Dreamcast PSU issues

So after finally obtaining the new GD laser for my dreamcast I plug it all in and the system just dies on me, the PSU has started making a high-pitched whining noise and the power LED won't come on. The system was working fine before (apart from not reading CDs).


I've tested the PSU outputs with a multimeter and I'm getting about 0.4V on the 3.3V line, 1.5V on the 5V and about 4.4V on the 12V.


The capacitors all look okay but I'm going to replace them anyway, I can't see any broken/shorted tracks either.


Does anyone know much about PSUs and what could be causing this please? I'm looking at the transistors but they're not easy to test.


Thank you

 

try with out gd-rom drive and ihope you put the plastic protecter back to stop sorting thepsu if its 220v or 240v its the caps replace all of them.

 

I kept the plastic in place

It is the 240V PSU, are the caps a known issue with this board? I'll try replacing the caps and then if that doesn't fix the problem I'll try it without the GD drive. Thank you

 

I wonder if this is a somehow common issue.
I too have a system died on me just like that. It was working fine (even gdrom), I opened it apart for a full cleaning (flea market, yeah), reassembled it all together, full tested and everything was ok. Shelfed for a couple of weeks and next time I plugged it in it just fried... That same sound, the led going weaker and in a couple of seconds no life sign at all.

The console was powered on when I plugged the AC cable. It was also attached to my lcd throught a cheapo vga box. I tested the box on another dreamcast and the same setup and everything was ok...

 

No VGA box for me, RGB scart lead. Was yours 240V as well? And did you manage to repair it?

 

Yes, a PAL model. I'm not skilled enough to attempt a repair but I'll keep it anyways.

 

GD-ROM drives might have issues with the motors as they're NOT brushless (stepper motor) type.

The brushes wear down causing the motor to short circuit. That in turn causes the BA5986FM to short circuit. That in the end shorts the power supply, causing it to whistle like that.

So you power the Dreamcast, the orange led flashes and it turns off a second after.

Edit: Another common issue is sparking (ESD discharge) on the A/V RCA plugs short out the VIDEO DAC.

That short circuits the +5V line and again has the same effect as the shorted out GD drive. The only difference is you can't disconnect the video DAC as it's part of the motherboard.
That also is a variation of the "has no video but has sound" fault.

 

If the shorting were to happen external to the power supply wouldn't the power light still be on and the PSU supply the correct voltages?

 

No, the shorting is inside the Dreamcast. Only the A/V short circuit which is caused by ESD overvoltage: It causes something called "punch through" which results on the diodes which protect the DAC output get shorted to both GND and +5V which in turn cause the PSU to short as well.

The shorting on the GD drive happens due to overcurrent from the motors being shorted up.

edit:The power supply is designed to react to overcurrent conditions (short-circuit) shutting itself down so the device don't catch fire.

 

Well I was just testing the rectifier diodes and the transistors, I'm getting an uneven reading (25% lower resistance from one side) on the D1276 transistor.

I'm not great with electronics but could this be a problem?

 

The power supply has a circuit in it called "shunt". It's a load which drags the output down a bit when it has no load. That prevents the oscillator from running free, making the voltage rise too much which would eventually cause the power supply to explode.

That oscillator controls the switching transistor on the primary. That pulses control the voltage at the output and a loopback circuit with an optical insulator returns the feedback. That's how the power supply work. I don't think there's anything wrong with it.

Measure the pins marked +5V and GND (the marks are on the power supply PCB) with the ohm meter on the Dreamcast motherboard. Do the same with the +12V and +3V. It's likely that one of these three are shorted to GND.

 

I'm measuring the transistor out of the circuit, aren't they meant to be roughly the same when measured on both sides?

I'm getting about 0.4V on the 3.3V line, 1.5V on the 5V and about 4.4V on the 12V, so I don't think they're shorting otherwise one of them would be reading 0 wouldn't it?

 

Dude, take that PSU to another DC and see if it works.

 

I would if I knew anyone else with a DC

EDIT: Misread your last post, you're suggesting I check the power pins on the DC side without the PSU connected to see if any are shorted to ground? I'll get on that now.

 

I'm getting around 660 Ohms from 5V to ground pins, 1000 Ohms between 3.3V and ground and no connection between 12V and ground.

So that could be shorting in the video DAC?

 

Hi itxi,
i'm speaking without checking by my self, just what i think we can expect from...
5v and 3,3v seem to be in range, the missing connection on 12v sounds particular. Maybe you have to scale up the ohm range of your multimeter, also a 20k ohm is possible.

You can check the psu by taking it out of the case, connecting and switch on the power button and reading the voltage on 3,3v, 5v and 12v. It is not a deep test because there are not loads, but it is a starting point...
Be careful where you sit the psu, the bottom side has to not touch conductive table or metal things, and be aware that high voltages can kill your self.

 

I'm getting 1.5 Mega Ohms between 12V and ground, I'm guessing that's a bit high?

Also, I lost my piece of paper where I'd written down the capacitor values for the PSU before removing them all. I don't suppose anyone can let me know which capacitors go where please? A rough diagram would be really handy

 

Okay, got the capacitors all replaced and tried the board separate from the console, it's still making that whining noise but the 5V and 12V lines are good (bit high but I assume that's normal with no load), the 3V is only reading about 0.3V though so I guess that's the problem.

 

Check the rectifier diode ( the thing on the heatsink which look like a transistor but has diode symbols etched in it. If that's shorted it will surely do the whine you're talking about.

 

I can find 2 rectifier diodes on the heatsink:

FCH06A10: Not leaking current, reads 5.3 KOhms across the diode on both sides when conducting.

MA10799: Not leaking current either, reads 1.35 KOhms on both sides when conducting