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Old 22nd May 2017, 7:50 am   #1
Ted Kendall
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Default Ancient microwave oven - repairable?

The Sharp Quartz Chef combi we bought two decades ago expired last week - I'd guess the magnetron has gone pop as there was was an episode of excessive cooking followed by no output.

This frankly isn't my thing at all - but is the thing repairable, practically speaking?
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Old 22nd May 2017, 8:17 am   #2
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Default Re: Ancient microwave oven - repairable?

Quote:
Originally Posted by Ted Kendall View Post
This frankly isn't my thing at all...
I think that's your answer, Ted!

Microwave ovens are cheap to replace, and messing around with old ones can be potentially fatal, as you know. If the mains transformer has failed, it will be a write-off. Magnetrons etc. can be very expensive if available at all. An excellent company called AWI, which supplied a huge range of genuine and pattern spares, went out of business about 5 years ago.

However, doing an autopsy can be interesting, and many 1980s/1990s vintage machines are of superb quality and well worth repairing if the fault is relatively trivial. For example, my parent's late-1980s Toshiba is still going strong after I replaced the turntable motor which cost less then a tenner.

There were a couple of short articles published in Television magazine which gave a very good grounding in the subject of operation and repairs. I think they were scanned and published here, though I can't find them at present.

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Old 22nd May 2017, 8:25 am   #3
Dave Moll
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Default Re: Ancient microwave oven - repairable?

Nick, I don't suppose you remember when in the run of Television the articles were. If you can narrow it down to a year or so, I'll see whether I can find them.
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Old 22nd May 2017, 8:28 am   #4
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Default Re: Ancient microwave oven - repairable?

At a guess, Dave, it was probably late 1990s but I can't be sure. I will have a rummage when I get a chance.
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Old 22nd May 2017, 8:59 am   #5
John Caswell
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Default Re: Ancient microwave oven - repairable?

A lot of Microwave oven problems were down to the magnetron heater pin connectors failing after may cycles. Might be worth a look.

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Old 22nd May 2017, 9:59 am   #6
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Default Re: Ancient microwave oven - repairable?

What is the actual model number of your oven? Quartz Chef is just a range.

Sharp service manuals are well written, and the microwave manuals show you how to 'cold check' the HV parts - voltages in microwaves are extremely dangerous.

I have attached a Sharp R209 service manual for you to look at, I probably have the manual for your oven, and we still have a lot of parts in stock.

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File Type: pdf R209 SM.pdf (524.4 KB, 506 views)
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Old 22nd May 2017, 11:41 am   #7
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Default Re: Ancient microwave oven - repairable?

Always discharge any capacitors before doing anything. Older models often weren't fitted with bleed resistors and can retain dangerous voltages for days.

As Nick said back in #2, microwave ovens can be a great route to the crematorium unless you know exactly what you're doing. If this oven needs a new magnetron then it's likely to be BER anyway.
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Old 22nd May 2017, 11:51 am   #8
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Default Re: Ancient microwave oven - repairable?

AWI went, but one of the brothers continued as
http://www.magsells.co.uk/
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Old 22nd May 2017, 12:07 pm   #9
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Default Re: Ancient microwave oven - repairable?

That's reassuring, Richard, thanks for pointing us to them.
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Old 22nd May 2017, 12:31 pm   #10
Dave Moll
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Default Re: Ancient microwave oven - repairable?

Was this article from the September 1999 edition of Television the one you were thinking of?
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File Type: pdf servicing micrwaves.pdf (694.5 KB, 133 views)
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Old 22nd May 2017, 12:50 pm   #11
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Default Re: Ancient microwave oven - repairable?

Hi Dave,

It wasn't that one, actually.

The one I was thinking of was definitely aimed at domestic machines and had much more of a practical feel to it, and included a circuit diagram.

It was the old-style typeface and layout, I think, if that helps date it.

N.
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Old 22nd May 2017, 1:50 pm   #12
Dave Moll
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Default Re: Ancient microwave oven - repairable?

OK, I'll continue working back to see what else I can find.
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Old 22nd May 2017, 2:22 pm   #13
Al (astral highway)
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Default Re: Ancient microwave oven - repairable?

