Microwave/Grill Combi Fuse Issues

Thread Starter

Acrimonious Mirth

Joined Jul 11, 2018
11
Hi all!
Hope everyone is looking forward to a good Christmas break.
We have a microwave/grill combi, the fuse blew some months back and the microwave stopped working (but the grill still does). Anyway, we swapped in a new fuse just yesterday and tried it again. The microwave still seems to be offline and the grill works as before.
Now, my theory is that the surge that blew the fuse also managed to blow the magnetron (or maybe originated there) but dad’s tasked me with finding out what else it could be.
Any suggestions?

Thanks!
 

MaxHeadRoom

Joined Jul 18, 2013
28,669
If the fuse is now hanging in? Pull the secondary lead(s) off of the transformer first,
Use insulated pointed nose pliers etc. Power off of course.
Check to see if you can hear the transformer power hum, or carefully feel the lamination's for vibration under power.
Max.
 

spinnaker

Joined Oct 29, 2009
7,830
This is another, if you have to ask you shouldn't be messing with it sort of thing.

I would take it for repair or buy another unit. I have never seen a microwave grill combo.
 

ian field

Joined Oct 27, 2012
6,536
Hi all!
Hope everyone is looking forward to a good Christmas break.
We have a microwave/grill combi, the fuse blew some months back and the microwave stopped working (but the grill still does). Anyway, we swapped in a new fuse just yesterday and tried it again. The microwave still seems to be offline and the grill works as before.
Now, my theory is that the surge that blew the fuse also managed to blow the magnetron (or maybe originated there) but dad’s tasked me with finding out what else it could be.
Any suggestions?

Thanks!
Apart from the obvious that looks pretty much covered - the door safety interlock has a failsafe set of contacts that short it out if both the break contacts weld. The most usual cause of interlock damage is door slamming. The contacts bounce and may arc - not enough to blow the fuse, but enough to compromise the contact surfaces. Its not worth attempting to repair that unless you REALLY know what you're doing. You need to use all new microswitches and alignment while fitting is critical.

A tranzorb type device became fashionable on the secondary side, failing short circuit isn't exactly rare. The HV rectifier can short, and occasionally the magnetron. On one case of fuse blowing; I found a small "pimple" on the doubler capacitor - the dielectric had broken down and debris had hit the inside of the aluminium can.

The 2kV secondary is no second chances lethal - don't mess with it unless you know what you're doing!!!
 

MisterBill2

Joined Jan 23, 2018
18,421
My experience with a microwave oven high voltage transformer was that with the tube disconnected the voltage was higher and it immediately started to arc to ground from the high voltage terminal. But checking the switch contacts is a very good idea. There would be either two or three switches. two in series and that one to short things and pop the fuse. So disconnect the PRIMARY of the transformer and then check that the switches provide continuity with the door closed. And if they are similar switches you may be able to swap positions of the shorting one with a failed series switch. I have done that. Of course, I have also opened the switch bodies and filed the contacts smooth. Tricky but it can work.
 

ian field

Joined Oct 27, 2012
6,536
My experience with a microwave oven high voltage transformer was that with the tube disconnected the voltage was higher and it immediately started to arc to ground from the high voltage terminal. But checking the switch contacts is a very good idea. There would be either two or three switches. two in series and that one to short things and pop the fuse. So disconnect the PRIMARY of the transformer and then check that the switches provide continuity with the door closed. And if they are similar switches you may be able to swap positions of the shorting one with a failed series switch. I have done that. Of course, I have also opened the switch bodies and filed the contacts smooth. Tricky but it can work.
Pulling the secondary spade terminal is the first check to see if the transformer has shorted turns - arcing all over the place isn't common IME.
 

Thread Starter

Acrimonious Mirth

Joined Jul 11, 2018
11
This is another, if you have to ask you shouldn't be messing with it sort of thing.

I would take it for repair or buy another unit. I have never seen a microwave grill combo.
Thanks for the advice but I’ve been messing around with machines since I was in diapers. I’ll keep on messing around with them until I either blow myself up and do my engineering degree an injustice, or die of old age.
Replacing it was certainly my advice.

If the fuse is now hanging in? Pull the secondary lead(s) off of the transformer first,
Use insulated pointed nose pliers etc. Power off of course.
Check to see if you can hear the transformer power hum, or carefully feel the lamination's for vibration under power.
Max.
Yeah, the fuse is doing fine. I’d need to double check the circuitry to be sure but I’m quite certain the transformer feeds both the microwave and the grill, there wasn’t a secondary one. So if the interface works, the grill works, the turntable motor works... I think the transformer probably works.

