CORRECTING GTIA-MODES & VIDEO OUTPUT ==================================== (By Jiri Bernasek - BEWESOFT) After many years of using an 800XL, I decided to buy a second Atari 8-bit computer (for a kind of software-testing). I got the last version Atari 800XE, but unfortunately, the displaying on TV wasn't fully OK. There were two problems, that I already came across at some computer-meetings, but now I had a chance to examine it a bit deeper. Finally I've found a way to correct both the problems. If you've the same problems - keep reading, the full documentation is coming! The first problem is bad displaying of so called "GTIA modes" (GRAPHICS 9, 10, and 11 in Basic), that occurs probably on all these last XE's made in China. There's a bug inside the GTIA chip, mixing bright dots or vertical lines into 16-shade graphics (mostly new demos). If you don't know how your computer display these modes, just type (in Basic of course): 10 GRAPHICS 9:FOR X=0 TO 15:COLOR X:PLOT X,0:DRAWTO X,64:NEXT X 20 GOTO 20 RUN You'll see a square, black at the left side, grey in the middle, and white at the right. If the brightness is just increasing from left to right (step-by-step), then your computer is OK, and you needn't to read the following part of this text. But when there are white vertical lines (mostly in the left part of the square), then your GTIA is bad. The best solution in this case would be to replace your GTIA with correct one, but this is mostly impossible (unless you have another broken XL/XE under table). The upgrade described below will help without of replacing GTIA - you'll need only five cheap chips (available at every electronics' shop), and a few even cheaper things like some wires etc. The displaying will NOT be 100% perfect (still), but the most awful effects disappear, so at the first sight the screen will be OK. The described upgrade does correct the brightness of pixels (GRAPHICS 9 and 10), but not the color (GRAPHICS 10 and 11) - that's not so simple. In the other hand, it affects the PMG graphics a little - on moving objects (over GTIA modes) there's almost nothing strange, but stable PM objects may look a bit "flat" (pixel-pairs linked together). When there's a fine high density text in the PMG (it happens almost never, but still it's possible), it might be unreadable - that's why the upgrade allows you to install a disable switch. Graphics modes other than 9 and 10 are not affected. To make the expansion yourself, you'll need a few components from an electronics' shop, and also some fine tools, a good soldering iron, a bit of experience with precise electronics work, and quite a bit of patience. So, if you're not experienced with the soldering iron and complicated Printed Circuit Boards, better ask a friend for help. The author of this text is not responsible for any damage caused by incorrect installation of the upgrade, or improper handling of tools and components! So, what you'll need: 74HCT00, 74HCT74, 74HCT75, 74HCT86, 74HCT138, Resistor 10 kOhm, Capacitor 100 nF, a small switch (optional), piece of an universal board (70x28mm), 1-2 metres of insulated thin wire, and a few centimetres of non-insulated thick wire (diameter about 1-1.5 mm). If you can't get the 74HCT... chips, you may also use the classic 74LS... (These chips only "eat" more current, the function is the same. Also 74ALS... are possible, and even 74HC... might work, but that's not sure. Old 74... and 74S... types needs quite too much current.) But the 74HCT... are probably the best, and are not more expensive than the others (around here). There are no special requirements about the resistor and capacitor - almost any small-size components should be OK. The board should be a kind of laboratory-type: with holes in a square raster of 2.5x2.5mm, each with an insulated soldering point around. You'll need a piece of 28x11 points. (It's also possible to make the upgrade without of a board, but this would be much more difficult, and such expansions are also easier to damage). The switch (optional) should be a small type (one way ON-OFF is enough), suitable for montage into back panel of the computer. The first step is preparing of the upgrade board, which is shown at figure 1. Cut the universal board to correct size, and insert the components, as shown (the chips, resistor, capacitor, and three pieces of insulated thin wire). Solder the leads of each component (at the other side), and cut off the unnecessary protruding wires. Then, make all the shown connections on solder side. You may use a thin wire (non-insulated, soldered to the board), but it's also possible to link the points only with the solder. Leave the numbered points free - you'll solder them later. All wires and other metal parts (on the components side) must be hidden under the level of chip covers. At the end, make the two "legs" for fixing the board in its place. Take about a centimetre of thick wire, bend it to right angle, and put it onto the solder side of the board, so that one half may be soldered onto a few soldering points on board, and the second half is protruding about 5mm above. Then check twice, that everything is OK. Note that the 74HCT... chips are electrostatic sensitive