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Other Vintage Household Electrical or Electromechanical Items For discussions about other vintage (over 25 years old) electrical and electromechanical household items. See the sticky thread for details.

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Old 30th Apr 2021, 9:30 pm   #21
Lancs Lad
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Default Re: 32 year old Sharp microwave oven...

Thanks, Richard!

So those plastic rivets aren't likely to snap and fall apart if I attempt to dislodge them, then?

I'll be able to just push them back into place when I've examined the other side of the waveguide cover, and want to replace it?

I don't want to cause any unnecessary damage!
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Last edited by Lancs Lad; 30th Apr 2021 at 9:39 pm.
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Old 30th Apr 2021, 9:34 pm   #22
Richard
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Default Re: 32 year old Sharp microwave oven...

Yes they usually survive.
You do not need the cover in place for testing.
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Old 30th Apr 2021, 11:58 pm   #23
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Default Re: 32 year old Sharp microwave oven...

A word of warning - Behind the cover is an opening cut in the oven's 'ceiling'. The edges of that opening are very sharp ! If you poke your fingers in with a piece of fabric or tissue to clean up into the waveguide there is every risk you will cut your fingers on those edges. Guess how I know this.

In the case of my oven the arcing around the cover was pretty impressive. I don't know how long it might have been going for before the fuse went - I didn't see it as only Mrs GJ (and her tea) were present. Having replaced the blown fuse I saw it arc again, but I probably switched off in a second or so. Also, fuses do get 'tired' over time if they're run near maximum capacity. It may be that the original fuse was close to end of life anyway.

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Old 1st May 2021, 12:32 am   #24
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Default Re: 32 year old Sharp microwave oven...

Thankyou, GJ, for the cautionary warning. I seem to remember reading about razor sharp edges under waveguides!

And, yes! My first hopeful thought was that it might be that the thirty-odd year old fuse might have just had enough and failed!

Unfortunately, that doesn't seem to be the case...

Nothing's ever that simple, is it?
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Old 1st May 2021, 9:16 am   #25
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Default Re: 32 year old Sharp microwave oven...

Discharge HV cap & remove magnatron wires Ohm the magnatron filament to ground and the HV conn to ground. If it shows leakage, mag is bad. You can ohm the diode for a short, as well as the capacitor for shorts.
Once in a blue moon you can get a shorted transformer. Think I have seen 2 in the last 30 yrs.
Growling is a major short. I would guess mag or diode, off my hat.
Be sure to clean any grease out of the wave guide and as was said, clean or replace mica. Arcing to metal can be repaired by sanding it then using the special microwave oven paint. Works quite well. Clean door seals as well and beg, borrow or steal a microwave leakage tester.
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Old 1st May 2021, 4:45 pm   #26
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Default Re: 32 year old Sharp microwave oven...

Quote:
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Discharge HV cap & remove magnatron wires Ohm the magnatron filament to ground and the HV conn to ground.
Does a microwave contain a giant valve by any chance? I never realised the magnetron had a heater, all this has now become very intriguing.
Learn something new everyday I suppose.
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Old 1st May 2021, 4:51 pm   #27
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Default Re: 32 year old Sharp microwave oven...

The 'magnetron' (more precisely a 'cavity magneton') is a directly heated diode valve with a very specially shaped anode (containing the resonant cavities) and operated in a magnetic field.

I've been told that due to their use in microwave ovens that the cavity magneton is the type of valve produced in the largest quantity now
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Old 1st May 2021, 5:11 pm   #28
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Default Re: 32 year old Sharp microwave oven...

If it contains a heater, that might answer as to why radar devices display a message saying it's warming up, it usually takes 30 seconds but annoys the hell out of me, I have no patience.
At least it draws the landscape, the pictures are rather pretty to look at.
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Old 1st May 2021, 5:40 pm   #29
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Default Re: 32 year old Sharp microwave oven...

oven magnetrons take about three seconds before they oscillate, no idea about radar sets.

Click image for larger version

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Another fun fact, the diode, and capacitor form a Cockcroft–Walton voltage doubler, the magnetron itself being the second diode.
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Old 1st May 2021, 6:59 pm   #30
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Default Re: 32 year old Sharp microwave oven...

At first-sight, classic big-transformer microwave-ovens are confusing to some people.

The semiconductor-diode/capacitor/Magnetron network in particular.

Yes, it's just a voltage-doubler. The Magnetron is a diode, after all. But where is the 'load' for the voltage-doubler?

The 'load' is the Microwave side of the Magnetron. It's a half-wave-rectifier and a self-exciting UHF oscillator concealed in the same envelope.

Once you grasp this, things become more-comprehensible.

The transformers in 'classic' microwaves are also quite cunning: OK, you need power to heat the Magnetron's filament - but once it's up to temperature and churning-out 800W or so of RF at 2.45GHz the cathode becomes subject to heating by 'back-bombardment' which raises its temperature rather higher.

