Sega Saturn PSU Repair Question

Hi, the power supply on my PAL Model 1 Saturn died, well, kind of died. One day the power light came on, but everything else was dead. I opened the case, took a few measurements with the MM and all the rails came up short, there's 5.6V on the 9V, 3.2V on the 5V and 2V on the 3.3V rail. First thing I did was replacing the main filter cap on the primary side (400V, 33µF), still the same (I did expect that as the cap tested good with the ESR meter). Next stop was the TOP202YAI PWM Mosfet, changed to a TOP223YN, did nothing, still the same.

At this point I've started to desolder all the caps on the secondary side, tested them out of circuit and all came up with good ESR ratings, bummer. I would assume that something in the feedback loop died, but I can't find it. There's a shunt regulator from NEC, a C1093, which has a leg for 2.5V reference voltage, but it only get's 1.2V on it.

I've got no second psu to track down some test points to compare and I couldn't find a schematic for the Saturn PSU's.

Any help or suggestions would be greatly appreciated!

 

I don't have any basis for this, but check for continuity(since it open and you have your meter) between the two pins on the 14akss6ft.

 

This is entirely compatible with a bad main board that's dragging down the 5V line - have you tried this PSU in another unit?

 

What component do you mean, I can't find one labeled 14AKSS6FT?

I've removed the PSU from the case and it's sitting on my bench, I put a dummy load on the 9V and 5V rails so I could test it.

I've reflowed every solder joint under the pcb and visually inspected all components for signs of wear, everything looks good, except something must be faulty.

This is the 5-Pin PSU model:

 

I guess just see if there is continuity between the out and ground on the regulator

 

Just checked, no continuity. The TO202YAI regulator was the second suspect, so I've ordered a replacement and put it in, no change. Same with the 400V cap that sits in front of it, both are good.

 

You're able to test the psu on its own and you're getting low voltages?

 

Exactly. I took it out, sat it on the bench an put some dummy loads on the rails. None of the rails is getting full voltages on the output, around 40% is missing.

What I suspect is a problem with power regulation, since the shunt regulator (this 3-pin TO92 sitting next to the potentiometer) is only getting 1.2V on its 2.5V reference voltage, I can't tell if its faulty or just chasing its tail in the loop. The optocoupler which is next in line should be ok I think, since it isn't shorted and pulses are getting through, otherwise the psu would be completely dead (correct me if I'm wrong about that).

 

I won't be correcting you on anything, your Saturn is in capable hands. I just got mine yesterday:
At least you have it isolated. Aside from the caps and the regulators, there isn't much on the psu I know about at this point. It sounds like a short, but that should be easy to test for. Good luck!

 

That's a Model 2 PSU? I think there's an NEC PS2561-1 optocoupler on there, could you give me the diode readings in circuit from yours, in both directions?

 

Sure, I'm working now, but I'll do that tonight.

 

It's a p521 g. I'm not sure I'm measuring correctly. One side shows .685 and open. The other side shows .486 and .494

 

Sounds legit, seems like the diode on my optocoupler is halfway out the door. I got .345 in both directions between pin 1 and 2, since it's part of the feedback loop, I think there's a good chance this is the fault. I'll try to source a replacement for it and post back the results. Thanks!

 

For the record, I bought this Saturn as "untested" on eBay and it arrived before I had all the appropriate hook ups. So technically it is still very untested.

 

Update: Fixed

The problem was the NEC C1093 voltage regulator in the feedback loop, after replacing the C1093, everything worked.

 

Btw, they're quite hard to find nowadays, if someone knows a suitable drop-in replacement, please let me know.

Datasheet: http://www.bucek.name/pdf/upc1093.pdf

 

Found one. As drop-in replacement use the Texas Instruments TL431. Tested and works.

Datasheet: http://www.ti.com/product/TL431/datasheet

 

Hey, nice job. I'm glad you got it working.