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. 

 

Building a Replica Acorn System 5 Computer System - Part 1b, The 6502A PCB

 Soldering components continued.

The vertically mounted resistor now all in. The link headers fitted. The 20 way IDC latched header fitted. And the electrolytic capacitor used in the reset circuit also fitted.

So that's nearly all the soldered components fitted. Only the main connector for the backplane left outstanding. I need to dig through a different box of parts to find those...

 

You may spot that I wired up and soldered the link wires for the EPROM configuration (LK9 on the PCB silk screen).

Building a Replica Acorn System 5 Computer System - Part 1a, The 6502A PCB

Finally after the long process of buying all the parts, well, all the electronic parts at least, construction has started 😀.

Here I have started fitting mostly some of the passive components, such as resistors and the one and only 1N4148 signal diode to the 6502A processor board.

The resistors used here are all 250mW or 600mW metal film types. A mix of 5% tolerance types (gold band) and 1% tolerance types (brown band). Most of which I keep in stock.

I normally start with the lowest profile (height) components and work my way through the various sizes finishing with the tallest last.

Progress:

 

Next I moved on to fitting the various 100nF ceramic "decoupling" capacitors. I normally use 100nF even if the original design lists 33nF or 47nF because I buy the 100nF in hundreds and keep in my own stock. The sightly higher value is fine. And modern multilayer ceramic types are smaller than the older disc capacitors. Most modern designs use 100nF as standard.

As I was using a low profile 24MHz crystal, that went in next. Then the resistor network (the long black thing on the right). This is eight resistors connected to a common pin.

 

 

Then it was on to all the zillions of DIL sockets. Okay, not quite that many... But this is one of the most time consuming parts of construction as there are lots of solder joints to be made.

I also fitted the 22µF axial electrolytic capacitor (on the far right of the board). I had bought two different lots, from different suppliers, as here the issue was the physical size.  I selected the smaller of the two types, but it still only just fitted.

 More tomorrow 😉

 

Building a Replica Acorn System 5 Computer System - The PCBs

 The PCBs arrived 😀

 

 Well, okay, they actually arrived last year, but I forgot to post the photos 😳