This excerpt from the user's manual can explain the problem of your mica sheet / waveguide cover.
"... Remove the packing material and any protective sheeting applied to the surface of the housing. Do not remove the brown glossy light cover plate that is screwed into the baking chamber to protect the magnetic field tube. Heat-resistant glass Non-heat-resistant glass Yes No No No Heat-resistant glass Yes Ceramic Microwave-resistant plastic dishes Kitchen paper Kitchen paper Metal shelf Metal plate Aluminum and aluminum trays Yes Yes No No No Yes No No No Yes No has sufficient space for ventilation of the appliance. After carefully cleaning the interior of the oven, residual grease from internal pipes and heaters can be removed by performing the recycling cycle on page 65. Accessories Accessories should be cleaned with a mild detergent and then dried . SPECIAL NOTE FOR THE ROTARY TABLE SUPPORT Always clean the turntable support after cooking, especially along the rollers...."
BLUESKY BMG900ESL23, BMG900ESL220, BMG900ESL2255, BMG900ESL22110, BMG900ESL22165, BMG900ESL22220
Manufacturer's part price is £10.91 Discount for bulk (starting at 10 units), £10.55 per unit
This manufacturer part is recomended by BLUESKY, which gives it a 10/10 rating.
This spare part is recommended for your BLUESKY BMG900ESL23 microwave oven .
All spare parts are new and come with a 1 year warranty
① Carefully check your BLUESKY microwave oven's product number
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Here are examples for partial reference for microwave oven :
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