Power Circuit (Commodore 16)

power enters the c16 through the dc jack on the right of the console and should be powered by 9 5v dc supporting up to 0 8a ground and vcc pass through the line filter inductor l3, through the power switch, and then through fuse f1 before making its way to the 5v ldo regulator vr1 use a multimeter to verify you have 9 5v (or slightly higher) coming in the dc jack to the right side of the l3 inductor (ground top, vcc bottom) then check you have 9 5v on the right side of l3 (ground top, vcc bottom) or measure in continuity from bottom left to bottom right, and top left to top right (with the power off) to check the inductor is good if all is good, your input power should flow from the dc jack, through the l3 inductor, through the power switch when turned on, and to the start of the fuse on the right side after that, check your fuse for continuity or measure for voltage on the left side of the fuse if the l3 inductor is blown, replace it or bridge the top and bottom pins across if the fuse is blown, replace it with a 2a or close fuse, or temporarily bridge it 5v regulator after the fuse the input vcc goes into the vr1 ldo regulator (a standard 7805) nothing special here if it's working, you will have 9 5v, and 5v out if the regulator is faulty, or there is a short on the output or input it won't work and you need to find the cause the regulator also has an external diode some old ldo's had a need for external diodes to prevent when vout is greater than vin such as when power is turned off and the charge in the capacitors on the output side have voltage this diode is d11 there is also a power resistor between the in and out of the ldo the reason for this appears to be to pass additional current to the output stage from the input removing some load off the ldo it relies on the resistance limiting the current causing volts drop from the source voltage to 5v or lower if this is the purpose, in my eyes that's a bad design the resistor gets super hot, and if the ldo ever fails, the resistor is powering the entire system, which will receive the full 9v until enough current is drawn to lower the voltage it clearly wasn't a failure point as these computers still run today and i've not seen one fail, but the danger is there i would remove it and upgrade to a better ldo another guess for the resistor is to discharge the capacitors on the output side of the regulator after power off, in an attempt at preventing reverse current damage but if so, this is not the way to do it