Monstermug
Active member
Think I got lucky with this fix.
The problem I had was there was graphics glitches. This board had been sent to Phil before so before I started working on it, I had asked him just in case he knew what might be the cause of it. As I didn't expect him to reply instantly, I decided to do a bit of investigation myself.
As you can see the dragons tail is not missing but draw in the middle of the screen rather connected to the head. In fact it was isolated to this middle section of the screen.
Also some of the sprites characters were missing on the animated section of the dragon breed logo as shown above. (Also noting that the missing sprites were in the same vicinity)
As I have heard, M72 boards are notoriously difficult to work on so I spent some time to analyse the problem as doing so could help me isolate the problem and save hours of chasing tails.
From the glitches I can see that it's definitely something to do with the sprite circuitry as background and foreground graphics are fine as shown above after the animated section of the dragon breed (sprite controlled). Also in game background and foreground graphics looks fine. Roms are unlikely to be the culprit here but as it was so easy to verify roms, I did this to confirm this to be true.
Next step was to isolate which boards were the cause of the problem. Lucky for me I have many M72 games and swapping boards isolated the problem to the main board.
The big board in the middle is the main board. It contains 6 customs chips with loads of buffers and other circuitry.
Normally I would take my probe out and start probing the address lines of the rams but because the CPU was connected to the top layer board this was impossible to do. I had thought about using some DuPont wires to separate the top boards but I didn't have enough of them! It uses two 50 idc connectors which I didn't have any either.
I manually traced the Sprite roms back to Custom chip KNA72H010 914) @ C46 and this goes out to some buffers to a bank of ram @ C53-C58. I deduced that the this custom chips replaces many of the addressing circuitry for the sprites and that the bank of rams were sprite rams. I think this customs also controls some of the sound effects too.
Lucky for me these bank of rams were all socketed from a previous repair. I knew not all of them were bad, maybe just one if not just a few, judging from the glitches I had observed from the start. Swapping the main banks of chips around I got nothing until I started swapping out Rams @ C53/54. These caused the glitches to change!
Got the dragons tail back but some other sprites corruption elsewhere.
I didn't have any spare 6116 chips so I ordered some.
At this point I got an email from Phil confirming it could be the ram (or custom chip that's at fault) so I was pretty assured that this was the fault. Many days later (today) a bunch of 6116s arrived.
Put two new Rams in at location C53 and C56 (was previously located at C54) and got a perfect picture. I decided just to replace all the ram as I have many 6116 rams now =)
Without any further glitches I deemed this one another successful fix!
Think I got lucky with this one being that it was not a custom that was the problem. They aren't easy to get hold of and can be quite expensive to if you can find one.
I'm a happy fella.
The problem I had was there was graphics glitches. This board had been sent to Phil before so before I started working on it, I had asked him just in case he knew what might be the cause of it. As I didn't expect him to reply instantly, I decided to do a bit of investigation myself.
As you can see the dragons tail is not missing but draw in the middle of the screen rather connected to the head. In fact it was isolated to this middle section of the screen.
Also some of the sprites characters were missing on the animated section of the dragon breed logo as shown above. (Also noting that the missing sprites were in the same vicinity)
As I have heard, M72 boards are notoriously difficult to work on so I spent some time to analyse the problem as doing so could help me isolate the problem and save hours of chasing tails.
From the glitches I can see that it's definitely something to do with the sprite circuitry as background and foreground graphics are fine as shown above after the animated section of the dragon breed (sprite controlled). Also in game background and foreground graphics looks fine. Roms are unlikely to be the culprit here but as it was so easy to verify roms, I did this to confirm this to be true.
Next step was to isolate which boards were the cause of the problem. Lucky for me I have many M72 games and swapping boards isolated the problem to the main board.
The big board in the middle is the main board. It contains 6 customs chips with loads of buffers and other circuitry.
Normally I would take my probe out and start probing the address lines of the rams but because the CPU was connected to the top layer board this was impossible to do. I had thought about using some DuPont wires to separate the top boards but I didn't have enough of them! It uses two 50 idc connectors which I didn't have any either.
I manually traced the Sprite roms back to Custom chip KNA72H010 914) @ C46 and this goes out to some buffers to a bank of ram @ C53-C58. I deduced that the this custom chips replaces many of the addressing circuitry for the sprites and that the bank of rams were sprite rams. I think this customs also controls some of the sound effects too.
Lucky for me these bank of rams were all socketed from a previous repair. I knew not all of them were bad, maybe just one if not just a few, judging from the glitches I had observed from the start. Swapping the main banks of chips around I got nothing until I started swapping out Rams @ C53/54. These caused the glitches to change!
Got the dragons tail back but some other sprites corruption elsewhere.
I didn't have any spare 6116 chips so I ordered some.
At this point I got an email from Phil confirming it could be the ram (or custom chip that's at fault) so I was pretty assured that this was the fault. Many days later (today) a bunch of 6116s arrived.
Put two new Rams in at location C53 and C56 (was previously located at C54) and got a perfect picture. I decided just to replace all the ram as I have many 6116 rams now =)
Without any further glitches I deemed this one another successful fix!
Think I got lucky with this one being that it was not a custom that was the problem. They aren't easy to get hold of and can be quite expensive to if you can find one.
I'm a happy fella.
