WTB - Macintosh SE "ROM High"

[PS2it]

For the first time, I found a vintage computer at a garage sale. It was a Macintosh SE dual 800k floppy. Upon boot, it was giving me a checkerboard pattern and an interesting buzz from the speaker. While swapping the ROMs from my other SE's I ended up finding the "ROM High" chip seemed to be bad.

It's probably a long shot, and I could probably buy a replacement board, but I figured I'd save some money by checking here first. If anyone has a junk board or something they could steal one off of that'd be great.

Let me know!

Thanks,
PS2it.

 

[clh333]

Upon boot, it was giving me a checkerboard pattern and an interesting buzz from the speaker.
PS2it.

Odd sounds from the speaker and / or display issues usually suggest a problem with motherboard capacitors - a common problem with Macs of this era. Before going after another ROM you might want to rule that out first.

-CH-

 

[PS2it]

usually suggest a problem with motherboard capacitors - a common problem with Macs of this era. Before going after another ROM you might want to rule that out first.

Thanks for the reply!

I didn't think that was a common problem with the regular SEs as they didn't have the SMD caps like the later Macs (SE/30, Mac II, etc) did.

I'll try seating the "bad" ROM in a different board and seeing if I get anything new.

 

[PS2it]

Well, I tried the ROM in another working SE and got those same lines.

Still looking!

 

[PS2it]

Bump.

 

[ldkraemer]

Masked ROM Replacement

Masked ROM Replacement

From the little bit of searching I did on the MAC SE, It has a ROM that is 28 Pin's and it uses A15 on Pin 22. The
Schematic is confusing because it has the ROM socket labeled as A{0..15} while the address lines are A{1..17}.
That means A{0..15} is 16 actual address lines. 2 ^ x = 2 ^ 16 = 65536 bytes of 8 bits for the OEM Masked ROM.

So, a 27512 EPROM should do the trick.

Motherboard MASK ROM Socket:
10..A00............Vcc..28
9....A01............O0...11
8....A02............O1...12
7....A03............O2...13
6....A04............O3...15
5....A05............O4...16
4....A06............O5...17
3....A07............O6...18
25..A08............O7...19
24..A09............CE*..20
21..A10............OE*..22
23..A11............A15..1
2....A12............A14..27
26..A13............GND.14

I've located the "B2E362A8 - Macintosh SE.rom" file, but need to know if the ROMS are split EVEN/ODD,
or just HIGH Bytes & LOW Bytes. (I've no clue on the MAC's.)

The Linux xdd command will strip out the Even or Odd Bytes very easy.

EVEN BYTES:

xxd -p -c 2 -s + -l 65536 mac-se.rom | cut -b 1-2 | xxd -r -p >ev-rom.rom

ODD BYTES:

xxd -p -c 2 -s + -l 65536 mac-se.rom | cut -b 3-4 | xxd -r -p >odd-rom.rom

Then all you need to do is to program the EPROM and replace the defective MASKED ROM after
making a couple of changes for the following Pins.

EPROM Pin 20 (CE*) needs to be bent slightly at the EPROM so it doesn't go into the Motherboard socket. Then it needs to
be tied to GND (LOW).

EPROM Pin 22 (OE*) needs to be bent slightly at the EPROM so it doesn't go into the Motherboard socket, then tied to the
Motherboard's Socket U60 Pin 20 (CE*).

It should work fine.


Thanks.

Larry