Hello! I bought a faulty Dreamcast on eBay to try and repair. It has no video output, but it powers on okay and it sounds like it's reading the disc. Control pad VMU powers on and displays an image. I took it apart but I cannot see any obvious damage on the motherboard. No broken traces that I can see. A quick visual inspection of the AV port seems okay, no obvious bent pins or anything. The thermal pads on the CPU had some hairs and dust on them, so I presume it has been taken apart before. I replaced them with some fresh thermal pads I had lying around. I have checked the GD-ROM drive and power supply in another Dreamcast I have and they both work, so I suspect whatever is wrong is going to be with the motherboard. It's not the PSU not giving out enough power or something. Is there anything else I can or should check? Does anyone have experience with the Dreamcast who can let me know how fixable this sounds? As I have stated in previous posts, I'm really new to repairing things as a hobby so please excuse any daft questions. Just trying to learn. Thanks!
If the OP is not getting sound either then it could suggest the problem is further upstream from the DAC. I've got a dreamcast with similar symptoms but I haven't had chance to do a full troubleshoot.
My main objective here is to learn as much as possible, so if you don't mind me asking, what would be your approach to troubleshooting this? What steps would you take and what tools/equipment would you use?
I had this exact issue with two Dreamcasts at an event this weekend. I took 5 Dreamcasts (only 4 were being used) and I brought a spare just in case one decided not to work on the day, and as if by magic both the back up and one of the intended display ones would give neither a VGA or an RGB signal! 3 out of 5 ain't bad...!
If you have audio but no video it's almost certainly a blown video DAC - this is quite a common failure on the Dreamcast. It's very easy to diagnose if you have access to a scope - just check to make sure that you have the sync signals and video clock on pins 52-54 of IC401 and digital video signals on pins 56 and 1-11 - if they are all there but there is no video then the chip is blown.
I know about the DAC issue, but what's intriguing is when there's neither audio or video - almost like nothing is being sent out through the AV port.
Yeah, if there was no audio either then it would suggest some other problem - but the OP didn't mention that, he just said there was no video.
An unplugged/still fan make the DC boot to a blank black screen, but I don't remember if the vmu and GD-Rom act like in a normal boot.
I would approach it in as much the same way as you have. I eliminated psu and GD-rom first as I had a known working DC. I knew it was upstream from DAC and AICA due to no video or sound. I had recently attempted a piggyback bios install which didn't work so I may have bricked the BIOS. I installed a new dev bios but still no joy so my next step will be to look at HOLLY or PLL chip. Use the block diagram https://gamesx.com/wiki/lib/exe/fetch.php?media=schematics:dreamcast_block_diagram.pdf to help plan your search but really you will need an oscilloscope or logic analyser to go any further. Good luck.
Don't think it's that - the display on the VMU is still working, which suggests the CPU is running normally - if the fan is broken, it gates off the clock to the SH4.
Although, it's unclear to me if he means that the vmu shows its own icon, which might happen only with power on regardless of CPU, or if it shows the game icon.
Sorry, I could have been clearer in my original post. I'm getting no sound from the unit either, no signs of life at all. The TV doesn't act like it's picking up a signal of any kind.
