JMC 8100ADB How can I determine if I need to replace the capcitor or magnetron? Unit blows internal 20a fuse -

Based on you information, if you disconnected magnatron and unit ran with no fuse problem, I would suspect magnatron. You should consider the age of unit it, may be be a better alternative to replace it.

You would better served replacing unit. Throwing parts at it is an unnecessary step.

More than likely the magnetron is the only fault.

Magnetrons usually can't be tested at home. A certified tech with proper equipment can do this. How old is it?

The magnetron is shorted, the keypad control board is bad, blowing fuses. If less than 7 years old it has a inverter board that can be trouble, But with the level of work and parts it will easily exceed the cost of a new microwave oven.

It doesn't have a thermal fuse, but it has a 20A ceramic fuse and on or around the magnetron it has a thermostat.

The 20A fuse (if blown) can be had in local appliance parts store, and places like Lowe's Home Depot, and Wal-Mart.

For the fuse and/or thermostat, you can find helpful exploded view diagrams and order parts by entering your full model number from the tag on the oven here.

If the fuse is blown and it blows again, it may be a loose fuse holder, shorted high voltage capacitor, or door switch trouble.

If the thermostat failed, it's due to a magnetron overheating due to the internal cooling fan's not working, or just the magnetron itself going bad.

As a bonus, you can find your owner's manual online here free.

If you need further free help, let us know.

We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.

sometimes just the internal units fuse blows from age/use it is called a ceramic fuse,you need to remove the outer cover to replace this,carefully before touching anything inside the unit take 2 insulated screw drivers and discharge the hi voltage cap,then replace the fuse and try it,if it blows again,then suspect a shorted hi-voltage capacitor(especially if there was no arc during shorting it with the screw drivers) or a hi voltage diode shorted or the magnetron unit has begun drawing hi current or is also shorted

There's an internal fuse - about 20 Amps. It usually blows when the magnetron dies. Unplug the unit from the wall and access the internals. You should plainly see the fuse holder - remove and check the fuse. Spares are available at any ACE hardware store. If the unit is more than 5 years old, I wouldn't mess with it.

Disconnect the connector to the primary winding of the high voltage transformer. Put a new fuse . If the fuse is not blowing the problem may be in the secondary side of the high voltage transformer. Chek the highvoltage diode, capacitor, magnetron respectively. you can also check the primary winding of the low voltage transformer.

First things first, check the big cigarette can type capacitor. OHm it out should not be shorted, meaning reads zero ohms. use ohm meter, set to ohms, put leads across 2 terminals (after your short them with a screwdriver)should not read zero..

next replace magnetron, it's easy, call appliance repair for price if it makes sense and your a gambler replace it.