Repair and restoration of retro consoles, 8 bit computers. In this blog I hope to show you how to repair, retrore and modify old consoles and handhelds, from manufacturers such as Atari Amstrad Acorn Sinclair Commodore Dragon Sony MSX BBC Playstation Sega Microsoft xbox Spectrum ZX81 Vic 20 VIC20 C64 Amiga Binatone Grandstand.
Apologies for the delay with this entry; varied reasons include school holidays, two birthdays and some minor repairs of some retro console birthday gifts! Please feel free to use my PCB and 3D printed housing designs found on my Github and Thingiverse should you wish to make your own joystick adapter. Here is the TI994a working with the FlashRom99 and my Joystick Adapter circuit. Here are all the components required for this mod. The gerber files for my PCB design can be found on my Github. https://github.com/Alleged-Geek/TI99-4a-Joystick-Adapter/ And you are looking for the zip file named below if you are going to send the gerber file off to be manufactured. ti99-4a-adapter_2022-07-27.zip My github also contains stl files for printing the housing. And here is my version of the schematic diagram. You will need 10 X IN4148 signal diodes which can be found on eBay. 1 X DB9 D-SUB 9 Pin Female connector, again found on eBay. 2 X DB9 D-SUB 9 Pin Male connector al...
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ZX81 Composite Video Mod and internal 16K Ram Mod - Part 1
Populating
the Composite Video Mod PCB
Following on from the success of the video modification
I did to the ZX81 won at auction, I decided to do the same to my original ZX81.
As this is my second ZX81, I decided to have more fun with it as well and
install an internal 16k Ram chip. Both modifications are fully reversible and
no destructive modifications are done to either the circuit board or the
computer casing.
This entry will go into more detail about the video mod
and will include a parts list, PCB designs (which can be found on my Github
site) as well as a component by component population of the video modification
PCB. I will also add a video of the whole process at the bottom of this entry.
So let’s make a start
The Composite Video Modification.
I would like to thank JoulesperCoulomb from zx.zigg.net
for supplying the schematic for the circuit.
Although the original PCB I designed worked
flawlessly, I decided to tweak the design a little and order some circuit
boards from JLPCB as I wanted a professional finish to the circuit board.
Below are the pictures of my updated design.
The above schematic is actually MK3 PCB, in this
schematic I added C5 in parallel with C3 as I was having trouble finding 820pF
ceramic disc capacitors. I actually ended up using a 680PF and a 220pF which
when put in parallel gave me 900pF (I really need to replenish my ceramic
capacitors stock) a 680 and 150pF would have been the ideal choice!
Below is a picture of my Eagle board layout
Below is a parts list and ordering information from
www.rapidonline for the circuit board (There are enough components for five
circuit boards as the minimum order of PCB’s from JLPCB is 5
10 X 81-0256 - CDIL 2N2222A NPN Transistor
General Purpose RF
@ £0.457 each
5 X 11-0230 - Forever RB 22U 25V 85°C Radial Lead Aluminium
Electrolytic Capacitor@
£0.041 each
10 X 73-0077 - Royal Ohm CFR01SJ0103A10 10K 5% 1W Axial Carbon Film
Resistor@ £0.098
each
5 X 73-0062 - Royal Ohm CFR01SJ0680A10 68R 5% 1W Axial Carbon Film
Resistor@ £0.098
each
5 X 73-0114 - Royal Ohm CFR01SJ0560A10 56R 5% 1W Axial Carbon Film
Resistor@ £0.098
each
to JLPCB and then you will
get the following images from their gerber viewer
This is an image of the
component side which has all the values and relevant information printed as a
part of the silkscreen
Below is an image of the
solder side
After a couple of weeks, the circuit boards should turn up and look
something like this
Note that there is only one space for a capacitor on the bottom right of
the PCB; I redesigned the board so that you can put two capacitors in parallel
to get the required value of 820pF.
Now let’s have a look at the actual components and tooling required to
do this mod.
Before soldering please ensure that you are wearing suitable PPE; I
recommend safety glasses and a well ventilated room.
To make things easier for you when soldering, start with the smallest
(in height) components then populate up to the tallest component last.
Pick up one of the 1N4148 signal diodes then bend the legs at 90° as
shown in the picture below
The black stripe on the diode indicates that,
that is the cathode side of the diode; it is important that you get this the
right way round in the circuit. Below is a picture of the correct positioning
of the first signal diode.
Holding the diode flat against the component side, turn the PCB over
then bend each leg out by around 45° (this prevents the component from slipping
down from the component side)
Then solder each leg
When soldering, place the very tip of your soldering iron on to both the
component leg and silver pad (via) to heat up the parts so that they are both warm
enough to melt solder; I recommend about 3 seconds, then feed the solder on to
the opposite side of the tip of your soldering iron and component leg and it
should simply melt.
It is worth noting that too much solder will result in a blobby solder
joint and too little could result in gaps between the component leg and via.
