Yes there is a short circuit. First I'd like to make sure the right regulators are on the right power rail. This was the reason I wanted the voltages seen as the regulators. There isn't that much on the supply line sides of the regulators. These regulators have output short protections that can take an output short for some time.
I'd really like to see the voltages on the two regulators. under the conditions I'd stated, with only the +16V and the lamps for protection. While the resistors are good, incandescent lamps are good constant current sources ( feature of a filament in a gas ).
I suspect the power leads may have been swapped or someone installed the 79L12 and 78L12 in the wrong locations. Swapping the input voltages is the more likely case of original failure ( consistent with blowing a trace ).
Measuring is the first step in trying to determine the cause. Please don't get offended by my roughness. I'm just trying to get to the resolution ( we are now at over 100 posts ). I'm 74 years old and grumpy but still know how to trouble shot.
Dwight
With no drives attached and no ICs on the board, just the +16V power supply with the 14V 3.8A bulb: The measurements are +16V on the power supply side of the bulb, 550mV on the other bulb lead, 58mV at the +16V power supply lug, 57mV on the input of the 78L12, and 2.5mV on the output.
The 78L12 and 79L12 are in the correct positions and orientation on the board (I even got in there and photographed them to double check the numbers). C1, C2, and C3 have all been replaced and are in the correct orientation. C4 is original and is part of the +12V power derived from the 78L12. It was working fine under the old linear power supply
I built this machine in 1978 and it worked beautifully for years. It sat in storage between about 1986 and 2020, when I switched it on. It worked nicely for a few weeks and then quit functioning. The old linear power supply was supplying all three voltages but something was causing the -16V line to blow out along the trace to pin 52 on the connectors. Although the power supply was powering the lines I got suspicious about its reliability. That's when I decided to install the three MeanWell supplies instead.
As this thread got going I saw that the 78L12 had blown up, and I also found a burnt out resistor (R25) on the -16V line to the second serial connector. I replaced it, but now I think I should give the area around that row of resistors a good look under magnification. Maybe something's shorting out there.
I'm fine if you're grumpy with me lol. I'm 72 and grumpy because I can't figure out what's wrong with this thing. I've never run into such a trouble shooting nightmare before, and I suspect the solution will be so obvious in the end I'll end up banging my head on the table