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ShootTheCore

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Great job troubleshooting! First up, all the times I answered you before I was at work without an actual PCB in front of me and I went off of the schematics. Now that I'm at home, I pulled out one of my working System 16B boards and checked things out for myself. It turns out that with the B Board, the schematics are just outright wrong. I'm not sure if they were written for a different B Board revision or what, but they don't match the B Board you and I both have. So I apologize for giving you incorrect advice on the traces to check for continuity before.

As you noted - I also forgot that all of the custom chips are through-hole mounted on this board so ignore my earlier statement about checking for lifted pins on the custom chip(s). However, the three custom chips are installed in sockets, meaning that you can carefully pry them out one edge at a time with a thin screw driver and swap them with the equivalent custom on your working System 16 board. This would be a quick and effective way to test if one or more of your custom chips have failed.
153878BB-E644-4609-BA06-DE52AA45849D.jpeg

Okay, now that I've said all that, let's step through your latest work.

From your video:
- You are correct that it looks like Chip Enable (Pin 20) is holding high. That line is Active Low, so that's why the output lines aren't triggering.

- You are also correct about it looking like Output Enable (Pin 22) is also holding high. That's another one that's Active Low, so that's another reason the output lines aren't triggering.

Since we know your CE and OE lines need help, let's ignore the schematics and figure them out for ourselves. Here's what I found from probing my board:

- The CE line (Pin 20) on the ROM at location B1 you've been checking shows continuity to Pin 15 on the 74LS138 chip located in the bottom-middle of the B Board.
- The 74LS138 chip is a decoder - it's responsible for activating a particular output line (CE signal) according to an enable signal and a select signal that's fed into it across 6 pins. There's a datasheet with a Truth Table for how the lines are triggered on the 74LS138 here: https://www.electroschematics.com/wp-content/uploads/2013/07/74LS138-datasheet.pdf

- You should find continuity on the Pin 20 CE lines to the output pins on the decoder as follows:
ROM B1 Pin 20 = Pin 15 74LS138
ROM B2 Pin 20 = Pin 14 74LS138
ROM B3 Pin 20 = Pin 13 74LS138
ROM B4 Pin 20 = Pin 12 74LS138
ROM A1 Pin 20 = Pin 11 74LS138
ROM A2 Pin 20 = Pin 10 74LS138
ROM A3 Pin 20 = Pin 9 74LS138
ROM A4 Pin 20 = Pin 7 74LS138

- The Select signal for the CE outputs on the 74LS138 come into Pins 1, 2 and 3. Continuity on these pins traces to
Pin 1 = CN2 Pin B9
Pin 2 = CN2 Pin A10
Pin 3 = CN2 Pin B10

- The Enable signal for the CE outputs on the 74LS138 comes in on Pins 4, 5 and 6. Continuity on these pins traces to
Pin 4 = Tied GND
Pin 5 = Tied GND
Pin 6 = Tied HIGH

Since the Enable signals are all tied and don't change, the Select signals are the ones you should focus on for your CE troubleshooting.
The three Select signals on the CN2 connector route to three pins on the 315-5196 Custom. I marked them in red on this photo:
IMG_9579.jpg

Check those Custom pins for continuity to the CN2 pins. If you're careful, you can also probe the pins while the board is running with your logic probe to see if signals are being generated on those lines. According to the 74LS138 Truth Table, the CE line for ROM B1 (Output 0) will only trigger Low if Pins 1, 2, 3, 4 and 5 are all Low and Pin 6 is High. We know Pins 4, 5, and 6 are tied and won't change, so look for activity on Pins 1, 2 and 3 - they should all show High-Low alternating activity if the 315-5196 Custom is functioning properly.

The B1 ROM OE Line (Pin 22) comes from Pin A9 on CN2, which then goes back to the same 315-5196 that drives the CE lines. I've marked the pin in white on the photo above.

Hopefully this information will be much more helpful for you. I should have just pulled my own board out and checked things over myself a while back instead of relying on the schematics. Sorry about that.
 
