Resurrecting a Levco MonsterMac Macintosh 128k

Context

At some point my Dad came into possession of a Mac 128k. I don't remember ever seeing him use it, it just seemed to appear in my parent's basement at some point. I remember trying to power it on 10-15 years ago, saw the "Sad Mac" screen and gave up quickly.

Fast forward to now, I discovered this old nugget again and decided to take a stab at fixing it. It seemed like an interesting device as it had a sticker on the back that says "MonsterMac" and has an extra DB25 connector where the battery should have been. This one was purchased from "Victoria Personal Computers Ltd", a computer store located in Victoria, BC, which is located at the complete opposite end of the country.

What is a MonsterMac?

MonsterMac was a product by the San Diego based Levco Enterprises. It was an addon board that added 1-2MB of RAM to 128k and 512k Macintoshes. It also came with additional optional features such as a SCSI interface, 256K PROM and a piezoelectric fan. Levco went on to create other Mac upgrade products before eventually being sold to Scientific Micro Systems. There is an interesting LA Times article about Levco here.

MacUser Magazine (June 1986, Page 128) contained a half page advertisement for the MonsterMac:

My MonsterMac has a DB25 SCSI interface on the rear where the battery compartment is supposed to be.

This is the Mac 128k motherboard with the MonsterMac 2MXS board sandwiched on top. Note the dwarf inspired chip names... The black wire (overlay) connects to pin 6 on BMU1 IC located at D2. The red wire (clock) connects to pin 2 on LS04 IC located at D4.

This is the Mac 128k motherboard with the MonsterMac board removed.

A closeup of the "Chip Clip". I really hate this clip. It's really hard to line up all those pins when you need to reattach the MonsterMac board to the Macintosh motherboard. My go-to strategy was to raise my standing desk up so I could get a straight on view of the pins so I wouldn't bend any of them.

Troubleshooting

My Mac was sad... at least it wasn't happy according to the "Sad Mac" error code presented when turning it on.

My specific error code was 030201. What did this mean? According to Apple Technical Information Library Article 7748, the error indicated a memory write error on the RAM chip located in position G6. You can find a copy of the technical acticle here. There is a handy coordinate system printed on the PCB that you can use to reference the position of specific ICs.

As a troubleshooting step I desoldered the RAM chips in G6 and G7 and installed sockets in their place. I then swapped the chip from G6 with the one from G7 and powered up the system. The system powered up and provided a new Sad Mac error code of 030401 indicating bad RAM in G7.

With this knowledge, I went on AliExpress and ordered 10 KM4164B-10 RAM chips for $10 shipped. A few weeks later they arrived and I swapped out the bad chip.

I then had another Sad Mac error code of 030001 indicating the RAM chip in position F5 was bad. I desoldered that chip, installed a socket, added a new chip and powered up the system.

It's alive! After realizing I needed external power for my BlueSCSI storage, it booted into MacOS 6.

The system was working more or less now but I noticed every now and then the system wouldn't boot. I measured the 5V rail and it was only at 4.5V. I followed a great video on YouTube on how to adjust the voltage:
https://www.youtube.com/watch?v=My5DJIC61_k

For now, I am more or less done with fixing up the system. I do think a recap is in my future as I couldn't quite hit 5V when doing the adjustments (4.95V) and the system will shut down after being on for 30 minutes or so. The latter is more than likely a dry solder joint I'll get around to fixing someday. To be honest, I'm a bit scared of the analog board and the CRT so I'll likely let it rest for a month or so unplugged before doing any of that work.

SCSI Termination Power Modification

The DB25 SCSI connector on my specific MonsterMac did not provide SCSI termination power on pin 25. I wanted this to power my BlueSCSI device without external power. Fortunately, there is a handy source of 5V next to the ribbon connector on the PCB. I connected the anode of a 1N5817 diode to 5V and the cathode to the pin connected to pin 25 on the DB25 connector.

Note: I'm not sure if there were multiple revisions of these boards. You'll want to double check the connections to confirm 5V is still available from that pad and which pad connects to pin 25 on the DB25 connector.

Additional Items

I dumped the PROMs from the MonsterMac PCB and uploaded them to Archive.org. I have also uploaded a copy here:
MonsterMac 2MXS Rev D PROM dumps

The Mac 128k / Mac 512k have recessed interrupt and reset buttons that you could push using an addon plastic piece that most people have lost. You can find a 3D printable model on Thingiverse. I have archived a copy here in case that disappears. Thanks to Thingiverse user Gael_FR for creating this.

According to the Journal of Washington Apple Pi (April 1986, page 53), if you press the interrupt button during the power up "bong" noise, the system will disable the RAM upgrade and revert to be a 128k / 512k Mac. I am unable to fully confirm this as when I do that, I am unable to boot from SCSI. I do not have any floppy boot disks for this system to confirm the amount of available RAM but it seems like it may work.

Resources

Classic Mac Repair Notes
Macintosh Repair & Upgrade Secrets (1990)
The Dead Mac Scrolls (1992)