Building a Replica Acorn System 5 Computer System - Part 3a, The 80 x 25 VDU Video PCB

This is the Acorn 80 character by 25 line text card based on a 6845 type Cathode Ray Tube Controller (CRTC) chip. It provides all the timing and address signals.

The card is provided with 2K bytes of on-board RAM that stores the ASCII text to be displayed.

The pixel data for the characters is contained in an EPROM. It is text only so has no graphics capability.

Acorn intended it for use in "professional" applications.

It's normally found in System 5s. But can also be used in a System 3 or System 4.

This is another PCB from Chris Oddy. On this version he has made a minor change. It incorporates additional circuitry to correct the Cursor and Dot Enable signals.

As normal, I have started by fitting the 1N4148 signal diodes and the resistors first.

 Next up, the DIL sockets.

 

You may have spotted that I missed off one socket. Also one resistor is missing! I was building this card while attending an Acorn user meet-up called ABug South, that was being held at at The Ely, near, Blackwater, Camberley.

Needless to say, I had to bring all the parts with me, but I did not bring enough 82 ohm resistors. I'll correct this when I do some more work at home.

Well, I remembered ordering various tubes of DIL sockets from Rapid Electronics. But when I checked in my box of parts from Rapid, I found I had ordered 24 pin 0.3" sockets not the 24 pin 0.6" type that I needed...😞

Darn, another part that will have to wait until I get home. Hopefully I have some in stock.

But a little later, while I was looking through the bag of parts that I bought from ESR Electronic Components Ltd for another part (some ceramic capacitors), I discovered that I had ordered some 24 pin 0.6" sockets from them. Panic over 😂

Having added the 24 pin 0.6" socket, I continued by fitted the various capacitors, the single 2N2369A transistor and the pin headers for the various selection links.

 

The only bit that I find fiddly, is the two pin headers. It's just so easy to solder them in, then turn the board over and find that the one you have just done is wonky and leaning...

 

You may notice that I have not fitted the two presets (variable resistors) or some of the capacitors. That's because these form part of two "local" oscillators used for timing. I don't need these "local" oscillators because I'm using an Acorn 6502A CPU card. This version of the CPU card provides all the clock signals that this card needs, hence no need for any on-board "local" oscillators.

Time to dig into another box and grab the DIN 41612 AB 64-way connectors.

With that connector fitted, that's it for today. More soon.

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 😳