maandag 29 april 2019

NEC 8201-A Portable PC

Advertised on eBay as 'one of the first laptops', the NEC 8201-A is actually a surprisingly nice computer, even today. It's got an 8x40 characters LCD screen, built in Basic, Word-processor and Terminal software and  a decent keyboard. With  three serial ports,  a printer port, Cassette interface and a Barcode Reader interface it also has plenty of connection possibilities. And the most appealing feature of all : it can run for almost 20 hours on 4 AA batteries !

The battery pack seems corroded, though not beyond repair. When cleaning there is a lot of green stuff coming off the board. Probably copper oxide, but it could also be remains of leaking batteries.
But after inserting a fresh set of batteries the computer does not turn on. Checking the connection from the battery to the board I soon found out the contacts were still bad. So I first just soldered the pack directly to the board.
The computer turns on, and the display shows text, although hardly visible. Turning the contrast control does not change this except maybe at one end, where I could see an almost black screen sometimes. So it looks like this control may be bad. The schematic shows its a 50K potentiometer so this might need cleaning or replacing. Looking on the internet for details on the control, I also found some notes from people that found the control to be OK, but a fault in the -5V power which could cause the same problem. So I first measured all voltages on the power supply.
On the picture I indicated the GND point used, how the connector is numbered and the voltages that should be present. All seem pretty good, except maybe number 18, which is not really 9V, but this could be due to the fact that it is battery powered.
Anyway, the -5V (which as far as I can see is only used for the LCD contrast) is present, so it must be a problem with the potentiometer.  Following is a detail from the schematic showing how the control is used.


Since I could not figure out from the footprint of the potmeter how it was connected I first measured connections from the adjacent  components R10 and R77. From that I found that R10 is not 100K but 50K. The two big solder squares are connected to the wiper of the potmeter, and the two other small contacts are the outer ends. Since I did not have a 50K model I used a 10K trimmer to test it. And indeed the screen now was readable. The contrast range was still limited so I added a 20K resistor in series with the trimmer and got a good result from that.
Obviously the trimmer that I used was different from the original so there was no way it would fit into the original PCB.


Also the knob would not fit the trimmer. So I decided to print one on my 3D printer. Then I connected the trimmer to the bottom of the PCB using wires, and glued it on the top using hot-glue. All this worked out really nice so and the computer screen is useable now.


Price [Original] €25,- [US$699,-]
Processor Intel 80C85 @ 2.4576 MHz
RAM 64KB
ROM 32KB
Programming Microsoft Basic 1.0
Why ? One of the first laptops.

dinsdag 18 december 2018

Philips G7000 - Videopac

 
My first game console was a Philips Videopac G7000. My father bought it, and I spend hours blasting spaceships, eating square 'dots' and playing tenpin bowling... I remember there were games I played so much that I could finish them with a perfect score every-time (games were not that complicated in those days). And, lets not forget, this computer actually started my programming career. Videopac cartridge nr. 9 turned this game console into truly programmable computer. Programming was limited to 100 bytes, output was a single line of 16 characters and you had to enter your commands in hexadecimal code. Or you could actually enter them in as 'assembler' text, but you could not list your program as such so it was easier to just enter the code .






I spent a lot of my savings on games and as far as I remember we had a box full of them in the end. Then, after not using it for a few years (since I got into PC gaming), we sold it.
But as with everything from this era, the nostalgia kicks in and I felt an urge to add one to my 8-bit collection. Fortunately more than a million were sold at the time so they are not rare and relatively easy to get.

I bought mine for €50, including 6 games which is not bad. The unit definitely looks used. One of the joystick knobs is missing and it has some dents and scratches, but not much more than the one we had 30 years ago.
The Dot Eaters site has a nice story on the Videopac or actually the Odyssey2 as it was known in
other countries. Though many were sold, it is not a hugely popular collectors item these days, and information on the internet is widely scattered with lot of websites that look like they were made in the nineties. There is a list of games on Wikipedia, but only few of them have an actual article or description.

