VDP RAM wrong
If you have a problem with the 4116 video RAM and you don't have the oscilloscope, you can basically use these photos to find the faulty RAM. This help is useful if one of the RAMs has the DOUT pin locked (broken ram).
The videoprocessor use in the TI-99/4A comes in three flavors, that differ uniquely by the kind of output signal they send to the monitor port.
The TMS9918A issues a 525-lines, NTSC signal for US television format.
The TMS9928A issues a 525-line signal, in the form of a B&W luminance/sync signal and two color differential signals. This is meant to drive a RGB monitor, with a minimal external circuitery.
The TMS9929A issues a 625-line signal, in the same format as the 9928A. It is meant for use with the european PAL system.