SE/30 with simasimac and happy chime

[AstroFirebird]

Hey everyone, I got my SE/30 and it has a classic case of simasimac. When doing research I found that the symptoms may vary depending on the cause, but I have a hard time finding anything in regards to my symptom. Along with the screen most people usually get no sounds or a bad chime sound. My Mac gives me the screen with a normal startup chime. I recapped the logic board to replace the leaking caps and gave it two alcohol baths. Unfortunately I’m having the same exact problems. Simasimac screen with a normal startup chime. I checked for any opens between the ROM and CPU along with the CPU and glue chip. I checked out UE8 with an oscilloscope and I don’t believe it to be an issue. At this point I’m leaning towards something being wrong with the VROM or PAL chips. Has anyone had one of these boards with the same symptoms I’ve had? I’ve attached a picture of my screen.

 

[mikes-macs]

The usual first step to check is a bad RAM configuration or ROM seating. Make sure all RAM matches and in correct configuration and capacity. Re-seat all, including ROM. Memory test takes time.

 

[Phipli]

The usual first step to check is a bad RAM configuration or ROM seating. Make sure all RAM matches and in correct configuration and capacity. Re-seat all, including ROM. Memory test takes time.

Hum, I've seen that pattern, but I can't remember what caused it. But bad memory would cause a sad mac, rather than a happy chime and that screen.

Can you check the voltage, I feel like this might be what you see when the 5v is too low? The 5v is available on the floppy port. 5th pin in from the top left is 5v, put the negative probe on one of the SCSI port screw fixings or similar.

 

[AstroFirebird]

I’ve had the ROM and RAM out of this thing numerous times. I’ve used two different sets of RAM with same results. I don’t believe the ROM is a problem given the fact that I’m getting a normal start-up chime so it seems like it’s communicating.

 

[AstroFirebird]

Hum, I've seen that pattern, but I can't remember what caused it. But bad memory would cause a sad mac, rather than a happy chime and that screen.

Can you check the voltage, I feel like this might be what you see when the 5v is too low? The 5v is available on the floppy port. 5th pin in from the top left is 5v, put the negative probe on one of the SCSI port screw fixings or similar.

Just checked. Right at 5 Volts on the dot.

 

[Phipli]

Bingo, this is when I saw it :




I'd added lots of cards to my SE/30 and the PSU was struggling. I fixed it with a PSU recap and setting the voltage.

 

[Phipli]

Just checked. Right at 5 Volts on the dot.

Strange. Exactly 5.0v? I'm stumped then. But I'd start by investing the video circuit.

 

[cheesestraws]

Yes: you probably want to be looking at the write path into the VRAM, at a guess.

 

[AstroFirebird]

Strange. Exactly 5.0v? I'm stumped then. But I'd start by investing the video circuit.

Yes that’s where I am and have been confused about. I’m getting video signals from UC6 and UC7. They get to UE8 and it appears to spit out a signal from pin 13 and make its way to UG6. After that I’m kinda lost on where to go. From my basic understanding of how this computer works, the screen displayed is simply gibberish stored in the VROM itself, right? I‘m not sure where the signal goes or how the VROM is even initialized.

 

[Phipli]

Yes that’s where I am and have been confused about. I’m getting video signals from UC6 and UC7. They get to UE8 and it appears to spit out a signal from pin 13 and make its way to UG6. After that I’m kinda lost on where to go. From my basic understanding of how this computer works, the screen displayed is simply gibberish stored in the VROM itself, right? I‘m not sure where the signal goes or how the VROM is even initialized.

Do you mean VRAM? That's where I'd be looking rather than the video ROM.

Perhaps start by verifying that good power is getting to the VRAM chips.

 

[imactheknife]

I had a board that had that problem, was a broken trace right in front of first memory bank.

 

[AstroFirebird]

Do you mean VRAM? That's where I'd be looking rather than the video ROM.

Perhaps start by verifying that good power is getting to the VRAM chips.

I’m confused on which pins would supply power to the VRAM.

 

[zigzagjoe]

Curiously, the apple schematics don't bother to list the VCC/gnd connections on a lot of the logic. Here's a pic from https://github.com/mishimasensei/macse30mlb/tree/master/hardware/pcb/mlb/output

Note this schematic has errors, so don't take it as 100% truth (same thing goes for Bomarc schematics). It's reasonably accurate in most cases, however, and the easiest to read.

 

[AstroFirebird]

Curiously, the apple schematics don't bother to list the VCC/gnd connections on a lot of the logic. Here's a pic from https://github.com/mishimasensei/macse30mlb/tree/master/hardware/pcb/mlb/output

Note this schematic has errors, so don't take it as 100% truth (same thing goes for Bomarc schematics). It's reasonably accurate in most cases, however, and the easiest to read.

