12V

Which seems okay if you look at the spec in the sheet.. my diode is P6KE9.1A
http://www.datasheetcatalog.org/data...gd/P6KE10A.pdf

I assume that rest of the 11.78v is dropped across the resistor. But either way the zener seems fine, even though there is burning on the board.

[ben7]

Re: Help me troubleshoot this power supply. FSP043

Havent tried it without the zener diode in the circuit.. I took out he zener diode and connected up a little circuit here with a couple of batteries.. When I put 13.2V across it, the zener diode reads 9.12V

Which seems okay if you look at the spec in the sheet.. my diode is P6KE9.1A
http://www.datasheetcatalog.org/data...gd/P6KE10A.pdf

I assume that rest of the 11.78v is dropped across the resistor. But either way the zener seems fine, even though there is burning on the board.

Take out D260 and see if both diodes measure good.

[JonathanAnon]

Re: Help me troubleshoot this power supply. FSP043

Take out D260 and see if both diodes measure good.

I've tested this in circuit and it seems fine.. the contact are VERY close together so I dont fancy taking it out to test.. I was trying to find some way of testing the input side of D260 as this would give me an indication as to whether it's working (as I know that there is 11.78v DC on the output side).. but I have tried switching the MM to AC, putting the positive on the D260 input and negative to the cold ground (case), and the MM doesnt show anything..

I thought anything on the secondary side could be tested using the cold ground.. What am I doing wrong

[ben7]

Re: Help me troubleshoot this power supply. FSP043

I've tested this in circuit and it seems fine.. the contact are VERY close together so I dont fancy taking it out to test.. I was trying to find some way of testing the input side of D260 as this would give me an indication as to whether it's working (as I know that there is 11.78v DC on the output side).. but I have tried switching the MM to AC, putting the positive on the D260 input and negative to the cold ground (case), and the MM doesnt show anything..

I thought anything on the secondary side could be tested using the cold ground.. What am I doing wrong

Your multimeter probably can't read high frequency AC.

[ben7]

Re: Help me troubleshoot this power supply. FSP043

What do R230, R231, R233, and R234 measure resistance wise?

[JonathanAnon]

Re: Help me troubleshoot this power supply. FSP043

Your multimeter probably can't read high frequency AC.

ohhhh.. that old chesnut again.. I've actually bought a new multimeter, just wired a guy the money last week...I forgot to check what frequency AC it measures..

Is there any workaround to enable me to find out the voltage at this point.. If I can find that the voltage is okay on the input to the Schottky diodes, then I establish that the secondary side is working fully.

[JonathanAnon]

Re: Help me troubleshoot this power supply. FSP043

What do R230, R231, R233, and R234 measure resistance wise?

Those four resistors are SMDs on the other side of the board. I measured across them but the measuring is being messed up by back circuits I think... Let me see what I can manage when I get my new multimeter. And I will try again to get a lend of that good monitor.

http://omg.wthax.org/fourresistorssmd.jpg

[ben7]

Re: Help me troubleshoot this power supply. FSP043

Those four resistors are SMDs on the other side of the board. I measured across them but the measuring is being messed up by back circuits I think... Let me see what I can manage when I get my new multimeter. And I will try again to get a lend of that good monitor.

http://omg.wthax.org/fourresistorssmd.jpg

Very nice macro there!

Code-wise:
R230: 250Ω
R231: 4.3KΩ
R232: 30KΩ
R233: 5.9KΩ
R234: 6.49KΩ

Ofc they will read differently on the circuitboard, but if any of them is off, they might cause the voltages to change.