Falonn

"Hopefully in a week or so" was probably too optimistic! But I have some exciting progress to report: I've built the TMS doc schematic on a breadboard, during which I discovered a half-dozen mistakes in the text and schematic (some minor, some not so minor). Most of the problems had "easy" answers (specifying a 0.01 pF cap across the supply pins of one op-amp when the other two have 0.01 uF is pretty straightforward, etc.) After reconciling all of that, doing the smoke test, and tuning the pots as described, I connected it through my Framemeister and lo and behold: a beautiful, crisp, sharp, bright, lovely image... with a very strong blue cast! The original, nearly 40 year old, built exactly-as-specified TMS doc circuit doesn't solve the "there's too much blue" problem! So much for putting that one on a pedestal. Lots more to come, but here's the first bit of evidence from the scope: An H-sync pulse (of an all-white screen) showing the C-sync out line (in yellow) and Blue out: Instead of a proper colorburst, it seems to drop below the 0V level (which is presumably tricking the TV into believing the blue output range is much wider, making 0V an already-strong blue). The same, but with all four output channels labeled: I suspect if we can find a way to hold the color lines at 0V for the 5us after the pulse ends, the blue problem will be solved. Curiously, just on both sides of the V-sync pulse (2 lines before and 12 after), those extra negative-going bits on the Blue output seem to disappear: It was late last night, so I wasn't able to investigate as thoroughly as I would have liked. If anyone has any requests for traces they'd like to see, just point me at the appropriate net on the schematic and I'll try to capture something for you.

That's awesome. At fifty cents, I wouldn't mind clipping a little bit of plastic off each one either! hehe. Is your local shop part of a chain? It'd be cool to check around and see whether I've got a shop around here like that.

Yeah, that $9 is the biggest drawback. Hmm, they do look very similar. Is the square border on the top a little... I want to say "rubberized"? I think this particular model is incidentally IPX rated to be a little waterproof (which probably goes a long way toward explaining the price), so there is a little gasket on the underside and the top has a slightly rubbery feel. (I'm also curious what that texture will mean for dust collection over time.) The "1" or "I" direction is rotated, too, but I think that's an option at DigiKey. There is another part number (one digit off from this one) that is the same switch with the labels rotated. I picked the one where it would follow the same reading direction as the words on the chassis next to it.

Are you sure they're the same model number? Without any modification on either side, mine snapped in nice and tight with no play or other wiggling at all. (It can still be removed fairly easily by squeezing the two grippers from the underside.)

I was having trouble desoldering mine, so I ended up cutting the switch's legs off. After that the desoldering was easy but the switch was beyond repair. So your assumption of competence might have been bigger than you expected. 😅

I wasn't making any assertions, just reusing the wording I've seen in dozens of forum topics and tutorials. All I can vouch for personally is that this particular ColecoVision developed its power switch problem in 1984 and has been in a box (stored indoors in a cool, dark place) ever since. My father wasn't much of a troubleshooter, so he just bought a second system at the time and used that one instead. Almost 40 years later, I pulled "the broken one" out of the box, saw the graphics corruption (described everywhere I looked as a poorly functioning 12V rail due to the switch), bypassed the switch, and now it runs perfectly again. That one only lasted 2 years, not 30, which is decidedly less impressive. I agree the original look/feel would be best. I tried to find a replacement board-mount switch with similar'ish dimensions (even if it meant a tiny 3D-printed "adapter" that needed to be inserted between the switch and the original plastic insert. I wasn't able to find anything with the right contact spacing or throw distance, unfortunately. I suppose a small adapter PCB could solve the problem of the contact spacing. That might not be a bad way to go to get the best of both worlds.

Digging through all the tutorials on replacing the original, prone-to-corrosion Coleco power switch, they all seem to mention the same Radio Shack part number (275-691). Opinion-wise, the red doesn't match as closely aesthetically for my tastes. Worse, the tutorial videos show that the nut on the back of the switch requires you to cut some tabs off the inside of the case. I wanted to find a switch that wasn't red and didn't require any case modification. After dredging through the switch catalog at DigiKey, I found this part that snap fits perfectly into the original case with no modifications required. Here's the 10 second video with the (satisfying) sound of the switch snapping into place. Once it's been clicked in, it's nice and solid with no jostling. It also seems to be a little closer to flush with the case vs. the Radio Shack part. The part is JWMW21RA2A from NKK Switches. The DigiKey part number is 360-1718-ND. Hopefully this will spread the word if anyone else is looking to keep their case in its original condition!

