Sega Mega-Tech Repair Log

[yavuzg]

Hey guys,

Recently I was working on a rare (at least for me) arcade board and wanted to share my experiences with it; Sega Mega-Tech.

First let me tell you the background story of this...

How I got this PCB is rather a sad story. The local ex-operator found this Sega Mega-Tech arcade cabinet in a this condition in a junk yard out of town.

Athough in this condition, he still promoted this to his "high roller" customer out of town (who is always interested in original arcade cabinets). The high roller customer got interested and wanted it to be restored but he wanted it to be converted into a MAME system which the second small monitor on top be utilized as a dynamic marquee. Probably he watched youtube videos of a Sega Mega-Tech cabinet converted into a hyperspin & hypermaquee setup...

Anyway, I didn't have the money to grab this cabinet and fully restore it as it is (I hate original cabinets converted into MAME systems) but I could only convinced the ex-operator guy to trade the PCB and the cartridges with a hyperspin setup.

I did prepared a hyperspin & hyper-marquee setup for this guy but that's another story

Anyway, after I gave him the hyperspin setup I prepared and he gave me the Sega Mega-Tech setup, shall we say, this junk saying it is "partially working"

This is the repair story of this junk...

 

[yavuzg]

Some of you may already know but let me give you some basic info about what a Sega Mega-Tech system is. It is basically Sega version of a Nintendo Playchoice system.

Sega took the similar path like Nintendo and made an arcade system which is based on their game console; Sega Mega Drive (Sega Genesis for the US market).

This system has 8 cartridge slots which you plug your game cartridges and let the customer insert coins, buy certain amount of time to play a selected game. To simplify the investment in the system, Sega used the same game code in the cartridges but made them as pin-incompatible with the game console versions. There are however projects to make custom mega-tech game cartridges by using standard mega drive ROMs...

Anyway, here is the PCB I've got...

As you can see, it is not JAMMA and has a special harness specifically designed to fit this into the Mega-Tech cabinet.

The PCB had some patch wires on the back which seems factory made...

In order to get this thing at least powered up in my JAMMA bench-setup I needed to first make a conversion harness. and put this filthy old harness away...

I've made a basic harness which basically consists of power cables and video cables. For now, this should be enough to power the PCB and let me see whats going on on the first and second monitor.

 

[yavuzg]

I plugged the harness to the PCB. There are two video connectors on this PCB. First one is for the main monitor which the game plays and second for the small top monitor which you have the list of games and make your selection, read the basic instructions of the game etc.

I first plugged the video cable to the main monitor connector and wanted to see how "partial" this PCB is working (as the ex-op guy said)... And I fired up the PCB...

Nothing!

Well, I wasn't expecting much on this PCB but wasn't expecting a totally dead board either... Anyway, I switched the video cable to the second monitor connector and tried again...

Better, but again, the pattern I saw seems to be an indication of a major malfunction...

 

[yavuzg]

Moving on, I read further about this arcade system. I found out that this PCB consists of two major regions;

1) There is a menu system which brings up the game list on the second monitor and contains the "commerical" logic such as game selection, coinage, time keeping etc. And this portion is based on Sega Master system and contains a Z80 CPU, RAM and a ROM containing the what might be called as a "BIOS".

1) The other region is the actual "Mega Drive" part which runs the game and have standard componenst such as the Motorola 68000 CPU, Z80 CPU (for Master System Games and sound for MD games), RAM, and custom Sega ICs.

Since I see no game list, I started my diagnosis from the first region.

The first region has a daugher PCB which the Z80 CPU sits on it.

Based on what I've read and pin-by-pin control, I found that the Z80 CPU is pin-by-pin connected to the main socket on the main PCB and the rest of this PCB only "listens" the bus. This PCB seems to be used as retrieving the credit data from the DB-25 parallel port and totally unnecessary for main operation of the PCB.

I just removed the Z80 from this PCB...

And plugged it directly to the socket which this daughter PCB was connected before...

I was hoping this daughter PCB (some say Book keeping daughter card - the manual says "Piggy BD") might be the problem...

Unfortunately I still got the screen with vertical bars...

 

[yavuzg]

I moved on and checked the rest of the PCB. I've checked the 68K cpu first...

The reset line was stuck low. It was briefly high when board first powered but moment later it just got low and stay there...

Next I check the clock. The clock was there... Around 8Mhz as expected...

Next I moved to the other Z80 cpu which is next to the 68K... Again checked the reset line and clock first...

Again the reset line was stuck low but the clock was OK. Around 4Mhz...

The address and data bus was totally static. They were all stuck HIGH.

While I was poking around the PCB, I felt a pain which my hand got burned with something...

These ICs were burning hot!!!

These are the two SONY chips (Sony CXD1095Q) which were called as "I/O port extender".

