But the cool white LEDs were permanently on at a very low level and did not respond to any of the brightness controls:
You can’t tell, but the cool whites are on in the first picture, too.
The symptoms suggested the driver transistor for the cool whites has failed partially on, although I’d expect it to be either a dead short or completely open.
The lamp being a year or more out of warranty and having come from one of the myriad Amazon sellers banned during the Great Paid Review Purge, there’s nothing to do but remove the four screws from the back of the control lump and see what’s inside:
How this was assembled I cannot say, because the three wires going to the LED head (on the far right) have less than an inch of slack. Maybe they pulled wire into the head while screwing things together?
I think the HC8T1212 microcontroller sticking out of the foam is a distant descendant of the Motorola (remember Motorola?) MC68HC05 family. I’m mildly surprised they didn’t use a 32-bit ARM / MIPS / whatever micro, with WiFi capability and a strong desire to siphon my private bits.
The two pieces of closed-cell foam seemed firmly glued to the PCB, but eventually yielded to brute force. Scraping brittle yellowish goo off the right end revealed the LED ballast resistors and the wire labels:
Note the bar-taut
Y- wire going to the warm-white (“yellow”?) LEDs.
The black foam left a mess over most of the PCB, but diligent scraping eventually revealed the driver transistors:
You can’t read it, but the topmarks were A6SHB: an old Siliconix (remember Siliconix?) SI2306 30 V / 3 A MOSFET. Turns out you can get new-production SI2306 transistors from the usual Asian foundries through eBay, which I did.
It’s not the neatest soldering job ever, but it’ll suffice:
The colorful wires over on the right added enough length for a pair of Tek current probes:
The top (cyan) trace is the (repaired) cool LEDs, drawing 600 mA from the 10 V supply, so the 0.5 Ω ballast dissipates 180 mW. The bottom (green) trace is the warm LEDs at 500 mA through a 0.75 Ω ballast for 190 mW. That end of the control lump does feel a bit warm after a while, but nothing out of the ordinary.
Stuff the foam back in place, tuck the longer wires around the edges, snap the cover in place, reinstall the screws, and the lamp is at least as good as new.