recalcitrant microwave

[visionset]

I've got an oldish (~7 years) Panasonic microwave/grill/oven that has developed a bizzare fault. When the house temp drops below 15°C the microwave exhibits the fault. Microcontroller madness. Damned thing keeps beeping, and oscilating the display, 'pork...fish...' or somut. And refuses to actually let me microwave anything.

Just about to whip the back off, clean between pcb tracks etc.

Anyone suggest a more focused plan of attack?

I don't believe it is condensation related, the house is really dry. I guess some component has just tightened up its temp operating tollerances in favour of a warmer climate.

[RedOnRed]

Hmmm, that's a curious fault.

All I can say is that our Panny microwave gave up the ghost at around the same age but was due to something called the megatron or something. Which apparently is the main part in it.

We were advised that we should have vacced out the rear air intake more often as that being blocked had caused the fault.

Our local microwave warranty repairer wanted £120 to fix it when a new one was only £149! Needless to say we went for a brand new one.

[Harry Bloomfield]

on 07/02/2007, RedOnRed supposed :

It sounds vaguely like the oscillator for the CPU is failing the oscillate properly at low temperatures. I'm not suggesting you do this, but if you can find the crystal and put your finger end on it to warm it it might confirm the fault. The crystal will be a small flat silver metal cased component 3/8 inch square, or perhaps one similar to those you see in a digital watch - small silver metal cased and rod shaped, both with two wires.

Another possibility is moisture between the diaphragm of the switches, shorting the buttons, if it is that type of microwave control panel.

Magnetron (sp?). A common failing and the replacement part will be common to many ovens.

[visionset]

I'll give it a whirl in the morning, should be cold enough with all that snow about!

Thanks!!

It seems to be such an exact temperature the problem develops I'd guess the former.

-- Mike W

[visionset]

Nearest thing I could find was a 4mm long cylinder, 1.5mm diam, graphite coloured, with axial leads. There are 3 processors. 1 large I think is the display driver and 2 smaller ones. If I get it off again I'll take a photo.

[Harry Bloomfield]

visionset explained :

Be aware there will be some seriously high voltages in there, especially around the transformer and the magnetron.

[visionset]

Thanks, I'm used to dealing with the like inc TV's. Just basic faults I'm upto, no electronics engineer. I can replace the crystal if that's what it is and it's broke though.

I was querying is the one just above the green ribbon partially obscured. 'L10' is printed on the pcb next to it.

Now it is reassembled, it does similar though different logic wierdness. Jumped into the middle of a entry sequence at start up when it shouldn't.

[meow2222]

Theres no way its the crystal, couldnt cause that.

I'd disconnect the keypad and see if the fault recurs, if it doesnt then your keypad needs sorting.

Otherwise really it could be any of numerous bits, and you'd need to become an electronic engineer to sort it,

Hae you been cooking beef in it? Maybe it got BSE

maybe it needs limescale remover.

NT

[Andrew Gabriel]

This type of fault is easy to find if you can run the appliance live with the covers off (which you can't with a microwave, unless you can disconnect the HT transformer primary for safety). Wait until it's warmed up and working, and spray freezer spray on the components and joints until you find the one which stops it working again. If you don't have any freezer spray, a dusting can held upside down works the same way (but watch for the liquid leaking out over your fingers and giving you frost bite).

There is a range of small electrolyic capacitors which can fail when cold as they age. I can't remember the make, but they have no insulating sleave, and include a tiny plastic spacer between themself and the PCB which is integral with the capacitor, but can be separated when capacitor is not mounted on a board. I had one fail in an old VCR, and discovered this is a common failure.

Yes.

[The Natural Philosopher]

Oscillators tend to either work, or not. Probably a faulty set of chips that get logic faults in the cold.

Magnetron.

[gort]

The blue one, label ' cx1 ' is the crystal.

Dave

[Guy King]

The message from gort contains these words:

Probably a ceramic resonator rather than a crystal.

[visionset]

Okay, now I'm 95% certain that it is indeed the keypad. It is the diaphragm type - ie film pcb and pressure causes contacts. Unfortunately it seems the most in accessible part. It is basically stuck on to the front fascia, and completely moulded in from behind. Removing it will likely destroy it completely, and with little hope of getting a new one - I'll investgate the new part option. Although is is moulded behind, there are two small holes you can see the film pcb through. I poked these a bit to change the pressure status quo. On reassembly I left the keypad connection off and it started up fine. I then reconnected the keypad to the live system and it now operates fine... For now... I cleaned the contacts on the film pcb connector yesterday so I don't think it is that. If I can get a new pad I'll probably solve it. See how it goes for now. Thanks all.

[Harry Bloomfield]

visionset submitted this idea :

L would either be an inductor or a simple wire link, that component looks like a simple link.

Keyboard ! Try pulling the green ribbon cable out with the power off, then re apply the power. If the display still works - if not on that ribbon, then you will have confirmed the problem as the keyboard pad.

[Harry Bloomfield]

I agree, except it is not an actual crystal. They have used what is known as a ceramic resonator. They are a cheaper, though less accurate option to a crystal. It probably has three pins going into the PCB. Try warming that with your finger end when it is misbehaving.

[mike]

Was looking at a uW circuit the other day. Capacitor in there rated at

2.5 kV - keep away from that won't you.