Retired Texas TI99/4A engineer needs help please.
Its nearly 20 years since I stopped fixing 99/4As and now I'm stumped with a simple power board repair.
The board presented with LED lit, computer not powering up.
Examination revealed R6 100 ohms burnt O/C. A replacement went the same way, fast.
Component checking revealed Q2 TIP31 O/C, replaced with a tested TIP 31, D10 diode O/C replaced with a 1N4007, perhaps a 1N4148 would be better, C15 Hi, leaky? replaced, D9 and Q1 checked OK.
Now I have + & - 5 v and +12v with no computer connected but with the computer I have a partial boot to a distorted screen and Q2 begins to heat very quickly.
The computer is not faulty.
I'm stuck, forgotten all I knew about this power board, they were always too reliable. Any help gratefully received.
Schematic attached.
Sam.
Last edited by Boater Sam; 19th Oct 2018 at 7:11 pm.
Reason: Added
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