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TI99/4a power supply can electrify the computer? possible shock hazard?


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Hello guys,

reading this page i am curious to know if someone of you know this problem...




someone can tell me about this problem please ? i'd like to understand it better :P


in the history, TI stopped his production of TI-99/4A for this problem ?


thanks :P




Edited by ti99iuc
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Here is a (quick & dirty) list :) of 120 Volt PSU´s. The green tag means it has the "safety feature".

The rest I have to check, so, this is not 100%






Edith: I also have collected (and political correctly named) a mass of pics of each PSU I found ;)

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yes, i have the stuff---


attachicon.gif2017-03-25_214050.jpg attachicon.gif2017-03-25_213613.jpg attachicon.gif2017-03-25_213450.jpg



But i'd like some more details on a reald true story for this problem... someone worked in TI maybe ?


If memory serves me correctly, this problem came to light very early in the year (1983) when the transformers failed high pot testing (this is an extreme test where the secondary is intentionally shorted out and the primary (120vac) is left powered up. After about 45 minutes, (the exterior case would start to deform from the internal heat) the transformer would fry. I used to spend many nights doing this test, and they were very repeatable. US Underwriter's Laboratories had approved the transformer, but the Canadian (CSA I think it was called) Product Safety group flagged it as a potential problem if it wasn't grounded. The factory had to grind off the CSA stamp on the bottom of the transformers. (You can look at the bottom of one to see if it was affected by looking for the circular grinding marks.)


I never heard of anyone getting zapped by their 99/4A, though I have repaired TI computers where you could feel a slight tingle when your wrist lightly brushed against the aluminum trim. Anyway, this was just another major public relations headache at a time when TI was reeling financially by trying to price-cut their 99/4A to compete with the lesser capable VIC 20 -instead of the comparable C-64, which coast more. I don't remember what it cost the company to recall all the outstanding transformers, but it wasn't cheap.


Did it kill the 99/4A? No. But it did cast it in more of a negative light when it was under intense scrutiny to be profitable. J. Fred Bucey was a Lubbock native and CEO of the company then. He made the decision to pull the plug and left a lot of people unemployed in Lubbock during the fall of '83. I was lucky, and got out about six months before the axe fell.



Edited by CC Clarke
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  • 1 year later...

Hi, thanks for your reply. Checking if the trasnsformer cord, not the the power supply. It has 4 female jacks, one is not in use and the other 3 suppsed to have a voltage output when working correctly.

Attached picture.

I do not know how to check voltages on AC without a ground. When I measure with the red testing lead I do get voltages that do not match with the specs shown.

Again, sorry for my basic question. Thanks a lot.



Edited by alortegac
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I do not know how to check voltages on AC without a ground. When I measure with the red testing lead I do get voltages that do not match with the specs shown.


If you measure a transformer's output voltage at the secondary coils, you will not see the numbers in the spec.


An AC transformer will output much higher voltage on the secondary side if it is not connected to its intended load that is actively drawing power.

This applies to any transformer, not just TI-99's.

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