Microwave issue NN-C897WBBP

[llonen]

I don't usually have these in for repair since they are generally so cheap no one wants them repaired, anyways so this Panasonic (NN-C897WBBP) comes across my bench. Its a dual oven with microwave and fan conventional oven, when in microwave mode it will run for a random length of time before shutting itself down and immediately restarting, usually around a minute maybe a little longer.

On inspection as suspected the magnatron cooling fins and fan assembly were caked in grease and muck, a thorough cleaning and inspection of the thermal trip and micro-switches revealed nothing of significance. Reassembly and testing revealed the same behaviour, removing the HV supply to the magnatron showed up the same behaviour. Clamping the supply in showed about when I would expect for a current draw for this appliance. When it does remain on long enough it will boil a mug of water within two minutes.


thoughts please.

[stj]

Re: Microwave issue

check the pcb for bad caps or soldering.

[llonen]

Re: Microwave issue

I had a look at the main controller board, what few caps there are on there appear in good condition, there are a number of flat film connectors going out to the display and touch panel which I am not inclined to interfere with due to age. There is no problem with the convection part of the oven which seems to function well enough and that is what the client has been using for a while.

[R_J]

Re: Microwave issue

without pictures its hard to tell but are there any relays on the control board? if there are, resolder there connections.
When the mwo shuts down do you loose the display?
Monitor the voltage on the primary of the magnitron power transformer and see if you loose the voltage

[llonen]

Re: Microwave issue

I can get some pictures tomorrow, I am reluctant to disturb the main controller board since these flat film connectors are old and potentially brittle. There is a sub assembly with a a number of relays which I will be removing and testing for next step. when the MWO stops it looses display then immediately restarts asking for clock settings the primary of the magnitron power transformer drops from local supply (230-240) to zero and stays there although can be resumed by setting an appropriate power / time and restarting. from here it will run for another 1-2 minutes.

[R_J]

Re: Microwave issue

If the display is dropping out and the clock needs to be reset, you must be loosing the main a/c in somewhere.
I would check the connections and any thermal cutouts in the line from the cord to the control board. It could be as simple as bad connections on the low voltage transformer for the controller
I could not find a service manual for this model

[sam_sam_sam]

Re: Microwave issue

Check your main power lines inside the microwave oven for bad connection and look very closely at connection that are crimped have seen this before

[llonen]

Re: Microwave issue

Update and I am leaning towards a microprocessor clock controller fault of some kind.

Customer said shutdowns were random, they are not it shuts down precisely at time elapse 59 seconds as it rolls over. Additionally the shutdown occurs irrespective of microwave, oven, grill or combination there of.

Power-lines have been rechecked, filter board checked and re-flowed not that there were any obvious bad joints. Thermal trip function verified and removed, current monitored, on monitoring the voltage rails I don't see any interrupt of supply to the controller board

[sam_sam_sam]

Re: Microwave issue

Ok here is what I would do if I were troubleshooting this microwave oven and you think it the controller board

First question dose it have a small power supply transformer on this board check the diodes right next to the transformer and make sure that they are good or does this unit have a switching power supply if it does there should be a switching power supply regulator IC chip on the board it might have any where from 4 to 8 pins on it and in a 220 ic chip case or a dip ic chip or a surface mount ic chip

I would need the part number of that IC chip if it has one

I would recommend powering it from the controller board where the main power leads hook up to it with nothing else hooked to it and see if it still does the same exact thing

[R_J]

Re: Microwave issue

If you program 5:00 min, does it count down ok and reset when it reaches 0:59 or does it reset at 4:59, If you program 0:50 does it count down ok?
I suspect the microprocessor has an issue, I seem to remember something similar with the panasonic mwo's. Its been awhile

[llonen]

Re: Microwave issue

The control board is powered by a conventional linear psu since the reset / shutdown is highly predictable and not what the client described I feel it is unlikely the problem to be supply related. That said I will look into separately powering the controller board, I do find that setting 50 seconds it will complete the time but anything over 1 minute results in a restart / reset.

[llonen]

Re: Microwave issue

So powering the control board from a separate supply made no difference, unless anyone has any further thoughts on this I am going to inform the client that this is a controller issue and since I am unable to obtain replacement unit that this one is not going to be repairable.

[sam_sam_sam]

Re: Microwave issue

So powering the control board from a separate supply made no difference, unless anyone has any further thoughts on this I am going to inform the client that this is a controller issue and since I am unable to obtain replacement unit that this one is not going to be repairable.

This is best thing to do because if you power it separately from the microwave and you still have the same problem then I would have to agree with you that it has some type controller issues and not worth fixing it