devices, as well as the chips inside your computer. You should solder them with a good soldering iron - NOT the simple transformator-based devices (with the tip of bent thick wire). It's best to connect the ground point of a board you're working on together with soldering iron and your hand with a wire, to avoid an electrostatic discharge - or at least touch the ground point (with hand and soldering iron) every time you're going to continue the work after a pause. Don't wear clothes of artifical materials, and don't move too much on a PVC floor in your room. The chips might be also damaged by heat - so better don't solder for too long time on the same chip (when the case become hot, you should stop). If you'll ignore these advices, the chips may be damaged (it's possible that you'll be successfull 99 times, but still there's the danger that 100th time you'll destroy a friend's computer). Now, open your XE. Remove 4 screws at the bottom, lift the top cover, and carefully remove the keyboard. (You should disconnect it from a connector - be careful! These leads on flexible plastic are easy to damage, but almost impossible to repair.) Now remove another 7 screws around the edge of board, and lift it from the bottom cover. Open the metal cover, bending the 7 fixations to be straight. Look at figure 2, and try to find the necessary points on the XE-board. (Hopefully it'll match the picture.) Cut carefully the shown 3 ways beside the 4050 chip close to GTIA (one of these ways is on the other side!). Carefully remove the loose metal pieces away from the board. Now prepare 18 short pieces of insulated wire, and solder them to the numbered points 1-18 on the upgrade board. The length of wires should be adapted to the connection with points on XE-board, as shown on figure 2. (It's allways better to have the wires short.) If you want to install a disable-switch, solder another wire to the point 19 - this wire should be long enough to reach the future position of the switch (somewhere above the monitor connector). Find the points "+" and "-" as shown ("+" is connected to ANTIC pin 21, and "-" to pin 1 - better check it), and try to put the upgrade into its place. Bend the "legs" a little, until it perfectly match the soldering points on the XE-board. Check the height - the "legs" must touch the corresponding points, but the upgrade board (the top of soldering) may not be more than 1mm above the ANTIC (that's the large chip below). The soldering on upgrade board may touch the ANTIC's plastic cover, but must be far enough from its leads. Then solder the "legs" to XE-board. (Good quality of soldering is required here, because these two points will hold the whole upgrade board in its place.) Now connect all the wires to the corresponding points on the XE-board, as shown on figure 2. The long wire (19) for the switch will be connected later, now bend it away from the board. Check everything twice, then connect a monitor (TV) and power supply (don't connect keyboard - it's easy to damage), and try to turn the computer (board) on. It should work, and you'll see "READY". If the screen stay black for a longer time than normally - switch off, and search for mistakes, bad quality soldering, or a short circuit. When the computer works, try to connect a disk drive, and insert an auto-starting (boot) disk with a title screen in a 16-shade mode (GRAPHICS 9), or better HIP (contains both GRAPHICS 9, and 10). Turn on, and check the picture - it should be displayed correctly, without of these white dots and vertical lines (some very faint grey rests are sometimes possible - when a HIP picture uses more different colors). Try to carefully touch the ground (places for fixing the metal cover) with the long wire for disable-switch (if present) - it should return the function into the original state, so that you'll see the white dots and lines again. When it all works, switch off, disconnect all the cables again, and install your disable-switch (if you're going to do so). Drill a corresponding hole to the back panel of (empty) bottom cover, and fix the switch in place. If you want to do also the modification of video output (described later), do it now (or at the begin). Take the metal cover, put it back onto the board, and check that everything is OK. The wire for disable-switch should lead out of the cover somewhere beside the modulator (the small metal box with TV output), and it must be free - not pressed under the cover. Looking through the ventilation holes, examine the position of upgrade - it should be about 1-2mm below the cover, and all wires and metal parts must be far enough (otherwise try to put a piece of insulation tape onto the cover). Bend the 7 points to fix the cover in its place. (It's allways better to return the shielding cover into your computer - when possible - and not to throw it out, as many authors of different expansions are advising. It seems to be senseless, but when you'll try to listen for example a radio receiver close to your (unshielded) computer - then you'll understand what's a radio interference...) Now put the board into the bottom plastic cover (with the new switch), and fix it with the original 7 screws. Solder