Friday, 27 February 2015

ZX81 and ZX Spectrum keyboard membrane repair

One of the frequent problems with the Sinclair computers of the early 1980's in the keyboard membranes turning brittle. This results in loss of one or more rows or columns in the keyboard. These are split into 5 rows of 8, so if some columns are out, you may have keys working and some not. For example, keys 3,4 and 5 working and also 6, 7 and 8, but 1,2, 9 and 0 don't respond, or if rows are out, 1,2,3,4 and 5 work, but 6,7,8,9 and 0 don't etc. It's rarely a single key. In this case, it's all of them.
Sometimes this happens right by the connector, as this this one, the entire row tail is detached. If this is the case, and the rest isn't too brittle, it is recoverable. It often happens when someone decides to have a peek inside, normally just before it is consigned to the attic, or just after it is retrieved from there. This usually leaves a small piece of membrane in the connectors, a rough edge on the membrane, and an ebay listing which states 'untested, worked before it was put in the loft'.
It can also happen if they tails fold sharply when the case is closed. Sometimes there are cracks further up, as with this ZX81 membrane, it would need to be cut back to before this point.
I have considered repairing this sort of thing by clamping another piece of membrane with tracks on over the top of this, so it makes good contact and bridges the gap. I don't know how reliable it would be though, or how best to clamp it. If it's too far gone, the best option is to get a replacement membrane. The are some excellent ones available from RWAP.
This is also the point where I line up candidates for my USB keyboards. I've designed the boards to be able to cope with short membrane tails that would otherwise be to short and would have to be replaced.
These wouldn't be long enough to reach the normal ZX81 or ZX Spectrum board.
However, when there is just damage to the end, they can be recovered if there is enough non-brittle tail left.
First cut back the tail until there is a flat edge with no damage. There are two parts to each tail, one with traces on, and one clear. Cut back the clear one by about 10-15mm. I then carefully wrap the remaining parts (apart from the last 10-15mm) with clear tape to support what is left and stop it being damaged by bending.
If both parts are still long enough, it can be reinstalled. The 5 way tail on this one is a bit short, but should still reach.
To reassemble, first give the case a good clean.
Then install the membrane with the tails poking through the slots.
The new Spectrum membranes look much the same if you're using one of those.
Cover that with the rubber mat, these could sometimes benefit from a wash before refitting.
Next comes the metal faceplate. Replacement for these are available (also from RWAP) if yours is a bit scratched or dented. Remove the existing adhesive or tape, clean well and apply new double sided tape.
Carefully position that and press down along the edges and the lid section is complete.
Underneath, the repaired tails will be shorter, so it's sometimes fiddly to reattach them,
Remove any remnants of the old membrane tail end in the socket and then gently connect the new (or at least newly prepared) tails. The trick is to apply as little pressure as possible, and to always press directly into the socket, not at an angle, or it may bend and snap again.
With those attached, it is time to test again.
All keys are now working, just be careful when opening the case in future.

Sunday, 22 February 2015

Atari 2600 Jr Composite Video Modification

Following the Atari 2600 Composite Video Modification article last week, I've been asked if the same thing can be done on the Atari 2600 Junior?
The answer is yes, although very different in size and appearance to the original Atari 2600, the Atari 2600 Jr uses mainly the same parts, laid out a little differently.
It's all on one board, the sockets and switches that were on the larger board on the 2600 are visible here, the interesting stuff is all under the metal shield. The two momentary switches (game select and reset are now on a membrane connection on the top of the case. The power and 'TV type' select are on the left (still don't know why people needed frequent access to change the type of TV they owned in 1989).
Here are the same three main chips as the 2600, and the 4050 buffer used to improve the mixing of the video signal (which was missing in some of the 4 switch 2600 versions). Rather than the fixed RF cable used in the 2600, there is now a phono socket on the outside of the case for the RF signal, the same arrangement used on the ZX81 and Spectrum etc.
This one is faulty, doesn't turn on. This is a simple case of a dry joint on the power connector. There is no ground connection from the power supply to the board. With the solder reflowed on that, it will be fine.
The composite video modification is basically the same, stop the audio and video being combined, route the audio to a connector, and route the video via a buffer amplifier to another connector.
The audio / video circuitry is all on the bottom right of the board, Here, I've gone for a different approach to the previous modification, and removed the circuitry not required around the audio / video combination circuit.
The main parts to remove are the modulator and connector, Q4 and R56, but I've also removed C53, C53, L8 and L10. This leave that area clear, and I've reused the holes to mount the new video buffer components.
I've fitted two phono sockets in the holes used by the original RF connector, and installed a simple emitter follower video buffer.
The video signal is already biased via R42, so the base of the transistor (2N3094) can be connected to that point (the top of R56). The collector is connected to 5V via the lower hole of Q4. The emitter uses the right hand hole of Q4, which is connected to the pads of C54 and L10. These have been removed, so this trace is no longer in circuit and can be reused. L10 is replaced with a 75 ohm load resistor to ground, and the video signal is taken from that point to the yellow socket.
The audio is collected from the  bottom of R34 or the top of C49 and connected direct to the white socket. Ground on the two sockets is taken to the thick ground trace.
I've applied a little hot melt glue to hold down the sockets, and the covers fit back to cover up the rest of the mod.
The original RF output hole has been enlarged to allow both photo sockets to stick out, and the mod is complete. As before, the colour can be trimmed via a variable resistor (R9, middle left of the board).
I've only used a single audio jack (as the PAL TIA is only mono), but you could add a second audio jack wired in parallel if there were space. More testing required, Maze Craze this time, simple but addictive.

