It all started innocent enough with a work ram error. But ...
Board was behaving randomly. Sometimes it booted to crosshatch for around 5 minutes then click of death. Other times it booted to backup ram error or work ram error.
(One thing was constant though - a really annoying crackling from the speakers. Changed volume pot and this was fixed.)
When it was booting to crosshatch I tried the sound test in which it failed: silence.
This made me suspect the NEO-BUF at GA9. Put in a new repro and ... now it only booted to a click of death. LOL.
Checked the adress and data lines on the bios. D0 had a dc offset which probably messed upp the whole operation. These errors I usually hunt by lifting the affected line at each chip, one at a time.
Experience told me to start with the NEO-BUFs and sure enough, when D0 at Neo buf at ga8 was lifted the bias went away.
Changed this buf. Booted to cross hatch for 30 seconds then a click of death. Rebooted and got the backup ram error. So pretty much back were I began.
Put a socket for the bios and ran smkdan/ack diagrom. Now the board booted to a backup ram error each time. But adress-, data and enable lines (/OE, /WE and CE) all seemed to behave properly. After 20 minutes with the diagrom error screen the board did a reset and after that it showed a work ram error (upper).
Adress D00000
actual 4000 expected 0000.
To make a long story short I checked the adress lines and data lines again. Seemed ok. Then I did my math and took a closer look at D7. And while it was pulsing it behaved different, inverted, than its corresponding pin at the lower wram. Checked with a working board and sure enough, in comparison D7 on the project board was corrupted.
Same procedure. Started to lift pins and found another faulty neo-buf, this time at GA11.
Now I got "All tests passed". Tried the inputs and found that select was stuck on. This line showed low at the input controller chip. measured the resistance and found a broken resistor bridge. Changed this which fixed the problem. Unfortunately the traces for ply 2 select (which isn't used but I wanted it to work never the less) was damaged so had to run a kynarwire.
Ran the m1 section of the diagrom with a test cart.
Z80 SLOT SWITCH IGNORED (SM1)
SM1 OTHERWISE LOOKS UNRESPONSIVE
Actual: 00
Expected:01
Tried to force with start.
Actual: xx (usually 41)
Expected: C3 from the "Hello"-section of the test.
Got different readings each time. This led me to believe that the sound ram was bad. Socketed this and put in a new.
Did not fix the problem. I still got no slot switch and failed Z80 testing when forcing with start.
Changed Z80. Now I got "all tests passed". Tried to run a game but only got to crosshatch. Sound tests worked though ...
Put in Unibios but no change. Ran the memory viewer. I did't really understand the data being presented. But with a cart in I noticed that the system tried to read from Slot 2.
Rebooted a couple of times and then the controls went crazy ... managed to get to the input tests and was presented with a somewhat corrupted ... Mahjong controller screen. LOL!
This made me suspect the neo-f0 which handles slot select and dip switch readings (dip 3 is Mahjong). Input tests said that dip 3 was off. But still the system wrongly believed otherwise.
Then the all of a sudden the slot switch error returned. Long story short: changed sm1 - no change. Changed to another Z80 and got the all tests passed again.
Still the dip switch drama and not booting carts so decided to change Neo-f0. Since this the random Mahjong controller issue stopped. But still no boot.
Put in super sidekicks 2 and scoped ROMOE on the backside of the cartridge board (vertical oriented). This was pulsing on start up. Scoped all the pins on the progboard. When I looked at the data lines many of the looked suspicious. They were pulsing but the signals looked weird and/or had an offset of around 1 volt (sorry no pictures). I traced the data lines which led me to a NEO-BUF @ GA11. This had been replaced earlier but this time I used a repro from furrtek. Put in super sidekicks 2 and booted to a perfectly running game.
There is some trash talk about these boards, MV1A. But I like them. Yes, neo-buf is prone to fail... but there are cheap repros avaible (in contrast to the neo-c1 which also often is corrupt (probably because people put in the jamma harness upside-down)).
