I just pulled my previously working TI out of the box and tried to use it, but it isn't working. I'm using a composite cable, and I'm getting just a plain black screen and a high pitched tone on audio. I did try a different power adapter, but got the same result. Also tried another composite cable. Any ideas on what I should check next?
Unfortunately a black screamer can be caused by failure of almost any part in the machine from memory to control logic to the 9901 or 9900 or etc. The high pitch tone is your 9919's normal state at power on. It is not being reset so I doubt the power up routine is being executed nor any processing for that matter.
Some likely causes may be.....
Cart port is shorting out somewhere (Remove it and see if the machine boots.)
Power supply is bad (check for 5, -5, 12 and gnd at mobo. They are printed on the board by the brown wires.) Check this with the PS unconnected to the mobo and then when it is connected. Note any big difference. Be aware that a bad PS can show good voltages when unconnected but suddenly drop out when it is plugged in making it appear that the mobo has a short when it doesn't.. If it does drop out when connected and you are sure the PS is OK then check to see if a Cap is shorted on one of the rails (pull one end and check to see if it is good.) Feel for hot IC's indicating an internal short.
Stuck line(s) (check all control, clock, address and data lines in an attempt to find the culprit.) This one can be more or less a crap shoot tracking down the offender.