Fixing Sharp MZ-700

Time to fix the Sharp MZ-700, I have this computer for a while, and it was not working. I decided to fix it. I opened the computer and I found that the power supply was not working correctly. I decided to do the standard procedure, replaced suspects capacitors and voltage regulators. Here are some pictures of the process:

This machine was manufactured in 1982, and it was a very popular computer in Japan. It was used in schools and homes. It has a Z80 CPU, 16KB of RAM, and 16KB of ROM. It has a built-in tape recorder, and it can display 40x24 characters in 8 colors.

C128 RGB Video 80col cable

The C128 seems to be lossing the brightness signal in the Composite output, the video is fine but the colors are not as bright as they should be.

I’ve decided finally do the cable to connect the C128 rgbi output using composite video. The cable is a simple DB9 pin to phono RCA cable, ground to pin 1 and video signal to pin 7.

ftp_server / telnet_server

Warning

These tools are quick-and-dirty solutions, not meant for serious or long-term use. It also serves as a subtle reminder that they should be used cautiously due to their lack of security features :-)

I wanted to have two fast programs to communicate with my old computers, from my linux box, I have three old pcs and I’m using mtcp on them. mTcp includes an ftp client and a telnet client, of course, I could use any of the existing ftp servers or telnet ones, but I don´t want to have more services running, also I thought that would be an opportunity to learn something.

CPM Disk Tool

I’m currently working on implementing a CP/M for my small Z80 CPU. As a reminder, this involves a single Z80 CPU connected to a Raspberry Pi, with the latter emulating RAM, ROM, and IO devices.

I spent this weekend diving into the workings of CP/M disks. While there is a Unix tool available to create CP/M disk images , I’ve decided to create my own version using Python.

I’m restoring this wonderful IBM XT 5150, machine powered up, but there wasn’t os. Floppy drive was`t working, I tried to boot from a 720k floppy disk, but it was impossible. So looking for a solution, I found a ISA-PicoMem (https://github.com/FreddyVRetro/ISA-PicoMEM/tree/main) card, a wonderful card that allows you to boot from a SD card, add memory, and is able to connect to a network using wifi and ne2000 drivers, of course you need install mTCP.

I have a long history with computers, starting with a Sinclair Zx81 that I still own, followed by other models and others that I wished for, as time went by.

A while back, I found an old box with some old processors, x386, x486, a few Pentiums first generation and two Zilog Z80. Then I got a kind of retro-feeling in my mind and I watched some YouTube videos about them. One of them was the classic nop-test, I had fun with this for a whole afternoon.

This morning the Dragon 32 power supply broke. I have a Dragon 32 that I bought one time ago. But when I tried to turn it on, it didn’t work. I checked the power circuit inside of the Dragon 32 and found that the regulator was broken.

It was a LM340T 7812, 12v regulator, so I replaced it with a new one. But when I tried to turn it on, it didn’t work.

I have been repairing the IBM 5150 motherboard.

Tantalum Capacitors are known to fail, :-) This is the famous C56

Almost each memory chip had a tantalum capacitor.

Also ISA slots had tantalum capacitors.

This ones, near the power supply connector, were also tested.

I have been repairing the model m keyboard, but not lucky this time. The membrane is broken and I have to replace it. I have to find a replacement membrane.

I have been repairing the Dragon 32

When I powered it up, the screen was all garbled. I opened it up and found a capacitor that was bulging.

Nude Dragon

Replaced Capacitor

First attempt to fix it was to replace the bulging capacitor. I replaced it with a new one, but after powering it up, the cursor was missing.

Finally I have had to replace most of the capacitors on the board.