Need help repair dead G-Tech SFX-500 power supply

[mikey5791]

Hi all,

Many years back while using my Pentium 4 desktop computer, this china made power supply (G-Tech SFX-500) suddenly produced a loud bang with spark. After shut off the 250V mains and opening the psu case, I saw the 5A glass fuse blacken and the black thermistor NTC5D-9 chip off on its side (this I replaced with green thermistor SCK-053). Tested the SCK-053 on diode test reads 6.3 ohm.

After some component testing, I started replacing two shorted RL207 diode, and on heatsink "1" two pieces shorted Bipolar Junction Transistor (E13007 & J13007) replaced with ST13007A (8A 400V NPN BJT). I hope the ST13007A can replace the E13007 & J13007 as equivalent. I attached some pictures of top & lower board as img 1114 & 1124.

For input protection and learning from momaka on this forum, I added a white square X2 safety cap (label 100n J4) & a common mode choke extracted from an old tv board. Then I connected a 60 watt light bulb in series to the fuse connection (red and yellow wire), shorted a wire on green and black cable on the ATX molex connector. The pictures of the board back & front after modify is img 1131 & 1132.
Last night I tried a power on, the psu fan did not spin but the 60 watt light bulb glows brightly (refer img 1129) indicating there is still fault some where.
There is no 5V standy power on purple wire nor any output DC power on the molex connector.

I am seeking help on how to check for the fault and currently own a 40-70 watt soldering iron, digital multimeter and a digital capacitor meter. I have a basic soldering skill and have moderate repair experience replacing capacitors in monitor power supply.
Thanking in advance for any help and suggestions.

[budm]

Re: Need help repair dead G-Tech SFX-500 power supply

What resistance do you get between the Line (Black) and Neutral (White) Incoming AC wires?

[mikey5791]

Re: Need help repair dead G-Tech SFX-500 power supply

Hi budm,

With AC mains off, the resistance between the Line (Black) and Neutral (White) Incoming AC wire reads 120 ohm onward and keep increasing on resistance scale 200k ohm. While setting the resistance scale on 2M (the highest on my DMM) it reads 0.176 onward and keep increasing.

BTW, there is no continuity between the Line (Black) and Neutral (White) on diode/continuity scale.

Hope for further assistance to revive this dead PSU. Thanks.

[stj]

Re: Need help repair dead G-Tech SFX-500 power supply

that just means the short is after the bridge rectifier.

[mikey5791]

Re: Need help repair dead G-Tech SFX-500 power supply

Hi,

Since stj pointed out the short is after the bridge rectifier, i unsoldered one of the primary cap which measured low as 2uf on my cap meter and found the lower part of cap is corroded with rust like material.(refer img 1146 & 1148). Off circuit it still measured 2uf on cap meter which confirmed is bad. It is of brand SUNNY-DZ with capacity 330uf 200V (105C). I hope this bad cap is the only fault this PSU not working.

For replacement & testing which one or combination can I use from the following caps extracted from a junk board ?
1pc Capxon 150uf 400V (85C)
1pc YST 100uf 400V (85C)
1pc Wincap 150uf 400V (85C)
1pc Teapo 470uf 100V (85C)

The cost of buying on line or getting one primary cap from the store is not feasible at the moment on my limited budget.

BTW, when I did my diode test on circuit below the transformer "STR-EE-19E" I got abnormal reading. So I decided to unsolder one leg of the three diodes on D50,D34 & D35 (refer img 1141). On diode test on DMM shows all diodes tested over limit (reads 1) and reads 468 to 471 after swapping test probes. This confirmed the diodes are good.

Appreciate for any suggestion and kind assistance in making good this PSU.

[stj]

Re: Need help repair dead G-Tech SFX-500 power supply

the brown shit on the cap is glue,
you need to scrape that off the board.

