Sega System 16b: Altered Beast has no sprites and garbled speech; Tetris is... fine?

[Pr3tty F1y]

Trying to troubleshoot a System 16b main board that I purchased as part of a 'not working' Altered Beast set.
I converted the whole thing to the decrypted FD1094 bootleg version of Altered Beast and the rom board works perfectly fine on OTHER known good System 16b main boards, so I'm 99% sure that the roms/rom board a not the issue.

However, the 'non working' main board has issues. I swapped in a good 10mhz 68K and I pulled the D7008AC Z80 out of the board and had it run as the CPU for my Arkanoid board and the D7008AC ran Arkanoid fine. So both the main and sound CPUs would seem to be good.

Using the Altered Beast 171-5521 board (the one with the capacitor jumpers and not pin jumpers), the game powers on fine, I can see the title screen it, coins up, and the first level starts with the background rendering fine and FM audio plays without issue

However, there are NO sprites at all and the "rise from your grave" speech is just garbled static. There's a few light grazes on the solder side of the board, but - thus far - haven't found any truly broken traces via multi-meter. Everything that has a scratch has continuity.

Now, swapping in a 5704 rom board hosting decrypted Tetris roms... everything runs fine (???). Game is fully playable. There's some slight static crackles to the audio, but the FM synth is perfect, there are sound effects, and the game is fully playable. Maybe the whole thing is running on background tiles with no sprites???

Either way, I ran the service mode RAM test for both Altered Beast and Tetris and all RAM types report back as good. There are a couple 6116 srams that have been socketed and replaced on the board. They also tested fine on my xGecu T56 (but I know that's not a full blown sram test). I reflowed the solder on each of the pins for the sockets and still no dice. The remaining srams are largely Toshiba, so... ugh.... I'm not sure how robust the service mode RAM tests are, but the boards are reporting no issues with ram. And the crazy thing with the sound test in Tetris is that FM synth and the PCM audio sound effects all sound fine... so maybe it's just the D7759 chip having issues with the speech from Altered Beast???

It's a real weird combo of issues, but being that the game actually runs and the board looks fairly clean, I'd really like to get this back into fully working order.

As usual, I'm going to continue to poke around at it - but if anyone out there has any thoughts, I'd appreciate hearing from you!

 

[SB_Electronics]

Any progress on this?

 

[Pr3tty F1y]

I was waiting on receipt of some supplies to make 4 x 50 pin IDC-based ribbon cables as extensions between the main board and the ROM board as I have a feeling my issue is going to be everything under the ROM board and it would be easier to probe around if I could have the ROM board on extension cables.

Unfortunately, the manufacturing of said ribbon cables has not been a one-and-done. While I do have the correct tools for crimping cables and have successfully made other cables previously (e.g., for Midway T and Y unit boards), these 50 pin ones are being flaky and there are connectivity issues. I'll have to go over them again to see if the ribbon cable approach is going to work at all or if it's just been a waste of time.

 

[SB_Electronics]

So to recap -

Decrypted Altered Beast doesn’t work on current (problem) board, but works on another system 16B mainboard. What decrypted rom set did you use and what ROM board version?

CPU and Z80 tried in Arkanoid - worked. Though Arkanoid isn’t a system 16B so may not rule them out completely.

Altered Beast 5521 works but has issues - no sprites, garbled speech.

Tetris 5704 runs fine - static on audio, does it have a speech ROM?

I’ve just finished converting the hard drawn service manual to Eagle schematic so I will have a look for any clues of where to check next. In the meantime time, the static and audio issues could be faulty caps around the analog sound section. Check the electrolytics and film caps in that area. In fact I would check all the electrolytics, given their age.

 

[SB_Electronics]

Ok, so possibly 2 separate faults. The missing sprites points towards the 5196 sprite generator and surrounding SRAM. Check the SRAM at G2, G3, pins CE, WE, OE. Do you have a logic analyser or oscilloscope?

You don’t need to use ribbon cables to extend the ROM board, you can probe the pins from the underside of the motherboard. You just have to suffer all the pins reversed.

 

[Pr3tty F1y]

Thank you so much. I sincerely appreciate your help on this.

RE: My finding with Tetris (as I just had it easily laying around and I knew it worked), I'm thinking that the gameplay is all background tiles and the samples that it plays aren't speech, so no garbled audio.

