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Panasonic microwave oven repair

I have a very nice (read: expensive) Panasonic microwave oven in for service. It will actually end up being mine, when it's completed. It uses a SMPS and has some nice features. The display and control systems seem to work just fine. Anyway, it throws the earth leakage detector switch when attempting to cook. Once of the power transistors measures S/C, so my intention is to replace both transistors (with OEM parts), along with the bridge rectifier (which is suggested by the manufacturer). The question is this:

If I replace the power supply parts, how likely is it that the magetron is faulty and the power supply will, again, be destroyed on power up?

Should I replace the magetron on spec? I checked the terminals to earth and there appears to be no leakage, but you can never tell with these critters. I guess it's safe to Megger test the magnetron (500 Volts)?

I don't usually do microwave ovens, so this is new territory for me.

TIA

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Trevor Wilson
www.rageaudio.com.au
Reply to
Trevor Wilson
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remove the magnetron from circuit and fire-up the oven and see if the safety switch drops out if not then magnetron is faulty............first measure the OHMS across magnetron if OK should read under 1...ohm if over 1 ...ohm then replac, it's faulty.get it from WES

I repair microwave ovens amongst other things.

Max.vk3jin

Reply to
Max Harding vk3jin

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The couple of times I have had to fix the microwave, it has been a dead shorted capacitor on the EHT side of the transformer, a couple of diodes are involved also, to form a voltage doubler.

These were basic units, where the microwave power is regulated by turning the magnetron on and off with higher power meaning a longer "on" period and a shorter "off" period. Medium might be 10 seconds on and 20 seconds off.

If it has a substantial SMPS, (not just a tiny one to run the clock and electronic control) it is probably an "inverter" type microwave This setup might be totally different, as the magnetron runs all the time, just the power to it is varied. This would likely be PWM.

Finally, magnetrons have 3 connections, heater, common and EHT.

You will see only 2 wires, the third connection will be via the chassis, so beware removing anything on the HT and powering the thing up, as it might be required to have a chassis ground to work.

Reply to
kreed

**Then read what I wrote. I KNOW the power supply is buggered. One of the inverter transistors is S/C (Short Circuit). I will replace both devices, along with the bridge. I am curious to know how often a magetron fault causes the power supply to fail. I don't wish to repair the power supply, only to find that the magnetron will cause the detruction of the power supply again.

Clear?

--
Trevor Wilson
www.rageaudio.com.au
Reply to
Trevor Wilson

**Did you bother to read what I wrote?

The display and timer control systems are fine. These are powered by a small auxiliary tranny. The main power is supplied by a SMPS (aka: Inverter system). The SMPS is buggered. One of the transistors is S/C. I will replace both devices and the bridge. My question is that is the magnetron likely to have caused the damage to the power supply?

**Thank you. I am well aware of that. I've serviced enough old style microwave ovens to realise how the system operates. It is certainly simple enough. This SMPS one, is far more complex and. given the cost of the power supply components, I'd prefer not to destroy them at switch-on.
**Of course. 
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Trevor Wilson
www.rageaudio.com.au
Reply to
Trevor Wilson

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ok, i wont bother in future then

Reply to
kreed

**I had no intent to insult you (or anyone else). I just ask that people read what I've taken the time to write, so answers can be meaningful and useful. I apologise for being blunt, but that is my style.

If you have no experience WRT: SMPS microwave ovens and the potential for the magnetron to destroy those power supplies, then I accept that.

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Trevor Wilson
www.rageaudio.com.au
Reply to
Trevor Wilson

One point I should have made, (regarding the 3 connections - one being the chassis) is not to go measuring the heater without knowing that it is one, and then thinking it is a shorted magnetron.

I don't know if magnetrons short out, arc internally or do other things that short circuit the supply.

Reply to
kreed

**They're funny things. Years ago, I had a mate sharing my workshop and he got a microwave in for service. He asked about testing the power supply. I figured that 10 X 240VAC, series connected, 100 Watt incandescents would be a good load. WRONG! We blew every single lamp. From that time on, I developed a very healthy respect for the damned things. Fortunately, I don't see many in for service. Particularly nowadays, due to the low cost of new models. The complex electronics of the Pana interests me though. It looks nice and has some useful features.
**I am aware that sometimes the heater shorts to chassis. I am also aware that if overheated, the magnet can crack, thus causing several different failure modes (including excessive PSU demands). No one (so far) seems to know if, at high Voltages, the magnetron can short to earth. I'm starting to think that I'll just plug the damned thing in and cross my fingers. BTW: The inverter transformer is an impressive piece of work. All Litz wire and respectably large (about 80mm cube). Lead wires are approximately equivalent to around 3mm CSA. This puppy can pack a wallop! Kinda scarey stuff. The manual carefully states that power supply repairs are not recommended. I can understand why. 
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Trevor Wilson
www.rageaudio.com.au
Reply to
Trevor Wilson

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