Board was behaving randomly. Sometimes it booted to crosshatch for around 5 minutes then click of death. Other times it booted to backup ram error or work ram error.
(One thing was constant though - a really annoying crackling from the speakers. Changed volume pot and this was fixed.)
When it was booting to crosshatch I tried the sound test in which it failed: silence.
This made me suspect the NEO-BUF at GA9. Put in a new repro and ... now it only booted to a click of death. LOL.
Checked the adress and data lines on the bios. D0 had a dc offset which probably messed upp the whole operation. These errors I usually hunt by lifting the affected line at each chip, one at a time.
Experience told me to start with the NEO-BUFs and sure enough, when D0 at Neo buf at ga8 was lifted the bias went away.
Changed this buf. Booted to cross hatch for 30 seconds then a click of death. Rebooted and got the backup ram error. So pretty much back were I began.
Put a socket for the bios and ran smkdan/ack diagrom. Now the board booted to a backup ram error each time. But adress-, data and enable lines (/OE, /WE and CE) all seemed to behave properly. After 20 minutes with the diagrom error screen the board did a reset and after that it showed a work ram error (upper).
Adress D00000
actual 4000 expected 0000.
To make a long story short I checked the adress lines and data lines again. Seemed ok. Then I did my math and took a closer look at D7. And while it was pulsing it behaved different, inverted, than its corresponding pin at the lower wram. Checked with a working board and sure enough, in comparison D7 on the project board was corrupted.
Same procedure. Started to lift pins and found another faulty neo-buf, this time at GA11.
Now I got "All tests passed". Tried the inputs and found that select was stuck on. This line showed low at the input controller chip. measured the resistance and found a broken resistor bridge. Changed this which fixed the problem. Unfortunately the traces for ply 2 select (which isn't used but I wanted it to work never the less) was damaged so had to run a kynarwire.
Ran the m1 section of the diagrom with a test cart.
Z80 SLOT SWITCH IGNORED (SM1)
SM1 OTHERWISE LOOKS UNRESPONSIVE
Actual: 00
Expected:01
Tried to force with start.
Actual: xx (usually 41)
Expected: C3 from the "Hello"-section of the test.
Got different readings each time. This led me to believe that the sound ram was bad. Socketed this and put in a new.
Did not fix the problem. I still got no slot switch and failed Z80 testing when forcing with start.
Changed Z80. Now I got "all tests passed". Tried to run a game but only got to crosshatch. Sound tests worked though ...
Put in Unibios but no change. Ran the memory viewer. I did't really understand the data being presented. But with a cart in I noticed that the system tried to read from Slot 2.
Rebooted a couple of times and then the controls went crazy ... managed to get to the input tests and was presented with a somewhat corrupted ... Mahjong controller screen. LOL!
This made me suspect the neo-f0 which handles slot select and dip switch readings (dip 3 is Mahjong). Input tests said that dip 3 was off. But still the system wrongly believed otherwise.
Then the all of a sudden the slot switch error returned. Long story short: changed sm1 - no change. Changed to another Z80 and got the all tests passed again.
Still the dip switch drama and not booting carts so decided to change Neo-f0. Since this the random Mahjong controller issue stopped. But still no boot.
Put in super sidekicks 2 and scoped ROMOE on the backside of the cartridge board (vertical oriented). This was pulsing on start up. Scoped all the pins on the progboard. When I looked at the data lines many of the looked suspicious. They were pulsing but the signals looked weird and/or had an offset of around 1 volt (sorry no pictures). I traced the data lines which led me to a NEO-BUF @ GA11. This had been replaced earlier but this time I used a repro from furrtek. Put in super sidekicks 2 and booted to a perfectly running game.
There is some trash talk about these boards, MV1A. But I like them. Yes, neo-buf is prone to fail... but there are cheap repros avaible (in contrast to the neo-c1 which also often is corrupt (probably because people put in the jamma harness upside-down)).
Last edited: