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Power board has exploded diode.


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Hi all,

 

I am new to the TI-99/4A and recently acquired 2 non working examples.

An original and a later model beige.

 

The first thing I have done is opened them up to do a quick check of the power boards to ensure the correct voltages are leaving it.

 

For the original model, the power board appears OK with the correct voltages appearing at the connector that goes to the mainboard.

 

With the beige model, I immediately observed one of the diodes on the board (CR2) was in half with a big air gap.

 

I have attached photos. I replaced the diode with an equivalent type, but as soon as I flicked the power switch it started smoking.

 

So I turned it off before it popped like I assume the other one did.

 

Any ideas on the culprit. I do not want to turn it on again just incase I cause more damage (I did have it disconnected from the mainboard though).

 

Cheers,

Shane

post-37708-0-21811700-1463273576_thumb.jpg

post-37708-0-73333700-1463273580_thumb.jpg

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Using a DMM, set to diode or continuity then check to be sure the other diodes nearby are not stuck shorted or open. Sometimes need to remove diodes from circuits to properly test, but I think in this case, should be fine leaving on the board.

 

Can replace the caps too as a matter of course, BUT… extremely rare that I've seen caps short out or go bad without signs of leakage or bulging. Still and since you have this stuff all torn apart, I'd go ahead and replace them anyway just because.

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That is pretty awesome. I love seeing the results of catastrophic failures.

 

Only a guess, but that diode is probably part of a bridge rectifier, and one of those large capacitors is probably there to smooth the edges of the output. I propose that one of those capacitors is shorted.

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That is pretty awesome. I love seeing the results of catastrophic failures.

 

Only a guess, but that diode is probably part of a bridge rectifier, and one of those large capacitors is probably there to smooth the edges of the output. I propose that one of those capacitors is shorted.

 

+1.

 

The TI power brick puts out AC (8V, 16V, and 5V).

 

This part makes it into -5V, +5V, and +12V. See a nice write up here: http://www.mainbyte.com/ti99/hardware/power_supply.html

 

I'm not sure what each component in the system each takes (someone probably can spout them off), but this tri mode PSU could eventually be replaced with something like this: P25A13A-R1B (https://www.meanwell-web.com/en/product-info/ac-dc-power-supply/adaptor/0-25-w/p25a-b/product/P25A13A-R1B)

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