tnt23

So I decided to try S-Video: lifted C28 and fed it via spare THS7316 channel to pin 4 (C), and connected Composite output to pin 3 (Y): Blue looks absolutely gorgeous. Red calls for help, and white background is decorated with some green hints. Vertical lines test is also great on Blue. I think that's because the top half of BURST mux E3 outputs zero when Blue is to be displayed: While for Red and Green (and black) 3.58MHz subcarrier is present on BURST line. Still I don't understand this.

Did a better grounding, and indeed it results in wide horizontal color bars. Cool for hacking, don't know what I could use these for Regarding empty bits 0, 1, and 4 on 74145, I guess they default red background to black maybe?

Yes, on my PCB 74157 is soldered, not 9322. As far as I understand, test points marked A5-xx and C6-xx relate to empty sockets on the PCB. Handy if you want to check certain signals, but also can be useful for debugging. For example, TP A5-14 can be grounded, and should select 8V and 16V clocks to define BG0 and BG1 colors instead of those coming from VRAM. (Did not work in my case, don't know if it was me or 74LS298) Interesting to see I1C and I1D inputs routed to VRAM but QC and QD outputs not used. Some rudiments for more colors perhaps? Similarly, TP A5-14 can be grounded to disable pixel data coming from VRAM, forcing all zeroes. TP A5-15 disables Burst Gate flip-flop D4 (and probably ruins video sync), which also disables palette data storing in C4, and so on.

Judging by schematic, CA1 should be 1 for Red, just like CA0 for Green and Blue. CA0..CA2 are produced by 9322 mux (also marked as 74157 on Luxor schematic). CA1 is either FG1 or BG1 depending on FG ENB signal, and the latter is active if FG0,FG1 pair is non-zero.

(sorry for extra noise in the thread) Another combination of shots showing CA0 and CA1 lines with respect to output composite signal (yellow). I don't quite like the CA1 to be honest.

You are right, chroma indeed shall be everywhere on the line. I got some comments from a friend, it appears that BURST and CHROMA in fact are I and Q components from NTSC theory, and get mixed or combined down the line. Will try replacing the 74153 mux however. BTW this is the picture without Chroma (L7 lifted). I'd say everything is just about perfect on Luma side of things.

Here are shots of BURST and COLOR signals (cyan channel 2) sampled at pins 9 and 7 of 74153. Yellow channel 1 is Composite output. CHROMA + zoom: Chroma looks good. I believe different shapes of chroma carrier pulse trains are due to phase shift for Red and Green vertical lines in the beginning of the scan line. BURST signal + zoom: Now there's something strange about BURST. I would expect the burst to be active only in the beginning of scan line, on back porch of HSync. Yet it seems that burst is also active on visible part of the line. Can it be that these extra pulses (with 0 phase shift) get mixed to CHROMA ones (with non-zero phase shifts for different colors) resulting in that odd color distortion? Exception is Blue, third vertical line of the test. I have also removed the RF modulator shielding and was able to locate and disconnect L2. Not much improved on color picture, but BW-wise extra shades of grey to the left and to the right of dark vertical lines are almost gone. burst.bmp

Thanks for your input e5frog. Since I haven't separated chroma yet, I cannot tell if its amplitude is OK. I have played a bit with chroma outputs, shorting them to ground with 0.47 cap. Disabling BURST turned the whole display black and white, however ghosts were still there. Then I have shorted CHROMA the same way, and ghosts were gone. I'll post some scope shots later. Checking ceramic caps sounds like a good idea. For the output, I have hacked a quick video mod using THS7316. Trimmed its input level so that output is around 1Vpp, display appears a bit dim but that's OK for the time being. It would be handy to have a PCB drawing showing at least components positions. Most caps and passives have their references printed on the PCB right under them.

Greetings all, what a relief it is to find a topic on Channel F, and even more joy to discover it is live I've been fixing a first generation unit for a friend recently, replaced an electrolyte or two and got composite video out. The picture I got had quite a bit of ghosting, and colors were clearly not looking right. My friend then hacked a video test binary for me to ease the debugging. The test displays white vertical stripes one pixel wide on black background, or R, G, B stripes on white background, and toggles between them with any button. From what I see, and judging on some oscilloscope shots I took, looks like chrominance sneaks too much into luminance, effectively making single pixel line into three or four colored ones. I will try to separate the two to see if this is the case, until then if somebody has any ideas please chime in.

Seems like .STX format details have never been published by PASTI? I would like to load .STX images directly in my floppy emulator. I've already added support for .MSA disk images, thanks to the fact that .MSA format is well documented. If this is not the case - oh well, will have to stick to .ST and .MSA.

Could anybody please point me to the description of the .STX floppy image format? I've got a bunch of those, and would like to use them as is, without need to convert to .ST or .MSA.

Great piece of knowledge, thanks!

OK, what is this additional decoding logic needed for 2.06 to be put into MegaST? While waiting for my 1M chips to arrive, I've dug out a bunch of old 512K chips to try with English TOS 1.02. As these are twice as big as original 256K ones, there probably could be a way to use 4 of them instead of 6. The W2/W3/W4 table from the service manual only mentions 256K and 1M chips. Looking at the schematics I can see that W3 and W2 are responsible for A15 and A16 address lines, respectively. For 512K chips I guess only W3 can be switched to 2-3 position?

Thanks tjlazer, I'll first try to use those 27C010 chips I've already ordered

Well, I won't mind running English 1.04 Are those GAL chips in IDE interface for Mega STs about this additional decoding?

Hello all, I've got a Mega ST4 with German TOS in it (6 ROM chips). I would like to replace it with English version. Sysinfo says it is version 1.02, and I've thought about upgrading to 2.06. I have ordered a couple of 27C010 chips, and now am slightly confused as these are 32-pin DIP chips, and ROM sockets on ST4 take 28-pin DIP ones. I understand I will have to switch some wire jumpers (W2, W3 and W4) and possibly install U12 chip which is missing on my motherboard. Could anyone please give me directions on how to wire 2 32-pin chips for this setup? Thanks!