On 14/05/2021 13:31, Chris Bacon wrote:
So it was making an odd noise, and not heating the food, light comes on,
steel turntable revolves, beeper sounds at end of cook time.

Check HV fuse. Blown. Aha, replaced, it's done that before, should be
OK... but no! Food still cold, fuse blown again.

Do you mean the fuse in series with the transformer, ie 230V?

Check the diode (CL04-12) using a multimeter on 20V setting and 9V
(actually 8.34V) battery. 0V one way, 3.26 the other. A bit low?

This may be a number of diodes in series, so 5V drop may be expected.

Capacitor: No connection (meter on 2000k ohms setting) between either
terminal and case. Between terminals resistance increases from zero to
infinity over a second or so.

Sounds good. If it was o/c there would be no HV power getting to the
magnetron.

Magnetron: Meter set to 200 ohms. 0.5 ohms between terminals either way.
Meter set to 2000kohms, no connection between terminals and case.

I assume 0.5 ohms refers to the cathode heater, which sounds ok.

The case is the anode, ie at ground potential.

High voltage transformer: Primary, isolated, about 2.2 ohms. Secondary
150. There is another winding that seems to go to the magnetron, I don't
know what this is.

There should be 2 wires to the magnetron heater from the transformer,
one wire will be attached to the HV generator.

Initially I thought 2.2 ohms sounded very low, but the ratio between
primary and secondary is consistent with 2kV output. ie a square function.

So any help on what's up, and what to do about it?

If you disconnect hot side of the secondary and turn it on, does the
fuse still blow?

Do take care. There is a lethal amount of energy at a lethal voltage in
a microwave.