The new schematics are reversed (incredibly thoroughly) from a Rev J Coleco PCB, over here.

I'd gotten as far as "disconnect when external video is enabled" while staring at it, but I'd lost the thread in the tangle after that. Again, now that you've filled in the blanks, that seems plausible. A "clean Y" output seems to be the only thing missing from my scheme of "just attach the mod to the RF riser pins". I suppose mounting on the back of the VDP and including three extra/duplicate coupling caps isn't the worst thing. The external video case will be broken either way. (And it's beyond the scope of the simple YUV'ish-to-RGB conversion I'm interested in to try and de-composite the external video from Expansion Module #1 to work through the same RGB output.)

Yes, of course you are correct. Thanks! Checking just now in ChildOfCv's new Coleco schematics, they are R40, R41, and R42 on the "A/V Switcher and Preprocessor" page. Every time I spend an hour sleuthing and decide I've discovered some new, key secret it seems to only take one sentence to bring my house of cards down. 😅 It's tricky operating in a subject where your assumptions and ideas are still wrong about 80% of the time. I suspect I'm in way over my head. Looking at that preprocessor page, there's already something sync-strippy-looking going on there. It would be a shame to lose the option of mounting right to the back of the VDP, but I wonder if a similarly-easy install point wouldn't be on the J4 ("Conn_01x08") connector that runs to the RF board? All of the (post-filter, post-coupling, cleaned-up, etc.) signals appear to run there already. Ah ha, thanks! Now that you made the connection for me, I vaguely recall reading about needing to activate RGB mode in those TVs somehow. My plan is to target the same 8-pin mini DIN Retro Access cable that Citrus mentioned (which was coincidentally delivered in the middle of typing this post)! I just popped the SCART end open and there's a 180ohm resistor splitting off the +5V line over to another pin. I haven't continuity tested it, but I bet it goes to pin 16. So it is no longer an open question! With that cable, the resistor doesn't need to be on the board. Thanks again for clearing that up.

One more interesting detail: While digging around in the TMS doc some more, I noticed this curiosity on all three (Y, R-Y, and B-Y) input lines. While trying to figure out what sort of purpose it might have, I found this line in section 3.4.2 (page 3-6) of the 9900-series VDP datasheet: "The internal output buffer devices on these pins are source-follower MOS transistors that require an external pull-down resistor to VSS. A 330ohm resistor is recommended." Despite the 330 recommendation, the diagram on the same page shows 470. The 390 in the TMS doc gives the impression it doesn't matter much. In section 3.4.1 (for the 9918A) just prior, it mentions "A lower resistor value [on the composite output] gives faster fall times and a sharper picture". I wonder if the same isn't true for the component'ish outputs. In any event, that pull-down is missing in all the other known RGB converters. Do you suppose this additional "out of spec" detail may have helped contribute to some of the problems experienced by the people trying to use these things in the past?

While waiting for my DigiKey stuff, I wanted to get all my ducks in a row. Back on page 8, ChildOfCv mentioned a missing capacitor between the Hackaday schematic and Citrus' PAL board. After spotting two other parts missing from the BOM (vs. Hackaday), I wanted to go through with a fine-toothed comb to see what else was different. So I also reverse-engineered it and then compared each net between the two (rather painstakingly, hehe). Citrus3000psi's board implementation of the Hackaday schematic is almost identical with the following differences (highlighted in red in the attached schematic): Citrus is missing the ferrite bead on the 5V input line. This seems reasonable given the board's mounting style and resulting short/non-existent trace length from the graphics chip. Citrus is missing the 270R on the 5V output. I'm not sure why it was there in the first place. Maybe something MSX-specific? SCART specific? This is still an open question. Citrus is missing the 0.1uF coupling capacitor between the Y input and the LM1881's Video-In pin. This is what ChildOfCv noticed before. Right in the LM1881 datasheet, it says "The only required external components [...] are the composite input coupling capacitor at pin 2 [...]," so this definitely seems like a mistake. The Citrus board has a typo: the resistors near U5 are labeled "R27 R26 R27". One of the two 27s should be a 28. Luckily, the resistor values are also on the silkscreen and they are correct. So, if someone is building one, they should confirm both the part name and values there. The component designators are all different. I can't imagine how this would be useful to anyone, but I've attached a CSV with the mapping between the two. Given the missing cap is a mistake according to the datasheet and that it's the only substantial difference between the two, I don't think I'll track them as separate variants when recording data for the upcoming RGB mod shoot-out. So the plan is still to test the three circuits that I know about: from the 40 year old TMS doc, from the Hackaday post (equivalent/identical to ColecoRGB PAL), and Citrus' ColecoRGB 1.2 ("NTSC") board. ColecoRGB PAL BOM comparison.csv

