Faulty Plus logic board?

[zeem]

I'm trying to work out what might be wrong with my third Plus, which is now made out of the 'worst' bits now I've swapped everything round to make two other 'nice' ones.

It starts to boot from the floppy, but it never gets past the "Welcome to Macintosh" banner, it just stops. The disk and drive are fine.

It also seems that it won't boot from SCSI. Pressing the interrupt button while the banner's on the screen gives me a prompt, but it doesn't seem to recognise any keyboard input. The mouse port does work.

I've tried rearranging the RAM in the slots, and cleaning the contacts of the RAM and the ROMs. There appears to be some odd localised corrosion around pin 32 of the SCSI chip - the rest of the board looks fine.

Am I just looking at replacing the logic board, or is there something obvious I could do to fix this one? Because this is made out of the worst bits, it's also got a wavy pulsing screen that's too bright, until about 30 minutes of warming up, after which it's fine. And it smells because the mains RFI filter capacitor's exploded, leaving goo on the analogue board (I'll clean this up if I can get it working).

 

[Franklinstein]

May want to clean and/or repair that analog board before you blame the logic board.

Otherwise, run fairly warm water over the logic board while lightly brushing with a medium toothbrush to remove light corrosion or anything else that's not supposed to be there. Then let it dry for a week before inspecting for damage such as broken traces, broken pins, missing stuff, etc. If it doesn't work after a thorough cleaning and any required repairs are performed, then you'd have to go much further in depth to discover the source of the problem.

Or you could set it aside to use as parts. Sad fact is, even solid-state integrated circuits will die, and there's nothing you can do to repair them; the only course of action is to replace the affected chip or the entire assembly, depending on how comfortable you are with sensitive work and if you can find replacement antique microchips.

 

[zeem]

It's not the analogue board, because this faulty logic board started off in another Plus. I swapped the boards over and the fault moved with it. The Plus that this board came out of is now working perfectly.

The analogue board in this one needs some work anyway, because of the pulsing video problem. C1 has expanded, there is also another capacitor looks suspicious. I'd like to fix this up, but there's no point when it doesn't boot!

I've repaired BBC computers to component level, but they're a bit simpler and I had a copy of the service manual.

Otherwise, I'll keep an eye out on eBay for Plus logic boards, preferably in the UK.

 

[equill]

... I'd like to fix this up, but there's no point when it doesn't boot! ...

Are you not putting the the hort before the carse? It may very well be that you need to fix this up in order for it to boot. 'Fixing' includes minute board checks and cleaning, as well as necessary replacement of replaceable components.

RAM: identical SIMMs of the same value in groups of four (so as to allow the possibility of 32-bit operation) in each bank, and speed enough (120ns or faster), with the larger value, or only SIMMs, in Bank A. Only in the case of 17MB total do the larger SIMMs go in Bank B.

Failure to pass the splash screen, about signpost five in the boot process, is indicative of specific faults in the HDD (and mebbe SCSI bus). You need a spic'n'span HDD in the long run, and a working boot floppy disk in the short term. Port-switching and chaining the mouse and kb will soon identify faulty ADB ports and peripherals.

Faulty capacitors must be replaced, preferably with tantalum electrolytics for the SMD units. Caps on the power-sweep board need to be in topnotch condition and spec, and the soldered connections of all components renewed. trag offers handy parcels of replacement caps for the MLB. In that connection, the distilled wisdom about repairs and upgrade from many SE/30 parents is available by searching these forums: far more than I can summarize in a single post.

Welcome to the SE/30 Parents & Friends Association.

de

 

[zeem]

Just an update with something interesting I've found. After finding some old system files in a folder on a 40MB hard drive attached to another Plus, I made up a floppy to try out this old software (I've never run anything earlier than 6 before). Randomly, I decided to try this disk in the faulty Plus.

It booted fine. I added a copy of Kid Pix to the disk, and spent 10 minutes or so playing about with it. The Finder was version 5.3, I'm guessing the System is 3.2.

I think I just need a new logic board for it. Just a guess, but I wonder if someone's plugged a parallel printer into the SCSI port and blown the chip.

Somebody's done something to it recently, because all the letters on the keyboard are covered with white paper stickers. I'm thinking that someone's tried to learn touch-typing with it. The stickers were clean, white and unworn, like they'd just been put on.

The unit also came with a HD20. The interface board and drive had all the internal connecting pins bent. I straightened them out, but the drive wouldn't spin up. I took the lid off to have a look, and the drive was full of water and rust! I think that's only any good as a case to build a SCSI drive in to.

 

[porter]

RAM: identical SIMMs of the same value in groups of four (so as to allow the possibility of 32-bit operation)

On a 16bit 68000?

 

[equill]

Guilty as charged, m'lud. Was still thinking SE/30 while writing about a Plus. I wonder what I wrote in the other post?

de