By “underside light” do you mean the ''Surface light''?
If so you press the Surface light button once for a bright light, twice for the night light and a third time to turn the light off.
If this is the light you mean and it stays on regardless of how often you press the button, is the light bright or dim?
If bright there may be a problem with Relay RY8 on the control board, (if dim it is relay RY 9) being permanently operated or its’ contacts may have fused together.
Here’s a link to the [https://www.appliancefactoryparts.com/content/pdfs/126409-8.pdf|service manual] for your microwave oven.
On p.5-1 you’ll find the overall circuit diagram and on p.10-1 you find the schematic for the pcb.
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Since changing the two capacitors didn’t make a difference you could check out the 7.5V Zener diode ZD3.
Hi @rsmith
By “underside light” do you mean the ''Surface light''?
If so you press the Surface light button once for a bright light, twice for the night light and a third time to turn the light off.
If this is the light you mean and it stays on regardless of how often you press the button, is the light bright or dim?
If bright there may be a problem with Relay RY8 on the control board, (if dim it is relay RY 9) being permanently operated or its’ contacts may have fused together.
Here’s a link to the [https://www.appliancefactoryparts.com/content/pdfs/126409-8.pdf|service manual] for your microwave oven.
On p.5-1 you’ll find the overall circuit diagram and on p.10-1 you find the schematic for the pcb.