If the fuse blew almost instantly after you press the start key, that is almost always a shorted high voltage capacitor.
if the fuse blew while you were opening or closing the door, you should check the door switches and their mounts.
if it blew several seconds or a few minutes after hitting start, then it is probably a shorting high voltage Transformer.
The service manual is at my website here:
http://www.microwavecontrol.com/r1870_r1871_sm.pdf
you should look at this base service manual as well:
http://www.microwavecontrol.com/r1850_sm.pdf
If you have other questions, please feel free to reply back or email me.
Ed Miller
[email protected] Microwavecontrol.com
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