zondag 14 mei 2017

ZX Spectrum+

Following the highly successful Spectrum was the Spectrum+. Basically exactly the same machine but slightly bigger and with a better keyboard.  The Ad on Youtube shows it all.
As with the original Spectrum this one needs the composite video modification to be useful. Easy, since the internal electronics are exactly the same. This time I thought I'd be extra careful, and not damage the keyboard flat-cable as happened with the ZX-81 and Spectrum. So I opened the housing very gently, moved the keyboard part only just as far as required to reach the video section, and avoided touching the cables.
Yet, while looking at the cables I noticed that the top and bottom plastic layer seems detached from the middle one, which carries the conductive traces.Still, they looked undamaged when I closed the housing after installing the composite video mod. 
And..No Luck... The letters B, G and T (all in the same vertical column) do not work. So this proves again that it is just not possible to work on these units without damaging the keyboard. So I'll have to order a new membrane, again from RWAP.

vrijdag 28 april 2017

ZX Spectrum

The ZX Spectrum (or 'Speccy') is the successor to the cheap, but underpowered ZX-81.
Though still not a real powerhouse, the 16 or 48K RAM and slightly better keyboard than the ZX80 and ZX81 made this a moderate leap forward for Sinclair. Well, you can Read the full story on the Register. And definitely look at the Flickr page of Rick Dickinson with all the images showing how the Spectrum was designed.
This computer was one I already owned before starting my collection. It was given to me years ago by my father in law, and was stored somewhere in a dark closet ever since. After bringing it back to the light, one of the first things to do was modifying it for composite video output. There's an extensive description on the retrogamescollector website that I followed. But instead of connecting straight to the video signal I used the same setup as used to modify my ZX81, with a transistor and a 100 ohm resistor.
Transistor on the left. 100 ohm resistor from centre pin to the metal shield.

Unfortunately the Spectrum suffers from the same problem as the ZX81: the plastic of the membrane keyboard has become brittle, and it almost immediately broke when I removed the cover.
Crack.. Oh No.. Not again...
And this is a lot worse than the broken cable on the ZX81 which could be fixed using a pair of scissors and some patience. The ribbon cable broke at the edge of the membrane foil itself. So I tried to connect some bare wires to the remaining traces using a soldering iron, but this is useless. The plastic and the metal trace just melt and there is no connection between the wire and the keyboard. Probably something could be done using conductive glue, but it's way easier (and probably even cheaper) to buy a completely new membrane. I got mine from RWAP Software, through the SellMyRetro site.
It's easy to remove the keyboard
Removing the metal frontplate and the rubber key-pad was easy. Just bending up the copper notches and carefully lifting the metal plate using a flat screwdriver worked for me. But I've read in several places that the metal plate may also be glued so be careful !
New Membrane
Then it's just a matter of replacing the broken membrane and re-assembling the keyboard.

DC- 2.1x5.5mm Socket
Since the unit was open now anyway I decided to replace the DC-input. During the first tests I already noticed this was really bad, and just moving the Spectrum often caused a reset because the power was interrupted. The connector on the board is a very standard 2.1 mm DC power socket, so getting one of these was easy. Replaced it, connected the keyboard and the power supply, and it worked !. For 30 seconds...
Then the screen went blank, and a terrible smell came from the power supply. After opening the PSU housing I found I could barely touch the transformer since it was really hot. and the rectifier diodes did not look good either.
Closer examination of the Spectrum mainboard revealed a short circuit that was caused by some solder that I dropped when removing the power socket. And since it seems that there is no protective fuse anywhere, this caused a total burn-out of the PSU. While looking for a replacement, or maybe even just a new transformer I found the Spectrum repair Guide, which saved my day.
Here I learned that the transformer actually is protected by a thermal fuse. And even though this fuse is not replaceable it is possible to mount a 160mA fuse in parallel so the transformer will work again.
Now that's not as easy as it is shown in the repair guide. You cannot really just 'solder a fuse'. If you use a standard 5x20mm glass fuse it will break as soon as you try to solder something to the metal caps.  So I used a fuse-holder, which will just fit next to the diodes, and connected it to the transformer using two wires. Then I just replaced all the diodes and the PSU was up and running again.
Next I incorrectly assumed the centre pin of the PSU was positive voltage and the outside was negative, so the Spectrum still did not work. But after correcting this it finally came back to life.

