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 😳

Building a Replica Acorn System 5 Computer System

Earlier this year I came across a website all about the Acorn System computer (a single board computer or SBC) and the rack mounted versions.

The site owner had various replica PCBs available that he had recreated based on the original Acorn versions. Well, I couldn't resist, so I bought a load of them...!

Over the past two weeks I have been ordering parts from various sources. So far I have received parcels from over ten suppliers with more on the way! And that's just the electronic components! Getting everything has turned out to be a bit tricky.

I already have a rack/case that I got second hand some years ago. But I will dig that out of storage later on.

More details to follow at the weekend...

Extracting or removing a DIL chip from a socket

Or indeed, inserting a DIL chip in a socket

Sounds easy right? Unfortunately many people who are trying this for the first time quickly find out it’s not as easy as it sounds.

You have to be careful when trying to removing a socketed DIL chip from it’s socket. The socket grips the chip pins/legs tightly. If you don’t use the correct technique, one end will come out easier that the other, with the chip rotating as it comes out, resulting in the last few pins at one end being badly bent.

You may come across IC removal tools advertised as chip (or IC) extractors or pullers. But most of these when used, still have the problem of resulting in bent pins.

The recommended method, if you can get easy access to both ends of the chip, is to ease it gently out from both ends.

You can use a suitable small to medium size slotted flat bladed screwdriver or blunt knife for this. When using either, the object is to get it between the plastic of the socket and the body of the chip. Do not lever against the PCB, as this may result in damage to the PCB tracks.

Do take anti-static precautions before starting. Professionals will want you to use a anti-static wrist strap, mat etc. But for the most part, doing the following is good enough for most chips used in 1980s computers: wear cotton clothing, slightly moisten your fingers and touch an earthed/grounded metal item before touching or handling any chips. Keep doing this regularly, and always if you have got up to walk elsewhere and have returned.

It’s also good practice to earth/ground metal tools before using them on or near chips.

When handling chips, where possible, try to hold them on their case, and try to avoid touching the pins. Both to reduce the chance of static electricity damage and to avoid contamination of the metal pins with your natural oils.

When holding PCBs, hold by the edges and avoid touching components, the contacts of the connectors or the PCB tracks.

Working alternatively from each end, gently lift the chip a bit at a time. Do this until the middle pins are almost completely free. When the chip is nearly out, keep a finger on each end to help prevent the chip from jumping up and rotating (if it does, at least one pin will be bent).

Once out, if any pins are bent, carefully flatten them and carefully bend them back into shape using flat-nose pliers.

When reinserting DIL chips, ensure that there are no bent or broken pins. Check by looking along the chip from both ends, that all the pins are in line and straight. Check the spacing width wise to see if they will line up with the socket. New chips, as supplied by the manufacturer are likely to have their pins splayed apart at an angle rather than being at a right angle from the package.

There are special tools to align and bend the pins to the correct position, or you can do this manually. You want multiple pieces of paper or a magazine, or similar on your work surface (to protect the surface). Hold the chip by its case body at both ends (use both hands). Turn it so that the body is lengthwise and at a right angle (vertical) to the work surface, now press the row of pins (their sides) into the paper/magazine so that the pins bend slightly inwards towards the chip body. Rocking it while applying pressure helps. But don’t go too mad!

Do the same for the other row of pins. Inspect, then compare against the socket. If you bend the pins too much, you will have to use flat-nose pliers to bend the pins back.

When inserting chips, carefully inspect each and every pin, looking for damage and dirt. As well as the alignment.

If pins are dirty or contaminated, clean first with IPA (not the drink!). An eraser designed to remove ink or a PCB eraser block or a glass fibre cleaning pen may be needed if the pins are really bad or have tarnished.

Line the pins up with the socket, then apply even force across the chip body and push it in. They normally require quite a lot of force, so ensure the PCB is on a flat work surface with a wad of paper or a magazine to protect the surface. If a chip needs very little force, it’s likely that the contacts in the socket are bent or damaged. If the socket contacts don’t make good contact, this will cause various difficult to diagnose problems.

Always check each socketed chip to ensure that they are all fully in their sockets. Check each pin to see if any have been bent and are under the chip or missed the socket.