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NLG Users Repair Logs and Other Ramblings. Request your very own topic. Just ask any site staff. => Channelmaniac's Arcadecomponents' Old School Repair Logs => Topic started by: channelmaniac on May 04, 2014, 05:46:19 PM
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Symptom: powers up but is dead
Checked the Address Bus lines with a logic probe. Some were stuck low. Replaced defective ROM IC.
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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.
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Model: TI-99/4 (NOT 4a)
Symptom: Missing characters and minor garbage on screen.
Computer still booted, but had some minor corruption. Checked the power supply and replaced a bad 4116 DRAM that had its output stuck low. Tested.
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Model: TI-99/4a QI version
Symptom: Dead keyboard
Keyboard was physically broken. Replaced with one from a parts machine and tested.