Dell DA2 12V 18A PSU repair P/N M8811

[simontay1984]

I have aquired a Dell SX280 from a friend that wasn't working.
I opened the PSU (part number: M8811) and found that some SMD transistors on a small daughter board are fried.
Its part of the feedback circuit. I think some pins on it got shorted to the metal shield/heatsink.
They're so badly burned I can't identify the part numbers.
I bought a replacement DA2 on eBay for £10 with the intention of looking inside to try and identify the components so I could fix the other PSU but the little daughter board in it is different.   
The part number on the case of the working one is N112H. The computer works fine with the new PSU.
Does anyone have a working DA2 PSU with part number M8811 that they don't mind opening to get the part numbers of the components on the little daughter board. That would be much appreciated I have attached some photos of it showing the fried components.

[simontay1984]

Some more photos of board attached cos of 2000KB limit.

[simontay1984]

An update:

The side with the 4 pins and the burnt transistors is on the primary high voltage side.
They're not likely to be just any 2SC, 2N or BC548/558 type. There are 100s of different transistors out there.
They probably need to be high voltage ones. Q201, Q202, Q203, R201 and ZD201 are what is burnt. R201 is completely black. The value of that resistor matters. I couldn't find any schematics on the web of this PSU. OEM is Delta. The model number of this little board is STDC-003.

The other working PSU doesn't have those tiny transistors. The components on it's daughter board are completely different. There is an IC and an opto-isolator as well as the transformer. The side with 2 pins on this is 3 pins on the other working PSU.

[Anthem]

Well I've got a Dell 12V 18A DA2 like yourself. Maybe you can reverse engineer it. I looked and measured my own and comparing got some clues.

The Primary Side has 4 pins. The grounded innermost pin is the grounded 0V reference and the pin closest to the PCB edge is rectified mains voltage from the supply capacitor. This looks the same on your PCB. We have 220~230VacMains  which translates to 311Vdc to 325Vdc, I didn't measure the two inner pins on my Dell supply, but  they seem to be the sense pins to see the if the Mains supply is present even if the Main supply cap is fully charged  What is your countries Mains Supply voltage ?

So your Q202 would suite a BRS1N80, which is an N-Ch FET 800V 1A in a TO-92 package, which is the only real high voltage component you need concern yourself with. My PCB has two types of transistors, the one is a 2N7002 which is an 60V/300mA N-C FET in a SOT23 package with SMD marking code 702. Which could be your very burned one. Just drawing out this simple circuit will reveal its function. Then the other transistor on my PCB uses is the MMBT4401 with marking code 2X.

The ZD201 Zener diode is a bit tricky as I can't quite see the colors properly. For instance, ZD202 is a 12V zener in a mini-MELF or LL-34 package. ZD201 looks like it may have been a red/purple green which is a 24V zener (RLZ24B). So that leaves R201 which looks like a 2 suffix. So 1k to 8k2 it must be supplying the base of Q201 which is normally pulled down by R202 which keeps it switched off. Looks like you could ID Q201 marking code with a toothbrush/dish washing liquid and a toothbrush dipped in water to brush off the carbon and then rinse in water to clean off the soap and look sparkly. Just dry with hairdryer. I would change the caps too if you can measure them out of circuit. You can see how the circuit failed. It looks like Q202 failed and high voltage burst through the gate and struck Q203 onward.

Looks like the 2 pin side is just an AC supply which will get rectified off the PCB and get regulated to the circuitry it powers up.

I would test it off the main PCB with a 220Vac/60W incandescent light bulb limiting the rectified mains + supply. Then the transformer has pin 5 connected to the OV of the 4 pins (inner pin) and pin 4 of the transformer goes AC straight out of the pin next to the +V rectified Mains pin (outermost pin). The last pin to resolve its function the one connected to ZD201. The Zener cathode is blocking the voltage arriving on the pin and so it means that Q201 is an NPN transistor. The MMBT4401 is thus an NPN transistor [60V 600mA MMBT4401 should be OK] as the gate of the main switching FET is probably 12V. This transistor looks like it switches off when the mains is ON and then releases the gate of Q203 to go into oscillation, with control from Q202 [it needs clean up on its base or gate to see what it is doing]. Also it means you can connect power to the two mains pins and leave the two mid pins disconnected and the circuit should oscillate under test, when you are ready. Q202 though, maybe used for voltage limiting Q203 gate with a square wave of 12V peak via a connection to the +12V ZD202 which limits the gate voltage of Q203 to 12V, which would likely make Q202 a PNP. [MMBT4403 would work OK here then]

Hope this helps after all these years. Unless you got it going in the meanwhile !

[soldar]

Dell SX280? Pentium 4? You sure it's worth the effort?

[Anthem]

Its a great 12V PSU and by changing the voltage divider control it can output 13.8V and make a fabulous Battery Charger. The Remote pin can be used to detect the presence of a battery or not and switch off the terminals so that they don't short with a simple optocoupler circuit. Or be used as a powerful 12V bench top supply.  So it is worth it in imo.

[soldar]

Its a great 12V PSU and by changing the voltage divider control it can output 13.8V and make a fabulous Battery Charger. The Remote pin can be used to detect the presence of a battery or not and switch off the terminals so that they don't short with a simple optocoupler circuit. Or be used as a powerful 12V bench top supply.  So it is worth it in imo.

Ah, OK, I get you now. Still, the high amp rail is going to be the 5V and the 12 V is going to be limited.

[Anthem]

This PSU only has a 12V Supply.  See attached pin usage.