SEGA SC-3000H Video problems

[firmgrip]

I have recently pulled my old SC-3000H out of the cupboard and am trying to get it running again.
I found some really useful info on this forum about issues others have had.

Firstly I plugged everything in and tuned to computer to my TV and got a black screen with a buzzing noise. After looking thorough forums I found that that was a good sign the system was functioning buy not connecting to my cartridges.

I cleaned everything up and still no joy. I through I had better check the PCB for any damage. I found small cracks around the connector pins for the cartridge slot. Mostly in the front row, and at the ends. I guess from inserting and removing the cartridges. I re soldered these and re tested. The computer worked.

I tested all of my cartridges. BASIC LEVEL IIIB, CONGO BONGO, STAR JACKER, SAFARI HUNTING, YAMOTO, AND EXERION. They all worked

After playing for around 30mins i noticed small errors appearing on the screen. In another 10mins the screen was unreadable. I have read on another forum that the basic cartridge does memory checks during loading. I reinserted the BASIC cartridge (which worked well the first time)and got a black screen with three beeps. According to the other forum that means video ram error. Could this video break down be related to issues with the video daughter board as well?

I have searched online for the MB1881-12 chips and didn't find many around. SC-3000.com have a tech report for replacing the VRAM they mention using MCM4517 chips as a direct replacement they appear to be more available.

Does anybody on here know if the chips are compatible?
I have also noticed the VRAM chips I have found sometimes have a different number after the dash, will that effect their compatibility?
Will I need to replace all of the original chips with the MCM4517 chips or can I just replace the damaged ones if I can identify them?

Thanks in advance

[bhabbott]

MCM4517 is a direct replacement for MB8118 (most other 16kx1 DRAMs need +12V and -5V as well as +5V, and so are not compatible). I recently bought a set of MB8118's from electron-bg on eBay. Only $16 for 8 chips which appear to be brand new - unlike the re-labeled pulls that most Chinese venders are selling!

The numbers after the dash represent the address-to-data access time (-10 = 100ns, -12 = 120ns). The TMS9929 requires access time of less than 200ns.

It's unlikely that more than one RAM chip would go faulty unless something is stressing them (eg. over-voltage on +5V line, faulty TMS9929). If one RAM chip goes you lose 1 bit per byte, which usually shows up as vertical lines on the screen. A bad chip may also draw more power and get hotter. For an example of this fault see http://www.sc-3000.com/index.php/Repairing-VRAM-on-SC-3000.html.

The SC3000 has a single sided circuit board, so the holes are not plated through and dry joints develop readily. Before pulling chips I would go over the whole board and re-solder any suspicious joints. If you decide to replace the video RAM then use sockets so you can swap chips easily.

[firmgrip]

Thanks for the info.
I'll open it up again and check for more faulty solder joints.

I've had it running since I posted after it had cooled down.
The basic cart was working and I was doing some basic coding.
I started to get weird artifacts, and letters began to pop up over others I had typed. After a while almost the whole screen was black with artifacts. Is that related to video ram or should I be looking elsewhere?

[bhabbott]

I started to get weird artifacts, and letters began to pop up over others I had typed. After a while almost the whole screen was black with artifacts. Is that related to video ram or should I be looking elsewhere?

If you could still type stuff while the weird artifacts were happening then it probably was either video RAM or the TMS9929 (if it was main memory or CPU etc. then it probably would have crashed or frozen).

It's possible that one of the video RAM chips (or the video controller) misbehaves when it warms up (though dry joints are often also temperature sensitive). You could 'soak test' the computer by leaving it on for an hour or so. Then if it starts to play up, cool down the chips one by one until the the fault clears. An infrared thermometer is useful for monitoring chip temperatures without disturbing them. You can also use your finger as a crude thermometer and a cooling device!

To check for dry joints try twisting the case (which flexes the motherboard), lift it up slightly and drop it, or tap on various parts of the motherboard. If the fault comes and goes or you see momentary effects during this testing then try to find the most sensitive area by tapping or prodding individual parts. Then inspect that area for dry joints.