[mikey5791]

Re: Need help repair dead G-Tech SFX-500 power supply

Hi,

To replace the busted primary cap of brand SUNNY-DZ with capacity 330uf 200V (105C), is it safe to connect my two pieces cap(1pc Wincap 150uf 400V (85C) & 1pc Teapo 470uf 100V (85C) in parallel (piggy back like negative side to negative side)? As of now, I only have these four spare caps that are available for use:
1pc Capxon 150uf 400V (85C)
1pc YST 100uf 400V (85C)
1pc Wincap 150uf 400V (85C)
1pc Teapo 470uf 100V (85C)

My noob understanding on capacitor parallel connection will boost up total capacitance but the voltage rating will reduce by half. In above parallel setting, do I get total capacitance 520uf and 150V ?

Hope for anyone who has done this type of modding or knowledge to assist kindly.
Thank you for your help.

[stj]

Re: Need help repair dead G-Tech SFX-500 power supply

wrong,
parallel caps will have the total capacitance of both caps combined, but the voltage will be equal to the lowest voltage cap.
(voltage stays the same)

so use the pair of 150uf caps in parallel to get 300uf at 400v

[stj]

Re: Need help repair dead G-Tech SFX-500 power supply

btw, the white wire in your 3rd foto of post#5 is about to break off.

[mikey5791]

Re: Need help repair dead G-Tech SFX-500 power supply

Hi stj,

Many thanks for your advice and feedback. Following your tip, I will use the pair of 150uf caps in parallel to get 300uf at 400v to replace the busted primary cap of brand SUNNY-DZ with capacity 330uf 200V (105C). I just hope the pair of caps doesn't get too crowded at that cramped area.

Also, before doing this I will scrape off the old brown glue from the board and maybe look at my old wire pile for a new white wire (for neutral line). The white wire has gone thru some strain from me flipping the board back and forth while checking components.

After all this mod and fingers crossed, I will hope for the best that the PSU will work.

[stj]

Re: Need help repair dead G-Tech SFX-500 power supply

i usually desolder the power wires and take the board right out of the case.

[mikey5791]

Re: Need help repair dead G-Tech SFX-500 power supply

Hi stj,

After scrape clean the brown glue, then I attached a hard glued and wrapped in electrical tape pair of 150uf caps soldered in the old cap spot. I also make sure there is no exposed wire or wire touching any metal surface.

Turn on the main power and VOILA ! The light bulb glows briefly then light off, the psu fan spins means the psu is working. Checking the purple wire for 5V standby power,orange wire for 3.3V,red wire for 5V and yellow for 12V. All readings on my DMM seems within normal range after running for an hour.

I will keep this unit as spare. Thank you so much for your suggestion and patience in making the psu working again. One less unit goes to our landfill !

[TELVM]

Re: Need help repair dead G-Tech SFX-500 power supply

^ The ghettoness is strong with this one.

[momaka]

Re: Need help repair dead G-Tech SFX-500 power supply

Yay! Congrats on the fix

^ The ghettoness is strong with this one.

But it works!
That's how you know you are a true engineer.

Also, I think mikey5791 repaired this PSU only for testing and/or learning purposes. Or at least that was the case with the last one we helped him repair.
.
.
Now, just to answer some questions below that may have been unanswered...

After some component testing, I started replacing two shorted RL207 diode, and on heatsink "1" two pieces shorted Bipolar Junction Transistor (E13007 & J13007) replaced with ST13007A (8A 400V NPN BJT). I hope the ST13007A can replace the E13007 & J13007 as equivalent.

Yes, that's fine. Any of those transistors with "13007" for their part number are pretty much identical and can be used interchangeably.

For replacement & testing which one or combination can I use from the following caps extracted from a junk board ?
1pc Capxon 150uf 400V (85C)
1pc YST 100uf 400V (85C)
1pc Wincap 150uf 400V (85C)
1pc Teapo 470uf 100V (85C)

This actually depends on the PSU you have - particularly, its topology.

Just like the previous PC PSU you repaired, this one is a half-bridge design as well. Without getting into too much detail, all I will say is that for half-bridge PSUs, you always need two caps in *series* on the primary side (like your PSU had originally) and each cap must be rated for 200V or more. The caps must have equal capacitance *and* voltage ratings. Otherwise, the rectified AC voltage will not be equal across each cap, and this can cause problems with the half-bridge operation, especially near the maximum load for the PSU (which I assume you won't try to push, because this is another not-so-greatly engineered PSU again .) That said, you should use the Capxon 150uf 400V and Wincap 150uf 400V - which I see you did, so that is good. Note that for *two* caps in *series*, the capacitance across the whole pair becomes half (i.e. 76 uF), but the voltage doubles (800 Volts DC max, in this case). Because the capacitance is so low, I do NOT advise you load this PSU with more than 50-75 Watts.