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rcboosted

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Great job troubleshooting! First up, all the times I answered you before I was at work without an actual PCB in front of me and I went off of the schematics. Now that I'm at home, I pulled out one of my working System 16B boards and checked things out for myself. It turns out that with the B Board, the schematics are just outright wrong. I'm not sure if they were written for a different B Board revision or what, but they don't match the B Board you and I both have. So I apologize for giving you incorrect advice on the traces to check for continuity before.

As you noted - I also forgot that all of the custom chips are through-hole mounted on this board so ignore my earlier statement about checking for lifted pins on the custom chip(s). However, the three custom chips are installed in sockets, meaning that you can carefully pry them out one edge at a time with a thin screw driver and swap them with the equivalent custom on your working System 16 board. This would be a quick and effective way to test if one or more of your custom chips have failed.
153878BB-E644-4609-BA06-DE52AA45849D.jpeg

Okay, now that I've said all that, let's step through your latest work.

From your video:
- You are correct that it looks like Chip Enable (Pin 20) is holding high. That line is Active Low, so that's why the output lines aren't triggering.

- You are also correct about it looking like Output Enable (Pin 22) is also holding high. That's another one that's Active Low, so that's another reason the output lines aren't triggering.

Since we know your CE and OE lines need help, let's ignore the schematics and figure them out for ourselves. Here's what I found from probing my board:

- The CE line (Pin 20) on the ROM at location B1 you've been checking shows continuity to Pin 15 on the 74LS138 chip located in the bottom-middle of the B Board.
- The 74LS138 chip is a decoder - it's responsible for activating a particular output line (CE signal) according to an enable signal and a select signal that's fed into it across 6 pins. There's a datasheet with a Truth Table for how the lines are triggered on the 74LS138 here: https://www.electroschematics.com/wp-content/uploads/2013/07/74LS138-datasheet.pdf

- You should find continuity on the Pin 20 CE lines to the output pins on the decoder as follows:
ROM B1 Pin 20 = Pin 15 74LS138
ROM B2 Pin 20 = Pin 14 74LS138
ROM B3 Pin 20 = Pin 13 74LS138
ROM B4 Pin 20 = Pin 12 74LS138
ROM A1 Pin 20 = Pin 11 74LS138
ROM A2 Pin 20 = Pin 10 74LS138
ROM A3 Pin 20 = Pin 9 74LS138
ROM A4 Pin 20 = Pin 7 74LS138

- The Select signal for the CE outputs on the 74LS138 come into Pins 1, 2 and 3. Continuity on these pins traces to
Pin 1 = CN2 Pin B9
Pin 2 = CN2 Pin A10
Pin 3 = CN2 Pin B10

- The Enable signal for the CE outputs on the 74LS138 comes in on Pins 4, 5 and 6. Continuity on these pins traces to
Pin 4 = Tied GND
Pin 5 = Tied GND
Pin 6 = Tied HIGH

Since the Enable signals are all tied and don't change, the Select signals are the ones you should focus on for your CE troubleshooting.
The three Select signals on the CN2 connector route to three pins on the 315-5196 Custom. I marked them in red on this photo:
IMG_9579.jpg

Check those Custom pins for continuity to the CN2 pins. If you're careful, you can also probe the pins while the board is running with your logic probe to see if signals are being generated on those lines. According to the 74LS138 Truth Table, the CE line for ROM B1 (Output 0) will only trigger Low if Pins 1, 2, 3, 4 and 5 are all Low and Pin 6 is High. We know Pins 4, 5, and 6 are tied and won't change, so look for activity on Pins 1, 2 and 3 - they should all show High-Low alternating activity if the 315-5196 Custom is functioning properly.

The B1 ROM OE Line (Pin 22) comes from Pin A9 on CN2, which then goes back to the same 315-5196 that drives the CE lines. I've marked the pin in white on the photo above.

Hopefully this information will be much more helpful for you. I should have just pulled my own board out and checked things over myself a while back instead of relying on the schematics. Sorry about that.
Some really good information again! I will sketch these out and ponder a bit to make sure I understand before I proceed. Thank you for tracing all of these out, I know it takes quite a bit of time. Don't worry about the bad schematics, I was starting to wonder about it too. Then I saw it has a board number on the B board, it didn't match the board number on the schematic, but since this is all we have, I went with it.