So the 'heavy iron' transformer is designed to approach saturation at normal load and so reduce the amount of power supplied to the heater in order to compensate [crudely] for the back-bombardment heating.
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Old 1st May 2021, 7:52 pm   #31
Cruisin Marine
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Default Re: 32 year old Sharp microwave oven...

That info. was a most interesting read G6Tanuki- thank you.

What is a typical voltage and o/p current for a microwave transformer, in case it was possible to use to supply a linear amp?
Thanks
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Old 1st May 2021, 10:03 pm   #32
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Default Re: 32 year old Sharp microwave oven...

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What is a typical voltage and o/p current for a microwave transformer, in case it was possible to use to supply a linear amp?
Thanks
Around 2100V, 0.3A in your average domestic microwave.
One end of the HT winding is connected to the laminates/earth which makes using them in power supplies tricky.

The older commercial microwaves had transformers without with a fully isolated HT winding, and current up to 1A, working with full wave rectifiers.
I have got a load, no idea what I will ever do with them.

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Old 2nd May 2021, 10:01 am   #33
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Default Re: 32 year old Sharp microwave oven...

-There's always creating Lichtenberg figures, but potentially highly dangerous of course.
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Old 2nd May 2021, 4:54 pm   #34
Lancs Lad
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Default Re: 32 year old Sharp microwave oven...

Do magnetrons become less powerful or effective with age?

I made scrambled eggs last night in this new Argos microwave, which, incidentally, is fifty watts less powerful than my original Sharp oven, and yet they cooked significantly faster than I've been used to.

And I mean a LOT faster! Three whipped-up eggs ready in under a minute and twenty seconds.

Obviously stopping and starting a few times during that cooking time to take the bowl out and give it a stir.

The same process in the Sharp had been taking near enough five minutes.

It was a good job I was keeping an eye on them, otherwise they'd have been incinerated.

Any thoughts?
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Last edited by Lancs Lad; 2nd May 2021 at 5:19 pm.
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Old 2nd May 2021, 5:03 pm   #35
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Default Re: 32 year old Sharp microwave oven...

They DO degrade, it's noticeable when a new one is purchased.
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Old 2nd May 2021, 6:13 pm   #36
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Default Re: 32 year old Sharp microwave oven...

I think my A-level Physics practical exam involved making some sort of power measurement using the temperature rise in a known volume of water in an ordinary polystyrene disposable cup as the sensor. I can't remember the details, but discussing it afterwards my physics master said that as long as the temp rise doesn't start causing large evaporative losses the water really does lose very little heat if you take its temperature quickly. Maybe this would be a way of at least comparing the power delivered by two different microwaves ?

Cheers,

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Old 2nd May 2021, 6:29 pm   #37
Richard
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Default Re: 32 year old Sharp microwave oven...

Hi

The original test was: -

1L of water in a plastic jug, measure temp in 'C, heat in microwave on full power for 1min 3 seconds (the 3 seconds is to allow the Magnetron to cut in), give it a stir and measure temp again. 'C rise x 70 = power in Watts.

Newer ovens they moved the posts, now rated according to IEC 705.
Add 15% to get the EEC version of power in Watts.

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Old 3rd May 2021, 4:18 pm   #38
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Default Re: 32 year old Sharp microwave oven...

Quote:
Originally Posted by paulsherwin View Post
The UK has quite tight liability laws applying to sellers of secondhand goods, and this has the effect of discouraging charities from selling electrical items, with some items (like microwaves) completely blacklisted. They just don't want the potential hassle. US thrift stores and UK charity shops are broadly comparable, but there are significant cultural and regulatory differences.

You sometimes find working microwaves dumped in skips (US:dumpsters) or on Freecycle/Freegle.

Back to the OP's Sharp please.
I fail to realize, rules and regulations of other countries. It even differs here in some localities.
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Old 3rd May 2021, 4:49 pm   #39
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Default Re: 32 year old Sharp microwave oven...

When it comes to American rules, there are 50 states, each state can have it's own rules within reason, although most rules are the same with a slight variation state-by-state.
Back on topic, microwaves do seem to lose output as time goes on, I couldn't help noticing that when using new machines.
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Old 3rd May 2021, 6:31 pm   #40
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Default Re: 32 year old Sharp microwave oven...

Although newer Microwaves are often more powerful 800W+
Whereas my > 35yr old (used daily and original Magnetron) Sharp is only around 600W, but doesn't seem to have noticeably dropped-off in performance.

Using more-powerful ones, when used to using lower-powered ones, can take getting used to when trying to gently heat something. As power-control is just on-off duty-cycling over a 'heat & cool-down' long-period, unless it's an expensive PWM Inverter type that early ones frequently failed..
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