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Old 25th Oct 2018, 11:16 am   #18
cmjones01
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Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 1,911
Default Re: Retired Texas TI99/4A engineer needs help please.

The original 5V regulator based around the 723 is terribly noisy and inefficient by modern standards. The quickest, easiest fix is almost certainly to replace it with one of the ubiquitous LM2596-based buck regulators available for a pound or two on eBay (lots of them seem to have displays attached, but there are some without). Connect the input to the TI's unregulated supply, set the output to 5V and wire it to the 5V pin. Remove Q1 and Q2 and that's the job done.

Chris
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