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.

ZX Spectrum+

Following the highly successful Spectrum was the Spectrum+. Basically exactly the same machine but slightly bigger and with a better keyboard.  The Advert 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.

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.

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.