Which RAM chip is bad?

[+DamonicFury]

I've got a Colecovision here with some scrambled text. I've read here that this is a sign of a bad 4116 RAM chip, and that the text can be a clue as to which one is bad. Can anyone decode this for me, or point me in the right direction on how to do the decoding?

 

[san-d-2000]

Why dont you replace them all with this set

http://console5.com/store/kits/memory-kits/colecovision-5v-memory-upgrade-kit.html

 

The original video RAM used in the Colecovision requires +5v, +12v, and -5v to operate correctly. If everything isn't "just right", graphics will be corrupted or gameplay will be incorrect. Worn and dirty power switches only serve to exacerbate the problem.

This kit replaces the original 8 video DRAM chips with +5v only RAM, allowing your Colecovision to operate more reliably and run cooler!

This kit comes with:

[nanochess]

It is pin 4 (AD6) from TMS9928 going to pin 2 of a 4116, the Colecovision schematic that I have reads that the corresponding 4116 is U13

 

Check with a multimeter the connection from TMS9928 to 4116 just to check that is the right VRAM.

 

Where you see TUPL (ASCII 54, 55, 50, 4C) it should read TURN (54, 55, 52, 4E,) in Z80 that is bit 1, but Texas Instruments reversed the bits for its chips, so it is bit 6 of TMS9928.

[NIAD]

It is pin 4 (AD6) from TMS9928 going to pin 2 of a 4116, the Colecovision schematic that I have reads that the corresponding 4116 is U13

 

Check with a multimeter the connection from TMS9928 to 4116 just to check that is the right VRAM.

 

Where you see TUPL (ASCII 54, 55, 50, 4C) it should read TURN (54, 55, 52, 4E,) in Z80 that is bit 1, but Texas Instruments reversed the bits for its chips, so it is bit 6 of TMS9928.

I think you need to change your name to DR. NANOCHESS.

 

BTW, here's an AtariAge Retro +1 x 1,000,000 for all the incredible help that you have provided since joining the CV Scene.

[+DamonicFury]

Yes, many thanks, Dr. Nanochess! :-) I was indeed able to confirm that U13 is connected to pin 4. Will get it replaced as soon as I have time. Many thanks!

[ApolloBoy]

I would suggest replacing all the RAM, it makes a world of difference in making the CV more reliable.

[+DamonicFury]

I would suggest replacing all the RAM, it makes a world of difference in making the CV more reliable.

 

My soldering skills just aren't up to that kind of job... I'm really struggling to get just the one done right!

 

I also have another CV that will probably need to have the 40 pin video processing chip replaced... No idea how that's going to get done either. :-(

 

Guess I need to stop buying these "untested" lots on Ebay! If only Coleco had socketed these chips... 2600's are SO much easier to fix!

[nanochess]

There is a quick and dirty solution for these difficult PCB. Just cut chip pins from over and solder the new memory over that.

[+DamonicFury]

Thanks for the tip! Actually seemed to be fixed for a minute... But now it just makes this screen...

 

 

Ugh.

[nanochess]

Thanks for the tip! Actually seemed to be fixed for a minute... But now it just makes this screen...

 

Ugh.

I'm glad it looked fixed at least for a minute

 

Check with a multimeter each pin from VDP to that VRAM chip in special, it's probable one pin is doing false contact.

 

Don't forget to do cleaning with alcohol and a toothbrush around the VRAM and VDP.