As for testing out the custom chip from the good board, that is no longer an option. :( In my test with the multi, I sent 12V into the 5V rail by accident. I want to blame the molex connector for letting me connect it backwards, but ultimately it's my own fault. At least I can practice taking out the custom chip using the fried board. The chip and the socket are so closely pushed together, it looks like it was soldered directly. Test results to come.
 

ShootTheCore

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Aw shoot - sorry to hear your other System 16 board is fried. Still, the 315-5196 custom off of it might still work, so I'd try swapping them with this board just to see if it makes any difference.
 

rcboosted

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Great job troubleshooting! First up, all the times I answered you before I was at work without an actual PCB in front of me and I went off of the schematics. Now that I'm at home, I pulled out one of my working System 16B boards and checked things out for myself. It turns out that with the B Board, the schematics are just outright wrong. I'm not sure if they were written for a different B Board revision or what, but they don't match the B Board you and I both have. So I apologize for giving you incorrect advice on the traces to check for continuity before.

As you noted - I also forgot that all of the custom chips are through-hole mounted on this board so ignore my earlier statement about checking for lifted pins on the custom chip(s). However, the three custom chips are installed in sockets, meaning that you can carefully pry them out one edge at a time with a thin screw driver and swap them with the equivalent custom on your working System 16 board. This would be a quick and effective way to test if one or more of your custom chips have failed.
153878BB-E644-4609-BA06-DE52AA45849D.jpeg

Okay, now that I've said all that, let's step through your latest work.

From your video:
- You are correct that it looks like Chip Enable (Pin 20) is holding high. That line is Active Low, so that's why the output lines aren't triggering.

- You are also correct about it looking like Output Enable (Pin 22) is also holding high. That's another one that's Active Low, so that's another reason the output lines aren't triggering.

Since we know your CE and OE lines need help, let's ignore the schematics and figure them out for ourselves. Here's what I found from probing my board:

- The CE line (Pin 20) on the ROM at location B1 you've been checking shows continuity to Pin 15 on the 74LS138 chip located in the bottom-middle of the B Board.
- The 74LS138 chip is a decoder - it's responsible for activating a particular output line (CE signal) according to an enable signal and a select signal that's fed into it across 6 pins. There's a datasheet with a Truth Table for how the lines are triggered on the 74LS138 here: https://www.electroschematics.com/wp-content/uploads/2013/07/74LS138-datasheet.pdf

- You should find continuity on the Pin 20 CE lines to the output pins on the decoder as follows:
ROM B1 Pin 20 = Pin 15 74LS138
ROM B2 Pin 20 = Pin 14 74LS138
ROM B3 Pin 20 = Pin 13 74LS138
ROM B4 Pin 20 = Pin 12 74LS138
ROM A1 Pin 20 = Pin 11 74LS138
ROM A2 Pin 20 = Pin 10 74LS138
ROM A3 Pin 20 = Pin 9 74LS138
ROM A4 Pin 20 = Pin 7 74LS138
I checked these lines, continuity are all good. Since these are on the known good B board, they should be good as I suspect. I didn't see A5, B5 and beyond, but I think these ROMs are used in tandem, so B1's CE is tied to B5's etc, and continuity test confirms.

- The Select signal for the CE outputs on the 74LS138 come into Pins 1, 2 and 3. Continuity on these pins traces to
Pin 1 = CN2 Pin B9
Pin 2 = CN2 Pin A10
Pin 3 = CN2 Pin B10

- The Enable signal for the CE outputs on the 74LS138 comes in on Pins 4, 5 and 6. Continuity on these pins traces to
Pin 4 = Tied GND
Pin 5 = Tied GND
Pin 6 = Tied HIGH
These traces confirmed as well. Here's what I observed. After power up, if I leave it alone, the screen shows dots and lines etc like there's video that's either severely squished or just not displaying properly as usual, Pin1 stays high, pin2 stays low. All other pins are as they should on input side. If I probe pin1 on power up and keep the probe on it, pin1 stays low, as does pin2, it still does the video dance, but still no video of course. If I probe pin2 on power up and keep on it, it starts low, then pops to high after 15 to 20 sec. Pin1 is also high. no video. I don't think these observations mean anything though. Just the probe interfering with boot.
Since the Enable signals are all tied and don't change, the Select signals are the ones you should focus on for your CE troubleshooting.
The three Select signals on the CN2 connector route to three pins on the 315-5196 Custom. I marked them in red on this photo:
IMG_9579.jpg

