[Lowfer] Another power supply 'head scratcher'

[Bill Ashlock]

All problem solvers,

This is a bit off-topic but since microwave power supplies were mentioned as a source of parts in the last 'head scratcher' thread I thought I'll throw this in the ring: Last night I got into the repair of a microwave oven of my neighbor's. Nothing happed (at least no display) when plugged-in and I figured the problem could be as simple as a blown fuse. My guess wasn't far off as it turned out that the fuse holder had not been fully contacting the 1/4" diam 20A fuse on one end and this resistance heated up the fuse to the extent that the internal solder connection within the fuse had opened up. The necked-down portion of metal link within the fuse where it is supposed to blow showed no sign of excessive current. So the oven came back to life, like new, with a little modification of the fuse holder and a new fuse. End of story?  NO!!

I happened to be looking at the HV circuitry that drives the magnetron and noticed that the HV rectifier diode, instead of returning to the neutral side of the line, connected to a lug that was screwed onto the chassis of the oven. I began thinking about what would happen if the safety ground (green wire) were to become disconnected or if someone used a 2-wire extension cord along with a cheater plug. I ohmed out the chasis and netraul lines and found no connection between them. Next I discovered a schematic of the oven glued underneath the metal U-shaped cover. The schematic showed the neutral and ground connections as they should be: IE: the HV diode returned to the neutral line rather than the chassis. 

This is a Kenmore microwave oven with a stock # of 60202 or 60209. No model #, serial #, or UL label that I could find. I'm about to google these numbers to see if there are any recalls. I would classify this oven as "lethal" and should not be used because without an external safety ground in place there is over 4KV from the metal cover to other kitchen appliances! Oh yes, I almost forgot: There is a warning on the back saying that it is dangerous to operate the oven without a grounded outlet. I'm sure the folks that have be knocked to the floor have a simple "geeee thanks" to offer for this :)

Any thoughts? Are there other microwaves out there wired like this?

Bill