Quote:
Originally Posted by Ted Kendall View Post
This frankly isn't my thing at all - but is the thing repairable, practically speaking?
Hey Ted,

I share your instinct and urge and interest in fixing anything that moves, should move, or used to move. And things that shouldn't move, too. I've opened tens of street-dumped microwave ovens over the years to salvage the main transformer, EHT wire and EHT caps.

I have to say it too, they are ridiculously dangerous, capable of sourcing minus 5,600V pulsed DC at almost 700mA for a short period on the doubler side, and a short-circuit (as in, drawing a continuous arc for many seconds) at 2,200 VAC on the high voltage transformer side.

I am intentionally using two transformers in a long term project, but I really would just walk away from trying to mend a microwave oven unless it was a simple thing like switch contacts, cooling fan etc.

Because of the paradoxical nature of global retail economics, we can see a new microwave oven in the larger supermarkets on offer for less than £50 from time to time -- less than the replacement cost of a transformer.

There is practically nothing on the high-energy side that can go wrong with one that can be fixed at home.

Models that pre-date the new EHT SMPS topology may or may not have a extra component that looks very much like the EHT rectifier (the typically rectangular cuboid or cylindrical black object on the doubler), but actually isn't one. It senses leakage current from the 2.2uF typically doubler cap and when foward biased, conducts, blowing the fuse. I think this was done to spare the EHT rectifier, but parts suppliers have grown wise to this and so in my experience, supply them together rather than individually!

These cost something like £8 together as a couple, tied to an 8kV EHT rectifier diode with attachment lugs. And then there's the cost of a new doubler cap, which is also very high compared to the cost of a new oven.

Personally, even with all my times of digging around for useful parts, I'd just walk away.
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Old 22nd May 2017, 2:47 pm   #14
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Default Re: Ancient microwave oven - repairable?

Having travelled back in time exactly twelve further months, I've found this one:
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File Type: pdf servicing microwave ovens.pdf (739.9 KB, 135 views)
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Old 22nd May 2017, 2:56 pm   #15
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Default Re: Ancient microwave oven - repairable?

I nearly killed myself playing around with a microwave oven transformer in 1998. Fortunately the zap went straight to ground via my elbow but it made me punch myself in the face. Instant self-education

I'd take it to the tip
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Old 22nd May 2017, 3:22 pm   #16
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Default Re: Ancient microwave oven - repairable?

Quote:
Originally Posted by Dave Moll View Post
Having travelled back in time exactly twelve further months, I've found this one:
Still not the one I'm thinking of, Dave

But it does look useful.
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Old 22nd May 2017, 3:25 pm   #17
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Default Re: Ancient microwave oven - repairable?

Safety-wise, if you can trust yourself to always unplug it from the mains and short the big cap before sticking your hands inside, the risks should be not unreasonable IMHO.

And as ever with risky machinery, always work in good light, in an uncluttered area, and when you're not feeling tired.

But if you can't trust yourself to do all these things religiously, then yes, treat yourself to a new one.
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Old 22nd May 2017, 3:28 pm   #18
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Default Re: Ancient microwave oven - repairable?

As long as you don't plug it in while it is in any state of undress, and apply a short across the capacitor all the while you are working on it, you should be fine. Note the EHT rectifier has several junctions in series, so a normal meter will not be able to get it to conduct -- use two PP3 batteries in series, an LED and a resistor for testing.

If the EHT fuse (which is a special size, 40*6.3 as it has to disconnect such a high voltage) has blown, replacement glass fuses can be had much cheaper than a replacement holder with a fuse already in it.
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Old 22nd May 2017, 3:29 pm   #19
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Default Re: Ancient microwave oven - repairable?

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...compared to the cost of a new oven.
I suppose it also depends on how costly and well-made the original one was. My parents' one cost well over £300, and it feels like it too. I would happily spend up to, say, £50 getting it going again if it ever fails again.
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Old 22nd May 2017, 3:47 pm   #20
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Default Re: Ancient microwave oven - repairable?

This is one of my favourite sites:
https://www.repairfaq.org
Among much else there is an article about microwave ovens.
http://www.repairfaq.org/sam/micfaq.htm
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