Saturday, 14 February 2015

Atari 2600 Composite Video Modification

Following on from the previous Atari 2600 black screen repair, I had been asked to modify it to composite video output. The original design has a UHF modulator which outputs the video and audio on analogue TV channel 36. Modern TV's no longer have analogue tuners, and those that do, don't cope with the signals from old computers and games consoles too well.
I've seen many versions of the composite video modification for a 2600, ranging from very simple (capacitor coupling the modulator input) to very complex (recombining the chroma and luma from the TIA in various ways).
Here I went for the fairly simple version, a single transistor emitter follower circuit to boost the video signal. Some circuits miss off the 75 ohm resistor, but it should be there to impedance match the TV input. The transistor is a basic NPN, I used a 2N3904.
The signal which goes to the modulator is actually video and audio combined. The TIA generates these separately, so it just a case of stopping the signals being combined. The easiest way to do this is to remove the parts which combine the signals. There are various suggestions as to how this is done, the chief part is Q202. The signal seems to be improved by the removal of L201 as well.
The audio is then tapped off separately, There is a spare pad to the side of C120 which has the audio signal on. Ground is taken from a convenient point nearby, I chose side of C238. This wire is then fed out of the metal box.
That is then connected to both the left and right audio outputs. That leaves only video on the signal going to the modulator. That can now be removed, and the 5 pins next to it carry 0V, 5V and video.
I built the circuit on a small piece of padboard and fixed that where the modulator used to be. I had left space on there in case I needed to go for a more complex circuit, but the basic one worked well.
You could drill holes in the case and mount phono sockets or a scart socket, but I went for running a cable out of back, and started with an old composite video lead, I think this was from a PS2?
Notice the large moulding at the connector end. This would normally be a ferrite to reduce interference. In this case, it appears they have just gone for the plastic shell to give the appearance of a more expensive cable. Nice touch.
That wasn't a problem here, as I just needed to cable, and it was a bonus that the ends were nicely tidied up so I just needed to remove the original connector and crimp on a 6 way header for my board.
The cable then runs out the case where the original RF lead went, quite a neat (and reversible) mod.
This now gives a very good picture on a modern LED TV. I had to adjust the colour timing slightly, I used Galaxian running on an emulator as a reference to get the rainbow colours right.
Composite will never give as crisp a picture as the high resolution digital connections like HDMI, but it's very usable.
Ah well, more testing on Galaxian to do, Atari Space Invaders seems very slow in comparison, Galaxian plays very well.
One Atari 2600, repaired and upgraded ready to go.

Atari 2600 Black Screen Repair

Here we have the classic Atari 2600 Video Computer System. This is a six switch model from 1980 and is suffering from the good old black screen problem. That covers a variety of problems, and nothing at all being output and a correct video frame being generated, just with nothing on it.
This is the second case, tuning an analogue TV into the signal from the unit finds a black screen on channel 36. There isn't much other than noise on an analogue TV these days, but it's easy to test by turning the 2600 off and the screen returns to static, power back on and it goes fuzzy black again. OK, so there is power getting in, at least enough for the modulator to be outputting a signal. Time to open it up.
Inside you can see they multipart construction, there is a large single sided board with the switches, voltage regulator and TV modulator, and a large metal box containing the rest. Testing this, there is 5V being generated by the 7805 regulator.
I've been asked if it is worth replacing this with a swich mode regulator, as I have done on ZX81 and ZX Spectrums (7805 regulator replacement on a ZX Spectrum Plus). This 2600 draws about 350mA in operation. So at 9v input, that is (9-5)*0.35 = 1.4 watts. The heatsinks probably equates to something like 20°C/watt, so they will end up 28°C above ambient, not too much of an issue. Given this one is working, no need to do it here. The ZX81 with a RAM pack is two or three times that and becomes noticeably hot, and so is worth doing on a ZX81.
The capacitors can also be points of failure, but these tested fine, so no need to replace those either.
Checking the other side of this board, the modulator. Looks similar to the UM1233 modulator used on many 1980's home computers. There are only three connections, ground via the case, 5V via a filtered input, and the video input. With a known working cartridge installed, checking the video on the scope shows only a flat line.
Time to go inside the big metal box. Here is the heart of the 2600, three chips. At the top is the MOS Technology 6532, otherwise known as the RIOT (RAM Input / Output Timer). 128 bytes of RAM (yes, bytes), 2 8 bit I/O ports and a timer. Atari uses their own part numbers, so this is marked C010750. The big chip the bottom is the TIA (Television Interface Adapter), the chip responsible for generating the video and audio, This is the PAL version, marked this C011903. The smaller chip in the centre is actually the CPU, a MOS Technology 6507, a cut down version of the 6502. Atari code C010745.
Reseating the chips often helps, but in this case, it made no difference. Failure of any one of these could lead to a blank screen, but the usual culprit is the TIA.  As is the case with Sinclair ULAs and Commodore PLAs, this is a difficult part to source. The choice for replacements being basically another 2600. The donor in this case is a spare 4 switch 2600 board with a faulty RIOT chip (another part that can only be gleaned from another 2600).
By this stage (1983), the design had been reduced to a single board. Two of the switches (difficulty A and B) had been moved to the back. I would have through the TV type would have been the least used, you don't often switch the type of TV you own.
With the TIA replaced, the screen now shows Space Invaders, in glorious black and white analogue TV.
The signal wasn't too bad on an analogue colour TV either.
The owner had requested a composite video conversion. I'll cover that in a separate article (Atari 2600 Composite Video Output modification).
The final steps were the usual clean up the case and clean the switch and connector contacts and then it just needs extensive testing. It's a hard job playing video games, but someone has to do it.