Check those Custom pins for continuity to the CN2 pins. If you're careful, you can also probe the pins while the board is running with your logic probe to see if signals are being generated on those lines. According to the 74LS138 Truth Table, the CE line for ROM B1 (Output 0) will only trigger Low if Pins 1, 2, 3, 4 and 5 are all Low and Pin 6 is High. We know Pins 4, 5, and 6 are tied and won't change, so look for activity on Pins 1, 2 and 3 - they should all show High-Low alternating activity if the 315-5196 Custom is functioning properly.
All these continuity checks out too. I tested from chip to CN2 on A board, chip to CN2 on B board, as well as chip to LS138. I tried tying both pin1 and 2 on LS138 to GND to force it enable, but I still got no video. I saw that pin3 went to high when I did that, so I grounded all 3, but of course that did nothing. I'm just forcing the LS138 to do something but if 5196 is bad, it won't do anything. It did get both CE to OE to read low though.
The B1 ROM OE Line (Pin 22) comes from Pin A9 on CN2, which then goes back to the same 315-5196 that drives the CE lines. I've marked the pin in white on the photo above.

Hopefully this information will be much more helpful for you. I should have just pulled my own board out and checked things over myself a while back instead of relying on the schematics. Sorry about that.

I looked closer at the custom chips, it appears both of my boards are indeed not socketed. I'll attach a pic of the close up, in case I'm just inexperienced in how they should look. I could try and unsolder the chip and swap, but it might be difficult without destroying vias. I wonder if I can test the fried A board with the B board again to see if it still works. Don't want to fry the B board due to some shorts that may have been created during the overvoltage. But from what I've seen so far, continuity are all good, LS138 is good, it's the 5196 that isn't enabling the ROMs? Which means that chip is bad?
IMG_20210828_141607416.jpg
 

rcboosted

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I desoldered the 5196 chips and swapped it. I'm hesitant to solder it back on though since I don't have a socket and if I solder it and the chip is bad, I'd have to desolder it again. Not sure if the board can handle another job like that. I did try to power it on while pushing down on the chip. The screen now show all white. Not really sure what that means. Sometimes it'll be the same as before, lines and dots. If I probe for OE, probe had no activity at all, CE shows high. Pin 1 2 3 on LS138 also had nothing at all. This could all mean nothing since the chip is not soldered down.

I assume a spare socket could only come from a parts board from someone.
 

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rcboosted

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I feel like eliminating the encrypted CPU as a possibility for troubleshooting. Would I need to replace all of the ROMs on the B board in order to use regular 68k cpu or just the OBJ? Which EPROMs would I need? The OBJ ones are 27C256, I see some variants from ST, such as M27C256B and TS27C256, or AMD has simply AM27C256, not sure what speed I'd need either.

I've got 2 more 16B boards coming, I hope they aren't DOA as well. So if I can revive this board, I'm in a better position.
 

ShootTheCore

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You have to replace the two CPU program code EPROMs at positions A5 and A7. The correct EPROMs to use with those are 27C2001, such as an NEC D27C2001D or a ST M27C2001. I'd use the 150ns speed-rated chips or faster (lower number).

Also remove the encrypted CPU on the A board and install a standard 68000 CPU that will fit the socket and is rated for 10mhz, such as the MC68000P10.
 

rcboosted

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You have to replace the two CPU program code EPROMs at positions A5 and A7. The correct EPROMs to use with those are 27C2001, such as an NEC D27C2001D or a ST M27C2001. I'd use the 150ns speed-rated chips or faster (lower number).

Also remove the encrypted CPU on the A board and install a standard 68000 CPU that will fit the socket and is rated for 10mhz, such as the MC68000P10.
Thank you, I will give that a shot since it's just 2 EPROMs for Tetris. I have 1 more 12mhz 68k cpu to try as well.
 
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