Microoven POOF after heating up metal kettle, repair attempt

[kurland]

Hi,

My kids broke the micro, so no pocket money this weekend...
Metal kettle with frozen food inside for 15+ minutes and thing went poof , and *very* hot. These things should have a thermal switch

I opened it and confirmed a blown 10A fuse. I took the inverter out and also found that the middle LGBT (GT60N90 - 900V @ 60 A) as well as the FBR at left was toast. The right LGBT measured good.



I figured this would be an easy fix and ordered and replaced all three components. Before that I also testet the magnetron and it measures ok using ohm-meter, i.e. shorted between terminals and open circuit with reference to ground.

I replaced the fuse and testet again, without the magnetron connected. Poof, the fuse went again. Taking out the inverter again I realized that the the LGBT got toasted immediately. The new LGBT I used had somewhat lower current rating (50A) but 1200V CE rating, so should be ok.

I managed to find a schematic for the inverter.


I desoldered the step-up transformer and measurements show:

Primary winding P701-P702 <0.1 ohm
Secondary winging S701-S702 4.8 ohm
Secondary winding H701-H702 < 0.1 ohm

I figured the transformer primary side is shorted? But I'm not sure.... What would be a normal resistance for a primary winding in a HV step-up transformer? The primary winding is a beefy 3mm stranded wire.
In step-down transformers the primary side usually has higher resistance than secondary side, but this is a step-up transformer... I might try to unwind it for fun.

Does it make sense that a shorted primary side would burn the Q701 LGBT ?

I have taken out all the HV caps and they're fine (using cap tester). I also took out Q703/4/5 and they all tested good.


Any thoughts on my analysis? From the schematic, and the fact that Q701 blows, does it make sense that the transformer is gone?

And yes, I know that the secondary side is 4kV, deadly stuff.

Cheers

[kurland]

Re: Microoven POOF after heating up metal kettle, repair attempt

To help others I should mention that this is a Bosch innowave microoven, and the inverter is a Panasonic A66454T08Ap

[kurland]

Re: Microoven POOF after heating up metal kettle, repair attempt

Btw, I also measured all diodes and resistors, they seems ok

[R_J]

Re: Microoven POOF after heating up metal kettle, repair attempt

The transformer will be ok and not the problem.
This is a common failure with this inverter, there are other components that also need to be replaced.
Both output high voltage diodes (D701,d702), two zener diodes Zd703,zd704, two or more resistors, and maybe the thermistor listed as D710.
I suspect that the magnetron could also be bad and is what caused the power supply to blow, Remove the magnetron and inspect the output for any discolor or signs of arcing.
The zeners might check ok in diode mode but will be bad and zener at around 6v and not 10v.

There is also a modification for the A606Y4T00CP, If R713 has a jumper wire, remove it and replace it with a 4.7Ω 1/2w and change R710,R711 from 39Ω to 30Ω
There is a kit available for the transistors and the hv diodes depending on the board model.

The last similar Panasonic inverter mwo I repaired, after repairing the power supply I tried the oven for 10 sec and it worked, tried another 10 sec and it worked, tried 20 sec and the magnetron arced, shorted and blew up the repaired power supply.
Here are the last 2 magnetron's that blew this type of inverter

[kurland]

Re: Microoven POOF after heating up metal kettle, repair attempt

Thanks for tips, I agree with regards to the transformer.
I will check HV diodes, I think they have FV at around 10v.
I will check zeners out of circuit as well, although I cannot see how a lover ZV would yield such a problem, I mean wouldn't that just leave the base unbiased turning off Q702 ?

But thermistor D710?? I've checked twice and cannot find any such thing?

[kurland]

Re: Microoven POOF after heating up metal kettle, repair attempt

Also, it's not the magnetron, because as I said the thing blew again after replacing components, while magnetro was disconnected

[kurland]

Re: Microoven POOF after heating up metal kettle, repair attempt

Regarding D710, I found it on the board, but it's not populated, and no sign of it in the schematic.
Do you know this board from before R_J ?

[R_J]

Re: Microoven POOF after heating up metal kettle, repair attempt

Also, it's not the magnetron, because as I said the thing blew again after replacing components, while magnetro was disconnected

The magnetron was the likely cause of the FIRST problem, the inverter blew again (with mag disconnected) because of other defective parts. The zener diodes are not the HV diodes. You can NOT check the HV diodes, the need to be replaced. The two small zener diodes can not be checked with a meter, they will also check ok but will likely be bad

[R_J]

Re: Microoven POOF after heating up metal kettle, repair attempt

This is likely slightly different than the Panasonic inverter, D710 is not populated. Please remove the magnetron and at least inspect it.

[kurland]

Re: Microoven POOF after heating up metal kettle, repair attempt

just saw you edited your post. Thanks for additional info, greatly appreciated!!

R713 is populated and measures ok. I will dissassemble magnetron as well and look for arcs.

TBC

[R_J]

Re: Microoven POOF after heating up metal kettle, repair attempt

I uploaded a service bulletin from Panasonic, The mwo's I have repaired are for 120vac not 240vac line so they are likely slightly different

[kurland]

Re: Microoven POOF after heating up metal kettle, repair attempt

> The magnetron was the likely cause of the FIRST problem, the inverter blew again (with mag disconnected) because of other defective parts.

Ok thanks, will check magnetron

> The zener diodes are not the HV diodes.
Of course, I know that.

> You can NOT check the HV diodes, the need to be replaced.
So verifying their forwards voltage does not mean anything?

> The two small zener diodes can not be checked with a meter, they will also check ok but will likely be bad
I know, I will test their ZV using a serioes resistor

Again, thanks!

[stj]

Re: Microoven POOF after heating up metal kettle, repair attempt

in my opinion, LGBT's can never be good!

[kurland]

Re: Microoven POOF after heating up metal kettle, repair attempt

haha oops .... IGBTs .....

[kurland]

Re: Microoven POOF after heating up metal kettle, repair attempt

Hmm, frustrating, cannot find any problems...

Zener diodes are ok, both 10V
HV diodes seems ok, both have Vf ~7 volt

Magnetron looks fine, no visual damage

[stj]

Re: Microoven POOF after heating up metal kettle, repair attempt

so is the heater element in the magnetron o.k.?
and not shorted to ground??

[kurland]

Re: Microoven POOF after heating up metal kettle, repair attempt

By "heating element", do you mean the cylinder tip in the photo? I haven't testet that specifically. I did however measure the contact leads into the magnetron, and those have infinite resistance to chassis ground. Will test further tonight.

Thanks for your suggestion.

[R_J]

Re: Microoven POOF after heating up metal kettle, repair attempt

The heater for the magnetron is across the two contacts and will read less than 1Ω, The resistance from those pins to the case will read infinite, unless the heater is shorted to the case which can happen. I almost always had to replace the bridge rectifier as well when these blew up.