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MVS 2 Slot Repair Help

ack said:
It looks like that one is ok, but you need to check them all. There are darkened traces all round the battery area need to be checked. I would advise cleaning them up, which involves removing the solder mask, tinning them, then covering them back up with enamel or epoxy.
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I tested all the lines like this. all lines are ok


how should the corroded areas be cleaned?
 
remove the solder mask over the darkened part of the traces, tin the copper traces with solder, verify continuity, cover the trace with enamel or epoxy
 
I did a little debugging in mame with unibios. Im pretty positive that one of the 74AS245s on the lower board is not getting enabled or is dead.

When the 68k cpu tries to read something from the data bus and nothing responds it will usually results in the next read thing being used for the missing data. In this case after trying to read the p rom data, the next thing on the data bus is the cpu reading the next instruction from bios rom which happens start with 0x6100. The upper byte on the p rom data read responds with nothing and is getting filled in with that 0x61 from the next instruction.
 

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ack said:
I did a little debugging in mame with unibios. Im pretty positive that one of the 74AS245s on the lower board is not getting enabled or is dead.

When the 68k cpu tries to read something from the data bus and nothing responds it will usually results in the next read thing being used for the missing data. In this case after trying to read the p rom data, the next thing on the data bus is the cpu reading the next instruction from bios rom which happens start with 0x6100. The upper byte on the p rom data read responds with nothing and is getting filled in with that 0x61 from the next instruction.
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I took them out and put them in eprom writer. it showed no errors in the ttl test function. unfortunately, I don't have another AS245 ic at home. I put in a 74LS245 as a test. did not start 0x6100 left
 

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did not start with 0x61xx meaning its working?

I would check the enable pins on them with a logic probe to make sure they are getting enabled.
 
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r67055 said:
can you tell me which pin it is? and what should I see there?
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The enable pin on 74x245's is pin 19. When you are in the memory viewer looking at that 000100 address it should be pulsing.
 
I measure both 74AS245 pin 19 en HIGH levels. from switching on. the signal level does not change
 

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Im a little unclear with your wording. They would only pulse if you were doing something to access the program roms on the cart.
 
I just confirmed both enable pins should be pulsing when you are in the memory viewer. I was also able to reproduce the 0x61 issue by forcing the 74AS245 @ B8 to always be in disabled state.
 
ack said:
Im a little unclear with your wording. They would only pulse if you were doing something to access the program roms on the cart.
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I'm really sorry.
It was already late last night.

I put the ttl logic probe on point 19 of the 72AS245. I placed the PCB under current. The logic probe shows a High level..
I enter the system memory map menu. I will go to address 100. the signal level does not change HIGH.
the logic probe does not pulse. only shows low level
 
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If pin 19 was stuck low the board would crash as it would cause a bus conflict. When you are using the logic probe make sure you are listening and not just watching the LED on the probe. Pulses can be fast enough it wont register on the LEDs but you can hear them.

I think what you should see based on the issue, when you are in the memory view at 000100 is that pin 19 on 74AS245 @ B7 should be pulsing, and pin 19 on 74AS245 @ B8 is likely stuck high.
 
ack said:
If pin 19 was stuck low the board would crash as it would cause a bus conflict. When you are using the logic probe make sure you are listening and not just watching the LED on the probe. Pulses can be fast enough it wont register on the LEDs but you can hear them.

I think what you should see based on the issue, when you are in the memory view at 000100 is that pin 19 on 74AS245 @ B7 should be pulsing, and pin 19 on 74AS245 @ B8 is likely stuck high.
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yes, pin 19 is stuck at a high level
sorry, I don't know why, but I wrote it wrong at the end. not low but high.
 
r67055 said:
yes, pin 19 is stuck at a high level
sorry, I don't know why, but I wrote it wrong at the end. not low but high.
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yes, I have experienced that the logic tester had a faint pulsing sound. I have an oscilloscope. I'm not sure if it works well.
Is it possible that the CPU is faulty?
 
its not going to be a cpu issue or the board would be stuck in watchdog. My suspicion is this will end up being related to the battery leak damage.

So I just wanna make sure we are on the same page.

At memory viewer address 000100

PIN 19 on 74AS245 @ B7 = faint pulsing
PIN 19 on 74AS245 @ B8 = stuck high

correct?

As you drew in the previous picture, PIN 19 on 74AS245 @ B8 goes to PIN 6 on that 74AS08. A 74x08 is an AND gate, so PIN 6 (output) will be set high if PIN 4 (input) and PIN 5 (input) are both HIGH. You will want to check PIN 4 and 5 to see what they are doing. I'm not sure where they come from.
 
ack said:
its not going to be a cpu issue or the board would be stuck in watchdog. My suspicion is this will end up being related to the battery leak damage.

So I just wanna make sure we are on the same page.

At memory viewer address 000100

PIN 19 on 74AS245 @ B7 = faint pulsing
PIN 19 on 74AS245 @ B8 = stuck high

correct?

As you drew in the previous picture, PIN 19 on 74AS245 @ B8 goes to PIN 6 on that 74AS08. A 74x08 is an AND gate, so PIN 6 (output) will be set high if PIN 4 (input) and PIN 5 (input) are both HIGH. You will want to check PIN 4 and 5 to see what they are doing. I'm not sure where they come from.
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""At memory viewer address 000100

PIN 19 on 74AS245 @ B7 = faint pulsing
PIN 19 on 74AS245 @ B8 = stuck high

correct?""" Yes exactly.

I made a beautiful drawing :D
 

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Just so there is no confusion. When I say B7 and B8 I'm talking about the location of the chips on the board and not pins on the chips. If you look above the middle part of the jamma connector there are letter A to S going up the board, and at the bottom/top of the board are numbers 1 to 11. So B8 is the chip at row B, column 8, which is the 74AS245 on the right.

For your issue you we need to work backwards from pin 19 to figure out why its stuck high. As you showed pin 19 connects to that 74AS08 on pin 6.

Next steps will be to logic probe pins 4 and 5 on the 74AS08 and see what they are doing. Pin 6 should only be high when pin 4 and 5 are high.
  • If pin 4 or 5 has faint pulsing, but there isn't faint pulsing on pin 6 it would point to the 74AS08 being bad (I doubt this is whats happening)
  • if either pin 4 or 5 is floating that points to a broken trace
  • if both pin 4 and 5 are also stuck high you need to take another step backwards and track down why
    • You show that pin 4 and pin 1 are connected together, so seems unlikely you need to look there or the issue would also be effecting the other AS245 too
    • So I would work backwards from pin 5 on the 74AS08 to see where thats coming from and see if that IC is working right
 
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