Building a Replica Acorn System 5 Computer System - Part 2a, The 40 x 25 VDU (Teletext ) Video PCB

This is the 40 character by 25 line text card based on a SAA5050 Teletext chip. It can show normal Teletext characters and semi-graphics.

It's normally found in System 2, System 3 or System 4 systems. But can also be used in a System 5. It's normally used in a system with a 6502 CPU running at 1MHz. I don't yet know if its possible to get it working if the 6502 CPU speed is 2MHz.

As normal, I have started fitting the resistors and the one and only 1N4148 signal diode first.

You may notice that I have not populated all the resistor positions.

Those left out are not needed, as they are optional.

The SAA5050 Teletext chip was designed for use in television sets. It was one of a set of chips and has inputs and outputs that were intended to be connected to the rest of the chips in the set. Plus inputs for use with a remote control.

Most of the unfitted resistors are for use with these unused functions. As are some of the unused link positions.

Next up, the DIL sockets...

  Not as many as on the 6502A CPU card 😊

 

And now for most of the remaining parts (capacitors, preset resistor and transistor).

I'll add the connectors later on.

This board has the pitch for the holes/pads for capacitors spaced at 7.5mm. It's tricky finding small ceramic capacitors in this pitch these days. So for the decoupling capacitors I used axial types. I think they fit quite nicely.

As does the transistor.

 

For the two small value capacitors for the 6MHz oscillator I used 5mm pitch types and bent the leads slightly wider.