I'm currently trying to repair a ColecoVision and my Google searches make me believe that this is the forum where the experts reside
Originally the CV just produced a black screen and made a weird droning noise. I found that one of the SRAM chips and one of the DRAM chips had heavily corroded legs. I also checked all the busses with my logic analyzer. CPU-side looked normal to me, but the data bus of the VDP just remained low aside from a couple of quick bursts at the beginning. So I decided to change all the RAM chips and do the 5V mod along the way. The solder at the legs of the corroded DRAM chip unfortunately was like stone and I did not manage to get it out without destroying a couple of traces and had to add a couple of botch wires. (Overall I have to say the CV is the worst soldering experience I had so far, I constantly have to work with 400dC+ and still have a hard time getting the solder to meld). Added the new chips (SRAM: Intel SAB2114PTL, DRAM: TI TMS4164-20NL), powered on the console and... garbled graphics... But well, clear improvement over a black screen.
First thing I thought is that I didn't solder properly somewhere. In the meantime, however, I checked for continuity and shorts 3 times and am fairly certain that everything is connected properly. So I assumed that maybe one of the DRAM chips was bad and replaced them one by one. Replaced the 4th, and.... SUDDENLY THE PICTURE LOOKED PERFECT! YAY!!!
Until I tried to switch it on again the next day. Back to garbled graphics...
Coincidentally I also had a C64 with bad memory laying around, so I replaced the chips in there with sockets, added a new batch of my TMS4164 and ran a RAM tester for a couple of passes - it did not find any problems.
Then I moved the whole batch of tested chips into the CV. Turned it on and... everything was fine again! Until I tuned it off and on a couple of times. Back to garbled graphics... So I moved the whole batch from the CV into the C64 and reran the RAM test. And this time, one of the chips reported as bad. This makes me think that the CV kills the chips somehow.
Voltages at the chips are too low rather than too high for my taste: 4.5-4.7V when turned on. PSU when CV is turned off gives about 5.1V. I found someone else in this forum reporting the same voltage drop problems after doing the 5V RAM mod, but there was no resolution (I unfortunately can't find the post anymore).
I checked for shorts and found that I have a resistance of about 340Ohms between GND and +5V, which seemed weird to me. But according to this post, it seems to be normal.
I did the 5V mod following this document. I'm not really comfortable with the A7 pin of the 4164 being connected directly to VCC. I'm not super experienced with electronics, but shouldn't there be a resistor inbetween to limit the current flow when pulling that pin high?
Any ideas about what might be going on are highly appreciated!