What does the ColecoVision use +12 and -5 VDC for?

[shadow460]

I read that the memory chips in the CV use both +12VDC and -5VDC. Since it's possible to swap the memory chips with ones that use +5VDC only, is there still a need for the other two voltages? Is there any reason not to ditch them like obsolete memory chips and run from a +5VDC power supply?

[jferio]

I do believe that some people have swapped the memory chips for ones with simpler power requirements... but it does require some adaptation work, as those replacement chips don't have quite the same pinout.

[+5-11under]

The video section uses +12V. The controller section uses -5V. The Atari expansion module uses at least one of those.

[shadow460]

Ah. Darn, I was hoping to eliminate the need for the funky power supply. I suppose it could be done by installing an ATX power supply within the unit, but i'm not going to try that until I get desperate. I've got it working well the way it was designed, so I'll probably just leave it stock, then.

 

Thanks for the info!

[ApolloBoy]

You can eliminate the need for 12V and -5V by performing the RAM upgrade and an alternate AV mod. You can either make a simple component video circuit like I did, or you can swap the TMS9928 with a TMS9918 and get composite video without an amp circuit.

[shadow460]

Now that would be valuable.

[wileyc]

I had similar questions a few months ago. I found that the simple transistor amp composite mod had serious issues talking to my TV, but the LM318 circuit did not. The power supply was flakey, but due to the LM318 +12V requirement two-voltage scheme was necessary.

 

What I ended up doing was performing the single-voltage RAM modification, then replace the stock power supply with a standard +12V wall wart, and fed the +5V rail via a DC-DC converter.

 

You could probably get away with using a 7805 to step down the voltage, but that's horribly thermal-inefficient.

 

Just my two yen.

[+grips03]

You can eliminate the need for 12V and -5V by performing the RAM upgrade and an alternate AV mod. You can either make a simple component video circuit like I did, or you can

 

I used THS7374 for component video, but still have faint vertical lines. I built on breadboard so that might be the issue. How is the one you made?

Edited April 12, 2013 by grips03

[shadow460]

I actually have some 7805s lying around and some old AMD retail heatsinks.

[matthew180]

You could also drop in an F18A to replace the 9928 and eliminate the need for the DRAMs entirely. Also, the entire RF section would no longer be needed and could be removed as well. Note, I don't know if the DRAMs were the *only* devices that required the +12V and/or -5V.

[ApolloBoy]

How is the one you made?

I actually built mine based off the datasheet for the TMS9928; it consists of 470 ohm resistors on each video line to ground, and then I also had 15 pF capacitors going from the video pins to the jacks. It's not exactly perfect as the blue is too strong, but if I turn the contrast down it looks much better and the overall video quality is incredible.

[channelmaniac]

RAM upgrade is simple to do. Instructions are on the download page on my site.