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Internal Power Supply Failure

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One of my consoles spontaneously died and it appears to be the internal power supply.  The issue follows the power supply board, so I know its gone bad.  When the unit went, R6 on the power supply started smoking.  The supply does regulate when the 5V rail isn't loaded but R6 starts smoking as soon as it is.  I suspect U3 or Q2 failed, making Q1 dump the unregulated DC intermediate rail across R6 continuously.  Anyone seen this before?

 

Ti-994a internal power supply

power_supply_schematic.jpg

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I can't help but wonder how many people have repaired power-supplies, instead of just swapping them out. I suspect not much analysis has gone into them. 

 

That schematic is super-interesting, I hadn't studied it before... I'm surprised that the LED is upstream of the bridge-rectifier. I wonder if a high speed camera would catch it flickering.  Or maybe it doesn't work that way... "White" is center tap, but still part of AC. So do all the '|li' symbols refer to a virtual ground that the op-amps in the regulators set up? Or do the polarized capacitors near the coil create the ground?

 

Power supplies are very (or appear to be) different from the digital side.

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"So do all the '|li' symbols refer to a virtual ground ..."

 

Those symbols are the common ground on the PCB. The common pin on each regulator is connected to it to give a common ground point, as is one side of the bridge rectifier.

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4 hours ago, jedimatt42 said:

I can't help but wonder how many people have repaired power-supplies

It looks like the boards are readily available at a very reasonable price, so I think you're right.  Time is better spent on other projects. 

 

https://www.arcadeshopper.com/wp/store/#!/Ti-994a-internal-power-supply/p/83577018/category=22342013

 

Was curious if this was a common failure and was simple to fix, but that's academic. 

 

Personally, I find this schematic terrible.  This must pre-date all the schematic  style guidelines I'm familiar with.  The negative tap of the bridge rectifier is ground.   Hard to tell since the net runs half way across the page before you find that ground symbol, and it actually crosses another ground symbol on they way there without showing a connection (a major schematic rule violation).

 

Yes, the LED will flicker at 60hz, and will light up even if the power supply is completely dead from rectifier on so somewhat useless.

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