Microwave woes

[RogerS]

We are great fans of the Sharp R-959M combination microwave. Virtually all our cooking is done on it. Been using them for many years. So much so we even bought a spare a year or so back as it was £140 end-of-line, checked it out and kept it in its box indoors. A bargain.

So...last night, while SWMBO was using it, the current one ceased to microwave. Oven fine. Grill fine. Just no microwave.

Discovered an internal fuse had blown. Replaced it and tried the microwave. Normal (if a bit vigorous) vibration-ey microwavey working sort of sound......for a second until the fuse blew. Nothing obviously black, broken or smoking in the microwave department.

So...magnetron? They are available and I have ordered one but it is debatable as to value for money as it costs £45 delivered and a new microwave is only £163....

Anyway, while waiting for that to arrive, took the new spare out of its box. SWMBO used it to cook her jacket potato for lunch using Combi mode (microwave and oven) and afterwards asks me if it's the same power as it took longer than the old one. Yup..says I..900w. Says so on the box. Go to defrost my curry for lunch and bugger me if the microwave isn't working either! Same fault. Fuse blown. Stick in a new fuse. Fire it up. Loud microwavey sound (just like the old one) but a bit of burning smell ...for all of a second until the fuse blew.

So I am a bit perplexed. A perfectly to all intents and purposes new microwave ceases to work out of the box. Do capacitors get unhappy if they aren't powered up? The microwave was made in 2011. Wondered if the mains voltage was doing something stupid but it's fine and where it normally is at 250v.

Am I missing something obvious here? Or is it just sod's law.

I've checked the magnetron in the new microwave and there are no nesting (cooked) mice inside it.

 

[Random Orbital Bob]

given the same fault appears to have occurred to two seperate machines...and that fault is unusual...I would look for what they have in common....ie your mains supply, the socket they're plugged into etc. That sounds logically more likely to be the weak link than the machines

 

[RogerS]

given the same fault appears to have occurred to two seperate machines...and that fault is unusual...I would look for what they have in common....ie your mains supply, the socket they're plugged into etc. That sounds logically more likely to be the weak link than the machines

A fair point but I did check the mains voltage and they were in two different sockets. I think Sod's Law.

 

[tekno.mage]

Capcitors don't usually mind if they are not powered up - but electrolytic capacitors do age and then fail. However I would not expect this to happen after only two years. This fault is very strange. However, if your mains voltage is measuring at 250V I'd say that was somewhat on the high side as it should be more like 240V. I once lived somewhere where the mains voltage was normally 248V and light bulbs really did not last very long there! I didn't have any problems with other equipment, however.

Alternatively, you have co-incidentally bought two microwaves with components from a bad batch inside - and one failed faster than the other. Or both your microwaves contain the rumoured "built-in-obselecence" component and yours was set to go bang in 2013

 

[andycktm]

Several years ago we had this fault,sound but no heat. we got a repair man out he showed me a electrical spade connection was not 100% connecting propely
he took it off and crimped it up refitted it and all was well. Strangley enough the other day different microwave same symtoms.Checked the connector and it was slightlty
black(burning smell),nipped it up working again.the wire was fairly thick so must be high current.

 

[monkeybiter]

I was told by a tech from our supplier [after complaining about voltage fluctuations] that they were allowed +- 12%, i.e. 257.6V max. [202.4V min] which seems pretty poor to me.

 

[RogerS]

I came across this useful website plus some circuit diagrams.

http://www.repairfaq.org/REPAIR/F_micfa ... MICFAQ_003

To be honest, there's not much to the microwave bit....just four components - magnetron, transformer, capacitor and high-voltage diode. It will be another thing sourcing them perhaps. Having Googled a little bit I came across this site in the US http://www.repairclinic.com/Sharp-Micro ... ?n=5&z=100 that lists quite a few components and I'm just wondering if perhaps their legislation is different to ours in that spare parts must be easily sourced. I am pretty sure that if I need a part from Sharp then it will have to be done through a repair centre...which, if they add a margin I really object to.

