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Hi all,
I'm new here and not 100% sure if I'm posting in the correct place but here goes:
I have a master system that's behaving very strangely. I have recapped it and swapped out a couple of chips but it's made no difference.
When I power up I get the Sega logo followed by the first screen of Alex Kid, it doesn't get as far as the boxes popping up and there is no music, it just kinda hangs there.

I've tried several other known working games:
Ghostbusters: Music works, game appears to work until you press start. You try and navigate to enter a code and the cursor goes everywhere by itself. Using the joypad makes no difference. The joypad works fine on another system.
Golden Axe: Seems to work with graphics and music, press start and it goes back to the first page.
Double Dragon: Sound and graphics, press start and back to the start.
Robocop V Terminator: Same thing getting a bit of a pattern here.
Other games get as far as the Sega logo then either go to a blank screen or hang on the logo.
Any ideas please. Sorry if I've posted in the wrong place.

Apart from Alex Kidd, sounds like things only start going wrong when you press a button on the control pad, is that right? If you don't press a button, do any of the games play a demo level or anything?

It's possible that there is a fault with the GA such that pressing a button causes a short across the power supply or something like that. That could cause the system to restart.

Another possibility is a fault on the address or data buses with the former being more likely.

If you have a multimeter my first suggestion would be checking for shorts between pins of the GA chip, in particular between the ones that connect to the control pad and +5v. After that I'd check for continuity between the address pins of the Z80 and the cartridge port.

You don't say what model of SMS you have, that would be helpful.

Thank you.
Yes the games that boot up do play demo levels.
Pressing fire doesn't reset the system it just goes back to the title screen of the game.
It's a EUR Master System 2 model 3006-05. It worked fine before but it's had several years in the loft.

I'll get to work with the multimeter tomorrow.

Mayhem wrote

Pressing fire doesn't reset the system it just goes back to the title screen of the game.

Hmmm, sounds like perhaps the button press is being misinterpreted as a soft reset, although the SMS2 doesn't have a soft reset switch. Again could be a fault in the GA chip (IC4).
Maybe take a look at pins 25-30 of IC4, which are the data pins, to see if either you can see any continuity, either short or some non open resistance between the pins, and also take a look at the resistance between each of those pins and gnd and each of those pins and +5v. Basically for anything unusual.

Given that the games do play demos fine, I suspect that there isn't a fundamental problem with your data bus, but it may be that when the console reads the controller buttons and a button is held it's reading something that looks like a 0 on D4, which is pin 28 which the game is interpreting as a soft reset. People have reported problems with the GA chip before where the active pull-ups have failed and sometimes that can be fixed by soldering an external pull-up resistor but better to do some more investigations first.

Something else that could be going wrong is that the two controller i/o ports could be being misinterpreted. D4 carries the soft reset signal on port $dd and the left button (TL) on port $dc so it's possible that when the console is reading from $dd to check things like soft reset and you press TL it's actually responding with the $dc results, so a false positive on soft reset. That could point to a problem with A0 which is pin 2 so maybe have a poke around there too. As suggested earlier, check continuity between all D and A pins from IC4 to the cart slot or z80.

Also check the physical condition of all the pins on IC4 - if there are any which look like bad joints, dirty, or with debris on or near them, consider cleaning and / or reflowing.

Thank you.

I'll have a look at the weekend and feedback my findings.

I've tried everything suggested and it's still the same as it was.

I have another board and I've swapped virtually everything over and the parts have worked on the new board. I even swapped IC5 - I won't be doing that again as it was an absolute nightmare :)

Transferring components from the new to the original board just give the original results.

Another thing I have noticed though is that some games completely refuse to boot on the old board but have no issues on the new one.

Hmm, well if as you say you've swapped lots of the parts over and they seem fine, it feels like it's looking like a problem with the board, maybe broken or damaged traces, or shorts from capacitor leaks or debris perhaps.

At least now you know the parts are good you can put them all back and keep iterating testing and cleaning or repairing suspect traces until you get it working.

Oh by the way, I was assuming Alex Kidd is the built in game and therefore that you're getting the same problem regardless of whether or not a cartridge is inserted. Let me know if that was an incorrect assumption!

