Mac 512k - bars leading to gobbledygook!

[em536716]

Hello all

recently picked a joblot of early 68k macs. 128/512/plus! To save time, I have resurrected the analog board from the Mac 512 along with its motherboard for a play.

issues appear to be when the computer starts up, a pop from the speaker is audible, and then junk on the screen. I will attach a video link demonstrating the issues. Sometimes I then get more periodic pops from the speaker.

so far I have gone over the analog board and checked everything I possibly can. Solder joints ok, I have measured and analysed the D.C. outputs. Nice clean DC, pretty much on the money on all rails.

I have cleaned connectors, reseated Rom chips and so on - no change.

the dead Mac scrolls do not mention such a fault.
I’m personally not suspecting caps as I’ve checked those on the logic board and appear to not give off terrible ESR and read within 10 percent of the values. I’ve probed

all voltage rails on the scope and confirmed no ripple.

I’ve heard people say these early ones have bad ram - how would one check that then?

thanks
Ed

 

[Nixontheknight]

There is a chip tester you can buy that should be able to test the RAM because I think it's all standard ram chips in the 128k and 512k

 

[bibilit]

more a Rom issue than Ram one.

 

[dochilli]

ROMs can be burned into 27512 Eproms.
You can download 512 Roms

Create Macintosh Plus ROMs

Create Macintosh Plus ROMs: This Instructable will guide you through the process of "ripping" EPROM images from from your Macintosh Plus ROM chips and (or) "burning" the images to new chips. The process will basically be performed twice to create both "hi" and "lo" ROM chips f…

www.instructables.com

All Macintosh ROMs (68K + PPC) - Macintosh Repository

This is an archive containing all of the most popular Macintosh models ROM files for emulation purposes, ranging from the first 64K ROM from the...

www.macintoshrepository.org

 

[dochilli]

AFAIK the plus ROMS will fit into the 512k board. Perhaps you can use them for a test.

 

[cheesestraws]

I’ve heard people say these early ones have bad ram - how would one check that then?

The 128 (and I think the 512) have RAM that is notorious for disintegrating, it's true. I had to replace quite a bit of it in my 128, and made a bit of a pig's ear of doing so.

That said, I think to get to that kind of failure, nearly all the RAM would have to be defective at the same time, which seems unlikely. I'd start with the ROM first, especially because...

AFAIK the plus ROMS will fit into the 512k board. Perhaps you can use them for a test.

Yes, they can—try swapping the Plus ROMs in before you start mucking about with RAM. The upgrade from a 512k to a 512ke back in the day was basically just the Plus ROMs and a new floppy drive.

 

[em536716]

Thanks for your replies everyone.

for context, 3 early compacts were purchased from a rather damp, horrid environment. Thankfully the owner not only had all the literature to go along with them, but nice add ons too! External drives, dust covers, and one of those swivel stands!
Also excited to know I have a 128k too!

anyway, I will try swapping ROMs with a plus rom as suggested. Wish me luck.

 

[em536716]

Sorry for the poor photo and shutter speed hoz line across the screen but this is the result. So what type of fault could this mean? This does appear on the dead Mac scrolls and indicates a bad. Now would this indicate there’s something iffy In the startup circuit?

 

[em536716]

Sorry for the poor photo and shutter speed hoz line across the screen but this is the result. So what type of fault could this mean? This does appear on the dead Mac scrolls and indicates a bad. Now would this indicate there’s something iffy In the startup circuit?

Oops apparently I can’t type!

this fault is mentioned in the Mac scrolls but only states that the ROMs could be the wrong way around, backwards and so on... so, what now? Are these ROMs faulty?

I’m almost certain this is part of the startup procedure on the Mac, perhaps it’s hanging

 

[Nixontheknight]

Oops apparently I can’t type!

this fault is mentioned in the Mac scrolls but only states that the ROMs could be the wrong way around, backwards and so on... so, what now? Are these ROMs faulty?

I’m almost certain this is part of the startup procedure on the Mac, perhaps it’s hanging

it most certainly is true that the roms are failing, I'd recommend replacing the roms with the Mac Rom-inator by Big mess of wires

 

[em536716]

it most certainly is true that the roms are failing, I'd recommend replacing the roms with the Mac Rom-inator by Big mess of wires

Is there a way I can confirm that the ROMs are bad in my case.

 

[Nixontheknight]

Is there a way I can confirm that the ROMs are bad in my case.

Take out the old roms and put them in an eeprom programmer and read them, if they read all or mostly Fs or 0s, that’s a dead rom

 

[em536716]

Take out the old roms and put them in an eeprom programmer and read them, if they read all or mostly Fs or 0s, that’s a dead rom

Unfortunately I don’t have one of those. I might have to send the chips to someone who has!

 

[dochilli]

You can read the rom with an eprom programmer and compare them to roms donwloaded from macintoshrepository. Where do you live?