Thursday, 7 November 2024

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...

Saturday, 4 November 2023

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.

Friday, 23 August 2019

ZX Spectrum 16K, 48K and Plus models - DC-DC CONVERTER MODIFICATION


ZX Spectrum 16K, 48K and Plus models

DC-DC CONVERTER MODIFICATION

Updated and corrected by 1024MAK. Version 7E3823_02.

This modification and upgrade is only needed on ZX Spectrum boards that are issue 4a or earlier where it has not already been performed.

The modification results in improved converter operation, prevents loss of the -5V supply if the input voltage of the DC supply to the computer rises above 11.9V and improves the voltage on the -5V power rail.

The following new materials are required:

Diodes:
D15 - BA157 (it may be possible to reuse the existing D15)
D17 - BA157
D18 - 1N4148
D19 - BZY88C 5V1 500mW 5.1V Zener

Capacitors:
C46 - 1uF 50V WKG 105°C Electrolytic or ceramic axial (only required on issue 1 and issue 2 boards)
C47 - 1uF 16V WKG Electrolytic axial or ceramic axial
C49 - 560pF ceramic axial (if not already this value)
C74 - 4.7uF 16V WKG Electrolytic radial (if not already fitted)
C78/C80 - 22uF 25V WKG Electrolytic axial
C79 - 1uF 16V WKG Electrolytic axial or ceramic axial

Resistors:
R60 - 220Ω 5%, 0.25W
R79 - 2.2kΩ (2k2) 5%, 0.25W

Remove the following components from the printed circuit board:
D12- 1N4148 Diode
D16 - 5.1V Zener
C47 - 22uF Electrolytic
C49 - If not already a 560pF ceramic axial
C77 - various values used
R54 - 100kΩ
R55 - 56Ω
R60 - 270Ω, various values used

Embody the modification as follows:

Remove the anode of D15 from the board.
Connect C78/C80 between the former anode PCB pad for D15 (the PCB track that connects to C49) and the now free end of D15 anode, positive end of C78/C80 to the PCB pad.
Connect D17 between ground (anode end of D17) and the new connection of C78/C80 to D15 anode.



Substitute components as follows:
Remove Substitute New Value
D12 R79 2.2kΩ 5%, 0.25W
D16 C79 (positive to ground) 1uF 16V
C46 (only issue 1 & 2 PCBs) C46 1uF 50V 105°C
C47 C47 1uF 16V
C49 C49 560pF
C77 None Open circuit
R54 D19 (cathode to ground) 5.1V Zener
R55 D18 (cathode to C46) 1N4148
R60 R60 220Ω



Install C74 if not already fitted between R58 lead (junction with R58/R59/TR5 base) and C34 positive lead (+5V connection) with C74 positive to C34 positive lead.


Circuit Operation
The modification is confined to adding/changing various components in the DC/DC converter circuit to improve reliability. Circuit function does not change.


Please take the notes above into account when reading the above diagram.

Sunday, 2 August 2015

Modifying a Sinclair ZX81 issue 1 - RAM

Modifying a Sinclair ZX81 issue 1

Upgrade the internal RAM to 16k bytes


This entry shows how I modified my issue 1 board ZX81. The pictures below show the steps I took.



The standard ZX81 as supplied by Sinclair Research Ltd only has 1k bytes of RAM (Random Access Memory). This is in the form of either a single 8 bit by 1024 bit (1k byte) SRAM chip (IC4, 4118) or two 4 bit by 1024 bit SRAM chips (IC4a and IC4b, both 2114 type). SRAM means Static Random Access Memory.

Now, if you are very careful, you can do useful things with only 1k byte of RAM. But you can do so much more with 16k bytes of RAM.

Back in the ZX81s heyday, the normal solution was to buy a memory (RAM) expansion and plug it into the expansion port (edge-connector) at the rear of the ZX81.

And you can still obtain used examples of the various different makes on the second hand market.