Yes Alex Kidd is the built in game.
That works strangely as well:

When you switch on you get the Sega logo then Alex Kidd comes on but there's no music and the little boxes that show up on the main screen don't come.

Pressing fire doesn't do anything it's just like a screen shot picture.

I've swapped the bios round and it plays fine in the other sms.

I've spent a lot of the weekend checking voltages and traces and I think I'm going to give up.

Just one thought though, I tried Sonic 1 today and when I powered up I had a screen of moving multi coloured bars not unlike early C64 loading screens.

The screen then went mostly blue but I could hear the music from the first level.

Could this be a memory thing?

I was thinking if anything this sounds likely to be faulty RAM.

My guess is that the title screen contains little data, enough it doesn't touch the higher regions where the d=fault may be, when you try to start a game it accesses a higher region and glitches.

I've finally sussed and fixed it.

It was IC3 - I thought that I'd already swapped that one over.
The original chip was a Sanyo LC3664R, the one I've put in is a KM6264BL.

It looks like I can get a replacement km6264bl from China via ebay - if it works.

All the games that didn't boot, now boot and games that partially worked now work well.

Thanks for all the suggestions.

Ah, RAM is is then! Good hunches both.
Really glad you've got it working, now you can enjoy it!

Author	Message
Mayhem Joined: 14 May 2023 Posts: 7	Bizarre Master System Posted: Sun May 14, 2023 7:17 pm
Mayhem Joined: 14 May 2023 Posts: 7	Hi all, I'm new here and not 100% sure if I'm posting in the correct place but here goes: I have a master system that's behaving very strangely. I have recapped it and swapped out a couple of chips but it's made no difference. When I power up I get the Sega logo followed by the first screen of Alex Kid, it doesn't get as far as the boxes popping up and there is no music, it just kinda hangs there. I've tried several other known working games: Ghostbusters: Music works, game appears to work until you press start. You try and navigate to enter a code and the cursor goes everywhere by itself. Using the joypad makes no difference. The joypad works fine on another system. Golden Axe: Seems to work with graphics and music, press start and it goes back to the first page. Double Dragon: Sound and graphics, press start and back to the start. Robocop V Terminator: Same thing getting a bit of a pattern here. Other games get as far as the Sega logo then either go to a blank screen or hang on the logo. Any ideas please. Sorry if I've posted in the wrong place.

willbritton Joined: 06 Mar 2022 Posts: 670 Location: London, UK	Posted: Sun May 14, 2023 9:19 pm
	Apart from Alex Kidd, sounds like things only start going wrong when you press a button on the control pad, is that right? If you don't press a button, do any of the games play a demo level or anything? It's possible that there is a fault with the GA such that pressing a button causes a short across the power supply or something like that. That could cause the system to restart. Another possibility is a fault on the address or data buses with the former being more likely. If you have a multimeter my first suggestion would be checking for shorts between pins of the GA chip, in particular between the ones that connect to the control pad and +5v. After that I'd check for continuity between the address pins of the Z80 and the cartridge port. You don't say what model of SMS you have, that would be helpful.

Mayhem Joined: 14 May 2023 Posts: 7	Posted: Sun May 14, 2023 9:30 pm
Mayhem Joined: 14 May 2023 Posts: 7	Thank you. Yes the games that boot up do play demo levels. Pressing fire doesn't reset the system it just goes back to the title screen of the game. It's a EUR Master System 2 model 3006-05. It worked fine before but it's had several years in the loft. I'll get to work with the multimeter tomorrow.

willbritton Joined: 06 Mar 2022 Posts: 670 Location: London, UK	Posted: Mon May 15, 2023 8:35 am
	Mayhem wrote Pressing fire doesn't reset the system it just goes back to the title screen of the game. Hmmm, sounds like perhaps the button press is being misinterpreted as a soft reset, although the SMS2 doesn't have a soft reset switch. Again could be a fault in the GA chip (IC4). Maybe take a look at pins 25-30 of IC4, which are the data pins, to see if either you can see any continuity, either short or some non open resistance between the pins, and also take a look at the resistance between each of those pins and gnd and each of those pins and +5v. Basically for anything unusual. Given that the games do play demos fine, I suspect that there isn't a fundamental problem with your data bus, but it may be that when the console reads the controller buttons and a button is held it's reading something that looks like a 0 on D4, which is pin 28 which the game is interpreting as a soft reset. People have reported problems with the GA chip before where the active pull-ups have failed and sometimes that can be fixed by soldering an external pull-up resistor but better to do some more investigations first. Something else that could be going wrong is that the two controller i/o ports could be being misinterpreted. D4 carries the soft reset signal on port $dd and the left button (TL) on port $dc so it's possible that when the console is reading from $dd to check things like soft reset and you press TL it's actually responding with the $dc results, so a false positive on soft reset. That could point to a problem with A0 which is pin 2 so maybe have a poke around there too. As suggested earlier, check continuity between all D and A pins from IC4 to the cart slot or z80. Also check the physical condition of all the pins on IC4 - if there are any which look like bad joints, dirty, or with debris on or near them, consider cleaning and / or reflowing.