zondag 26 maart 2017


One year after the release of the (now unobtainable) ZX80, Sinclair introduced the ZX81 in 1981. Almost 1.5 million units were sold, so they are not particularly rare. I bought this one for €25,- and it came complete with the original power supply.
The ZX-81 is small. Really small, as you can see in the picture above (although I must admit I have big hands). It has a tiny membrane keyboard which has four or five functions for every key. There is a lot of mode-switching when entering a program.

Since this was a real home-computer, it only came with an RF connector so you could connect it to your standard TV-set.Which is impractical in these days so I decided to modify it for composite video. Which is actually quite simple. The video signal is already there, it only needs a bit of buffering and you'll have to bypass the RF modulator. I got my instructions from here: "Adding a composite video output.", but of course I started with this  illustrated step-bystep instruction on taking it apart. And what happens to almost everybody also happened here: the keyboard flatcable broke when I turned the PCB over. :-(
The only solution is to cut off the broken section, reshape the end of the cable and push it in again. And the latter part is the hardest. The flatcable is fragile and flexible so it's very hard to get a grip and push it into the connector. I used a pair of flatnose pliers, grabbed the cable firmly, close to the header and pushed it it small steps at a time. It's not something you want to do often, so it's better to do this after finishing the video mod.

The video buffering circuit is really simple. It's just a NPN transistor and a 100 ohm resistor.
I chose the BC547, which is as one of the most common transistors in the world, and it worked fine.
On the side of the RF-box there are all the connections you need

After cutting off the resistor that is connected to the centre pin of the RF connector, I pushed the emitter lead of the transistor through the white hole on the side, and soldered it directly to the connector pin. I also soldered one end of the resistor to the pin, and the other end to the metal of the shielding.
I cut the 5V lead close to the box, soldered it to the collector and finally cut and connected the video lead to the base of the transistor. And this simple mod works surprisingly good:
Next I tried to write a line of code, and save it by recording the tape output on a laptop. After connecting the 'MIC' output to the microphone input of my laptop and trying to record the signals using 'Audacity' I found this does not work. Somehow the output signal is extremely weak, and it was impossible to record even the faintest sound. Apparently the output is intended for a specific type of tape recorder that supported a condenser microphone, which generates very small signals.
When checking with an oscilloscope I can see the signal is actually generated on pin 16 of IC1. But the filter, made up from R29, C12, R27 and C11, attenuates it tremendously. I tried removing R27, and replacing it with a 100K resistor, but that did not seem change much. (Nope. That makes sense with this 47nF capacitor still in place...) Probably the best solution would be to  pick up the signal from pin 16 and buffer it using an OPAMP.

Price [Original] €25,- [ £69.95]
Processor Zilog Z80 @ 3.25MHz
Programming Sinclair Basic
Why ? Iconic, as being the first super cheap computer.

maandag 19 september 2016


The Amstrad / Schneider CPC464. Again a perfect, though quite lengthy story on The Register about the origin and creation of this machine.
 It's a Z80 based computer with a built-in cassette-recorder. Quite amazing for it's time. The one I bought from eBay is in a reasonable condition. Its got some bumps and scratches and it definitely needs a good cleaning.

