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No power, fuse OK where else to look?

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wilykat

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There is no schematic of this anywhere, I've asked and even posted request on this forum a few days ago. The closest schematic I can get is for TurboGrafx-16 but they have different power design, Turbografx-16 only needs regulated 5v to run. Turboexpress that I am trying to fix has 3 different power rails, 5v, negative 29v for LCD, and another via inverter for CCFL for LCD backlighting.

I've cleaned the grey crud off the board where the power is and took a high resolution photo: https://i.imgur.com/wSxc2Ei.jpg there is still its of grey crud. (this is what it looked like before I cleaned it: https://i.imgur.com/q83SXRF.jpg the grey crud was likely applied to deal with those 2 inductors)

The brown plug and the black plug sticking out on bottom are both power in (brown jack from batteries, black jacks from external ac adapter) and goes through a few components toward the power switch (top left, DPDT switch but wired as SPDT)

When I power it, normally I should be getting about 350-400mA draw but this one is drawing only 20mA which means a few components are drawing power but for some reason it's not getting to the rest of the system. I checked at the switch and I am getting 9v on center and on whichever side the switch is set to.

I can't see obvious trace to follow thanks to ridiculously thick and dark solder mask, so it would be hard for me to track the power rail to try and find the cause of failure. There is no obvious voltage regulator either. I thought it was the large transistor marked B1182 but it checks good and there's no power to any of the 3 pins so the issue seems elsewhere. The inductors all checked 0 ohms so it's not open. Someone suggested maybe shorted MLCC but I can't find a short either, there is a few SMD caps to check with.

This is probably one case where a schematic would be very helpful. The warranty went out probably 20 years ago and I doubt NEC still have the information on how to fix this, they have merged with another company and seems to be in batteries these days.

Edit: I forgot I removed the audio out jacks, that's why there are empty spots below the battery in jacks. Those should not affect the power system at all.
 
From your (good) pic i would check continuity , L501 , L502 inductors could be open circuit
 
I've checked those inductors and a few other nearby, they all measured 0 ohms right away. I don't have the equipment to measure the henry value, not that it mattered the value isn't marked anywhere and no schematic to tell me what it should be.
 
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