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Strange SE/30 Simasimac


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Hello,

 

I am working on repairing one of my dead SE/30 boards, but I am having some problems with Simasimac (insert frustrated noise here). I have cleaned and recapped this board, and I am testing it inside a known good chassis with known good RAM and ROM.

 

When I power the machine up, I do not hear a chime, and this pattern is displayed onscreen. (Apologies for the strange camera angle.)

 

IMG_20200822_211449.thumb.jpg.92fadada5281900a169ed4965834dbe7.jpg

 

To troubleshoot this, I have replaced UA8 through UG8 with brand new chips. I then tested all of the connections to these seven ICs with my multimeter, but I couldn't find any breaks. I also confirmed that the connections to UB10 and UB11 are good. I checked the voltages on the board, and they are normal.

 

I'm not sure how to proceed, or what the pattern I see is indicating. Do I need to replace all of the F258 chips? Should I re-recap the board, and use my microscope to inspect for damaged solder mask under the capacitors? Is the connection to the ROM bad? -If so, would I have to check all 64 ROM pins? (Now that would be time-consuming!)

 

I would greatly appreciate help if anyone has ideas about this. :)

 

Thanks!

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  • 3 weeks later...

I'm not sure if anyone saw this post the first time around, so I'm bumping it in case anyone has some ideas about how to repair this motherboard. There are a few more things I could try (see above), but otherwise, this board has me stumped.

 

If anyone has suggestions about how to fix this logic board, I would greatly appreciate them. Have you seen a board with an issue like this before @Bolle@techknight?

 

Thanks!

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  • 2 months later...
On 9/17/2020 at 1:40 AM, techknight said:

Also make sure all of your address and data lines, plus control lines are good to the ASC, SCSI, SCC, etc... from the ROM socket or GLU as the origin test points. 

@techknight

 

It took me a long time to get around to it, but I just buzzed out the both the address bus and the data bus from the ROM on this motherboard. I used the matrices from the modernized schematics, which were a big help. Strangely, I did not find any breaks in either of the buses. I wasn't sure how to check the control lines, so I didn't do anything with them yet. I also checked the VRAM ICs, but all of their connections were good too. I know it has been a while, but if you or anyone else has suggestions, I'd love to hear them!

 

I became much faster at checking connections from the schematics, and I have to say, it wasn't really as hard as I thought it would be! :)

 

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2 hours ago, ronan said:

Did you check the address bus lines conductivity between the cpu and the glu chip ?

Yes -all of these connections are working correctly. Apart from connections going to and from the PDS socket, I tested every possible connection for both the address and data buses, and they were all good. @techknight mentioned checking control lines, but I'm not sure how to test these/where they are. (I think they have something to do with interrupts and reset, but I am still figuring out how computer architecture works in general) Believe it or not, I actually used your article from Hackaday as a guide to help with this process! I found the diagrams to be very useful, especially in conjunction with the modernized SE/30 schematics from GitHub. I really liked the labeled pins on the bottom of the ROM socket -they saved me a ton of time and hassle. Thanks for writing about your project. :)

 

I figure that there still might be a broken trace somewhere that I haven't checked yet, or maybe one of the chips has gone bad. I suppose cap goo could be a problem, but this board is pretty much squeaky clean (short of me buying a very expensive ultrasonic cleaner I don't need). I have almost checked out the entirety of the video circuitry with my multimeter, so I could technically finish that process and see what it reveals. I could also replace all of the F258 chips in hopes that one them is faulty. The only other thing I can think of doing (as a last resort) is re-recapping the board, for I last recapped this logic board before I purchased a microscope. Maybe there is something shorted underneath a capacitor? This seems technically possible, but is probably unlikely.

 

I feel like I am just guessing what could be wrong here, as I don't yet have the engineering knowledge to understand what is physically producing the errors I see onscreen. Hopefully, someone here understands better than I do, and is willing to enlighten me. :lol:

 

 

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6 hours ago, ronan said:

Did you check the address bus lines conductivity between the cpu and the glu chip ?

