Microwave fails to heat.

[shempert]

I found these forums because of Sam's excellent postings over on repairfaq.org.

I have a microwave that has just stopped heating. I followed the instructions on checking out the microwave and pretty much found everything is "working."

However, I got to the Magnetron and I became confused. I use to deal with vacuum tubes way back when and I don't know what to make of a two terminal magnetron. It has two wires, labeled F and FA on the schematic. Schematic also says third terminal is wired to ground... which is no wire, just four screw posts to the frame.

I have a Panasonic 2M167B-M10 magnetron. I measure between the terminals and find 0.2ohm. I measure from each terminal to the metal casing and in both cases it measures 37.6 ohms. I'm pretty sure this isn't right, but with only two terminals, something is "common" and I don't know how to proceed from here.

Capacitor - charges up from my ohm-meter. No short.
HV diode - Tests at 7.4 V with a 18V input and 250 ohm resistor.
Switches - work per schematic
windings - within spec, no shorts to core.

All other things work fine (no turntable). Rotator for microwaves on, cooling fan on, lightbulb on, panel on, beeper on. One of the wires to the magnetron did show some spots that had been slightly melted. No wire showing, but after 17 years, something in there probably got a bit warm.

small cup of water on for 90 seconds does not heat. The normal sound of the magnetron is missing.

Have I really narrowed it down to the magnetron?

Thanks in advance!

 

[Resqueline]

If you get a measureable resistance between the terminals & ground (in particular when they're disconnected from the transformer) then the magnetron is dud.
It's a thermionic radio valve/tube with the anode to ground and the heater (kathode) terminals at minus 1500V so it's supposed to insulate when inactive.
The heater supply is 3V at some 30A (good contact is vital). I guess the heater might have sagged inside and made contact with the anode (seen it in happen lasers).
The HV diode should not read the same both ways, else it's dud.

 

[shempert]

If you get a measureable resistance between the terminals & ground (in particular when they're disconnected from the transformer) then the magnetron is dud.
It's a thermionic radio valve/tube with the anode to ground and the heater (kathode) terminals at minus 1500V so it's supposed to insulate when inactive.
The heater supply is 3V at some 30A (good contact is vital). I guess the heater might have sagged inside and made contact with the anode (seen it in happen lasers).
The HV diode should not read the same both ways, else it's dud.

Thanks!

I figured as much but I'm at a loss to understand how the filament has a complete circuit... Grounded Anode: Check. Understand that.
Cathode input. OK should be open with respect to the Anode and run from the HV side... but what I can't seem to picture is how the filament gets a complete circuit with only one connection!

Looking at the schematic, one of the low voltage filament connections is shared with the high voltage Cathode. I guess since the filament windings are isolated from the high voltage windings, no current will flow and they can coexist, but my mind just does not quite grasp why.