Anyway, that's a job for next week.

Re the 250v I'm glad that it is so high as it means that when I start up the Sedgwick in the workshop the voltage available is still within spec....the power feed to the workshop is a bit too thin TBH.

 

[Eric The Viking]

I'm probably telling granny, but those caps can be VERY dangerous. There's usually a high-ish value resistor strapped across them to discharge them after use, but if the resistor fails, the microwave will still run, albeit with a lethal amount of energy (at a lethal voltage) in the capacitor.

If you haven't abused either unit by putting reflective things in it, making sparks, etc. the magnetron is probably fine. I'd make an educated guess it's the capacitor.

Electrolytics certainly die if not used regularly: they plate their insulator, which slowly dissolves back into the electrolyte if not used. The electrolyte is conductive, and if the insulating layer fails, the capacitor conducts at low resistance, leading to heat and explosion (or the fuse goes). Or they just dry up, but that's more common with the low voltage high value ones, as a faulty formula got into production some years ago.

I think the ones in microwaves *are* electrolytics, but I may be wrong.

BTW, don't forget that the magnetron has Beryllium in it - highly, fatally poisonous (and it's reckoned to be a nasty way to go, too).

E.

 

[RogerS]

Thanks for the heads-up re beryllium. If the magnetron was duff then I could quite easily see me opening up the old one. (Note to self...read up on how they work).

Got a price on a transformer if I need one...well over £100 and so if it is the tranny then no chance of buying a new one. Condenser or HV diodes are around the £24 mark which is reasonable. Must look out my trusty Avo next week and a few resistors to drop the voltage down so i can measure things ...4kv is a figure I have read....well, that's chicken feed compared to sticking your hand around the back of a colour TV and adjusting the static magnets with 25kv a few inches from your knuckles.

 

[Eric The Viking]

How many people have you heard of that've been killed by the HT on a telly? (me: nobody)

How many... by fiddling with a microwave? (me: a couple down the years)

I think like many things it's a combination of H(enough)T and the energy density wot does it. It's 20mA through the heart muscle, after all, not the voltage, that kills.

E.

As a kid I was thrown across the room by the anode connection to a black and white 19" set (Pye "Luxury 19") that Dad was repairing. I never even came close to touching it - the spark jumped about 2". I think it may have been the fumes from his soldering iron that didn't help. He just roared with laughter. Parents, eh?

 

[RogerS]

Who's the Daddy!

Following the excellent website I linked to earlier, testing the transformer, HV diode and capacitor (non-electrolytic) showed them to be OK (at least as far as low-voltage testing went....no telling if they'd break down under HV).

So fitted the new magnetron and Bingo! Working.

Sod's law says that one of the grill elements went open-circuit but I've got a spare one of those. And to top it all, the oven light went phutt.

So tomorrow I'll take a look at the new one and see what progress I can make there.

Dead chuffed though....time for a cold beer.

I also highly recommend kitchenwareonline.com for spare parts...very comprehensive list and good website

 

[RogerS]

Well blow me down but the new magnetron in the new oven was also faulty !

 

[Eric The Viking]

I should go to that site - how do you test a magnetron, or do you simply eliminate everything else?

E.
(ever curious - probably ought to bathe more often)

 

[RogerS]

section 7.11 and 7.12 discuss it in detail but in the final analysis they end up saying 'try a new one' when all else seems OK....exactly as you surmised !

Have to confess to rather enjoying getting out the old Avo Mk 8 again after all this time.

 

[Eric The Viking]

I scored a Farnell 2ch bench PSU on fleabay t'other week, and recently also got the full manual for it. The actual PSU was ex-Bush house, but the manual was ex-MOD contractor, so had every circuit diagram for the series. I'm looking forward to calibrating it properly, as I've got a number of amps etc. to service.

My dad reminded me not to use the AVO to do this, as it's only 1%. Happily I also have a rather nice mains DVM, so I should have it pretty much dead-on shortly. I have to change the electrolytics though, so it won't happen overnight.