TRS-80 Color Computer (1, 2, and 3)

[channelmaniac]

Model: Color Computer 1 or 2
Symptom: Random garbage on text screen or stuck/flashing/incorrect pixels on high res graphics.

System will have a defective DRAM chip. Unfortunately the CoCo doesn't have any built in diagnostics however there are 2 options. The first is to simply replace the ICs 1 by 1 to find the defective chip or chips. The second way is to download the Diagnostics cartridge ROM image from the internet, burn it on an EPROM, and place the EPROM in a cartridge. Use this cartridge to run the memory diagnostics and isolate the bad RAM.
 

[channelmaniac]

Models: CoCo 1 and 2
Symptom: Orange screen filled with '@' characters

This is a problem with either all of the RAM chips, power supply, or with the SAM memory controller IC.

Depending on the model and symptoms it could be multple problems.

The 4K and 16K DRAM ICs in the CoCo 1 take +12v and -5v in addtion to the +5v power. Check for power supply problems and fix as necessary. If power supply is good then replace the SAM IC. (MC6883 or 74LS783 on CoCo 1)

64K CoCo 1 and the 16/64K CoCo 2 computers use +5v only DRAM. Replace the SAM IC. This will either be the MC6883/74LS783 IC on CoCo 1 and early CoCo 2, or the 74LS785 IC on the later CoCo 2 models.

An odder problem could be a failure of every DRAM chip in the computer. This can happen on a 64K CoCo 1 if the RAM select jumpers are moved by someone. If the wrong jumper setting is chosen it could put +12v or -5v on the 64K DRAM ICs which will destroy them.

[channelmaniac]

Model: Color Computer 1/2
Symptom: Keyboard not working

If it is a single key then replace the keyboard as it is not a serviceable item. If it is missing rows or colums of keys - replace the bad keyboard controller (6821 or 6822 PIA) IC.

[channelmaniac]

Model: TDP-100 (Tandy Data Products version of a Color Computer 1)
Symptom: Missing keys

Keyboard mylar was scratched. Replaced keyboard with one from a parts machine and tested.