New Member - working on Sega G80 X/Y - stuff...

xxyyzz

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Donor 2012, 2015
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Hello - wanted to introduce myself...
I've been a collector (on and off) of early 80's Arcade Games since late 1997.
I like just about anything and as of recent got back into trying to ressurect some old SEGA G80 - X/Y - stuff.

My current problem (and this is where I need some G80 expertiese), is that I have an X/Y - Control Card,
which as of recent, continues fails with the following ' faults ' :

(1.) Upon power up - screen shows random lines (drawn / blanked / drawn all over the place).
(2.) Connecting Output(s) to an O-Scope yields same problem / random lines being drawn.
(3.) Depressing the ' Test ' switch on CPU - Card - indicates - repated U27 Ram - Fault
(4.) Replacing every RAM (2114) chip - U21 -> U34 - has no effect / no change,
(5.) Removing RAM (2114) chips - has no effect - U27 Fault continues - as if Ram is not present.
(6.) Problem is isolated to the Control Card itself, (verified after Control card swap).
(7.) Problem seems to point to ' inability ' to either write to or perhaps access Ram by the CPU?


In an effort to try to resolve this issue, I have also gone in and replaced the following Parts(s)...
(please note: These were replaced one at a time - Board Powered after each instance - with no change).
(I have also added some notes - based on my limited knowledge of this particular ' Board ' )

U2 - Color Word is Read & Applied to RGB / DA - Converters (U5 and U1 - T) and associated Diodes/ Resistors.
U5 - Draw - Blank On / Off -
U3 - Compensates for Electron Beam - Deflection -
U10, U11, U12 - 74LS161 <- Vector Address Counter - Circuit - Loaded with First Address of the Line Instructions -
U14 - CPU DataBus <- D0 - D7
U15, U16, U17 - 74LS191 <- Vector Line Length Counters
U18 - Signal End - 74LS10 <-
U24 - U31 <- 2114 (RAM) -
U33 - Advances - 74LS283 <- Program Counter - by 10 counts - (if instructed Not to draw) -
U48 and U49 - 74LS244 <- Multiplexers - Allow CPU to Address Video Memory - U24 - U31
U50 - 74LS20 - Decoder <- Selects 1 of 15 outputs - goes >Low< & stores Video Words - in memory one at a time.
U51 - Sequences U50 <- through its 15 - count - cycle. At count 14 - Pin 16 of U50 - goes >Low< - to - DRAW.
U33, U34 and U22 Allow Character Words to be taken from Memory as the Program Counter Addresses
them via U36, U49, U35 and U21 -

Looking any information on how ' RAM ' on the G80 - Control Card is initially accessed (upon power up), by the CPU.
Appreciate any comments, suggestions or ideas that anyone may have.


Thanks,
-xxyyzz
 
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Sorry forgot to mention that - I did replace the Z80 on the CPU Board and even swapped CPU Boards (since I had another one handy).

Unfortunately no change.

Thanks,
-xxyyzz
 
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Try a PM to Mongo on this forum. He might be able to troubleshoot. He knows a lot about these boards.
 
Thanks, will PM him...

Regards,
-xxyyzz
 
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-Ok after doing some tracing with a Logic Probe (not that easy since I have to view
the Control Card from the backside) here is what I found / where I am at:

U42 - Pin 3 - MemW - Low (U42 was replaced)
U42 - Pin 4 - MemW - Pulsing -
U41 - Pin 5 - High - (U41 was replaced)
U41 - Pin 6 - High - connects to Pin10 (All RAMS)
U24 - U31 - Pin 10 - Low (All 2114 RAMS replaced)
U38 - Pin 9 - MemR - High (U38 was replaced)
U38 - Pin 8 - MemR - Low
U40 - Pin 1 - Mux - High (U40 was not replaced)


Open to any ideas / will keep looking, in the interim.

Regards,
-xxyyzz
 
Do you have access to another XY control board? I would try another known working board to see if the problem goes away.

Also, did you change out the Cap near the edge connector on the XY Control board? - if that board was made in 1982??? then the Cap is 30 years old. Sorry, I don't remember the value of it off hand..

-Muel
 
Yes, I sure did (happen to have 1 good - spare control board) and after swapping the problem one with the good one, game displayed a normal picture.

Also swapped out the capacitor near the bottom of the control board, (I believe it is a 10uf 25 or 35 volt if I recall correctly) but I have seen both on various control cards.

I'm thinking that since the Output of ' MemW ' - U42 - Pin 3 - is always ' Low ' and since the Output of ' MemR ' - U38 - Pin 9 is always ' High ' perhaps the problem is that Memory Write is ' stuck ' for some reason and is causing issue with being able to Write?

But that is just a guess? Since I do not know all the ins and outs of how vector memory access works on the control card - I.E. Initial Loading / Reading / Writing etc... during power up as well as normal operation.


Thanks,
-xxyyzz
 
Update

After replacing U40, here is where I stand as of recent....

U38 - Pin 9 - MemR now seems to indicate as Good - High - Low - High - Low etc...
U42 - Pin 3 - MemW indicates a constant High - as if Writing is not possible?
U24 - U31 - (Memory Chips) - Pin 10 - continue to indicate a constant High -

Pressing Test Switch on CPU Card - continues to point to Defective U27 - which I believe is ' false '

Does anyone know what the actual purpose of the two Transistors (2N4401 and 2N4403) located near the bottom of this Card is? I.E. are they for Voltage Regulation? or ?

Thanks,
-xxyyzz
 
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