It came without a manual, but this is available at the CPCWiki.

dinsdag 6 september 2016

Thomson MO5

This one was advertised on eBay as 'ORDINATEUR THOMSON MO5 EN LOOSE'. Yes, it came from France. While collecting several types of computers from the eighties I slowly realised that every country has it's own history of retro computing.In the UK it's mainly Acorn and Sinclair, Germany was mostly Commodore minded, TRS-80 and Apple in the States. And in the Netherlands we had a mix of all of these, plus some additional Philips computers like the P2000.
But France it was slightly different. As the French government needed a computer to be used in schools they obviously wanted to have a French product. So Thomson hastily produced the MO5, which had to compete with the ZX-Spectrum and Commodore 64. But it was actually already outdated when it was released so they were mainly sold to schools. In large quantities I presume, because they are not very rare on eBay. Almost none have found their way to other countries, most likely because they were only produced with an AZerty keyboard.

For me there are a few challenges. First it came without any accessory, so I'll have to find a 17VDC power supply and a connection cord for the cassette recorder. And it only has a fixed SCART cable so that somehow has to be connected to a monitor.
Extra challenging is that the most detailed information is only available in French, which is not my best language...

Connecting the monitor.

The MO5 comes with a fixed SCART (or 'Péritélévision ' as it is called in France) cable. Although it's not used much nowadays, the standard is still quite common.
Connections are as follows (from Wikipedia):

Pin Name Description

1 AOR Audio Out Right

2 AIR Audio In Right

3 AOL Audio Out Left + Mono

4 AGND Audio Ground

5 B GND RGB Blue Ground

6 AIL Audio In Left + Mono

7 B RGB Blue In

8 SWTCH Audio/RGB switch / 16:9

9 G GND RGB Green Ground

10 CLKOUT Data 2: Clockpulse Out (Unavailable ??)

11 G RGB Green In

12 DATA Data 1: Data Out (Unavailable ??)

13 R GND RGB Red Ground

14 DATAGND Data Ground

15 R RGB Red In

16 BLNK Blanking Signal

17 VGND Composite Video Ground

18 BLNKGND Blanking Signal Ground

19 VOUT Composite Video Out

20 VIN Composite Video In 

21 SHIELD Ground/Shield (Chassis)

 But that does not tell me which outputs are actually supported . Looking for 'Video signal MO5' does not get me anywhere, but after googling 'signaux video MO5' I finally found 'Christophers's lair', a site with some technical information.
From this I can conclude that the unit will support RGB output, something that I can easily interface to my GBS8200 converter.


The only label on the unit says 'Power Supply 17VDC'. It's not clear what the tolerance or power consumption is. The technical manual contains a diagram of the (very basic..) power circuit:

The circuit shows that the this 15 or 17 VDC is not very critical at all, and I actually think it would work fine on 12 V. What is important here though is that the power jack has ground on  the centre and positive voltage on the outside, something that is not very common in standard power supplies.

Price [Original] €25,- [€541,-]
Processor Motorola 6809E @ 1MHz
Programming Microsoft Basic
Why ? The first French computer in my collection.

maandag 29 augustus 2016

Philips VG-8020

The VG8020 is one of the few computers released by Philips in the MSX line. This unit came as console only, so no cables or peripherals. Fortunately it has an internal RF-modulator and a built-in power supply so a 220VAC power cord and a standard TV-set is all that is needed for basic operation.

Video connection.

Since using the RF output is not an option (I do not have a TV set...) we'll need a cable to connect the video output to the video monitor, or even better, to the GBS8200 video converter card.
The connections are shown in the user guide:

Very good resource for all things MSX: the MSX Info Pages by Hans Otten
Price [Original] €25,- [€455,-]
Processor Zilog Z80 @ 3.58MHz
Programming Microsoft MSX Basic V1.0
Why ? I really wanted at least one Philips product in my collection, to remember the good old days when they were still the main producer of quality consumer electronics.

zaterdag 27 augustus 2016


So far the only computer in my collection that meets the '2 bytes' criterion. The TI99/4a is a 16 bit home-computer made by Texas Instruments. It took a while before I found one at a decent price (€25,-) but I finally got one from another collector who was clearing out his collection.
It came complete with power supply and RF-modulator. The RF-Modulator was a bit of a surprise. So far I had not encountered a computer that used an external one, and certainly not such a huge, heavy black box as this one. Fortunately the output leading to the modulator carries standard YUV (or Component Video) signal, which is supported by my GBS8200 Video Converter Board.

