Looking for arepair manual for LG MS 428MD micro oven

I don't recall ever needing a repair manual to repair a microwave oven apart from part numbers to order spare parts from.
Is it dead?

Door switches

If one of the door interlock switches is not switching properly or there is a condensation problem on the sensor control panel, either can cause the turntable/light/oven to come on even though the "Start" button has not been depressed due to the wiring of the switches.
Check for faulty door switches and/or contaminated actuators.
I hope that helps.

Error F3 control board faulty.

It sounds like the magnetron may have either failed or is not being supplied with the extremely high voltage required to run it.

YOUR SAFETY IS PARAMOUNT

This is a job for a professional but if you are safety clued up, here's instruction.
Make sure the !!!!any capacitors are discharged!!!!! before attempting any sort of repair.
Check the door interlock switches first then the high voltage diode with either an AVO model 8 or moving coil meter on high resistance range for short circuit, (DVM's won't show the fault!), the capacitor can fail and go short circuit, the feed fuse on the primary of the high voltage transformer and then finally, the magnetron is best checked by substitution.
Hope that helps.

Load test.

A simple safety test.
Place 1/2 litre of water in a glass container. Measure the temperature of the water.
Place in microwave and heat on full power for 1minute.
Measure the temperature after it has stood for 1minute.
If the temperature difference is more than 9 degrees Celcuis the magnetron is working and producing about 700Watts of heat.
If your microwave is labelled with 900 or 1000Watts. That is about right.
If the temperature is less then the magnetron is not working properly and may need replacing.

This a only a rough guide and any doubts you should consult an authorised service agent.

Magnetron used with no load.

It sounds like the magnetron may have either failed or is not being supplied with the extremely high voltage required to run it. Using the microwave with nowhere for the energy to go results in magnetron meltdown!
Make sure the !!!!capacitors are discharged!!!!! before attempting any sort of repair.

The magnetron may be all that is required but the diode pack may also have failed. These components are Safety Critical Items and MUST be replaced with identical units for continued safety.

Spare parts are available through the Manufacturers Authorized Agent. Go online to find a supplier near you.

Hope that helps.

Replacement of the lamp should be carried out by a qualified appliance repair service.

YOUR SAFETY IS PARAMOUNT!

Attempt this only if you are competent.

But here is a basic method if you are mechanically minded and have electrical safety knowledge.

The lamp is sat in a housing and its position is adjacent to the vent holes that it shines through.
If you feel confident, disconnect it from the mains power, remove anything in the cavity.
Remove the microwave cover. Purchasing a new replacement lamp will be useful as it will give you an idea of what it looks like. Locate the dead lamp remove the lamp and fit the new one. Replace the covers. Plug it back in and try the lamp by opening the door.
Hope this helps.

Replacement of the turntable motor should be carried out by a qualified appliance repair service.

YOUR SAFETY IS PARAMOUNT!

Attempt this only if you are competent.

But here is a basic method if you are mechanically minded and have electrical safety knowledge.

I am assuming the plastic coupler is not split or broken and causing the problem.

The motor assembly is fixed to the base of the oven usually with two screws and access is obtained usually by removing the inspection plate directly beneath the motor. It is usually a plug in synchronous motor that revolves at a few revs per minute but the plastic cogs in the motors gear box fail and the motor assembly must be replaced. They are fairly inexpensive and obtainable usually from a domestic appliance repair shop.
If you feel confident, disconnect it from the mains power, remove anything in the cavity and turn it upside down if it is a tabletop model, a little more complicated if it is built in!
Remove the inspection plate, sometimes it must be cut out with sharp cutters, and disconnect the motor. Remove the motor and go and get a new one. Fit the new motor and fit the inspection cover back on the base by turning it to fit and using an extra screw fix it then put the microwave back in its position. Plug it back in, with the turntable and plate. Put a cup of water on the plate inside of the oven and heat for 20 seconds to check the plate goes round.
Hope this helps.

Replacing the display unit.

Disconnect from any power source. Remove the outer case. Press to open the door and just beneath the open button there is a self tapping screw holding the front unit down and in the slots. Remove the screw. Remove the plugs making a note of where they all came from! (A mobile phone camera is very handy for this job).