@MrPix That's starting to sound like you might be heading toward FPGA territory (along the lines of the F18A mod or even the CollectorVision Phoenix). With things like the OSSC (and Framemeister, etc.) making RGB input more accessible to everyone, my own interest would be to keep the mod as unobtrusive/simple to install with the smallest reasonable BOM. Those upscalers can already do a lot of the fancy cleanup work afterward, so it seems like a duplication of effort to add that sort of thing to the console. In other news: last night I placed the DigiKey order to build all three (TMS doc, Citrus, and Hackaday) RGB boards using their (99%) originally specified parts. The two 2N2907's from the TMS doc are even in the little metal can package. Hopefully in a week or so, I'll have some Epic Colecovision RGB Board Showdown screenshots from my scope. Then we'll be able to quantitatively compare the performance of these things (against an NTSC board).

I meant the oscilloscope-required, 1.0V peak-to-peak on each R, G, B line tuning described in the TMS document and used on the citrus board. Since page 4, MobiusStripTech has been cautioning DIY'ers without a scope that it's "basically impossible to get perfect [by eye]". It'd be cool if the Vref out from the TL081 in the Hackaday circuit was doing some sort of auto-scaling. Even if that still meant something like a single pot that you can adjust by eye ("raise it until you see clipping, then lower it a little"), that'd make it a lot more accessible to hobbyists. Hmm, re-reading the line on page 2 of the TMS doc, it sounds a little less vital: "Variable resistors R52, R53, and R54 have been included in the circuit in order to accommodate monitors requiring more (or less) than one volt peak-to-peak of drive, or to allow colors to be adjusted to suit a particular application". I vaguely recall someone (earlier in this thread?) describing that they'd bottomed out one of the pots on the citrus board and still couldn't get the colors right. Maybe that was more due to the missing DC restoration step than a finicky, ~40 year old graphics chip?

Oh, hey, that schematic is lovely. It neatly matches the TMS document but with the 1881 supplanting the DC Restoration and Sync Separator blocks (while still using the same 4066's to drop the color burst). The only thing that creeps me out a little is the lack of trim pots to do any sort of fine tuning. I don't see any extra magic tucked in there that wasn't in the TMS doc (except that extra Vref output from the TL081 that is fed back into all the other amps?) and TI seemed to still believe they needed the ability to do manual adjustment. Hmm. Now I'm even more interested in doing a sort of head-to-head battle where I build all of the above and compare their outputs on the scope (and screen). Then some hybrid (probably this Hackaday version + pots), opensource board could be the ultimate solution that works for everybody in every region. It looks like I need to place a Digikey order and build one of those all-white output test ROM cartridges...

@MrPix Yeah, digging through the LM1881 datasheet just now, they even mention (in section 8.1) that you can use the burst gate/back porch pulse from the IC for some "DC Restoration" step for some potential RGB output, which makes it sound like that single chip fills in virtually all the missing pieces. I only glanced at the LMH1980 datasheet for a couple minutes, but at $10 and with so many mentions of HD format detection and tri-level sync detection, I wonder if it isn't overkill. The LM1881 seems like it's exactly what we need. I was thinking of grabbing a couple of them in the DIP package and tinkering around with some of this on a breadboard. After poking around with my scope for a couple hours last night, these video signals don't seem as scary anymore. If you needed any help, MrPix, with testing or just getting a confirmation that something works on someone else's board, I am happy to help! For my part, I only need/care about RGB output. I'm curious why you had mentioned buffering the RGB output a few posts back? Are there any benefits? All I can imagine is that you'd be introducing an extra source of latency. @ChildOfCv Only tangentially related: I've been repairing an old Coleco board that "stopped working" that we've had since I was a child. Upon opening it, the ground straps were already disconnected, all four power lines had been desoldered and were loose in the case, the heat sink on the 9928A was missing, the leg on one of the 0.1uF caps next to a VRAM was cut and partially desoldered, the resistor by the expansion port was missing... it was quite a crime scene. I eventually deduced that sometime between it "stopped working" and today, a (presumably teenager-at-the-time) older brother of mine decided to use it for (de-)soldering practice. I didn't know the values of the missing components, so I went searching for schematics. As a happy coincidence, I found your excellent schematics from your very first post here. That made repairing the interim damage much easier. The original problem turned out to be the usual corrupt graphics due to a corroded power switch---also an easy fix---so the board is back up to 100%. So, you've now inadvertently helped me one more time than you knew! Thanks again! hehe.