Mayhem Joined: 14 May 2023 Posts: 7	Posted: Tue May 16, 2023 11:12 pm
Mayhem Joined: 14 May 2023 Posts: 7	Thank you. I'll have a look at the weekend and feedback my findings.

Mayhem Joined: 14 May 2023 Posts: 7	Posted: Sat May 20, 2023 11:56 am
Mayhem Joined: 14 May 2023 Posts: 7	I've tried everything suggested and it's still the same as it was. I have another board and I've swapped virtually everything over and the parts have worked on the new board. I even swapped IC5 - I won't be doing that again as it was an absolute nightmare :) Transferring components from the new to the original board just give the original results. Another thing I have noticed though is that some games completely refuse to boot on the old board but have no issues on the new one.

willbritton Joined: 06 Mar 2022 Posts: 670 Location: London, UK	Posted: Sat May 20, 2023 12:36 pm
	Hmm, well if as you say you've swapped lots of the parts over and they seem fine, it feels like it's looking like a problem with the board, maybe broken or damaged traces, or shorts from capacitor leaks or debris perhaps. At least now you know the parts are good you can put them all back and keep iterating testing and cleaning or repairing suspect traces until you get it working. Oh by the way, I was assuming Alex Kidd is the built in game and therefore that you're getting the same problem regardless of whether or not a cartridge is inserted. Let me know if that was an incorrect assumption!

Mayhem Joined: 14 May 2023 Posts: 7	Posted: Sat May 20, 2023 1:29 pm
Mayhem Joined: 14 May 2023 Posts: 7	Yes Alex Kidd is the built in game. That works strangely as well: When you switch on you get the Sega logo then Alex Kidd comes on but there's no music and the little boxes that show up on the main screen don't come. Pressing fire doesn't do anything it's just like a screen shot picture. I've swapped the bios round and it plays fine in the other sms.

Mayhem Joined: 14 May 2023 Posts: 7	Posted: Sun May 21, 2023 7:30 pm
Mayhem Joined: 14 May 2023 Posts: 7	I've spent a lot of the weekend checking voltages and traces and I think I'm going to give up. Just one thought though, I tried Sonic 1 today and when I powered up I had a screen of moving multi coloured bars not unlike early C64 loading screens. The screen then went mostly blue but I could hear the music from the first level. Could this be a memory thing?

thatawesomeguy Joined: 25 Jul 2007 Posts: 730 Location: Melbourne, Australia	Posted: Tue May 23, 2023 12:16 pm
	I was thinking if anything this sounds likely to be faulty RAM. My guess is that the title screen contains little data, enough it doesn't touch the higher regions where the d=fault may be, when you try to start a game it accesses a higher region and glitches.

Mayhem Joined: 14 May 2023 Posts: 7	Posted: Tue May 23, 2023 3:38 pm
Mayhem Joined: 14 May 2023 Posts: 7	I've finally sussed and fixed it. It was IC3 - I thought that I'd already swapped that one over. The original chip was a Sanyo LC3664R, the one I've put in is a KM6264BL. It looks like I can get a replacement km6264bl from China via ebay - if it works. All the games that didn't boot, now boot and games that partially worked now work well. Thanks for all the suggestions.

willbritton Joined: 06 Mar 2022 Posts: 670 Location: London, UK	Posted: Tue May 23, 2023 4:04 pm
	Ah, RAM is is then! Good hunches both. Really glad you've got it working, now you can enjoy it!

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