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Author Topic: TI-99/4a Computers  (Read 3962 times)
channelmaniac
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« 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.
Logged

I have too many hobbies! Electronics, gunsmithing, Miatas, arcade games, metal detecting, etc...

http://www.arcadecomponents.com
channelmaniac
Surface mount soldering geek
Global NLG Site Moderator
Sr.Tech NLG Member 1000+ Post
*

Total Karma Storms: 568
Offline Offline

Gender: Male
Posts: 2126


Few things are better than fixing an old game...


WWW
« Reply #1 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.
Logged

I have too many hobbies! Electronics, gunsmithing, Miatas, arcade games, metal detecting, etc...

http://www.arcadecomponents.com
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