Colecovision 5V RAM Modification
Purpose of this modification: The 4116 DRAMs used by the TMS9928A video chip require -5v, +5v, and +12v voltage sources. These RAM chips produce more heat than chips that only use +5v, and problems with the power switch can cause the 4116 DRAMs to fail to operate properly. The result is glitchy graphics, and sometimes a complete failure to boot properly.
IMPORTANT!!! Once you have installed sockets, BUT BEFORE you have inserted the new RAMs, please perform the voltage checks as listed in Step 5. You do not want to send +12 or -5 into the new RAMs.
Please fully read these directions before proceeding
- Parts list:
- 16-pin 5V DRAM chips, such as the 4164 or 4516, quantity 8
- 16-pin sockets (optional, but highly recommended), quantity 8
- 1 bit of wire, like a clipped lead from a 1/4w resistor or something similar
- 1 Step 1: Disassemble the Colecovision
- 2 Step 2: Remove -5 and +12 coils
- 3 Step 3: Connect +5v to L3 via
- 4 Step 4: Desolder the original RAM chips
- 5 Step 5: Inspect pads and traces
- 6 Step 6: Insert sockets
- 7 Step 7: Install RAM and test
- 8 Troubleshooting
Step 1: Disassemble the Colecovision
Disassemble the Colecovision and remove the PCB shielding.
Step 2: Remove -5 and +12 coils
Remove coils L2 and L3, near the RAM and RF modulator.
Step 3: Connect +5v to L3 via
Observe the group of 3 ceramic caps just above where the coils were. On the underside of the PCB, connect a piece of wire from the L3 pad NEAREST to the RAM, to the closest lead of the center capacitor.
Step 4: Desolder the original RAM chips
Note: Desoldering components from a Colecovision requires patience as the traces seem to be prone to lifting. If you doubt your desoldering skills, or if you don't have the proper desoldering equipment, you can use diagonal cutters to cut the DRAM chips off and leave the old legs in place. Then, carefully solder the sockets to the old legs. It's a bit of a hack but it works amazingly well and can be far less frustrating than lifting and breaking traces.
Step 5: Inspect pads and traces
Colecovision traces can be lifted easily. Also, the bus wires from the VDP to the RAMs tend to snake around in a confusing manner. Before installing sockets, it's important to look for any damaged pads or traces that will be obscured by the sockets.
Step 6: Insert sockets
Solder in the 16p sockets. Be sure to install them with the notch facing in the correct direction.
You may also wish to install the sockets and leave a little extra height on the top side of the PCB. That way you can inspect and add solder to the top if necessary.
At this point, you might wish to do a quick voltage check. Power up the board and check the following pins on one the sockets:
Pin 1: ~0 volts. This is -5v on the original RAM and is now disconnected.
Pin 8: 5 volts. This was +12, and by connecting the pad at L3 to a +5 source, should now read 5 volts.
Pin 9: 5 volts. This is not connected internally on +5v-only DRAMs, so +5v on this pin is just fine.
Step 7: Install RAM and test
Install the new RAM chips, connect video and power, and check that your Colecovision is functioning.
If you're voltage checks pass but the system boots to blank / garbage, frequently it is due to an open circuit on a bus.
Coleco designed the board in such a way that many of the bus wires will hop from the PCB top to bottom repeatedly, usually where a RAM connection is made.
With your meter in resistance mode, take all measurements from chip leg to chip leg. Do not measure from the solder side.
Ground & +5v
On each and every RAM: Negative lead on Pin 16 Positive lead voltage checks:
Pin 1: 0v Pin 8: 5v Pin 9: 5v or 0v
Don't leave your negative lead in the same spot - move it to pin 16 of each RAM being tested.
Common Bus Checks
Check for continuity of the listed VDP pin at all 8 RAMs
Address & Control
R/W (Pin 11) - ALL Pin 3 AD0 (Pin 10) - Only connected to U16 (Pin 2) AD1 (Pin 09) - ALL Pin 13 AD2 (Pin 08) - ALL Pin 10 AD3 (Pin 07) - ALL Pin 11 AD4 (Pin 06) - ALL Pin 12 AD5 (Pin 05) - ALL Pin 6 AD6 (Pin 04) - ALL Pin 7 AD7 (Pin 03) - ALL Pin 5 CAS (Pin 02) - ALL Pin 15 RAS (Pin 01) - ALL Pin 4
Unique Bus Checks (NTSC Rev J)
Check for continuity of the listed VDP pin to the unique RAM listed.
Rev D or other PAL units may not map to the same RAM chip identifiers (Uxx), but the principle is the same: One wire to one chip only.
RAM data IN to VDP address bus
AD0 (Pin 10) - U16 (Pin 2) AD1 (Pin 09) - U14 (Pin 2) AD2 (Pin 08) - U12 (Pin 2) AD3 (Pin 07) - U10 (Pin 2) AD4 (Pin 06) - U17 (Pin 2) AD5 (Pin 05) - U15 (Pin 2) AD6 (Pin 04) - U13 (Pin 2) AD7 (Pin 03) - U11 (Pin 2)
RAM data OUT to VDP data IN
Check for continuity between each RD pin on the VDP to each RAM pin 14
RD0 (Pin 32) - U16 (Pin 14) RD1 (Pin 31) - U14 (Pin 14) RD2 (Pin 30) - U12 (Pin 14) RD3 (Pin 29) - U10 (Pin 14) RD4 (Pin 28) - U17 (Pin 14) RD5 (Pin 27) - U15 (Pin 14) RD6 (Pin 26) - U13 (Pin 14) RD7 (Pin 25) - U11 (Pin 14)
If you find a break in continuity
Add more solder to the pin where the break is located on every RAM. If the wire in to the chip is on the topside, there may not be enough solder to make the topside connection.
If you still read a break after doing that, run a jumper directly to the correct pin on the VDP (or nearby RAM if it's a bus line)