Bombed Macintosh Classic Failing to start after attempted fix + recap

[Kouzui]

All right. I did the repair, and this is what happened.

When I try to turn the Mac on, one of three things happens:

1. Nothing. Fan spins and CRT turns on but nothing else.

2. The screen alternates between two bars and three bars:


3. Garbage displayed on screen similar to two bars image, but covering right half of the screen. Makes the "drum machine" sound described in this video:

After a few seconds of this noise, it will go to 2 and behave as described.

I've looked over the board a few times and found a couple unconnected traces, but connecting them has not changed the behavior in any way. I will provide pics of the board if anyone wants to take a look, and I can take pics of specific things tomorrow if requested.




Does anyone know if Kai Robinson is on this site? I'm running out of ideas.

 

[joshc]

Kai doesn’t post here anymore, he’s on
tinkerdifferent.com.

 

[Kouzui]

After careful inspection, fixing a couple of unconnected traces, and separating a short, I hadn't made all that much progress.
Today, however, I made a bit of a breakthrough:

While I knew that R2 on the underside of the board wasn't populated, I hadn't realized that R6 and R8 were ALSO unpopulated.

After removing the 22ohm resistors I placed there, I get a gray screen. The mac does not chime, but you can hear the speaker pop on startup. You can see that there is no mouse, and it never goes to the question mark floppy screen.

I might be pretty close to fixing this thing, but I don't know what this screen is symptomatic of.

 

[Kouzui]

After some more fiddling, I discovered I can make the classic display a sad Mac if i hit the reset button:

Searching here on the forums, I found that it is a "data bus error":

but I don't really know what this means. Could it be a bad connection with the ram chips? I haven't put in a memory expansion yet.

I should note that the mac still doesn't chime, and still displays a gray screen if I don't hit the reset button.

 

[zigzagjoe]

After some more fiddling, I discovered I can make the classic display a sad Mac if i hit the reset button:
View attachment 60701
Searching here on the forums, I found that it is a "data bus error":
View attachment 60702
but I don't really know what this means. Could it be a bad connection with the ram chips? I haven't put in a memory expansion yet.

I should note that the mac still doesn't chime, and still displays a gray screen if I don't hit the reset button.

First off: Nice work! You've made a lot of progress.

That error is something we can work with. I just took a brief look through the ROM source. It tries to write successive bits to identified memory and verifies that all bits are correctly read/written. STCrittests.a is where DataBusTest is defined.

To translate the sad mac, it is saying test failed on data lines D8, D9, D10, D11 (0xF00 -> binary results in 111100000000). 4 bits is a common width of a RAM chip, so it sounds like you've got an entire chip incommunicado. Based on schematics, looks like UK4 and UK3 are responsible for those data bits. Looks to me like data bus test only is going to be testing one of these chips, not both, but I am not sure which lies where in address space, so you will want to check both chips as potential issues.

My thoughts on troubleshooting:

1. basics first: make sure VCC and GND are good on each chip
2. R19, R18 are populated
3. Verify R18 has continuity to Pin 17 of UK3
4. Verify R19 continuity to Pin 17 of UK4
5. Check continuity of at the other control lines such as OE, W, RAS.
   1. You should be able to cross check those signals against other RAM chips as they are all connected identically across each RAM chip. So check pin 3 of UK8 against UK4, and pin 3 of UK8 against UK3 - and so on. Any other RAM chip (not UK3 or UK4) should be fine.

Below is excerpt from the datasheet for those chips, because the bomarc schematics are painful to read.

 

[Kouzui]

After running through your suggestions, I found that this leg on UK4 was not actually making a connection to the board, despite looking like it was.

After resoldering it:

And plugging it in:


IT FUCKING WORKS!
Chimed, booted normally. I'm gonna test the ram expansion I have for it and see if I soldered the scsi and floppy connectors properly here in a bit, but it fucking WORKS!

Thanks for the help, everyone who did so.

 

[zigzagjoe]

View attachment 60733
After running through your suggestions, I found that this leg on UK4 was not actually making a connection to the board, despite looking like it was.

After resoldering it:
View attachment 60734
And plugging it in:
View attachment 60735

IT FUCKING WORKS!
Chimed, booted normally. I'm gonna test the ram expansion I have for it and see if I soldered the scsi and floppy connectors properly here in a bit, but it fucking WORKS!

Thanks for the help, everyone who did so.

Nice! Great job sticking to it.

 

[cheesestraws]

Well done!

 

[davidg5678]

Great work! Another Macintosh Classic is back in working order.

I'll have to try putting together one of these replica boards together once more are manufactured!

 

[Kouzui]

Everything works, btw. SCSI, memory expansion, ADB, etc.

Now I just have to wait for the keyboard I ordered to arrive...

 

[Fred1212]

Well done you

 

[chiptripper]

No better feeling than that! Great work.

 

[joshc]

Woo! Nothing better than hearing that chime and seeing everything work after you've spent so long on it. Well done.

 

[Kouzui]

Just wanna say real quick - I was looking at Mac Effect's site, and it seems like they have the reloaded board in stock again:

Macintosh Classic Reloaded Logic Board

Another MacEffects exclusive a Macintosh Classic Reloaded bare logic board (PCB) painstakingly engineered by Kai Robinson. These boards are made with Gold ENIG-RoHS. These are a limited production run. 100% of the proceeds, less selling and administrative costs will go directly to the creator...

maceffects.com

I remembered it saying "sold out" (for the SE too, which seems to also be back in stock). Maybe they made more.

Either way, just letting you guys know in case you want to do what I did.