I'd read through the Wikipedia entry on Norton amps, but didn't feel like I got much out of it. Following along with your example just now, that was very instructive! Thanks! I was poking around the "Sync Separator" block in the TMS document (unsuccessfully) trying to work out the analogous example to see if it would do any better, when I discovered a curiosity: for whatever reason, it looks like they're outputting C-sync twice. Worse, R30 and R31 appear to have the only unspecified values in the entire design and aren't mentioned anywhere in the text. (Is that some convention with resistor divider networks? Is that shorthand for "use whatever you like, as long as they're equal" or something along those lines?) Calling both of those "composite sync" is the part of circuit analysis that drives me a little crazy. With one coming after an additional transistor stage, they couldn't possibly be the exact same signal (and if they were, why bother with the second transistor)... so which one is the "right" one? If I wanted to build this thing verbatim, would it be safe to just leave out the R30/R31 leg and treat the circuit as only having a single C-sync output (coming off the R29/R32 divider)? Or is the right answer to let the LM1881 / LMH1981 worry about that part in a modified design? (There is a certain peace of mind knowing that TI graybeards effectively blessed the old schematic for our exact purpose. I'm half-tempted to build the old design exactly as specified, old part numbers and all...)

I feel like some variation of "if only we had a schematic..." has come up in this thread a dozen times now. My wife took my four year old to a birthday party today, so I had a quiet afternoon to myself. After following traces in a paint program for a few hours, then tinkering around in Eagle for a while, here is the reversed schematic for the ColecoRGB 1.2 board to the best of my (rather limited) ability. Now we've at least got something to look at. Things I learned: 1. Unless I'm missing something (I usually do when looking at these things), this is just the "R-G-B Converter" block. It looks a lot closer to the German forum post that citrus3000psi mentioned in post #3 than it does the TMS document. 2. Norton op-amps are strange. I'd never seen those I_set lines before. 3. Going board-to-schematic uses all different parts of your brain than schematic-to-board. It's not unlike putting together a jigsaw puzzle. hehe. It's (well) beyond my skill level to make any sort of meaningful comparison between what's going on (or missing) here vs. the TMS document. So, the best I'll hope is that by doing the tedious part, someone else might be able to make heads or tails of it a little more easily! ColecoRGB 1.2 schematic.pdf

I've been following this topic for more than a year and it's been exciting to see the flurry of new activity in the past week. Something ChildOfCv said finally made me decide to come out of lurker mode to ask some questions: I spent a few minutes cleaning up the image from the TMS document: removing the 3.58MHz clock, the "Composite Video Encoder", and the "PAL Encoder" blocks. This version is a lot easier to look at. (See attached.) Now for questions: 1. Was the LM1881 recommendation just a simple reduction in discrete component count? Or does the LM1881 do something better/smarter than the "Sync Separator" block in the TMS document? It certainly seems to do more, looking at the block diagram in the LM1881 datasheet. With V-sync (and almost H-sync) output, you'd nearly get VGA for "free" in addition to RGB, right? 2. Judging by component count (and the lack of CD4066Bs, transistors, diodes, etc.) it looks like the citrus3000psi board has only replicated the "R-G-B Converter" block from the TMS document. Is that correct? If so, why aren't the "DC Restoration" and "Sync Separator" blocks from the TMS document also needed? Their outputs seem to feed directly into the "R-G-B Converter" block's inputs. 3. Most/all of the R and C component values used on the citrus3000psi board are different than the TMS schematic. Even things like the LM318's take -5V and +12V vs. the single +5V inputs to the LM359M on the citrus board. Is this just because we have access to better rail-to-rail op-amps now? The Coleco board gives us easy access to -5 and +12, so hopefully it wasn't left out just to simplify the circuit. If we were to build exactly the circuit as presented in the TMS document (with the unnecessary composite blocks removed), would it perform better than the citrus board? 4. Kind of a bonus: wouldn't it be easier to get component video out? Couldn't we drop most of the "R-G-B Converter" block, leaving just the 1.14x and 2.03x correction factors but not adding Y to B-Y and R-Y with the op amps? Isn't that already a "native" component video signal? Thanks!