I'll check out the main board as soon as I have a chance. Between real life and the fact that I tend to get myself into too many projects, this one hasn't had the attention it needed (but, in my defense, I was fixing a busted Out Run - which I'm overjoyed about as it's one of my all time favorites - which had several chips which were physically cracked as well as it having a typical and as-expected bad Toshiba SRAM chip - and that Out Run Memory Test from aaldert.com was a life saver).

Also, while I typically don't have 3D spatial issues, flipping and lateral orientation breaks my brain with respect to pin numbering so I was really hoping to work on the parts side (hence my wonky cable approach) But I can probe the solder side. I may just need to make myself a cheat sheet

 

[Pr3tty F1y]

Ok, so possibly 2 separate faults. The missing sprites points towards the 5196 sprite generator and surrounding SRAM. Check the SRAM at G2, G3, pins CE, WE, OE. Do you have a logic analyser or oscilloscope?

You don’t need to use ribbon cables to extend the ROM board, you can probe the pins from the underside of the motherboard. You just have to suffer all the pins reversed.

I do have a logic probe and oscilloscope, but I'm out of my league with the oscilloscope. I played around with it a bit trying to troubleshoot a dead Midway Trog soundboard, but when I got to the point of finding what looked like an interference signal (i.e. a square wave of sorts with another gently curved wave hugging the x axis) I had no idea what to actually do or what that meant so major work-in-progress re: my skills there.

As far as the SRAM at G2 and G3 under the 5196 chip; the G3 chip had already been socketed and replaced. The remaining SRAM chips in that entire area are TMM2018 chips... so yay... Toshiba.

Both chips had the same read outs on the logic probe for WE, OE, CS:

• WE: pulsing/beeping with LED indicating high
• OE: pulsing/beeping with both high and low LED's lit
• CS: Solid low

While I was there, I checked all of the pins on the 5196 (not really knowing what I'm looking for) with the logic probe with some pins matching the slow beeping that I was getting on the SRAM chips, while other pins were much higher frequency. I didn't bother to trace them at all last night, but the chip seems to be operating.

And the board is still running the game logic/backgrounds/music fine:

 

[SB_Electronics]

You already narrowed it down nicely by using a known working system 16 and a game with and without sprites/ speech. I think you are right about Tetris. It is tile based, no sprites and no speech.

I’m the same, so many projects but no time. Well done on getting Outrun working. Great game. It looks like there are a lot of reports about the Toshiba SRAM failing so if G2, G3, G4 or G5 are Toshiba (they are Sony on my model) look to replace them.

Working upside down is a headache. I had to map out the ASIC pins and it was an absolute head fuck.

A logic probe is more useful here. The System 16 is a bus monster. Where the oscilloscope would be useful is if the logic probe showed activity but you wanted to confirm the quality (ie edge transition), or the analog section.

With the chip activity, I would expect the issue to be with WE, CE or OE. A stuck bit on D0-7 or A0-10 would create garbled sprites rather than remove them completely.

Try the Toshiba RAM on the 5196 next.

 

[Pr3tty F1y]

Pulled all of the SRAM's around the 5196 and, according to my XGecu T56 (not the best way to test a 6116 SRAM), all TMM2018's tested bad (like 5 of them, which surprised me). The single TMM2015 tested OK and the replacement 6116 SRAMs also tested good.

My concern is that my narrow-width 6116 SRAMs that I have on hand are 45ns and it looks like all TMM2018's original to the board are 35ns. I would usually brush of that difference, but it may be why the replacement MCM2018-45AN's have not really remedied the issue.

The craziest part is that I pulled the TMM2015 from the sound section as I had a random memory that some SEGA system (not sure if it was System 16 or not, but I remember reading it somewhere) that the sound Z80 occasionally was used for graphics/sprite management

And, even though that TMM2015 tested OK, I am getting some barely there shimmering sprites. So that is telling me that the sprite chip is trying:

So I currently have some components on the way to build a true 6116 SRAM tester as I just need to be able to do that in general as Toshiba SRAMs have been the bane of my existence, but many of them are still good.

But it may not entirely be the Toshiba SRAMs, so I'll continue to test and replace chips where it makes sense.

 

[SB_Electronics]

Good progress. You are right, this shows that the ASIC is working so we can leave it alone. Don’t worry about the timing, the service manuals show 45nS 2018 so 35nS may have just been available at the factory.

On the System 16B the Z80 just deals with sound. What speed is the 2015?

When you get your tester you can truly rule out the dodgy RAM. The shimmering could be a faulty 74LSxxxx logic. Once you have replaced the RAM with confirmed working 2018 RAM, check their /WE, /OE and /CE pins against the activity from your good system 16.