Apart from the obvious that looks pretty much covered - the door safety interlock has a failsafe set of contacts that short it out if both the break contacts weld. The most usual cause of interlock damage is door slamming. The contacts bounce and may arc - not enough to blow the fuse, but enough to compromise the contact surfaces. Its not worth attempting to repair that unless you REALLY know what you're doing. You need to use all new microswitches and alignment while fitting is critical.

A tranzorb type device became fashionable on the secondary side, failing short circuit isn't exactly rare. The HV rectifier can short, and occasionally the magnetron. On one case of fuse blowing; I found a small "pimple" on the doubler capacitor - the dielectric had broken down and debris had hit the inside of the aluminium can.

The 2kV secondary is no second chances lethal - don't mess with it unless you know what you're doing!!!
Thanks! It’s not the door because the grill still works and opening the door switches it off, naturally.
I do actually have microwsitches lying around from last time I made a microwave spot welder.

Will check the rectifier and capacitor, but as I say, grill and interface functionality is still operational. Pretty sure it’s the magnetron.
 
Last edited by a moderator:

ian field

Joined Oct 27, 2012
6,536
Thanks for the advice but I’ve been messing around with machines since I was in diapers. I’ll keep on messing around with them until I either blow myself up and do my engineering degree an injustice, or die of old age.
Replacing it was certainly my advice.
Microwave ovens are far more dangerous beasties - don't even think of sticking your hands in there without pulling out the mains plug.

The charge on the doubler capacitor is priobably only unpleasant - but I didn't make a habit of discharging them with my fingers.
 

Thread Starter

Acrimonious Mirth

Joined Jul 11, 2018
11
My experience with a microwave oven high voltage transformer was that with the tube disconnected the voltage was higher and it immediately started to arc to ground from the high voltage terminal. But checking the switch contacts is a very good idea. There would be either two or three switches. two in series and that one to short things and pop the fuse. So disconnect the PRIMARY of the transformer and then check that the switches provide continuity with the door closed. And if they are similar switches you may be able to swap positions of the shorting one with a failed series switch. I have done that. Of course, I have also opened the switch bodies and filed the contacts smooth. Tricky but it can work.
Thanks for the advice. However the grill functionality remains unhindered and the interface works fine too. The machine turns off when the door opens so I’m led to believe that the switches are okay.

Microwave ovens are far more dangerous beasties - don't even think of sticking your hands in there without pulling out the mains plug.

The charge on the doubler capacitor is priobably only unpleasant - but I didn't make a habit of discharging them with my fingers.
Certainly. I have taken several microwaves apart before, one for my spot welder that I used to make my 48V 50A LiPo battery for my own-design e-bike. Naturally before any fiddling with it, I make sure it’s unplugged, the capacitor has been grounded (if necessary, it’s well insulated), and there are no obvious electrical faults I can see by eye.
 
Last edited by a moderator:

MisterBill2

Joined Jan 23, 2018
18,421
The interlocking for the grill is not nearly as critical as for a microwave generation circuit, so they may have separate switches. If you have a circuit diagram you could verify that easily. If not, it is a bit more complicated indeed. But I would guess that they are switched separately, even though at the same time. Safety rules are funny about some things.
 

Thread Starter

Acrimonious Mirth

Joined Jul 11, 2018
11
The interlocking for the grill is not nearly as critical as for a microwave generation circuit, so they may have separate switches. If you have a circuit diagram you could verify that easily. If not, it is a bit more complicated indeed. But I would guess that they are switched separately, even though at the same time. Safety rules are funny about some things.
Thanks for the insight. I’ll see about giving it a closer look soon.
 

MaxHeadRoom

Joined Jul 18, 2013
28,669
but I’m quite certain the transformer feeds both the microwave and the grill, there wasn’t a secondary one.
Not familiar with a combo microwave grill, but highly doubt that that transformer would be used for both, the MW uses very high voltage, a grill is usually a simple element across the AC supply.
Max.
 

MisterBill2

Joined Jan 23, 2018
18,421
Our group once went through the basic concept development for a microwave barbaque grill. The feed horn would double as the smoke collector hood. The problem was powering the 120KW magnetron needed for the large cooking area. Also, excess heating of people nearby would have been an issue. But it was an interesting discussion.
 
@Acrimonious Mirth

The following assumes your cognizance of the not-inconsiderable safety issues involved -- Which being, IMO, reasonable based upon the monitions of other respondents taken with your own comments...

So... With no further ado, I offer the following (hopefully helpful) observations gleaned away-back in the daze (sic) of my youth when high-level RF was my very 'fave' toy and a 'fab' one it was!:D

Abrupt magnetron failure generally owes to one of the following (order insignificant):
1) Open filament
2) Filament (and, hence, cathode) to-anode short.
3) Broken magnet{s} (corollary to overheating).
4) 'Going to air' as a result of envelope penetration secondary to antenna-to-waveguide arcing.