What you are looking for is a nice curve (all the way round) up from the
via to the component leg. Once you have fed enough solder on to the joint,
remove the solder whilst keeping the soldering iron on the component leg and
via (for 2 – 3 seconds) then quickly remove the tip from the solder joint. If
you remove it slowly you could end up with a jagged edge on your solder joint.
Using your cutters, trim both of the surplus component legs off the
circuit board. Try to trim the legs just above where the solder stops as
cutting into the solder will result in a weak solder joint.
Now pick up D2 and bend the legs as before with your pliers
The cathode of D2 is facing towards the middle of the circuit board as
shown below
It is the same soldering process; press the diode flat against the
component side, bend the legs out 45°, solder then trim
Now pick up R1 which is a 10kΩ resistor
The colour codes are Brown, Black, Orange, Gold
And place it in the R1 position on the circuit board; it does not matter
which way round the resister goes but I like all the gold bands to face the
same way which makes it easier for me to read the values of the resistors in
circuit.
Now solder and trim the legs
R2 is next and that is another 10kΩ resistor (Brown, Black, Orange, Gold)
Place it on the circuit board then solder
R3 is a 68Ω (68r) resistor and the colour codes for that is (Blue, Grey,
Brown, Gold)
Place it in the circuit then solder
The last resistor is a 56Ω (56r) and the colour
codes for that are (Green, Blue, Brown, Gold) place it in the circuit then
solder.
Next up is the 82pF ceramic disc capacitor; for this type of capacitor
it does not matter which way round it is soldered into the circuit but I like
to place it so that I can easily read the value printed on it in this case the
82pF capacitor is C4 so I soldered it in place with the writing pointing
outside the circuit board.
For placement, it is the same as the diodes and resistors; try and get
it as flat against the component side of the PCB as possible, bend the legs out
by 45° then solder.
Below is a picture of the actual component with markings (82)
Which tells me that it is in fact an 82pF capacitor
Now solder it into the circuit then trim the legs.
Now we come to a tricky part; I couldn’t find an 820pF capacitor from my
supplier so I had to improvise. What I ended up doing here was to solder two
capacitors in parallel which will be easier in the PCB design I have uploaded
to Github as there is space for two capacitors. What I ended up doing was to
solder another capacitor on the vias of C3 on the solder side of the PCB.
I looked for the two values I recommended earlier but didn’t have them
in stock so I opted to use 220 and 680pF
capacitors in parallel, simply add the numbers together and that will give you
the new value of the capacitor in this case it will be 900pF (a wee bit high
but it should still work ok)
Look at the markings on the picture below
although very faint it says that it is a 221 capacitor which means that it is a
220pF capacitor.
Like the other ceramic disc capacitor, it does
not matter which way this goes round on the circuit board but again, I like to
be able to read the values once soldered on to the circuit board so I opted to
have the printed value facing outwards as you can hopefully see in the picture
below.
And here is a top view of the capacitor soldered into the circuit.
Now for the tricky part; soldering the other C3 in parallel. For this
capacitor I am using a 680pF capacitor which has the markings 680 on it as you
can see in the picture below. Isn’t it strange that some manufacturers put the
actual value of the capacitor on and some use the capacitor value code like the
220pF capacitor above?
Bend the legs out 90° as shown below.
Now we want to solder the capacitor on the solder side of the circuit
board on the same via’s (pads) as the ones we have just soldered C3 (220pF) on
to. Place the capacitor on the circuit board as shown below.
I then tinned the legs (added solder) of the capacitor at the bend on
each leg which will make soldering a lot easier then solder one leg on to one
of the via’s.
Now solder one of the legs on to either of the via’s; again note that I
have opted to have the printed value visible.
Solder the second leg on to the other via then
carefully trim the legs of the capacitor.
Now we are going to solder in the IC socket for the 8 pin 555 Timer
chip. If you look closely at the socket, you will notice a notch cut out of the
plastic in the middle of the plastic housing; this tells you that pin 1 is to
the top left hand side of the socket.
It is really important to solder this in the correct way as placing the
555 timer chip in the wrong way round will almost certainly destroy the chip
when powered up. It is also worth noting that you do not necessarily need to
use a socket and you can solder the chip directly on to the circuit board.
Now let’s have a look at the PCB; highlighted is the silkscreen outline
of the 555 chip and the corresponding notch.
Place the socket on to the PCB then turn the PCB over to solder. A wee
tip here is to solder opposite corner pins then check if the IC socket is lying
flat against the component side of the PCB. If not, simply hold the IC socket
with your finger, heat the corner pin which may be raised with your soldering
iron then push the IC socket until it is lying flat.
Once happy that the socket is seated correctly, solder the remaining
pins
The next components to be soldered are Q1 and Q2; they
are 2N2222 NPN transistors. There are three pins on these components, a
Collector, Base and Emitter. The emitter can be found by the tag highlighted in
the pictures below.
Below is a picture of the underside of the 2N2222; I
have highlighted the emitter tag
Place
the component into the circuit; a wee note here, don’t push the component flat
against the PCB, and ensure that there is about 5 mm clearance from the
component side to the bottom part of the aluminium casing of the transistor. Then
solder the transistor in place.
