Microwave oven loss of power

[Joe_Lorenz]

Hello,

now I have got our beloved Buderus Juno TM 8040 microwave oven on my bench, aka Philips M702. Bought in 1987 shortly after our wedding this device has been in daily duties ever since. Just now the best wife of all stated a noticeable loss of power. "Everything seems to take longer and longer" I was reported. So I removed the hood but found nothing to be wrong at first sight. "That" capacitor of 0.72 µF @ 2500 V would read 726.4 nF, Vloss 0.2%, ESR 1.0 Ohms.

Might the magnetron valve Toshiba 2M172AH suffer from degrading vacuum or what ever could possibly go wrong? I have been encouraged by my oldest son as "Can't be that bad, not many parts in there!" but I am afraid I haven't got exactly matching spare parts at hand and I have no good clue yet of how to measure produced HF.

Anybody already been there done this? Any thoughts appreciated!

Regards, Joe

[kalee20]

I have a Panasonic microwave oven, bought new 1994, that last year just stopped cooking! It had been my impression that cooking was taking a bit longer than it used to... anyway, it stopped. The light and turntable still did their stuff.

Running on 24V AC (via a Variac) confirmed that the mains transformer was OK, as was the rectifier diode and capacitor. The magnetron heater was intact.

So with only a couple of tenners to lose I bought a salvaged magnetron from eBay (searching using the magnetron type number as per its label). A good clean, and careful swap over, restored operation. That was about 10 months ago, and it's been working fine since! I suspect that cathode emission deteriorated to the point that oscillation just wasn't sustainable.

Don't forget that the HV capacitor can store enough energy to seriously bite, and possibly kill.

I did treat myself to a leakage detector afterwards, and went on a 'sniff' but the leakage was negligible.

[G.Castle]

The magnetrons do need a good emission to even start oscillation, so the oven will often appear to loose a little heating power, but continue to work, however then just suddenly stop. If supplies are still present a dead mag, is the most likely cause.

[G8UWM-MildMartin]

I agree with G.Castle.
(Provided there's not something else absorbing the "missing" energy such as gross rust or dirty deposits.)
The capacitor seems OK, the diode must be OK as it would dissipate a ridiculous amount of heat if other than s/c, o/c or OK, and the transformer is likely to be OK if not smoking!
Replacement magnetrons are available from a company in the UK that I have used, and they list equivalents for you to look for from German suppliers:
https://magsells.co.uk/product/magnetron-msm707/
They are:
A670 IH, 2M244 M32, 2M303 H and 2M258 M23
Also 2M172 HB , 2M172AH (DW) , 2M172AH (P) , 2M167 M14 , OM75 (21) , S107HB , 2M107A (94) and 2M107A 33 2M292 H 2M258 M32, 2M303H

[buggies]

Originally Posted by Joe_Lorenz

I have been encouraged by my oldest son as "Can't be that bad, not many parts in there!" but I am afraid I haven't got exactly matching spare parts at hand and I have no good clue yet of how to measure produced HF.

Anybody already been there done this? Any thoughts appreciated!

Regards, Joe

I think it is usually done with a beaker of water. The pics I took from the instruction sheet of my microwave detector might give a clue...

[Graham G3ZVT]

Not withstanding that different magnetrons have differing HT and heater requirements, some suppliers provide a number of "universal" magnetrons differing only in physical characteristics, ie mointing studs vs holes, and relitive oriantation of the terminal block wrt cooling fins and mointing bracket. In other words you just look at the pictures in the catalogue and choose the one that matches what you've got.
I used to salvage magnetrons and other parts from units I found on the street.

[paulsherwin]

I would say that fitting a replacement magnetron to a 1987 microwave oven is unlikely to make sense unless the appliance has particular sentimental significance.

[Nickthedentist]

…which this one does, if I understand Joe correctly

But take the utmost care. They are about the only domestic electric appliance which can kill the unwary repairer with ease!

[factory]

If the microwave is in good condition, with no rust and all the other bits are working, then it makes perfect sense to me. Must modern microwaves seem to die from the casing rusting away after a few years, unless your lucky enough to find one that is actually made from stainless steel.

David

[Cobaltblue]

That's the great thing about these 80's Microwaves.

We have a Sharpe still in excellent working order from 1987.

They tended to be made of stainless steel in the oven cavity meaning they would last pretty much forever most modern "cheepies" rust out in 2 to 4 years.

They also usually have much bigger cavity allowing much larger or multiple items to fit with ease.

David has posted pretty much the same reasoning.

Cheers

Mike T

[Joe_Lorenz]

Dear gentlemen,
thank you very much for your comments! These are very helpful!