Note that conditions 2 and 3 (above) represent 'permanent' low impedance failure modes, with resultant OCP deployment and/or failed rectifier and/or (too often) damaged transformer insulation -- Please note that should the rectifier fail 'open' the replacement fuse will remain intact and the appliance appear to operate 'normally' sans RF heating.

For all that, based upon the symptoms described in your posts, a faulty magnetron seems unlikely...

Assuming the appliance in question employs 'conventional' PSU topology (i.e. line-frequency magnetic transformer/half-wave doubler) please check the the following:

1) The rectifier: (Be advised that attainment of forward bias may require several volts).

2) The capacitor: against terminal-to-terminal and terminal-to-'can' shorts. --- Thence for correct capacitance (within 10% of component marking). Note: Inasmuch as said caps generally contain an integral 'bleeder resistor'-- moderately higher than expected ESR and rapid self-discharge are not necessarily indicative of defect...

3) Thermal cut out{s} (mounted on the magnetron and, often, the waveguide) -- Although designed to 'auto-reset', said components not-uncommonly 'stick' open...

4) The cathode current switching device (which being a thyristor or, occasionally, an electromechanical contactor).

/////
Should the magnetron PSU be of SMPS/power-oscillator topology (said schemes often marketed as featuring 'Inverter Technology' including --albeit not limited to-- most microwave ovens of 2005 and subsequent Toshiba manufacture --- A generally adequate 'sense' of PSU functionality may be gleaned via placement of a small discharge lamp (e.g. Ne2) near the perimeter of the high-frequency transformer's secondary winding during operation (via a 30cm or longer non-conductive rod/tube) --- A brilliantly glowing bulb out to 1cm or more from the transformer generally exonerates the PSU:)


Best regards and good luck
HP:)
 
Last edited:

ian field

Joined Oct 27, 2012
6,536
Magnatron. It sometimes sputters before it actually dies. I fit works at all, you can do the wattage test.
Found the occasional shorted mag, but most lose emission and take steadily longer to cook anything. Is possible that excessive use of defrost strips the cathode, the heater is quick start - but not that quick. Some top of the range ones have separate heater transformer.

Once I had an old mechanical timer one with low emission. Nothing to lose, so I jammed a screwdriver in the fan and cranked the timer all the way - it more or less recovered to a useable power output once it cooled down.
 
Once I had an old mechanical timer one with low emission. Nothing to lose, so I jammed a screwdriver in the fan and cranked the timer all the way - it more or less recovered to a useable power output once it cooled down
Yebut overheating the tube is verily inviting fractured magnets!:eek: Reactivation of the 'emitter material' is better attempted via briefly over-powering the filament (in the absence of 'B+')...

Some top of the range ones have separate heater transformer.
As do antique Amanas:) Of course good design practice will keep the filament 'at temp' throughout the entire cooking process ('keying' the anode/cathode supply only) -- But then I don't expect 'elegance' offers much in the way of competition to 'the bottom line' where consumer products are concerned:rolleyes:

Very best regards
HP:)
 

ian field

Joined Oct 27, 2012
6,536
Yebut overheating the tube is verily inviting fractured magnets!:eek: Reactivation of the 'emitter material' is better attempted via briefly over-powering the filament (in the absence of 'B+')...


As do antique Amanas:) Of course good design practice will keep the filament 'at temp' throughout the entire cooking process ('keying' the anode/cathode supply only) -- But then I don't expect 'elegance' offers much in the way of competition to 'the bottom line' where consumer products are concerned:rolleyes:

Very best regards
HP:)
What part of: "nothing to lose" did you not understand?
 
What part of: "nothing to lose" did you not understand?
Right off hand I'd say the "nothing" part!;) -- Granted! 'Rejuvenation' of power tubes tends to be a 'lossy' proposition under the best conditions - but then applying Ca. 8V @ ≈ 10A to the tabs for a few seconds requires only slightly greater effort than 'pinning' the blower during operation - and with notably better prospects (such that they are):cool:

Best regards
HP:)
 
Last edited:

ian field

Joined Oct 27, 2012
6,536
Right off hand I'd say the "nothing" part!;) -- Granted! 'Rejuvenation' of power tubes tends to be a 'lossy' proposition under the best conditions - but then applying Ca. 8V @ ≈ 10A to the tabs for a few seconds requires only slightly greater effort than 'pinning' the blower during operation - and with notably better prospects (such that they are):cool:

Best regards
HP:)
I just wanted to see something go phut, but it didn't - I ended up using it myself till it eventually quit.
 
Top