 

I'm thinking this as well.  Looks like only half of the VRAM is available to the main bus, but all of it is being displayed.  A stuck VADR line into the muxes can result in portions of the display repeating, but that's not what you have here.  I'd look closely at the lower A lines.

 

Are you checking these with a scope or DMM?  A continuity test won't tell you about a short to GND, etc.

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1 hour ago, ymk said:

Are you checking these with a scope or DMM?  A continuity test won't tell you about a short to GND, etc.

Unfortunately, I only have a multimeter to check these lines at the moment.

 

I'll have to double-check the CPU-GLU address lines tomorrow to confirm that everything is working correctly, but no problems were detected on my first pass today. 

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Can you give us a front picture of the symptoms instead of that impossible to see side-view? 

 

im curious if the gray patches are actually the proper checker pattern of the ROM initializing things, maybe a mouse cursor shows. I dont know. 

 

with my bad eyes, I can somewhat see the checker pattern, and i think its 8 pixels wide. 

 

So somethings wrong with the muxing somewhere either with the CPU writing to VRAM, or the display circuits reading from VRAM. because every other 8-bit block appears dead. The lack of sound at this point im thinking is coincidental, being another issue entirely. Although missing half the bus, it may not be. Hard to say without physical troubleshooting. 

 

 

Edited by techknight
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On 11/28/2020 at 3:11 PM, ronan said:

Did you check the address bus lines conductivity between the cpu and the glu chip ?

I just checked all of these address lines again, and I found no breaks.

 

2 hours ago, techknight said:

Can you give us a front picture of the symptoms instead of that impossible to see side-view? 

Here you go! Sorry about the previous photo's quality. I couldn't make out a mouse cursor or a blinking floppy on the screen.

PXL_20201202_145640905.thumb.jpg.4b99dfa695355956b0e4eb4cf3db4930.jpg

PXL_20201202_145640905(1).thumb.jpg.55f4bcc07dcefbbc823346e70205cca1.jpg

 

Edited by davidg5678
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Well it looks like the ROM is drawing out the checkerboard like a normal start up. 

 

But its locking up before switching over to the screen with the mouse cursor. 

 

There is something wrong with the bus somewhere. it could even be a bad IC, or connections between the GLU and all the other ICs. 

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34 minutes ago, techknight said:

There is something wrong with the bus somewhere. it could even be a bad IC, or connections between the GLU and all the other ICs. 

Do you think it would make sense to replace all of the F258 chips? I have already swapped the UA8 through UG8 chips. I also have another dead SE/30 to swap parts with. I'm not sure what I should try replacing, or which ICs are relevant here, though.

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1 hour ago, techknight said:

Ok as expected. 

Great! Is it okay that the error screen changed slightly between having RAM installed and not having RAM installed? It obviously remained nonfunctional either way.

 

If I understand what you are saying here, the F258s are probably not an issue.

 

Do we have a way to know if the Data bus or Address bus is the one that is broken? It sounds like it might be the Address bus, as that is what everyone has been focused on.

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45 minutes ago, davidg5678 said:

Great! Is it okay that the error screen changed slightly between having RAM installed and not having RAM installed? It obviously remained nonfunctional either way.

 

If I understand what you are saying here, the F258s are probably not an issue.

 

Do we have a way to know if the Data bus or Address bus is the one that is broken? It sounds like it might be the Address bus, as that is what everyone has been focused on.

 

Normal. its expected behavior if the CPU is running and is able to execute ROM. 

 

There is something going on in the bus somewhere. its going to take troubleshooting. 

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  • 2 weeks later...

Update on this board:

 

I decided to re-recap everything, as I wanted to make certain that there were not any shorts between the capacitors and traces nearby. I removed all of my capacitors, applied solder-mask to the many exposed traces I found, and put the capacitors back. I also removed a few crusty looking ICs, cleaned up underneath them, and soldered them back down. The board certainly looks neater, but remains very broken.

 

Interestingly, it now alternates between displaying the same screen as pictured above, and a blank gray screen. The gray screen shows no cursor or floppy, but at least it is something different. My understanding is that it means the bus is still messed up, because the ROM is not loading anything else in. I also tried some different RAM and ROM SIMMs, but they didn't make any difference.

 

 

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