the wire from the expansion onto one end of the switch, and connect the other end to ground (the large metal areas on the board, connected to the shielding metal cover you've just installed). Add the keyboard carefully, and insert its leads into the connector. (The leads must perfectly match the contacts. While inserting it, hold the flat cable close to connector, and don't use sharp tools or nails.) Finally add the top cover, and fix it with the 4 screws. Check if everything works, and... It's finished! Now, an additional information for more advanced guys is following. The basic idea behind this upgrade is that the bad GTIA chips are displaying the left half of each pixel correctly, and only the right half is bad (lower two bits ORed into higher bits). So, the upgrade just take the left half, and put it over right half. The schematic diagram is shown on figure 3. The luminance goes out from GTIA in the form of 4 digital signals (4 bits). These signals are going through the 74HCT75, which is a D-type register, giving a direct input-to-output connection during the whole time of a clock pulse. This feature allows easy disabling of the upgrade just by keeping the clock inputs active. Note that only 3 signals are connected to the upgrade - the lowest bit (LUM0) needn't to be corrected (remember - in GRAPHICS 9 it's correct, and in GRAPHICS 10 it's allways zero). The clock pulses (the first 1/4 of a pixel) are combined from OSC and Phi1. Because the GRAPHICS 10 pixels are moved to right, the Phi1 input must be inverted depending on the active mode. (In the corner of figure 3, you can examine different clock signals available in the computer. Phi2 may not be used here, because the delay between Phi1 and Phi2 is not stable enough - sometimes it might give false clock pulses.) When the highest two bits of PRIOR ($D01B) are the same (that's GRAPHICS 11, or no GTIA mode), or when the disable-switch is "ON", the clock signal is allways active, so that the 74HCT75 doesn't change the signals. The 74HCT74 is a 2-bit register, making a copy of two highest bits in the PRIOR. The circuit around 74HCT138 just detects the moment of writing data to the address $D01B. Unfortunately, the color signal may not be corrected in such a simple way, because it goes out from GTIA in the form of final PAL signal. ----------------------- The second of the two problems mentioned at the begin was TV synchronization. As far as I know, this problem occurs on all XE's. Depending on the type of TV/monitor you're using, it may happen that the screen is "flying" a bit, or even goes into something like diagonal lines - mostly when the displayed picture is bright. On most TV/monitors it doesn't happen, but it seems that my TV is extremely sensitive for this problem... The reason is inside the XE - the output signal is not fully correct. The worst case is with the video-signal on the DIN output, but also the "antenna" output is affected a bit. The monochrome signal (also available in the DIN connector) is OK, as well as all outputs from an XL-type computer. To correct this problem, you need one resistor 220 Ohm, and one PNP transistor. (I used BC556, because it's easily available and cheap around here, but almost any universal PNP type should work.) Open the computer (as described in the previous section), and look at the corner of XE-board, where is the TV modulator. As shown on figure 4, you should cut one way at the solder side, and then add the resistor and transistor. These components should be soldered onto the terminals of modulator, and connected with a wire to the other end of the way you've cut - as shown on the figure 4. Note that the components may not protrude above the level of modulator's top side - because of the shielding cover. After testing the function, close the computer, as described above. (You may also cut one of the modulator legs instead of the way on PCB, and use the rest on XE-board instead of that long wire. But since the legs are quite thick and hard, you'll need a very good tools in this case.) It should work without of problems. After installing this simple upgrade, the output signal from your XE is correct for all TV's you might meet at friends or computer parties, just like the output from an XL. More information: The problem of original XE output is that the size of synchronization pulses is reduced. The monochrome output (correct) is copied from the XL. But the original resistors for mixing color into the composite video output were replaced with a different type of TV modulator in the XE. It's a bit better solution (the color signals are not going back to the monochrome output), but there's another transistor inside the modulator (see the figure), so that the whole signal is moved more than 0.5V down. After that, the synchronization pulses should run into negative voltages, but since this is impossible, the output stays on zero - and reduce the synchronization. The added transistor is just moving the signal up (the mentioned >0.5V) before the modulator, so that the output is correct. ----------------------- P.S. If you've got this text on disk, you'll find the figures in separate files - in the form of GRAPHICS 8 pictures.