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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 21st Jul 2020, 9:16 pm   #1
Julesomega
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Default Microwave oven repair - aggravations, warnings and questions

Must have been just as the COVID19 pandemic was starting that our microwave oven let out a faint "Pheee-p" and expired. It's a Panasonic 'Inverter' type and is powerful (1kW), light enough to lift onto the eye-level shelf and is altogether rather wonderful and the wife loves the "Chaos de-frost" whatever that is, so my immediate thought was it must be very old, time to buy a new of the same make. Can't remember where it came from, I've never bought a new MW oven, people leave them behind when they move or people give them you. The only indication of age was what looked like an '08 date code on one component on the control board.

A quick look at Argos and Curry's found the equivalent power is only available in huge or built-in units which would require the kitchen to be rebuilt. eBay only had the odd same model "requires attention" or "collect only" so as a temporary measure I got a genuinely veteran oven out of the cellar where it has stood for years since I found it left behind or etc. It is a Moulinex and is from the same series as the one in the attic which the wife already had when we met in 1991, and it has enearing idiosyncrasies, and a lot of cleaning and a bit of oiling brought it back into service.

The same eBay search threw up a 'replacement inverter unit' from a dealer so it seemed worth a try. It is an updated replacement, smaller and of a different format and took a day in the workshop to make it fit and now works perfectly. I know that there are people on here who would repair a magnetron switch-mode inverter and I admire them for it so I took an easy way out. Even so it took weeks to do, what with all the plastic-bashing and looking for the perfect screws.
If anyone plans to repair a Panasonic appliance my advice is to contact their UK Service Centre, it looks like you can get parts and I expect they do a kit which includes a revised mounting assembly.

The major aggro is common to all MW ovens: it's a right PitA to refit the cabinet because of all the little lips that have to slide behind the edges of the base unit. Un-kink the case if bent, and use a kitchen knife (don't get caught) to ease the lips slightly, and make a mental note of where they are on the top and sides. And a warning: you can't do this with leather gloves on so have some micropore tape to hand for those fine cuts which will leave sanguine streaks on the cabinet. Expect to have to do it several times when you forget and leave parts out.

Another warning: if you're fixing a transformer type oven, discharge the high voltage reservoir capacitor with a resistor on a wooden stick or something. Not needed on an inverter type though, and this has a 107MΩ (eh?) discharge resistor. A final aggro that caught me out though it wouldn't catch anyone else: I chose to apply mains via a a series bulb just to check against any shorts on the control board, wiring or mains input filter. A 60W bulb lit up bright and I slowly disconnected every cable inside the thing, ending up with the mains fuse removed but still full bright. Aha! - must be one of those Class of mains caps that goes short when they fail, I thought, but the ohmmeter showed a small capacitor charging correctly. There was no problem with a 150W bulb

What had failed in the old inverter? There are only a few turns of the really fat Litz wire, making one tapped winding and one untapped. Even so the insulation has to cope with 240V ac. The HV winding must be inside somewhere, and the connections are o/c, so that explains it. Should be fixable if you have a 150W Solon iron or the like. It operates at ~37.5kHz which explains the Litz.

Hope you do better than me with your oven repairs!
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Old 28th Jul 2020, 9:42 pm   #2
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Default Re: Microwave oven repair - aggravations, warnings and questions

This sounds similar to the Panasonic MW oven as mine which failed last week after 15 years of service.

I also attempted to repair it. I found getting into the oven was fairly straight forward after realising that the cover slides back after undoing the rear screws.

I couldn't find any faults on the inverter unit so I turned my attention to the control pcb which was a real pain to remove from the front control panel.

As can be seen on the picture, the tiny diode D29 (best seen on the print side) serving as a half wave rectifier taking full mains voltage from CN1 to the electrolytic capacitor, had shorted out. This had blown a track, the TOP2?? switcher IC and consequently the micro-controller IC. Sadly I could not find a replacement control pcb and in any case similar pcbs were in excess of 60.

I ended up splashing out on the new equivalent Panasonic MW oven (NN-CT55). However I am surprised that Panasonic being one of the upmarket brands would use such a crude power supply circuit.
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Old 28th Jul 2020, 9:56 pm   #3
paulsherwin
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Default Re: Microwave oven repair - aggravations, warnings and questions

They are all made by subcontractors in China nowadays. Brands mean nothing sadly.
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Old 28th Jul 2020, 11:12 pm   #4
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Default Re: Microwave oven repair - aggravations, warnings and questions

It was probably the switching IC that failed and took out the rest. I've had one fail, but without the cascade effect. They're quite common now. One spike too many.
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Old 28th Jul 2020, 11:33 pm   #5
paulsherwin
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Default Re: Microwave oven repair - aggravations, warnings and questions

Just a reminder, as always:

Microwave ovens are potentially very, very dangerous!

If you don't know exactly what you're doing, you shouldn't attempt to repair a microwave. They contain very high voltages which can easily kill you.
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Old 29th Jul 2020, 3:16 am   #6
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Default Re: Microwave oven repair - aggravations, warnings and questions

Those inverters look like they would do 300V from a 12/24 volt battery with minimal modification.
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Old 29th Jul 2020, 10:13 am   #7
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Default Re: Microwave oven repair - aggravations, warnings and questions

Quote:
Originally Posted by AC/HL View Post
It was probably the switching IC that failed and took out the rest. I've had one fail, but without the cascade effect. They're quite common now. One spike too many.
Yes possibly, I was just surprised by the low spec diode used with half instead of full wave rectification.
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Old 30th Jul 2020, 2:53 pm   #8
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Default Re: Microwave oven repair - aggravations, warnings and questions

My Panasonic 1KW inverter microwave failed a month ago. I bought a new one but regret not just replacing the Magnetron which had a cracked magnet.

I still have the inverter which is probably OK if anyone needs one...

Best Regards
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Old 30th Jul 2020, 9:31 pm   #9
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Default Re: Microwave oven repair - aggravations, warnings and questions

Thats a shame, sadly my old oven with a good inverter and magnetron has now gone to the great microwave oven graveyard in the sky.
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Old 31st Jul 2020, 12:05 am   #10
dglcomp
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Default Re: Microwave oven repair - aggravations, warnings and questions

Mike Harrison (Mikeselectricstuff) attempted to repair one where the bulb blew and took out some of the circuitry making it scrap.

https://www.youtube.com/watch?v=Fmcg_cVO_1s
http://www.electricstuff.co.uk/
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Old 9th Aug 2020, 12:51 pm   #11
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Default Re: Microwave oven repair - aggravations, warnings and questions

Maybe this should go in a separate thread, but I have mused before that half the problems with electronic stuff is getting the mains supply to the units needs. Most items carry effectively a mains cable ( computer printer and laptops and some similar items dont as such), into a section of the item - adding weight, of assorted active and passive devices to step up the voltage, take it down, change the Peak RMS or whatever, make it AC, make it DC, split it to different voltages for varying applications. I suppose it would be unsafe to have always a power supply separated with plug modes to the next stage, but something like that, accessible, modular, repairable, would make life easier if a straightforward mains to application is not needed.
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