The TMS9900, one of the first 16-bit processors.
After powering up the screen indeed shows the TI99 intro screen. But it is unstable. Graphic characters just randomly seem to change shape. Pressing the '1' key does bring up the basic prompt, but that's about it. It appears a mere coincidence that the '1' key actually works, since almost none of the other keys do.

So it has to come apart..
Using the illustrated instructions on Mainbyte, this is easy. And I soon find out that tapping the 'power bar' that runs past all the video RAM chips causes flicker and the changing of the characters. After touching all solder joints with my iron and some fresh solder the video problems are greatly reduced.

Now for the keyboard. Initially I suspect the keyboard scanning chips and resistors. But after checking the signals with an oscilloscope it soon shows that the keyboard contacts are causing the problem.
The TI99/4a has been produced with several different types of keyboards, and mine appears to contain the worst of all: the 'Mitsumi' membrane based keyboard.
When searching for possible fixes I soon found out that most of them indeed don't work any-more and also that everybody more or less gave up on these keyboards. So there's a challenge: find a way to fix or replace it.
After posting this question to the Retrocomputing StackExchage, I followed the advice and  cleaned both sides with 'Chemtronic Pow-R-Wash', a contact cleaner the leaves no residue, and I gently rubbed the contact carbon using an eraser. The keyboard now seems to work great again, not sure how long this will last though.

Meanwhile the video failed again. And after a lot of tinkering the system does not work at all. It looks like the 5V is down, but when I disconnect the PSU the 5 V is correct. So it looks like there's a short circuit somewhere.
What puzzles me are the white bars, which I expected to be full metal bars carrying either power or ground. But that does not match the connections to the RAM chips. It looks like its bot conected to the 5V and the GND pins.
I found the following comment on the 'AtariAge' forum:

 "that the 'weird rails' are actually two metal strips with a paper separator, and actually carry both +5V and GND. By trimming it, you've possibly created a short circuit and/or you've only jumpered one side of it. I'd give that a careful inspection."

Right. I did not expect that. And this means I probably created a short somewhere. Looking further I found it's even worse. The bars may also carry +12V and 5V. The top one shown in this photo is the +5V / 12V bar. It supplies the video RAM chips, 12V to pin 8, 5V to pin 9.
1.With bar.  2.Bar removed.  3.New connections.
It the end I just removed the complete bar. Which indeed solved the short circuit, suggesting I was on the right track. So I just wired all 5V and 12V points manually using some solid wire. Which worked. Video is back again and the computer runs as normal.
Unfortunately it failed again after a week, same problem. So the short must be somewhere else. I suspect a capacitor, but finding that will be a lot of work.

A bare TI99 is nice, but limited. After typing a few simple BASIC lines, but not having any means of saving and loading programs the fun is more or less over. But I discovered the FlashRom99.
The FlashRom99 Module is created by Ralph Benzinger and uses a Secure Digital (SD) Card for storing ROM images. After power up the content of the SD Card is displayed in a menu. When a selection is made the ROM image is read from he SD card by the ATMEGA8515 CPU on the module and copied to the 32KB RAM chip. It's available as fully built cartridge, a complete kit, or just the PCB with a programmed CPU. I chose the last one. After all, building is 90% of the fun.
Assembly was really easy, and it worked immediately.
The .zip file that can be downloaded from the FlashRom site contains a bunch of sample games and programs so you can get started right away.

A set of useful utilities like an editor/assembler, ready for installation on the FlashRom can be found on the TI99 site of Fred Kaal 


Useful links for the TI99/4A

The MainByte site has got a lot of really good information.
Stephen Shaw's 'Getting Started' book online.
Ninerpedia. The Wiki for TI-99/4A, TI-99/8, Geneve 9640, and all related hardware and software.
The TI99/4A Forum on AtariAge
A recent article on The Register about the increasing popularity of the TI-99.