cdn2a Posted January 7, 2016 Share Posted January 7, 2016 Hello, I'm new here, I'm trying to fix my coleco. Maybe someone can help? I have no video, just a black screen. No coleco title screen, with insert cart. I started with all the usual stuff, power switch etc. With expansion mod 1 plugged in I do get video, and sound. I can assume the RF board is fine? I've followed the flow charts in the repair manual. I have both the 3m and 10mhtz clock signals. The sound is off, no lingering tone. I suspect the processor is fine and the VDP is shot? How does one verify this? To me the rest is all TTL logic, I have more than one board, so short of brute force and ignorance, socketing the bios, vdp and processor, and swapping until it works? hints, tips? Anyone? Thanks. Quote Link to comment Share on other sites More sharing options...
+nanochess Posted January 7, 2016 Share Posted January 7, 2016 Try a Frogger cart and check sound directly in SN76489 output with earphone. (it sounds better with a .1 uf capacitor in series) You can connect directly a composite RCA to the video output pin of TMS9928 for testing directly VDP, a good VDP always generates synchro right for NTSC. Quote Link to comment Share on other sites More sharing options...
Nebulon Posted January 7, 2016 Share Posted January 7, 2016 Some nifty reference info: http://www.colecovis...tries/tech.html http://www.colecovis...ision tech2.pdf Quote Link to comment Share on other sites More sharing options...
cdn2a Posted January 7, 2016 Author Share Posted January 7, 2016 Nanochess, You mean pin 36 on 9928? the Y signal? The screen does go from snow to black. Correct me if I'm wrong, but I thought the 9928 sent the y, r-y, b-y signal to the LM1889 to be encoded to be composite? Nebulon, Those are the exact manuals, I've been using... I'm on sheet 5, and sheet 9, both deal with video problems. On sheet 9, U9:34 low? = Mine is high... -> U7:13 high? Yes mine is -> = "Check U7/U1" That to me says change processor or U7, U7 runs the reset switch, and that seems to work properly. However on sheet 5, U9:40 good? yes, I have clock signal -> U9:36 good? -> No picture, so I think, no -> Change U9 (VDP) Soooo brings me back to is it bad processor or VDP? Thanks for your guy's input... Quote Link to comment Share on other sites More sharing options...
cdn2a Posted January 7, 2016 Author Share Posted January 7, 2016 Ok, so to update.... I have pulled and swapped the VDP, same issue. On to the processor... Quote Link to comment Share on other sites More sharing options...
+nanochess Posted January 7, 2016 Share Posted January 7, 2016 The Y signal includes synchro. If it isn't stable then your VDP is bad. Just make sure your VDP clock input reads 10.7 mhz before replacing. Quote Link to comment Share on other sites More sharing options...
cdn2a Posted January 7, 2016 Author Share Posted January 7, 2016 ok, so I have exactly 10.74mhz on pin 40 of the vdp... pin 36 seems ok, I get black screen, however no coleco logo or insert cart message. On the CPU pin 27 M1 seems to stay low, like it's stuck trying to read from memory. CPU then BIOS? Quote Link to comment Share on other sites More sharing options...
cdn2a Posted January 8, 2016 Author Share Posted January 8, 2016 did some more checking today pin 28 of the CPU RFSH does not cycle, always high. Quote Link to comment Share on other sites More sharing options...
cdn2a Posted January 8, 2016 Author Share Posted January 8, 2016 Sooo... Swapped out CPU have the same issue... So I guess It's on to the BIOS and the 2114 ram?... Buller... Buller... Anyone?... Quote Link to comment Share on other sites More sharing options...
+nanochess Posted January 9, 2016 Share Posted January 9, 2016 Check RAM temperature. Sometimes too high signals a bad RAM Quote Link to comment Share on other sites More sharing options...
cdn2a Posted January 10, 2016 Author Share Posted January 10, 2016 Thanks for the ideas Nanochess, I'm still plugging away at it, there's not too many threads out there, with people who have fixed black screen / no boot Colecos... This thing still has me perplexed... It looks like all the VRAM pin 14 goes low, so I believe the BIOS has attempted to clear them. I have exchanged the SRAM, with no change, and the BIOS, with no change. I guess onward to the VRAM, soon all but the TTL logic chips will be socketed... Not sure why there's still no BIOS screen... Quote Link to comment Share on other sites More sharing options...
Vectorman Posted January 10, 2016 Share Posted January 10, 2016 (edited) It can't be said enough - try a different power supply(s) before doing anything else. I've had dozens of CV's and the ones I often thought were dead(yes, black screen) ended up working perfectly when tried with a different PS. They're super finicky about PS's. No, it doesn't mean the PS you have is defective, your CV just might not like it. Edited January 10, 2016 by Vectorman Quote Link to comment Share on other sites More sharing options...
matthew180 Posted January 11, 2016 Share Posted January 11, 2016 Make sure the system is not being held in constant reset. 1 Quote Link to comment Share on other sites More sharing options...
cdn2a Posted January 11, 2016 Author Share Posted January 11, 2016 Thanks guys, I have tried 2 power supplies, same thing.... this weekend I pulled the pin and brought in a friends working coleco to test the chips on. (I know, scary) I confirmed the sram is good, audio chip, and the VDP chip is functional. I'm focused on Vram right now, however, on the last pages of the service manual, they show pictures of what it looks like when it's not working. It still shows the coleco bios splash screen. persistence is going to be the key... Quote Link to comment Share on other sites More sharing options...
