Sega Saturn v1 problem

[snejk]

Hi guys, my problem:

Sega saturn, mark 1. When I play for some time and turn it off and then on again white, moving horizontal stripes on the screen are visible. They look like a kind of noise; normal sega screen is visible and console responds to actions. When i put the cd in, the console cant read it (but i hear that it tries - rotating disc, moving laser).

I know that this problem is commonly known but as far as i can see no one has found the solution. So maybe we'll try to solve it together...?

I have found that:
- pressing power button SHORTLY when the cd is starting to turn removes the stripes and problem vanishes (after normal turn off/turn on stripes are visible again). Power button must be pressed off/on so fast that console wont notice it lost power.

- power source. three voltages; 3.3V, 5V, 9V. Actually they are 3.3V, 5.2V, 8.5V. Tried to trim the voltages with the trimmer built into PSU; I set precisely 3.300V. Other voltages are dependent on that trimmer also but I decided to set 3.3V precisely because it powers up processors.

There are three resistors heating up (about 64degC) when the console is turned on, but looking at the circuit - these resistors are intended to be hot. Any other part (even heatsinks) doesnt heat up above normal. I checked resistors with multimeter - their values are ok.

Capacitors look fine.

When i used oscilloscope 9V and 3.3V are ok. When cd tries to read there are some voltage drops on 9V line, but its normal imho.

!!! But on 5V line, only when the problem occurs (stripes are visible), there is strange ripple ~380Hz very slightly noising the line (very low aplitude of the noise). After pushing the power button (what i described previously) noise vanishes.

Looking at the schematics of mainboard there are no voltage stabilizers or voltage regulators; voltage from power source unit is drived straight to the chips. Noisy 5V VCC on the processors could be the reason.

But on the other hand, maybe faulty chip takes too much power and noises the 5V line?

Got any ideas what can cause that noise?

I cant find schematics of the power supply board - if anyone has them please let me know

[snejk]

Update guys; i checked this a bit deeper.

- ~380-400Hz noise shows up on every voltage line (3,5,9V)
- PSU emits silent buzzing sound when noise is "active"

- when PSU is cold there is no buzzing sound

If nothing else helps i desolder capacitors and check each separately. But thats last resort so be patient..

[grobda]

sounds like a dodgy cap, have you checked visually no caps are swollen or have leaked fluid? or you could try freeze spray to try and isolate it.

[snejk]

no leaked fluid whatsoever. all caps appear to be ok

[grobda]

on to the freeze spray then i'd say, you can get it from rs... maybe maplin, or online. comes with a nozzle and its a good idea to make a card 'shield' to help localise the effect of the spray. preoper electronics spray is non-conductive but id still be carefull around the AC parts.

[snejk]

it's definitely not the overheat issue.

[zyrobs]

I have the exact same problem with my VA3 (model 1) saturn. If I reboot the machine after extended gaming, the screen gets a horizontal waving line, the psu goes BZZZZZ and no games boot. I believe it's an overheating problem, because after installing a tiny fan next to the PSU, I did not get the problem again even after 5-6 hour sessions.

Absolutely no visible problems on the PSU either.

One day I'm going to try switching out the PSU to the one in my VA5 console, which is the exact same hardware just in a model 2 case without access led and mem reset.