Dreamcast Repair

[RobUK]

Hi All,

Recently aquired a Sega Dreamcast, I got it given to me becasue it's broken.  Upon first inspection the console turns on, screen stays black for a little while, then the dreamcast logo appears, it then stays on this for a few minutes, before loading the dashboard and displaying "Please wait while disc is being checked".  This is all it does, this message doesn't go away and I am unable to browse the menu or do anything else.  Even the time doesn't change, it's like the system has crashed.

Looking at the console it has been opened before, upon opening it I found screws missing on the laser unit and control ports.  The fixed lithium battery had been replaced with a holder, the holder contained at CR3020, not the rechargable kind.  So I removed that as I believe the Dreamcast battery recharges.  I tested the controller fuse, this is intact.

The laser unit wasn't behaving normally when searching, so I tested the cables and found the ribbon cable had a damaged trace, repaired that and now it seems to be acting like it should.  Before fixing the ribbon,  holding down the lid switch it would spin up the disc and stop.  Now it spins up, searches the disc and continues spinning.  It also looks like someone resoldered the lid switch.  The switch appears to be functioning normally as when I press it the laser unit starts working and when I release it, it stops straight away.

Upon booting the system with no battery and fixed ribbon, it still shows no output for a short while and then stays on the dreamcast logo for a few minutes, but then asked me to set the time, which I can do with no problems.  Once I hit select to continue, it goes to the dash and displays the same message and I am unable to do anything again. 

At this point I have run out of ideas, not sure if its hardware or software.  Visually inspecting the rest of the system, looks like the motherbaord has never been accessed before and the psu output is testing fine.

Any help would be appreciated.

Regards

Rob.

[RobUK]

Update.

Decided to use my own dreamcast to swap the laser unit over. Lucky no damage was caused and found some interesting results.

The good laser unit from my own DC in the broken DC showed the same symptoms as the original laser unit. The broken DC laser unit in my own DC tried to load the games, got the splash screen and partial load on a game. The laser pot probably needs adjusting. But the console booted fine with the broken laser unit.

So now my attention is on the motherboard. I am going to reflow the laser unit connector on the mobo as I have read the issues I am getting are laser unit related. The only other option is a corrupt flash. But not looked into that yet.

Any tips guys?

[shakalnokturn]

Usual tips... Is power clean? (Usually the first step to any diagnosis.)
Clean meaning with "reasonable" ripple, often people will check the DC voltage without measuring ripple, sometimes giving the false impression that power is good enough.

The mainboard uses some SMD electrolytics, possibly from the capacitor plague era. Intensive CD access combined with heavier CPU usage at one point during loading could cause enough ripple to hang the CPU if decoupling is insufficient.

How does playing a simple audio CD go?

[RobUK]

Hi thanks for your reply. 

You are correct, I have only checked the DC voltages as I don't have anything to check the ripple.  The voltages to the motherboard are all fine.  3.3 and 5v are spot on, even when under load.  The 12v line is a little higher at around 13v under load.  I have one of the DSO138 oscilloscopes, not built it yet, but not sure if it would be any good to measure accurately.  I am looking at getting a "proper" oscilloscope soon.

I will try and check the motherboard capacitors over the weekend. 

I am unable to get into the OS to try and play an audio cd. 

[Ordinaryman1971]

You can check ripple with your multimeter, switch it to AC mV range. Ripple is an AC voltage "imposed" on the DC voltage.
Scope is just nice visual way to confirm that there is one.
I wonder what your findings are. I have one of those Dreamcast consoles myself. It's waiting for its turn...

[RobUK]

You can check ripple with your multimeter, switch it to AC mV range. Ripple is an AC voltage "imposed" on the DC voltage.
Scope is just nice visual way to confirm that there is one.
I wonder what your findings are. I have one of those Dreamcast consoles myself. It's waiting for its turn...

Thanks for the info,  I will try that and report back

[RobUK]

Tested the 12v line and getting 0.049v on AC. Not sure what that means.

[maginnovision]

.049 is likely not an issue, to verify check the known good unit.

[shakalnokturn]

Should be OK, test similarly on other supply rails.
If you can also try repeating the AC measurement across a few decoupling caps on the mainboard before the system hangs, that will give you a clue to whether it is worth replacing the electrolytics.

[RobUK]

Hi all,

sorry it's been a while, unfortunately I've been ill and so not been doing much on this until now

So tonight I put the whole laser unit from the faulty console into my working Dreamcast, tweaked the laser and it is now reading all the discs I can throw at it, even backup cd's.  Putting it back in the faulty console made no difference, but I now know the the laser and laser board are fine. 

So I then put the PSU and controller board from my working dreamcast into the faulty unit, only the faulty units motherboard remained.  It exhibited the same issues as always.  So everything has now been changed, except the motherboard and it still doesn't work. 

I tried checking a few capacitors, for dead shorts, but all seamed well.  I didn't remember to check for AC ripple.  I will do that some other time. 

Is there anything else you guys can recommend I check to get to the bottom of this?

Regards

Rob

[stj]

first, be carefull swapping drives - early models are 5v, newer ones are 3.3v
it's stamped on the frame of the 3.3v ones.

check the esr of the psu caps,
they fail.

and if all else fails and you have the tools, replash the settings flash with defaults for your region.
i'm not sure if it self-tests the o.s. rom, but that can in theory cause your issue.
if it has the wrong checksum the ATA interface wont enable - it's a protection function.