But some designs do suffer from RAM pack wobble. That is, a slight movement of the ZX81 and the plug-in memory sometimes causes a momentary break in one or more of the edge-connector contacts and your program either becomes corrupt or the ZX81 resets and deletes your program :-(

However, now that technology has moved on, you can get a single chip that can provide 16k bytes of RAM. And it is not that difficult to fit it to a ZX81 PCB (Printed Circuit Board).

You do however need some skills. Requirements are: able to de-solder, solder and to have a steady hand when cutting the PCB tracks. I used a hand held PCB 12V drill with a cutting blade fitted.

Before proceeding, it is wise to confirm that your ZX81 does in fact work. Then you know the condition and status of it before you work on it.

Once you have opened the ZX81 case by unscrewing all the screws (including the hidden ones under three of the rubber feet), you can unscrew the small screws that hold the PCB to the case.

Be very gentle with the keyboard membrane tails, as original membranes go brittle. Then any crack that forms will break the electrical connections that form the wires and some or all of the "keys" on your keyboard will no longer work.

In fact, it is likely that the cracks have already started long before you get a screwdriver to open the case. So buy a new keyboard membrane ;-)

Once the PCB is free of the case and the membrane, have a good look and see if there are any problems. For example dry solder joints or poorly soldered joints. Resolder any that look suspect [unless they are on the RAM chip(s) IC4/IC4a/IC4b in which case this/these are being removed].

If you are very good at desoldering, now is the time to put this skill to the test. Desolder the RAM chip(s) IC4/IC4a/IC4b. Be very careful not to damage the PCB tracks (traces) and pads. Although lifting a pad is not terminal, it does mean you have to repair the damage.

If however, you are less experienced, as it is unlikely you will need to reuse the old RAM chips, instead use some electronic side cutters to cut each pin (leg) of the RAM chip(s) at the bend where it goes into the plastic case of the chip. Then you can de-solder one pin at a time. This is far easier.

Top of PCB after old RAM de-soldered
Bottom of PCB after old RAM de-soldered

Top of PCB after tracks cut

Top of PCB after tracks cut - close-up

Bottom of PCB after tracks cut - see circled areas

Bottom of PCB after holes de-soldered

Top of PCB after holes de-soldered

Bottom of PCB showing signal details

Bottom of PCB showing wiring

Top of PCB showing wiring

Top of PCB showing SRAM socket

Top of PCB showing SRAM socket and wiring

Top of PCB showing temp +5V wire link (the uninsulated wire)

Top of PCB showing link L2

Top view, finished 16k RAM

The test program I used

Passed test okay

Saturday, 29 March 2014

Modifying a Logic3 Arcade Controller designed for a Nintendo Wii to work as a Joystick on Retro Computers


A while ago I bought a Logic3 Arcade Controller designed for a Nintendo Wii from eBay for a reasonable price.


Having finally got around to opening it up and looking at the internal wiring, an idea formed in my head.

You see, there are a number of different joystick "standards" used on retro computers. The most common connection is a digital joystick known after the manufacturer that used it, now known as the Atari 9 pin "standard".

But some other manufacturers used either different connections (Amstrad with their Sinclair +2, +2A, +2B and +3) or instead used a analogue system. Acorn fitted an analogue system that used a 15 way D socket to their BBC B computer (also fitted to the successor 8 bit "BBC" models) and to the Plus 1, an expansion for their Electron computer.

So, could a circuit be put together so that a single modified Logic3 Arcade Controller could be used on a computer or games console with an Atari 9 pin D compatible connector, a Sinclair +2, +2A, +2B or +3 and an Acorn BBC or Electron computer fitted with a analogue 15 way D socket?

Well, not at first sight. But...
If a 25 way D connector (of which I have a few spare) was fitted to the Logic3 Arcade Controller, I could make up adaptor leads :-)

So on went the thinking cap...


Tuesday, 8 January 2013

Intro

This is an occasional blog of work I do on 8 bit and 16 bit computers.

It will give details of both repairs, maintenance and modifications that I sometimes carry out.

I will sometimes include peripherals and interfaces.

I hope you enjoy reading.

I also sometime post in the following forums:-