Computer Lab

Monotech NuXT v2

Created: 2020-07-22

Built around the new XT-compatible motherboard from Monotech and new off-the-shelf case and power supply.

I picked the case because it seemed compact while still potentially having space for a full-length CGA card. It also feels somewhat retro in the right context. I've modified it slightly: I removed the uneeded hard drive cage and front panel ports to free up space. I also replaced the LEDs with red ones to look more vintage and match the floppy drive. (The drive just fits, and only in the top bay, due to the non-modular power supply's cables being right there.) I wrapped the lid of the case in cheap woodgrain contact paper.

I went through three floppy drives to get a working one. The first was advertised as 360k but is some weird non-PC format. The seller sent a replacement, but "accidentally" sent a parts drive! They gave me a refund and let me keep both drives.

I then ordered a third drive that was definitely the correct model and intact, but not tested. First, the spindle motor didn't spin up. I found out a trace on the back of the main PCB had broken. My guess is that it got nicked, then overheated and peeled up. I patched it with a component lead and covered it with Kapton tape. That got it to spin, but it wouldn't read.

The main PCB has the track 0 sensor on it, and the screw holes are elongated for adjustment. I played with the ImageDisk alignment tool and figured out that the drive was reading the wrong tracks. I moved the PCB a bit, and on the next test it passed the test 100%. I was able to read and backup a Prodigy demo disk that I had been using for testing.

Then I tried to format a disk, but MSDOS told me the disk was write-protected.. The write-enable receiver was not seing the write-enable emitter. The emitter works, I can see it on an unfiltered camera. I could only get the receiver to trigger by tipping it way too far off-level. After messing with it for a while, I got the idea that it could be bypassed by opening the circuit. I desoldered one leg of the receiver, and sure enough, the drive can now write disks. Finally, I swapped the black faceplate from the first drive on to the working one to match the case better.

During the process of figuring out the problem with the first drive, I found out I needed to update the BIOS to get 360k drives working correctly. I took a chance and just flashed it in-system. As a small precaution, I flashed it on the half of the rom that comes with the V20 BIOS, using the timer feature of the flashing program. So the old BIOS is still there on the other half. I'd like to get an external programmer so I can customize and extend the BIOS in the future.

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