Happy New Year!

A project log for Novasaur Retrocomputer

Retrocomputer with serial and video built from only 1978-era TTL logic

Alastair HewittAlastair Hewitt 01/25/2020 at 02:510 Comments

As in the Chinese Lunar New Year! It's remarkably cheap to run prototype PCBs since the design works on a 2-layer board. I decided to ship the Rev. 3 board design last week to get it here before things shut down in China.

There's now four populated boards (2x Rev. 1 boards on top, Rev. 3 and Rev. 2 on the bottom of the picture)

Rev. 3 included a few minor updates to improve the ground planes and power distribution. A bridge rectifier was added and the filter capacitors were increased to handle AC power input. There was also an update to the horizontal control circuit to allow switching between 2 and 3 micro-second H-sync pulses.

The good news is the board worked first time. The bad news was the updated power supply generates too much noise when the components heat up. This is not a surprise though. Trying to put the entire power supply circuit on the same board was really pushing it!

The output filter cap was moved away from the main switching circuit due to space constraints. This adds inductance to the ground return path and increases the switching transients on the buck regulator. This causes sharp 20ns pulses riding on the power lines and some pretty horrifying EMC implications I would imagine. The board starts ok, but then becomes unstable as the thermal drift kicks in.

The Rev. 2 buck converter is working fine though, so the power circuit will be rolled back for the Rev. 4 board. Further testing seems to indicate a 33MHz dot clock is going to be stable and the the hardware abstraction layer is being designed around this (more on that in a later log). The true color video output is greatly improved in terms of supply noise. Not only that, but the video signal gets cleaner as things warm up. There's something strangely satisfying about that in a vacuum tube sort of way.