View attachment 61693

Thank you. That explains a lot. I’ve got 5 volts and good ground at both vram chips. I can’t say they’re 100% good but I know they’re putting out some sort of signal. I’ve been checking all the traces like crazy and to my surprise have yet to find a single one that’s open. Do the PAL chips run much hotter than the rest? Because after about 30 seconds of runtime all the PAL chips are pretty hot to the touch while every other component doesn’t even come close.

 

[zigzagjoe]

Thank you. That explains a lot. I’ve got 5 volts and good ground at both vram chips. I can’t say they’re 100% good but I know they’re putting out some sort of signal. I’ve been checking all the traces like crazy and to my surprise have yet to find a single one that’s open. Do the PAL chips run much hotter than the rest? Because after about 30 seconds of runtime all the PAL chips are pretty hot to the touch while every other component doesn’t even come close.

They generally run hot. Unless they are hotter than you can hold your finger on, I wouldn't worry about it being too hot. That said, 30 seconds to be /actually hot/ rather than just warm seems a little off to me. If you can't keep your finger on it, it's possible you've got some dead PALs which would definitely be able to cause these symptoms.

Another sanity check you could perform is hook up a floppy drive and insert any disk. If it trys to read the disk, odds are it's trying to boot off it - proving the boot process has continued (as expected) despite lack of display.

Otherwise, as phipli mentioned it looks like there is an issue with data getting into VRAM. Getting out of VRAM seems fine as you have a classic example of what uninitialized vram looks like. I'd begin by checking continuity between the VRAMs and the assorted control pins (not the numbered data or address pins RA/D/VID).

 

[AstroFirebird]

They generally run hot. Unless they are hotter than you can hold your finger on, I wouldn't worry about it being too hot. That said, 30 seconds to be /actually hot/ rather than just warm seems a little off to me. If you can't keep your finger on it, it's possible you've got some dead PALs which would definitely be able to cause these symptoms.

Another sanity check you could perform is hook up a floppy drive and insert any disk. If it trys to read the disk, odds are it's trying to boot off it - proving the boot process has continued (as expected) despite lack of display.

Otherwise, as phipli mentioned it looks like there is an issue with data getting into VRAM. Getting out of VRAM seems fine as you have a classic example of what uninitialized vram looks like. I'd begin by checking continuity between the VRAMs and the assorted control pins (not the numbered data or address pins RA/D/VID).

I’m getting continuity to all pins on the VRAM. I’ll definitely get a boot disk made and try that. Sounds like a good idea. Will probably pick up an IC tester as I‘m leaning towards that being the issue.

 

[zigzagjoe]

I’m getting continuity to all pins on the VRAM. I’ll definitely get a boot disk made and try that. Sounds like a good idea. Will probably pick up an IC tester as I‘m leaning towards that being the issue.

It doesn't need to be a boot disk. Any disk is fine. If it ejects the disk, that still proves it's willing to boot, which is the goal of that test - make sure it's actually a problem with video instead of an early failure in the POST process before video initializes.

Ic tester to test which ics?

 

[croissantking]

the screen displayed is simply gibberish stored in the VROM itself, right?

Yes, it's just the default state of the bits in the VRAM (not VROM) chips when they are powered on. These are NEC chips, but other brands such as TI will display black and white bars.

It's interesting to mix two different brands of chip and make completely new combinations of this pattern. Like a child of simasimac.

That's where I'd be looking rather than the video ROM.

A bad (or missing) VROM can cause this behaviour. It happened when I inserted a VROM chip in backwards and fried it. After correctly inserting it, the Mac would chime and boot normally, but I got this pattern or a variation of it.

t's possible you've got some dead PALs which would definitely be able to cause these symptoms.

I'd expect a black screen (no output) with dead PALs. The simasimac pattern is actually a good sign as it shows that the video circuitry is mostly operational.

 

[AstroFirebird]

Ic tester to test which ics?

The PAL chips and VROM as they’re the ones that’s socketed.

I'd expect a black screen (no output) with dead PALs. The simasimac pattern is actually a good sign as it shows that the video circuitry is mostly operational.

Yes at this point I am leaning towards a bad IC rather than something wrong in the circuitry.

 

[zigzagjoe]

The PALs can't be easily tested, I don't think any test vectors exist for them. They're programmable logic, each one with different behaviors. I think it is easiest to assume they are good until proven otherwise by elimination.

A programmer (like a TL866 or similar) can be used to verify the ROM contents. Or if you have an arduino or something handy, it'd be straightforward to breadboard something up to test it if you can manage the code.