As mentioned, the unit I got was definitely used, and showed some marks of that. So I cleaned it and the first thing I did was to add connectors for the joysticks. This series consoles came with the joysticks hardwired to the console, which is really annoying if you want to pack the system up. The weird thing is that the housing does contain the space for external connectors, they were just not installed, probably to save some money. So I just removed the two pieces of plastic that covered the connector holes and mounted two 9-pin D connectors.
There is a VideoPac forum, and one of the posts shows the wiring of the joystick in detail. The colours match the colours of the wires in the cable, so I just cut the cable and soldered it to pins 1 to 6 of the connector.

Next issue was the missing joystick knob. Which is not a big deal in these days of 3D printing so I designed and printed one that turned out remarkably nice. Its on Thingiverse now, so you can print one if you need it.
Then came the joystick itself. The Videopac joystick is a bit of a unique design, with 8 switches arranged in a circle which are activated by a disc that you tip to a side by means of a large spring.

The switches are just metal strips that you push down to touch another piece of metal on the PCB.
I set out to make a complete new base, now with 8 mini switches as shown here.
That did not really work. When I mounted the disc again and put the spring in place the disc just activated all buttons at once. Clearly the spacing was much more critical than anticipated, so I placed the switches on second board and put that underneath the first board.
Now only the buttons stick out, and this seems better. But it still did not work great. Now the problem was that the centre of some switches was not exactly under the edge of the disc. Since this is a rounded edge it sometimes just hit the side of the button and the switch would not fully close. So I ended up printing a new disc, slightly bigger than the original and with a flat ridge that covered all the switches.
And this works surprisingly well. After reassembling the joystick it works flawlessly, and I immediately used it to set my high score on Munchkin !




maandag 27 augustus 2018

Aquarius


The Mattel Aquarius is something I almost given up on, since most of the the systems offered on EBay are way over my budget. In fact I did not even expect to get this when I placed my €40,- bid for what I thought was only the console itself. To my surprise and great joy I did not only win the bidding, but found myself with a really complete spread. Including the tape recorder, a printer, all the original books and even four original games.
Normally I refer to the articles on The Register, but this time I also recommend reading this section of the RetroArchive since it's got the best story of the Aquarius. As both articles really make clear the Aquarius was one of the biggest flops in the 8-bit computer era.
Multiple sources on the internet report that the standard Basic is supplied even without a FOR NEXT loop, but that's not true.
There's not as much information available on the internet as for the Spectrum or Commodore C64, and it's much more scattered but I found the following sources very useful:
The Aquarius section on the website of M. vd Steenhoven.


Mattel Aquarius Thermal Printer

One of the many peripherals designed for use with the new Mattel Aquarius home computer system is its thermal printer. This is strictly a one-system printer; it connects directly to the Aquarius console without additional interfaces, as do the other Aquarius peripherals.

Mattel Aquarius Printer
The Aquarius printer offers 40, 5 x 7 characters per line (10 cpi) at the rate of 80 cps. It is capable of printing 256 characters including upper- and lowercase letters, numerals, and special graphics characters. The printer will accept special thermal paper 4-3/8 inches wide.
Although the printer looks really fancy and if it was specially designed for the Aquarius, it is in fact a Alphacom 42 VP thermal printer, which was also used for the Commodore C64 or VIC20.

Composite Video

As with all my retro computers, having a composite video output is one one of the first requirements. Geordy Rostat at notANAN has a description of his attempt to do this. Amplifying the existing video signal so it is suitable for a standard monitor is what I've done so far on all my Sinclair computer,so this seemed like a good solution. Ed Snider on 'The Zippster Zone' however mentions the possibility for a different solution, using the raw RGB signals. This seems like a nice option too, when combined with my GBS-8200 video converter board.
Third option would be the composite video modification found on the  Intellivision wiki.

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 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.


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.