Now, if you were not dealing with a half-bridge PSU *and* you live in a country with 220/230/240 V AC, you can get away with just using one 400V/450V cap (or two in *parallel* to double the capacity on primary for better filtering). You can do this with for single-transistor forward (STF) and double-transistor forward (DTF) PSU designs.

**EDIT**
Oh wait, I *just* saw the pictures and the repair. WOW! I thought you put the two 150 uF 400V caps in place of the old 330 uF 200V caps. But instead, what it seems like you did is you put the two 150 uF 400V caps in parallel and left the good 330 uF 200V cap in place.
Now that is truly creative repair, sir!

I think you can ignore my comment above about not loading the PSU more than 75 Watts. You can probably load it up to 150 Watts now. Of course, I still wouldn't advise you use it in any PC, even though you did properly add most of the necessary filtering components on primary and secondary side.

By the way, if you like these kinds of repairs, have a look at this thread:
https://www.badcaps.net/forum/showthread.php?t=23719
I think your PSU here is a good candidate for that. Heck, I say we should nominate this repair for the coolest PSU repair of 2015 .

[mikey5791]

Re: Need help repair dead G-Tech SFX-500 power supply

Hi momaka & TELVM, stj and all who contributed,

Awesome explanation in detailed and yet easy to understand term. I am thankful that many of you are able to shed different type of assist in your own way, and many of your suggestion does great things and generates ghetto way of doing stuff. Haha, that's how i see it.

Through your guidance and learning experience, i am able to deepen my understanding esp. On power supply and best thing is the excitement from seeing a dead psu becomes alive!
Also, the benefit to our mother earth is one less broken item to the landfill.

My final word, thank you for all your relentless effort .
P.s. help keep this forum alive forever

[TELVM]

Re: Need help repair dead G-Tech SFX-500 power supply

But it works!
That's how you know you are a true engineer.

Indeed! It was intended as a compliment.

[mikey5791]

Re: Need help repair dead G-Tech SFX-500 power supply

Hi momaka,

Referring to your comment on "I think you can ignore my comment above about not loading the PSU more than 75 Watts. You can probably load it up to 150 Watts now."

How do we calculate how much watt can this PSU load? FYI,the primary heatsink uses BJTs while the secondary heatsink uses schottky rectifier. I can provide the part no if you need them for your reference.

Glad if you could do the explanation. Thanks.

[stj]

Re: Need help repair dead G-Tech SFX-500 power supply

transformer size is the one thing you cant do much about.

[TELVM]

Re: Need help repair dead G-Tech SFX-500 power supply

Also '13007' switching transistors have a habit of exploding if more than about ~250W DC is asked from them.

[momaka]

Re: Need help repair dead G-Tech SFX-500 power supply

transformer size is the one thing you cant do much about.

Also '13007' switching transistors have a habit of exploding if more than about ~250W DC is asked from them.

^ Those, and a few more things.

The capacity of the primary caps is also important. For *two* 200V caps, you can generally use this rough estimate: max safe continuous output power = capacitance / 2
So if you have two 330 uF caps, expect about 150 Watts continuous and around 200 W peak
Two 470 uF caps: about 250 Watts continuous and 300 peak
Two 680 uF caps: about 350 Watts continuous and 400 peak
Also, I should note that this is not a "hard" limitation. You can actually pull more than 150 Watts from a pair of 330 uF caps on the primary, but you might start getting some nasty 60 Hz ripple on the secondary side outputs. And the PSU hold-up time will be reduced quite a bit, which will matter if electric grid brown-outs are frequent where you live.

Finally, there are the secondary side rectifiers. Calculations here will vary depending on design, but *usually* they can do more than what the primary side can.