Do the same for the second transistor but note that the
emitter tag has been rotated 90° clockwise.
Now place C2 into the circuit; this is a 10nF
capacitor. It has the markings 10n K 100; the 10n means that it is a 10nF
capacitor, the K is the tolerance and the 100 is the maximum voltage.
It does not matter which way round this goes but I like
to have it placed so that I can read the value easily so I placed it with the
text reading the same way as the silkscreen writing on the PCB.
Place it into the circuit then solder; again ensure
that it is seated flat against the component side of the PCB.
Now we come to the final component to solder and
arguably the most dangerous one as if fitted incorrectly it could explode when
powered up!
C1 is a 22uF electrolytic
capacitor; it is polarity conscious meaning it has positive and a negative legs.
The easy way to determine which leg is which is that when new, the longer leg
is almost certainly the positive leg. If you follow the shorter leg up to the
component, you will see markings along the side of the capacitor which should
have a minus (-) sign.
I have highlighted the positive leg. The markings on
this capacitor are 22uF 25V, this means that it is a 22uF capacitor with a
maximum working voltage of 25V. Any 22uF capacitor with a working voltage of 5V
or greater will work with this circuit but as the voltage increases, so will
the physical size of the capacitor.
Place the capacitor with the positive leg (longer one)
in the via highlighted below.
Then solder the capacitor into the circuit
Looking from the side, you should see that the negative
side of the capacitor is facing the IC socket.
Now we are going to solder the wires directly on to the
circuit board; I could have used header pins for this but there is little
advantage in that and it would increase the cost of the circuit board (and you
know how painful that is for Scotsmen and in particular Aberdonian Scotsmen
;o))
Firstly we need to prepare the wires for soldering.
Cut four wires to length, I think I used about 150mm
(it is always best to have the wires too long) then strip about 10mm of the
insulation off one end of each wire.
Below is a picture of the tools and wire needed for
this part.
I used 7/0.2mm equipment wire for this so to strip the
insulation off the wire, I used the 0.6 hole in my wire strippers. It is easy
to cut the actual wire when doing this so a good tip is to start with a larger
hole in your wire strippers and see if it removes the insulation; if it doesn’t
then go to the next smallest hole and try again.
Twist the frayed wires together tightly
Then tin the end of the wire (coat it with solder) this
will ensure that the wire doesn’t fray when putting it through the Via and it
will also make it much easier to solder.
There are a few ways to tin the end but the safest way
is to use some helping hands; hold the wire in the clip, take the tip of the
soldering iron up to the insulation, then add solder whilst running the iron
along the exposed wire until the end (continually feeding solder as you do so)
You will have excess solder on your soldering iron at
this point so it is a good idea to wipe the excess solder off using the sponge
or tip cleaner afterwards.
now place the wires into the circuit.
For the 5V Via I used red wire
For the Input Video Via I used white wire
For the 0V Via I used black wire
And for the Output Video I used yellow wire
Let’s start with the 5V wire; the via is situated on
the top left of the circuit board.
Turn the board upside down and bend the tinned wire to
about 45°. Then solder.
Then
repeat for the remaining wires. Below is a picture of all four wires soldered
into place. Please note that there is no exposed wire showing from the
component side upwards. If you do have any exposed wire, heat the via up with
the soldering iron until the solder melts then gently push the wire up until
the insulation of the wire touches the circuit board.
Here is a side view of the circuit board which shows
that there is no exposed wiring showing.
Now all we have to do is to fit the 555 timer IC. I
tend to keep most of my IC's in a static sensitive box but you should be ok
with the 555 timer as it isn’t really static sensitive.
When new, the legs of the 555 timer are spread out too
wide to fit into the socket so we need to straighten the legs. There are a
couple of ways to do this; I am lucky enough to have an IC leg straightener so
I simply run the IC along the track and through the bearings which straighten
the legs beautifully.
The other way to do this without the expensive leg
straightening tool is to simply place the IC on its side (as shown in the
picture below) and rock the IC until the legs are straight. Don’t use too much
force doing this as you could over bend the legs and if you are unlucky, you
could even break the ends of the legs off the IC.
Now
let’s take a look at the IC itself; I have highlighted the notch on the IC.
This tells us that pin 1 is to the top left hand side of the IC. Pin 2 is
directly below pin 1 and the pins count in a counter clockwise direction with
pins 1 (top left) – 4 (bottom left) being on the left hand side and pins 5
(bottom right) to pin 8 (top right) being on the right hand side.
Highlighted below is the position of the notch on the
IC socket on the circuit board.
Finally fit the 555 Timer chip into the circuit.
And here we have a fully populated circuit board.
A video of the population of this circuit board can be
found below
All we have to do now is to fit it into the ZX81 which
will be coming up in part 2 of this thread.
I hope that you have found this entry interesting; I
decided to go into more detail about the actual soldering of the circuit board
for this entry as I thought that my last one didn’t cover that part of the mod
enough. You may decide to use another circuit board for this mod which is fully
populated but I have included everything that you will need to make the PCB
featured in this entry.
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