Tagan 700W ATX PSU faulty - no 5V STBY. TG700-U25

[dicky96]

Hi guys
I got a beast of a ATX PSU here with a blown Standby section - Tagan TG700-U25

It takes a power surge at switch on to charge up the main electrolytics then goes to practically no current so I think the main section is probably OK

Problem with this one is the standby section has a discrete power FET which burnt some components on the PCB

Firstly does anyone have a schematic for this supply otherwise I doubt I have any chance of fixing it.

From what I can see the Standby supply provides 5Vsb and possibly the -12V supply rail (or at least the -12V comes of the same connector on the ATX pcb but I didn't trace it to see where it acually comes from). There doesn't appear to be a short on 5Vsb

I has occurred to me maybe I could use my bench PSU to provide 5Vsb and then see if the PSU would power up. Also it could be possible to squeeze a small 5V PSU inside the atx casing to effectively provide 5Vsb and 'bodge' it?

If i get it working I only wanted to use it in my own workshop on my soak test bench anyway, for motherboards and GPUs

[RJARRRPCGP]

Re: Tagan 700W ATX PSU faulty - no 5V STBY. TG700-U25

Why does the transformers look a lot later than the components on photo #2?

The transformers look like they're from 2010, week 49.

[R_J]

Re: Tagan 700W ATX PSU faulty - no 5V STBY. TG700-U25

There seems to be a bad electrolytic near the small standby transformer, It looks bulged in the picture (pic#1). How close is the corrosion (pic#5) to the cap? it could be that the cap leaked but the components around D2,R26 are still ok, It could be that the discoloration is due to heat over time, If the diodes check ok, clean up the section and resolder the parts. If the fet is not shorted I suspect the standby circuit would be ok after the leaky cap is replaced

[dicky96]

Re: Tagan 700W ATX PSU faulty - no 5V STBY. TG700-U25

Well spotted R_J I'll check that in the morning

I do think that the leaky electrolytic is on the opposite side of the transformer to the FET

What looks like corrosion around the burnt area is actually the remains of some resin type glue that was holding the ceramic disc capacitor in place. I bent the disc capacitor over the opposite way and cleaned the orange/yellow glue away as best I could to look at the area a bit more easily.

Also something has gone bang here as there was a 'explosion' mark on the plastic sheet under the PCB in that area. . I didn't check for shorts yet to be honest, though the STBY PSU is obviously open circuit on the primary side at the moment

[R_J]

Re: Tagan 700W ATX PSU faulty - no 5V STBY. TG700-U25

If the primary for the standby circuit is open, check that resistor in the heatshrink, Also under that diode (next to the diode marked D2), it looks like the circuit trace is blown, or it could just be the way the picture was taken.

[dicky96]

Re: Tagan 700W ATX PSU faulty - no 5V STBY. TG700-U25

OK guys I cleaned it up as best I could

The two diodes tested OK but after some more cleaning the one above D2 broke in half and part of the track from that diode to R26 came away from the board and broke off

The track above that top diode going to one leg of C18 (or c16) looks like it is broken but it tests good.

The track from the lower diode D2 going to the other end of R26 tested open circuit but after cleaning it now test good.

R1 reads about 500K in circuit and connects to the other resistor R71 which also reads about 500K in circuit. Possibly these two form a start up resistor

The other resistor R26 reads 250K+. All three resistors I couldn't get a good reading in circuit as there is a capacitor charging somewhere when I try to read these.

The MosFET IRH2N60 tests good out of circuit on my DCA55

What I assume is the PWM controller chip is marked AHABW

That 'resistor in the heatshrink' looks like an inline glass fuse to me, and it tests good

Rich

[dicky96]

Re: Tagan 700W ATX PSU faulty - no 5V STBY. TG700-U25

If I can determine the value of those two resistors and the broken diode I may have a chance to fix this. Unfortunately I couldn't find a datasheet for AHABW or possibly AHA8W

[diif]

Re: Tagan 700W ATX PSU faulty - no 5V STBY. TG700-U25

If I can determine the value of those two resistors and the broken diode I may have a chance to fix this. Unfortunately I couldn't find a datasheet for AHABW or possibly AHA8W

Doing a search for AAHBW suggests it could be a SG6848T.

[dicky96]

Re: Tagan 700W ATX PSU faulty - no 5V STBY. TG700-U25

AHA. . I was wrong lol

That reference circuit in the datasheet should help a lot. I'll try reverse engineer the circuit on my PSU

Looks like I was probably right about the two resistors in series being the start up - probably both 1M0

[dicky96]

Re: Tagan 700W ATX PSU faulty - no 5V STBY. TG700-U25

It's the Vdd circuit to the SG6848 that has fried

The red circle shows the burnt part of the circuit

The 2x 1MO resistors actually test OK

The resistor marked 237K R71 and it has is burnt. The diode feeding this is D2 near the burnt area, that tests OK. R71 should be ~10R according to the reference circuit on the SG6848 datasheet

The broken diode would be a zener, according to the SG6848 datsheet, Vdd should be clamped to 26V so that should be the zener voltage

I applied my bench PSU to SH6848 pin 5 and brought it up to 25V (the maximum Vdd rating) but it draws absolutely 0.00A and doesn't osciillate

It looks to me like SG6848 probably went short, which is what burned the PCB around R71, then went open

Guess I'll have to order some SG6848, replace R71 and the 2x 1M0 resistors which I can do using through hole components so they are no longer attached to the burnt are of the PCB. There is space between the PCB and the case to do that, as it is mounted on standoffs

As the FET is still OK, with a bit of luck I can just change the leaking electrolytic on the secondary of the 5Vsb supply and it may well lwork


Rich

[dicky96]

Re: Tagan 700W ATX PSU faulty - no 5V STBY. TG700-U25

I finally got the parts to repair this.

I replaced the AAHBW IC with a SG6848 as advised on this thread.

I removed R71 and one of the 1M0 resistors as that area of the PCB was quite charred. After cleaning the area as best I could I decided to 'bypass' that bit of the PCB by soldering through-hole resistors over the board. I replaced the broken diode with a 24V zener through-hole as I didn't have any 26V. I also replaced the bulging cap and powered it up, using a light bulb in series with the 220V just in case....

And I have 5V standby! I then connected the PSU to an old motherboard and it fires up and runs nice.

Just one of the two cooling fans is running lazily and these are two wire fans so they should run full speed all the time I would have thought. I can soon check that and replace if needed.

So that's another one resurrected

Not the tidiest job maybe (see pics) but this big heavy duty PSU will be very useful around the workshop I am sure.

[R_J]

Re: Tagan 700W ATX PSU faulty - no 5V STBY. TG700-U25

Good to here it is working. Are the two fans connected in parallel? if not the fan voltage may be controlled by a transistor/temp sensor circuit.

[dicky96]

Re: Tagan 700W ATX PSU faulty - no 5V STBY. TG700-U25

It was the fan - they both plug into little connectors on the PCB so I swapped them over and still the same one ran sluggish. I've changed it now and both are good

I doubt this was the cause of the the 5V STB supply failing

[momaka]

Re: Tagan 700W ATX PSU faulty - no 5V STBY. TG700-U25

Nice work!
Good to see another PSU saved from the landfill. Hopefully will serve you well.