During the 2-3 days before the microwave totally died, the display would randomly shut off. It would do this when the appliance was idle (not in use, nobody even in the kitchen or lights turned on, let alone any electrical equipment being used) and when it was in use. Sometimes unplugging the microwave and plugging it back in would restore the display, and the tenants could use the control pads to program the cooking cycle, but when they'd shut the door the display would blank out again.
That's a verbatim description of the tenant's observations/troubleshooting. I couldn't tell whether the door really had anything to do with the symptoms, or if that was just a vibration issue.
No blown circuit breakers. 120V is present at the line (input) terminals to the noise filter. The 250V 20A fuse appears to be fine and 120V is coming out of the noise filter's load-side (output) terminals. (The fuse's glass cylinder is wrapped in a white plastic that makes it impossible to see the filament, but it has zero resistance when pulled out of the board and checked with an ohmmeter. Besides, there's 120V at the noise filter's output.)
The control board looks OK-- no visible scorch marks, slagged solder, or other damage visible on either side. However the control panel display does not light up or make noises when the buttons are pushed, the microwave's interior light doesn't come on when the door is opened, and the vent fan doesn't run. Everything's deader'n a doornail.
I was able to measure an AC volt or two going onto the control board at the "Hi power secondary relay #2" and the "Low power secondary relay #1". So I'm assuming that voltage is getting to the control board.
Upon really close inspection with a magnifying glass, I finally noticed a small burn spot on the LED display module mounted at the end of the control board. It appears to be a sealed, integrated, & evacuated component. The scorch mark is a small 1/8th-inch circle on the lower right-hand side of the display next to the word "TIMER". The photo shows a couple arrows pointing to the scorch.
I think I'm going to need a new control board because this display module appears to have all 27 of its pins soldered directly to the board. The only way to replace it would be to unsolder the old display, yank it out of the board, clean out the solder holes, stick in the new display, and solder 27 pins. I doubt I could get it right on the first try, although my technique would probably be a lot better by the 27th pin.
Before I go board-shopping, is there any other troubleshooting I should check? Would any other part have been destroyed by this fault? Does anyone know if this control board has a recall or some sort of tech service bulletin or out-of-warranty repair guarantee? Is it worth replacing this control board, or has this microwave been "cooking" a lot of them?
I know that Kenmore is Sears' cover name for the actual manufacturer (in this case Samsung) but I'm having trouble figuring out how to search/order the "primary control board" or "smart board". What number do I use here? The microwave's model # (401.80089700), the card's label (00040RAS-SM7MGV-04), or the card's "part number" (DE41-00352A)?