Sega Saturn v1 - problem

[ci00rix]

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

[Hobie-wan]

I'm no expert, but even if the caps look fine, they might not be. The only way to be sure would be to unsolder and use a capacitor tester. If you have one or your multimeter has that function, maybe you can find one near the 5.5 volt source to check?

[Anthony817]

I seen this happen to a guys Dreamcast and 3DO, must be a similar problem then?

[ci00rix]

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.. Ah, and console is not mine so owner must agree with this method also;)

[ci00rix]

Okay, found the solution but not the problem.

I checked all capacitors and some elements one by one after soldering them out of the board. Capacitors were ok, hot resistors were ok too (they are intended to be hot), shottky diode (one with the radiator on the left) is okay also.

I decided to put a 2200uF 10V capacitor into the output voltage control loopback to extinguish the noise. I marked it green on the picture. It is electrolytic capacitor, minus leg solered to the ground, plus to the lright leg of photodiode.

That removed noise from the voltage control line and in consequence from the output lines.

Result; no more stripes and buzzing PSU even after few hours of play or heating console up under the blanket.

I havent found faulty component; i think it is probably TOP202yai PWM regulator IC. Its hard to obtain nowadays, but if stripes are visible again despite the new capacitor, i get it and replace.

If You have any ideas and suggestions, Youre welcome .

PS. sorry for crappy pic but forum allows only 256kB pix. If you want better res ask me.

[ci00rix]

I dont know how to attach a few photos in one post so... Hot points.

A - R8, designed to be hot
B - Shottky diode on 5V line (with radiator)
C - R73, designed do be hot
D - TOP202yai (with radiator)
E - R63, designed to be hot

A,C,E - no comments. Cools down fastly after turning off the power.
B - I replaced it with another diode, still control loopback was noisy.
D - PWM regulator, intended to be hot, but it generates pulses. Cools down relatively slower after turning the power off.

[ci00rix]

As You see, PWM is quite hot.

[ci00rix]

IC under CD drive. As You see, sometimes high temperature is normal. So PWM on previous picture might be okay... but still I suspect it to be faulty element .

[Hobie-wan]

Heh, you're way above most of us here at this point. But uh, nice thermal imaging and workaround.

[G-Darius]

I have known this before with some old consoles,compiuters and arcade boards i have repaired,usually a bad ground or the main largest capacitor (reservoir cap) is old and the dialectric (inner material) is loosing ability to store charge.as the main cap this ripple is carried and effects every stage after.
Another tell tale sign is an ac level on the dc voltage line,i like your thermal images very nice but to be honest if a component is getting to hot you can normally see signs of dicolouring,burned pcb or melted insulating plastic etc.