Panasonic microwave oven repair

[Trevor Wilson]

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

 

[Max Harding vk3jin]

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

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

 

[Trevor Wilson]

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.

**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]

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.

**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?

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

**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.

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.

**Of course.

 

[Trevor Wilson]

ok, i wont bother in future then

**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.

 

[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.

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

**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.