@ Martin: I have a 2M167 and a 2M303 as candidates for replacement
@ George: That's what I've been looking for, great! I will do so
@ Paul: Yes, I daresay we could afford a new oven ... But: The questioned one is of "Philips Space Cube" size and has been integrated into a brand new kitchen 2 years ago. Most but all of today's models won't fit so I'd have to do some carpentry work

And last but not least: We want a simple microwave oven, no combination ovens, no touch screens, no programmable digital clock, no IOT controlled device or anything unnecessary else!

Thanks again, folks, I hope I can report progress soon!
Joe

[Bazz4CQJ]

My beloved (and very simple) Sharp dates back to 1986, and is still in everyday use, as is the 1985 Hotpoint washer .

The test procedure given in post#5; I'm curious about why that specifies a polypropylene beaker. Beakers, as used in labs, usually have specific "form factors" which may be an issue in repeatability. Plastic is probably safer than glass. Any other issues involved?

B

[Richard]

Hi.
If the microwave still works, but with reduced power there are only a few options.
1 - Magnetron, it is a valve, they slowly die, reduced microwave output, more thermal heat produced in the body, removed by the cooling fins. When it gets really low power the round core in the fins may discolour and turn black, and the fins may become lose, which just makes things worse as they don't remove the heat effectively.
Everything else is fine, HT transformer, capacitor, diode - they all either work or they don't, there are no in betweens due to the high voltage and power involved.
As the mag loses power in will often "mode", which you may be able to hear as a regular sharp "tick" noise when it's cooking.

The other things worth checking before fitting a new mag are: -
1/ There is a "stirrer" fitted, being Philips most likely under the floor, often blown around by some of the mag cooling air, or in the roof, as the name suggests it stirrers the microwaves to help achieve an even heat patten, make sure it's turning ok.
2/ Check the oven is on full power, and actually is cooking continually.

The standard power test in place when that oven was made was
1L of cold water, plastic jug (not glass, it will absorb too much heat from the water)
Measure temp. in C
Heat on full power for 1min 3 seconds.
Give a stir and measure temp again.
Temp rise x70 = power in Watts.

Those ovens usually produced 600W - 700W.

Newer ovens use a different calculation, IEC705, basically add 15% to the above result.

2M172 = 2M107 = many other.
There will be various variations, with additional letters on the end, these all relate to: -
Type of fixing that hold the mag in place in the oven
Orientation of cooling air flow relative to the fixings
Orientation of the power terminals relative to both above.

Make VERY sure it is unplugged and the HT capacitor discharged before touching any HT system, it will kill you!

Richard.
Over 30 years repairing domestic and commercial microwaves.

[Joe_Lorenz]

Richard,
thanks for the hints, too. I love to learn something about this microwave subject I haven't seen the necessity to care about yet.

As it was already sitting on my bench I took the chance to inspect the apparatus closely. Astonishingly clean inside, not much dust to remove and all components look healthy but those two wires feeding the magnetron! Please see the photos. The wires (or the valve?) must have been very hot, starting to melt the insulation.

We will see how things will go with the magnetron swapped.

Regards, Joe

[Richard]

Hi Joe

The slightly melted wires are due to the filament current, 3.2V @ 10A ish.
I would clean them up and see if you can feed some solder into the crimps or fit new tags and solder them. Crimping onto single core wires is never a good idea.

Richard.

[G6Tanuki]

I would beg to disagree; *proper* crimping on to solid wire produces a 'cold weld' whose reliability and low-resistance is vastly better than you can achieve with mere soldering.

The aviation industry just doesn't do soldered connections in this sort of application, and I wouldn't want to use soldered joints here either.

[G.Castle]

This will probably not apply to Joe's machine, but on the subject of the solid wire that is a continuation of the transformer windings, in newer ovens the wire is often enamelled aluminium.

As the mag output becomes weaker they do tend to overheat, a clamped connection would be a lot more secure...

[kalee20]

The thread when I replaced a magnetron (it looks very similar to yours) is here.

[Hartley118]

I bear the (metaphorical) scars from clamping on to aluminium in safety-critical applications. The stuff creeps, leading to a relaxation of clamping pressure and hence the eventual loosening of the joint. Really bad, both mechanically and electrically, leading to things falling apart and/or catching fire.

Martin

[G.Castle]

This is quite true, more so than copper and it is not trivial.

The clamping must be done to constrain the cross sectional area of the conductor and also be arranged to maintain clamping pressure in the presence of creep and thermal expansion. There is also the consideration of keeping the interface of the clamped surfaces free from oxidation particularly where there is dissimilar metals present.

Unfortunately modern microwaves do use aluminium as a transformer conductor and the often just crimped connections do fail.

As I said it's likely Joe's machine was built before aluminium was widely used for the construction of their transformers.