I'm assuming you are saying that the 5 and 15 traces don't overlap each other on the opposite sides of the board, but that each one is duplicated on the opposite sides of the board, for some distance, and they share the total load for that rail. Obviously if you only have one trace it carries the entire current and you could calibrate for an absolute reading with the probe. More often you would probably not calibrate for absolute and just use it to find relative ratios between probed different locations to find where the heavy load was.
Another alternative would be to use a thermal camera and see what is getting hot.
Yes, they do overlap on the board. On the connector to the resistor board, they're sandwiched top and bottom.
As for heat output with a thermal camera, here's two photos from a series I took a few months back when I was studying replacing the fan. Eventually, I elected not to.
This is how I knew that the 600ohm resistor was getting above 300F. I don't know just how hot because that's as high as the FLIR will measure.
Here's one from last night. There is some offset between the thermal camera and the human-range camera, as well as some light reflecting from my bench lamps, but things get hot fast.
During my most recent tests, I was noticing that the switchmode whine was... irregular. Concerningly irregular. I also noticed a burning smell after only 90 seconds of operation, which made me uncomfortable enough to shut it off and not want to test the switchmode section any further.
I don't like these type of shotgun style parts replacements. They are a lot of work and expense and rarely solve anything. It may be better to carefully analyze the circuit and understand how it works. An LM723 based regulator is not too complicated although in this example DG engineers did a few odd things which I don't fully understand. With the limited understanding I would suggest the following simple tests with a load which takes about 1A (suitable light bulb or resistor):
I mentioned it before, but I'm not a fan of shotgun replacements either. Everything I replaced before I did so with a reason. It was either known failed, or suspected of drifting out of spec, and hard to test in-situ under load, so it was safer to swap.
An LM723 based regulator is easy to understand if it's being used for linear regulation applications, but these two are being leveraged as switchmode, which isn't exactly common. DG did it in a janky way, and I don't like the risk it puts on the computer part of the Nova.
I've done my fair share of load tests using indicator bulbs, checking ripple with my scope at various points in my test process, and it's just a weird design. The large capacitor across the 5V rail to smooth it is one of the few that's new, so why does it ripple that much? Who knows.
Here's from the most recent tests (after I powered down), where the PSU whine was outside of the expected pitch and range that I have come to expect either under test or operation in the machine, coupled with that burning smell. Probes go up to the scope, and dummy load bulbs are wired in to keep the switchmode section happy.
The point is that I just don't trust this power supply any further than I can throw it (which isn't far because it weighs a ton). It's just not worth fixing at this point. I've been talking with firebirdta84 and Nevets01 (who has a DCC-116, a Nova 1200 clone) about replicating the edge connector so modern switchmode modules can be installed and we're starting to devise a gameplan. I'll keep y'all posted on how that progresses.