There were two of these chips, one located near the menu region the other near the mega drive region.

These chips where sitting right on the busesses and were extremely hot and I suspected these might have been gone bad...

 

[yavuzg]

Luckily these chips are still available...

I've put the repair on hold and ordered some replacements from utsource and within couple of weeks I've got mail

I

I resumed the repair and replaced both of these chips...

And powered up the board...

Sadly, still got the vertical bars on screen.

Although replacing these SONY chips didn't made any difference, the chips were not getting hot anymore. There must be some other components that might have got fried...

Continued my diagnosis, I checked the BIOS...

I've cleaned the legs and replaced the socket which was totally rusted...

Didn't worked...

I've then checked the contents with ROMIdent which showed up as OK...

 

[yavuzg]

I then replaced the IC socket of the Z80 cpu which was not in good shape, although I was getting signals from the legs of the cpu which the problem might not be related with the socket itself...

I was getting a little angry since I was so convinced that replacing fried sony chips would fix the board. Then I moved on to the last "usual suspect" on the bus after checking the CPU, BIOS and the sony chip... The RAM...

There was a separate RAM on this region of the PCB which seems to be responsible with the video since it was connected to the video addresser custom Sega IC (found in Sega Master System consoles).

Tried to check it with my eprom burner TL866 which can also test standard SRAM chips.

It showed up BAD!

I was happy for a moment that I thought I nailed the bastard. But soon realized that this specific type of RAM used in this system looks like a standard 62256 type SRAM but it was indeed a Pseudo SRAM (some sort of SRAM which internally designed as a DRAM acting as a SRAM). My eprom burner cannot even test the replacements (which were pulled from a known-working CPS1 A board).

Anyway my eprom burner cannot test these Pseudo SRAMs fully but I found that the "Unit test" works for the replacement Pseudo SRAMs. I just limit the tests to only "Unit test" and re-tested the chip...

Ahaa! We are getting somewhere... The Pseudo SRAM was indeed faulty... Replaced it with the one I pulled from my parts CPS1 A board and...

Finally some progress!!!

 

[yavuzg]

Now I've fixed the menu region... But not I'm stucked to a situation that apparently is a very common fault with these mega-tech boards; cannot read carts...

No carts shows up on the list. Moreover, only 6 slots are displayed on the list. According to the various repair logs on the net this was normal anyway. If the PCB cannot read any carts it just displays 6 empty slots rather than 8...

I've read ALL the repair logs so far.

I even checked the transistors on the video out part which is said to be a reason when you get a black screen.

Although the transistors were cheked OK they were not the cause of my current problem anyway. I mean I've concluded that the real cause of the problem (that the carts won't show up on the list) was the complete "mega drive" region was not working at all on this PCB.

Anyway, I continued to diagnose why my data and address buses are stuck high and started to test the usual suspect line up;

1) CPUs
2) RAMs
3) Glue logic custom ICs

I've started with the CPUs and socketed the Z80 and 68K on the mega drive region and replaced them with known working ones...

No luck... Still no games on the list and black screen on the main monitor port...

Btw, I've washed the PCB and the cartridge connectors extensively... Still no joy...

 

[yavuzg]

Then I socketed and replaced the RAMs on the mega drive region, even the obscure shaped SRAMs...

Btw, the other Pseudo RAMs that Sega used on this board cannot be tested with TL866 eprom reader, not even the unit test works...

Now I ran out of options with the standard components and I focused on the custom Sega chips which makes up the "mega drive/genesis" portion of this PCB...

 

[yavuzg]

There are 4 sega custom chips on this PCB which one (or more) of them might be the real cause of the problem;

1) VDP: The PPU used as the main graphics processor on this PCB is a Sega 315-51313 custom IC.
2) Bus arbiter: Sega 315-5308 custom IC
3) I/O: Sega 315-5309
4) ???: Sega 315-5345

Detailed information regarding these custom ICs could be read from wiki.megadrive.org

Anyway, these are not off the shelf standard components but you can find them from a mega drive or genesis game console. The tricky part here is that Sega used many different designs during the life time of mega drive console production and used many different custom chips in these designs.

I've extensively investigated and hunted down these ICs in different revisions of the mega drive game consoles and found that you can find ALL of these customs in one concole revision;

Sega Genesis Model 1 with board revision VA2

It is said that early japanese Mega Drive 1 consoles having boadr revision VA0 and VA1 might also have these but the problem with hunting down such consoles is that there were no obvious external giveaways in japanese consoles indicating that the board revision inside is indeed VA0 or VA1.

But Sega Genesis Model 1 consoles having FCC ID on the back FJ846EUSASEGA are good candidates that they might have a VA2 or VA3 board inside...

