512k M0001ED sad mac 01743F - caps??

[RickNel]

Hi all,

I've been window-shopping here for a while and read a lot of threads, but now that I'm registered I can finally pose my questions.

So far, the 512 ED that I found recently stops booting with 01743F sad mac code, with underline pixels cycling below the characters "74". The Apple information asserts that 01 prefix indicates ROM read failure, and that any following digits are "insignificant". Since those digits appear with complete consistency, they cannot be random and therefore must indicate something. Supposedly, 01 prefix is not a RAM error.

The other thing I have noticed is that any thread I have found asking about sad mac 01..., on this or any other forum, seems to get 0 replies or a redundant quote from the Apple user guide.

I'm planning to replace electrolytic caps on the logic board. The values are not listed in the CAPS! threads here. In the past I replaced a lot of failed or exploded axial tantalums on S-100 boards from the 80s, so I may do the same for the axial tants on the 512ked.

Should I also be looking at the analog board? The display and PSU appear to be functioning normally.

Any pointers or advice appreciated, thanks.

Rick

 

[aurel6814]

I have two 128k boards with other problems than ram issues. And as you, I couldn'd find an answer on the internet. http://68kmla.org/forums/viewtopic.php?f=7&t=16269

You can try the caps (but there is just two or three electrolytics).

I have a book about repairing these boards and it talks about replacing the 6522...

The problem is the components on these boards are so difficult to unsold (without ruining the board...) . I didn't manage to do it.

I think you're analog board is OK.

 

[bbraun]

In the early mac ROM (128k and earlier, which displayed 6 hex digits for sadmacs instead of the two rows of 8 digits like later macs), the first 2 characters indicate the test being performed. The second 4 characters are information about the test being performed. For those following along at home, for 128k ROMS with checksum 4D1F8172, address 0x15C is where the printing of the sadmac error code happens. The bottom byte of register A4 contains the test being performed (1st 2 digits) and the bottom word of register D1 contains the last 4 digits.

Test 1 is the ROM test (stored in A4), this starts at location 0xD76 of the same ROM. Register D1 is used for computing the checksum. If the checksum is computed correctly, it moves to the next test, incrementing A4.

In summary, this appears to be what the Apple docs say: ROM failure. The lower 4 characters really are "insignificant" as they are just book keeping data from the checksum calculation. My suggestion would be to try replacing the ROMs with ones from a known good unit. I have personally used 27C512 EPROMs in place of the stock ROM chips. De-interleaving your ROM image into the 2 separate high and low ROM chips would be an exercise left to the reader.

 

[RickNel]

Thanks bb, that is helpful.

I have been able to read the two ROM chips into hex files, by identifying them as 27C512s.

Can I use those dumps to verify against a dump from a known good ROM set?

Are standard ROM dumps available for comparison and/or burning to EPROM?

I know so little about the particular machine at the moment, I am still wondering whether some other hardware issue might be stopping the ROM read, such as caps or address-handling chips.

Rick

 

[RickNel]

Aurel,

I read your other thread. The code beginning "02" is supposed to indicate a RAM error, as you are aware.

It seems that your problem is not centred in the ROM, though it may relate to other functional chips that are necessary to the reading of the ROM as well as the RAM. I plan to check that out thoroughly when I get enough information.

What is the book that you mention, about repairing 128k Macs?

Rick

 

[aurel6814]

-Macintosh, Repair and Upgrade secrets

-The Dead Mac Scrolls

Both by Larry Pina

 

[RickNel]

I've bought a copy of Dead Mac Scrolls and waiting for delivery.

The "Repair and Upgrade Secrets" seems harder to find, more expensive and fewer pages. Is there stuff in it that I won't find in the Scrolls?

Meantime, by swapping boards and ROMs with a Plus, I'm confident that the 512ke ROM(s) are corrupted, but the analog and logic boards are otherwise functioning OK. Now planning to replace with EPROMs (see other thread)

Rick

 

[JDW]

Regardless, you need to swap out those electrolytic capacitors as soon as possible. They have definitely leaked and that leaked fluid is slowly chewing on your board. I speak from firsthand experience as a fellow Mac 512k owner on this. It's much easier to swap caps on a 128k/512k board than doing the same on an SE/30, let me tell you!

Here's my 512k board:




You can see the 3 caps that need replacing from looking at my 128k motherboard photos here:

 

[RickNel]

Thanks for the pics. I have replaced those caps. I'm sure it was a good idea anyway, but it didn't solve the ROM problem.

I've proven the logic board works OK by swapping in ROM chips from a Plus - it boots fine.

Now just waiting for blank EPROMs to arrive so I can copy the Plus ROMs for this board.

Then I suppose I should replace the electro caps on my Plus board - though it is working fine so far.

Rick

 

[JDW]

Capacitor replacement is often most needed on the logic boards, but you shouldn't restrict yourself to that alone. In my case, I've never made the time to swap out all the caps on my analog boards, but I have encountered some keyboards that locked up the computer due to leaked caps. For example, the IIgs keyboard (beloved by SE/30 owners) has electrolytic caps that often need replacement.

Sounds like your main problem is not the caps, but replacing them is important. Hopefully the leaked fluid didn't eat through any traces. But the only way to test that is with schematics and a digital multimeter in Continuity Check mode.

 

[RickNel]

There were no signs of trace corrosion anywhere on the logic board. I had cleaned it thoroughly and inspected with magnifying glass. A little bit of green fur on the negative side tag of one of the caps was the only hint of leakage. I think I caught it before any damage.

Still a puzzle to me how the ROMS would have deteriorated - they should be quite stable. Can only guess that a spike of some sort got into the logic boards. I'd be interested to know if anyone knows what causes ROM failure.

Rick

 

[Trash80toHP_Mini]

Welcome aboard. :beige: