Nateo Posted January 18, 2022 Share Posted January 18, 2022 Hey all - got an issue with this ColecoVision I've been cleaning up, and it's behavior is unlike anything I've come across. And before you ask, yes, I've cleaned up the power switch and I'm getting the proper voltages ? The RAM I've heard can be suspect, but I kinda doubt RAM would be enough to cause the screen to wig out to that degree. Just wanted to see if anybody had some ideas for possible suspects. Quote Link to comment Share on other sites More sharing options...
Nateo Posted January 18, 2022 Author Share Posted January 18, 2022 Also, is it normal for Mostek RAM ics to have legs that look like this? Quote Link to comment Share on other sites More sharing options...
allansim Posted January 18, 2022 Share Posted January 18, 2022 I wouldn't say the stuff on the legs looks normal but I can't see how that would affect them working or being an indicator of a problem with them. Check continuity to PCB? Slowed down the video to notice this but it looks like its automatically starting a game at the options menu screen and when the game starts, the screen freakout happens. If you're sure its not the cartridge and you tried other games that do the same thing I'd remove the 2 ICs that control the joystick inputs 1 at a time to see if that stops the automatic selecting happening and then if it does (assuming one IC is still good) use the good IC to try to select a game start. If the screen craziness still happens then and with a different game my guess is either dirty cartidge slot, bad ram (this I think is likely), random cold solder joint on pcb. If the auto-selection and screen freakout happens without those ICs obviously they aren't the cause then. Tempted to say its the RF box or the display mod it but that wouldn't explain all the @'s up the side of the title screen. I'm also tempted to say its your power supply/switch but you say you've checked these. 1 Quote Link to comment Share on other sites More sharing options...
Falonn Posted January 18, 2022 Share Posted January 18, 2022 The pins on my (working perfectly) RAM ICs looked exactly the same. You can clean it off with IPA and some Q-tips... but I didn't see any evidence of it hurting anything. Quote Link to comment Share on other sites More sharing options...
Nateo Posted January 19, 2022 Author Share Posted January 19, 2022 Just checked the data lines going to VDP. They turn to garbage right after the BIOS, which makes me think that the 4116s are the next suspect to interrogate. Luckily, I have some working extras on hand that I can use as replacements. Man, I could really use a retro chip tester in situations like this... Quote Link to comment Share on other sites More sharing options...
Nateo Posted January 19, 2022 Author Share Posted January 19, 2022 I took out two 4116s, and they are far and away the most difficult things I have ever desoldered. Those things have rust eating through nearly every pin, and its making getting them off the board a giant pain. I've only managed to coax two chips to come out, and I've already pulled a few pads and a trace. In other words, the RAM is a basket case. I'm half-tempted to design a PCB daughterboard for the fresh RAM so I don't have to worry too much about damage done to the motherboard PCB while I take out those rust-eaten chips. 1 Quote Link to comment Share on other sites More sharing options...
+-^CrossBow^- Posted January 19, 2022 Share Posted January 19, 2022 12 hours ago, Nateo said: I took out two 4116s, and they are far and away the most difficult things I have ever desoldered. Those things have rust eating through nearly every pin, and its making getting them off the board a giant pain. I've only managed to coax two chips to come out, and I've already pulled a few pads and a trace. In other words, the RAM is a basket case. I'm half-tempted to design a PCB daughterboard for the fresh RAM so I don't have to worry too much about damage done to the motherboard PCB while I take out those rust-eaten chips. The easiest way to remove those 4116s from the CV is to clip the legs near the dip package and then use some tweezers and the iron to lift the legs out of the vias. From there, use the braid to remove the solder from the vias. Even with my desoldering iron, I still have to be very careful as the traces on the CV are quite fragile and that is especially the case on that vram section of the board. 2 Quote Link to comment Share on other sites More sharing options...
Nateo Posted January 19, 2022 Author Share Posted January 19, 2022 3 hours ago, -^CrossBow^- said: The easiest way to remove those 4116s from the CV is to clip the legs near the dip package and then use some tweezers and the iron to lift the legs out of the vias. From there, use the braid to remove the solder from the vias. Even with my desoldering iron, I still have to be very careful as the traces on the CV are quite fragile and that is especially the case on that vram section of the board. I imagine clipping out those pins is most easily done with diagonal sidecutters? If so, I should probably pick up a pair before I continue to add to my lifted trace/pad total. 1 Quote Link to comment Share on other sites More sharing options...
+-^CrossBow^- Posted January 19, 2022 Share Posted January 19, 2022 6 minutes ago, Nateo said: I imagine clipping out those pins is most easily done with diagonal sidecutters? If so, I should probably pick up a pair before I continue to add to my lifted trace/pad total. Yes... a pair of side/flush cutters is really essential in this kind of work. Not just for this particular situation, but also to help trim leads and cut shrink tubing... and.... Quote Link to comment Share on other sites More sharing options...
Nateo Posted January 19, 2022 Author Share Posted January 19, 2022 49 minutes ago, -^CrossBow^- said: Yes... a pair of side/flush cutters is really essential in this kind of work. Not just for this particular situation, but also to help trim leads and cut shrink tubing... and.... Well, I have sidecutters, but I didn't know if perhaps it would be good to have one that's angled in a certain way? Or if there was such a thing for such precision work? My sidecutters can't quite get all the way in-between the pins to cut them clearly. Quote Link to comment Share on other sites More sharing options...
+-^CrossBow^- Posted January 19, 2022 Share Posted January 19, 2022 Sounds like you are describing what I refer to as just wire cutters. But yes I mean the side flush cutters where one side is flat and really helps to get in there to tight spots to clip things. I use the Plato blue handled ones for 97% of my work that you have likely seen elsewhere. https://www.amazon.com/PLATO-170-SHEAR-CUTTER-piece/dp/B000AS3BLM 1 Quote Link to comment Share on other sites More sharing options...
ChildOfCv Posted January 19, 2022 Share Posted January 19, 2022 I've also thought about the possibility of using a daughter board to replace the individual chips with one big one. It would then have to fan out the data connections back to their data out connections at each individual chip site. And it would need the extra latching hardware to deal with the RAS/CAS/DATA-IN style of memory access. 1 Quote Link to comment Share on other sites More sharing options...
Nateo Posted January 20, 2022 Author Share Posted January 20, 2022 1 hour ago, -^CrossBow^- said: Sounds like you are describing what I refer to as just wire cutters. But yes I mean the side flush cutters where one side is flat and really helps to get in there to tight spots to clip things. I use the Plato blue handled ones for 97% of my work that you have likely seen elsewhere. https://www.amazon.com/PLATO-170-SHEAR-CUTTER-piece/dp/B000AS3BLM Now that's what I've been needing all this time! Thanks for the link and the recommendation. I did manage to get the rest of the chips out, and by the time I was taking out the last four chips, I got to where I didn't lift any traces or pads. Kinda sucks that I'm gonna have to do some major bodge work to get the circuit going again, but it was a valuable experience dealing with something both incredibly stubborn and incredibly fragile. I have extra 4116's from when I upgraded the memory in my CoCo 1, but I think I might just go ahead and do the 4164 memory mod. Might as well, eh? Quote Link to comment Share on other sites More sharing options...
Nateo Posted January 20, 2022 Author Share Posted January 20, 2022 51 minutes ago, ChildOfCv said: I've also thought about the possibility of using a daughter board to replace the individual chips with one big one. It would then have to fan out the data connections back to their data out connections at each individual chip site. And it would need the extra latching hardware to deal with the RAS/CAS/DATA-IN style of memory access. Should be plenty do-able. Thank God clear schematics for the CV are available. Quote Link to comment Share on other sites More sharing options...
Nateo Posted January 20, 2022 Author Share Posted January 20, 2022 I decided to bodge the points together that were lifted, since there weren't as many as I was fearing. And besides, I get to do my first trace repair! Fun! 1 Quote Link to comment Share on other sites More sharing options...
Nateo Posted January 22, 2022 Author Share Posted January 22, 2022 (edited) Update! I got the random character to go away once I replaced the RAM and repaired the traces. Cool! BUT - it still goes to garbage after the splash screen. My hunch is that the BIOS IC is bad as it is also a crummy Mostek chip. I can't help but wonder too about the 74LS138s, as I'm not getting any distinct waveforms out of them on their output ends, just pulling high or low. Shouldn't I be seeing some kind of data coming out of those multiplexers? I wish the CV technical manual had diagrams for how those chips are supposed to look. The Z80 and VDP looked OK when poking around on my oscilloscope and comparing with the diagrams in the technical manual. EDIT: I should also add that the controller ICs are good per comparison against specs outlined in the technical manual. Edited January 22, 2022 by Nateo Quote Link to comment Share on other sites More sharing options...
Nateo Posted January 22, 2022 Author Share Posted January 22, 2022 Also, I sometimes get a fixed tone on boot. Combing through the forums, I found that this might indicate the 74LS138s being shot. Is that worth replacing? I have a couple on hand from a previous repair project. Quote Link to comment Share on other sites More sharing options...
ChildOfCv Posted January 23, 2022 Share Posted January 23, 2022 74LS138s will pull a single line low when the enable lines are correct, according to the 3-bit index on the inputs. Only one output should go low at any time. 1 Quote Link to comment Share on other sites More sharing options...
Nateo Posted January 24, 2022 Author Share Posted January 24, 2022 4 hours ago, ChildOfCv said: 74LS138s will pull a single line low when the enable lines are correct, according to the 3-bit index on the inputs. Only one output should go low at any time. Good to know. Thanks for chiming in, I'll give it another test once I get the fresh ROM chip in (that one was also a Mostek, and looked like hell when I took it out) Quote Link to comment Share on other sites More sharing options...
Nateo Posted January 24, 2022 Author Share Posted January 24, 2022 UPDATE: IT WOOOOORRRRKKKSS!! The RAM definitely was bad, but that wasn't the whole story. Bad VRAM might have caused graphical glitches, but the major issue was the BIOS. It couldn't initialize past the startup screen. I ordered a Fast BIOS from Console5, installed it, and it works great now! What a thrilling victory! 4 Quote Link to comment Share on other sites More sharing options...
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