Also, the "High Definition Graphics" text is written on the front case...

Still, even with the above evidence, you cannot be sure %100 that the console have all the above ICs inside. But you'll find a VA2 or VA3 board inside for sure...

VA2 board has all these ICs. VA3 has all but 315-5308 the bus arbiter chip. Sega combined 315-5345 and 315-5308 into a single chip called 315-5364 beginning with VA3 revision.

Anyway, I didn't have the needed Genesis (had couple of MD2 consoles, VA6 and VA3 Genesis Model 1 consoles) found a console with a VA2 board on a friend. I traded my VA3 with the VA2 and finally got access to all of the chips...

Now I'm ready for a "shotgun" style repair

 

[yavuzg]

I call it shotgun style repair since I have no means of identifying which custom might be faulty. I mean they all connected to the same address and data bus and one of them is dragging these buses to HIGH. I have no idea of the inner workings of these customs either so basically no info in testing their pins and signals... My only choice is to replace the ICs one by one till I find the faulty one...

First I started with the bus arbiter chip 315-5308

I removed it from the Mega-tech board...

Used the known working one from the Genesis board...

Soldered the replacement to the Mega-tech board...

Did it solved the problem? No!

But I took the time to solder the 315-5308 chip I got from the Mega-tech to the Genesis just to see if it was OK or not...

Guess what... 5308 was OK anyway. Genesis worked without any issues...

I've also socketed and tested the Pseudo SRAMs on the Genesis...

They all checked out OK...

 

[yavuzg]

Moved on to the next custom; 315-5309

This is the I/O chip used in the mega drive/genesis console. Earlier my prime suspect was the bus arbiter chip 5308 but this custom has also have extensive address and data bus connections. Also I always suspect chips on the front line of the battle. Since this is an I/O chip, it has interface with external world and under more risk than others.

Anyway, this was my second suspect before I move to the larger VDP chip which has too many legs

Removed it from the mega-tech...

And before I solder the new one, I soldered the 5309 chip I removed from the mega-tech to Genesis.

... and powered up Genesis...

Ahaa!!! The same exact balck screen, now on the Genesis...

I guess I found the killer... Soldered the 5309 chip I got from Genesis to the Mega-Tech board and fired it up...

Yes!!!

 

[yavuzg]

I finally nailed down the component which stopped the mega drive/genesis region of the mega-tech PCB. So what I learned from this repair is that if you do not see any cartridge listed on the game selection menu, then it means there is something wrong with the mega drive/genesis region...

There are some repair logs (not too many though) which the culprit was a Sony chip or the bus arbiter 315-5308. Mine was the 315-5309.

No matter which chip, if one of them is fried it seems it totally brings down the address/data bus.

Anyway, after I fixed the board I completed my harness just to see if all controls are working and whether I can select games or not, hear the sound etc...

I then hooked up the board to my JAMMA test rig again and tested the games...

All of the games worked. All controls were fine...

Although I've sacrificed a Sega Genesis console to save this rare arcade PCB I'm still happy that it is back to life again. Definitely a more interesting piece of Sega hardware than an ordinary Genesis console.

Although I must say, the Genesis I sacrificed here is known as the "Non TMSS (TRade Mark Security System)" model which is also a "sought after" type of Genesis which there is no licence lock out system in these early models...

So if you are up to fixing a mega-tech PCB, prepare a Sega Genesis Model 1 having VA2 or VA3 revision main board around to use as a parts bin

 

[ShootTheCore]

Excellent sleauthing and repair- bravo! Thank you for documenting it for the rest of us. Hope you're enjoying the board!

 

[PascalP]

Very nice repair!
Wished I had those skills

 

[brizzo]

Dang, thats a lot of photos! 169mb to load this page!

 

[yavuzg]

Dang, thats a lot of photos! 169mb to load this page!

I forgot to optimize the photos. They should be resized now. Sorry

 

[Yippikaye]

Excellent and detailed repair log = Bravo
Thanks for sharing

 

[Gakman]

Amazing. Thank you for sharing!

 

[yavuzg]

... Hope you're enjoying the board!

You know, I still do not know what to do with this board.

One option is to consolize it, make a plexiglass case, put a small LCD on top of it for the game menu etc... Would be a good project but personally I'm not so into consolizing arcade systems. Also to just play games, my other Genesis + Mega Everdrive is more convenient than a 2-3 Kg bulky giant consolized mega-tech.

Other option is to plug it in one of my cabinets. This option sounds more logical to me for now. But the downside is what will I do for the second monitor? May be a monitor switch could be used?

Another option is to build a mega-tech repro cabinet from scratch (second monitor etc) and put this pcb in it. But that is the hardest path which I do not see enough motivitaion in me...

For now, I just packed it and shelved it