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Title: TI-99/4a Computers Post by: channelmaniac on September 16, 2008, 12:33:22 AM Symptom: powers up but is dead
Checked the Address Bus lines with a logic probe. Some were stuck low. Replaced defective ROM IC. Title: Re: TI-99/4a Computers Post by: channelmaniac on July 25, 2009, 04:54:49 AM Model: TI-99/4 (NOT 4a)
Symptom: Garbage on screen This was the older TI-99/4 computer. The screen background was the correct color but the foreground was scrambled. The computer would still boot since the CPU RAM (2 x 6810 SRAM chips - 128 bytes x 16 bits total) was good. This was bad video RAM. Checked the power supply and found it was bad. +5 volts was 4.45v, -5v was -5.63, and +12v was 11.65v. Replaced the power supply to fix voltage problems and the garbage problem was still there. Replaced all 8 DRAM ICs and tested. They all had random errors thanks to the bad power supply. |