+nanochess Posted January 12, 2016 Share Posted January 12, 2016 Clean around the crystal oscillator with alcohol. Check for 3.579545 mhz at pin 6 (CLK) of Z80. Check for reset pin of Z80 (it should be 1, only 0 when pressed) Check for activity in 74LS138 (the first port enable is to turn off SN76489A) Try Frogger, even if VDP doesn't work you should hear music at startup (using earphones in audio pin output from SN76489) Quote Link to comment Share on other sites More sharing options...
cdn2a Posted January 16, 2016 Author Share Posted January 16, 2016 Update, CPU Z80 is tested and confirmed working. VPU is tested and working. Audio 76489 is tested and working. Sram 2114 is tested and working. I have found a couple of bad Vram 4116 chips bad. Not sure if this would cause black screen, but once they are replaced, I'll update. Quote Link to comment Share on other sites More sharing options...
Jinks Posted January 17, 2016 Share Posted January 17, 2016 Mine has the same problem. I replaced vram chips as well still a black screen. Installed a Yurkie mod kit. Has sound and no picture well sometimes a black and white one without the rf daughterboard with the rf daughterboard the sound and picture are gone. Tried turning the pots on the daughterboard on rf and the screen changes but no sound or video. I will be interested in what you find. There must be a component that goes bad that causes this fault as no one yet has found. I have spent well over 50 hours on mine and am still at a loss. Quote Link to comment Share on other sites More sharing options...
cdn2a Posted January 18, 2016 Author Share Posted January 18, 2016 Hey, hey, another from the great white north? (With a picture your farther ahead than I am, if you have B/W pic then your into the RF section. I'd go after the LM1889) Update, So, I've replaced/checked and confirmed the vram is all working. I still do not get the bios splash screen, However, I do get some letters and random patterns, when pressing the reset button. They stay on the screen frozen. Since I know the chips are fine (tested on other machine) I'm going to check the board wiring/pin connections on ram. There are only a few chips left, and since this board is mostly sockets already, might as well keep going. Quote Link to comment Share on other sites More sharing options...
Jinks Posted January 18, 2016 Share Posted January 18, 2016 Yeah I tried tuning the pots on the rf and could get sound but no picture. Where to buy such chips? Glad your vram is good it sucks to replace. Wonder if a ay-3-8900 would work. Quote Link to comment Share on other sites More sharing options...
cdn2a Posted January 19, 2016 Author Share Posted January 19, 2016 The Vram had 2 bad chips, I pulled them all and socketed them. I had to pull them to test, (at least the way I could test) I checked the data bus lines and they all seem fine. I also swapped out U6 and U8, no change.... I did notice that U18 and U19 are connected to the data bus... D0-D7, I know those chips blow, I wonder if one of them is holding down the bus? I havent tried them, but it looks like the chips are available here... http://www.ebay.ca/itm/1PCS-LM1889N-TV-Video-Modulator-A615-LW-/171892149275?hash=item2805920c1b:g:7rAAAOSwu4BV4Tiq Quote Link to comment Share on other sites More sharing options...
+nanochess Posted January 19, 2016 Share Posted January 19, 2016 Man, check the decoders, the 74LS138 chip and 74LS04 Quote Link to comment Share on other sites More sharing options...
cdn2a Posted January 19, 2016 Author Share Posted January 19, 2016 Which decoders? U18 U19? I swapped out one of the 138 chips at U6, do you mean U5? I haven't touched the 74LS04 since I had a good clock signal, but I'll try that one next too... Quote Link to comment Share on other sites More sharing options...
+nanochess Posted January 19, 2016 Share Posted January 19, 2016 You're testing chips, but you need to check the decoders following this list and using a logic prober: * Z80, check the RESET pin working, CLK working, MREQ working. * BIOS chip (ROM), check that the chip selection is made. * 74LS138, check that the selection for SN76489 appears (first thing done in BIOS) * RAM, check that the chip selection is made. * VDP, check that the chip selection is made. One of these is failing, all are needed for the Colecovision BIOS showing the main screen. Typically if SN76489 outputs a fixed tone, then the BIOS isn't being read, so check decoders per schematic. Going on this from head, no time to check schematics. Quote Link to comment Share on other sites More sharing options...
cdn2a Posted January 19, 2016 Author Share Posted January 19, 2016 Thanks for your help Nanochess... I do have TTL logic probe... http://www.atarihq.com/danb/files/colecovision.pdf Z80 - Pin 6 - 3.58Mhz measured, Reset Pin 26 goes from Hi -> Low when reset button pushed. Mreq pin 19 has data. Goes high when reset pressed. Pin 22 seems to write data for a half second then stop... I'm not sure how to test chip selection?... 76489 chip pin4 has data too... There is NO tone... 10.74Mhz measured at pin 40 of VDP... There are 2 74ls138's do you mean U6 or U5? (I replaced U6 since it deals with the VDP) U5 pin 15 seems to have data is that bios chip selection? Quote Link to comment Share on other sites More sharing options...
omf Posted January 20, 2016 Share Posted January 20, 2016 try replacing u3 / u4 these were bad or one was in one of my machines and this caused blackness Quote Link to comment Share on other sites More sharing options...
Like us on Facebook
Follow us on Twitter
AtariAge on Instagram
Watch us on YouTube
Chat on Discord
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.