zondag 24 september 2017

Toshiba HX 10 MSX


The HX-10 was one of the first MSX computers to be introduced in Europe. MSX computers were the final initiative to get to a universal 8-bit computer platform. These Z80 based units were all equipped with Microsoft Basic. And just to illustrate that the HX-10 is really one of the first: it  contains version 1.0.

This unit was sold for a decent price (€20,-),but with the warning that some keys did not work. I expected that some cleaning might solve this problem, but when I tried the unit I found that Q, 3, R,F and V keys don't work. Since they are all close together on the keyboard it is more likely that something is wrong in the connections of the keyboard matrix. To my surprise there does not seem to be a 'Service Manual' or even schematics for the HX10 available online, so I'll have to figure it out myself.

Opening the unit is easy: there are only two screws left, the other four seem 'missing in action'. The keyboard is a very decent and sturdy construction. The key numbers are printed on the rear of the PCB so it's also very easy to find the location of the failing ones.

First I measured on a working key (the Esc key) how the keys are actually connected.
So top two and bottom two contacts are connected, and the switch connects bottom to top.
As with all keyboards from this era it must be some matrix. On the lower left side there are two connectors, a 9 and an 11 pin which are most likely the row and column connections. So I used the multimeter to check which keys each contact was connected to, which soon led to the following schema:

Key Numbers
1 0 1 2 3 [3] 4 5 6 7
2 10 11 12 13 14 15 16 17
3 20 21 22 23-. 24 25 26 27
4 30 31 32 33[F] 34 35 36 37
5 40 41 42 43[N] 44 45 46[Q] 47[R]
6 50 51 52 53[V] 54 55 56 57-[Z]
7 60 61 62 63[Caps] 64 65 66 67
8 70 71 72 73 74 75 76 77
9 80 81 82 83 84 85 86 87

The key numbers that have a letter next to them are the ones that do not work. From this I can see it's not a complete row or column that is missing, but just some section of a row. Closer inspection reveals that it indeed comes down to some broken traces.
So after adding just three wires, all contacts seem fine, and indeed the keyboard is working fine now.

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

ZX-81



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. :-(
Crack...
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
RAM 1KB
ROM 8KB
Programming Sinclair Basic
Why ? Iconic, as being the first super cheap computer.


maandag 19 september 2016

CPC464

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.

Power

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
RAM 16KB
ROM 32KB
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
RAM 64KB
ROM 32KB
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.