Monthly Archives: June 2018

Luggable overhaul: Part 1

I had set my Compaq portable up on the table in preference to the Macintosh, with an idea to use it more regularly. It started up okay but made a rather unpleasant burned smell after about twenty minutes so apart it came.

expansion slots

Nothing particular showing up, so lifted all the expansion cards out and then removed the motherboard.

motherboard

There’s a couple tantalum capacitors with cracks in, so they’ve gassed up and are not long for this world. The video output has been a little wobbly since I got the machine, particularly in low resolution mode. It’s all packed in fairly tightly.

vdu and psu

So, out came the VDU. Picture tubes always look so forlorn when they don’t have any cases or bezels fitted.

picture tube

There’s always one part that makes a bid for freedom. Tonight’s candidate was this washer:

washer

The power supply is all plugs, making a note of what goes where as there are quite a lot of them..

power supply plugs

Everything all lifted out quite easily.

all boards

These are the two boards that need the most attention. The power supply (the highest power most compact board designed to date at that time) and the video board, powered by the power supply on the 12V rail.

vdu and psu

The connector was showing signs of high current draw so it’s definitely time for a rebuild. The power supply is marked “upgrade” on the back, so that’s positive.

Cycle rectification

I had pulled all my test equipment out to align my radio again;

test bench

So, I started my bike up and checked the output of the generator with my oscilloscope.

scope on bike

The output was rather spikier than hoped for into the negative, so redoing the rectifier was in order. Possibly the vibration was too much.

circuit

I finished up the latest revision of the regulator board, with a new Zener to bleed off any excess input voltage, a resistance across the input capacitor to bleed off any stored charge, a mechanically better connection to the board and some adjustment pots.