Hi folks ,
As in the title , i have a VIDA stainless steel microwave oven that comes on but it doesn't heat anything up.
Its just over a year old and looks brand new but the warranty is out . I've checked the HV diode , HV capacitor , checked all the overloads , checked the windings in the HV transformer, everything is fine. I even replaced the Magnetron but it made no difference , im completely stumped , anyone have any suggestions as to what might be wrong with it ?

 

Hi,
first of all, take a multimeter and check the voltage on the transformers primary. Should read 230VAC , or 120VAC.

P.S. Don't touch the secondary while energized !

 

Is there some switch that detects that the door is closed? Or some other safety related feature, something like that can block the emision.

 

Hi,
first of all, take a multimeter and check the voltage on the transformers primary. Should read 230VAC , or 120VAC.

P.S. Don't touch the secondary while energized !

Thanks for responding Pyrex , i've since bought a new microwave ( it was on-sale) , but im still going to try and repair my old one, might bring the new one back if i do.
it may very well be the 120volt ac side of the transformer, i haven't tested it while it was live/running as im a bit weary of the high voltages, as you alluded to, that's what i will be testing next.

Additional info: my bro said he could sometimes hear a slight arcing sound, appears it was an intermittent problem that slowly got worse over time, my suspicion is that it may be the HV transformer winings , but the meter says its ok with a reading of ~137 ohms?

 

Is there some switch that detects that the door is closed? Or some other safety related feature, something like that can block the emision.

Thanks for responding Meth, i've tested the door switches and overloads in the line voltage part of the circuit and it all appears to be normal, but i have not tested it while it was running.

 

137 Ohms sounds like a lot for a transformer.

 

There should be a HV fuse in there as well. Did you check that?

 

137 Ohm is too much

 

I think the 137ohms will be the resistance of the fan in parallel with the turntable motor. Is there a fuse in series with the transformer primary ? (Or an additional safety interlock switchin series with the transformer primary.) Trace out a diagram of ALL the wiring inside the device.

Les.

 

Common fault - door safety switch?

A failing switch at line voltage can easily arc.

 

137 Ohms sounds like a lot for a transformer.

Fuses tested fine, the 137 ohms was the resistance reading across the HV winding . From what i've read , 50 to 150 ohms is normal for the HV winding? All safety switches, door locks etc and overloads all tested fine with the multimeter. I've tested between all windings and the transformer body and everything appears ok no shorts. But a have not tested it live yet, i do not have the equipment to test the HV side of the transformer , is there a way to do that safely ? My multimeter has a 2KV max setting but i don't want to damage my meter.
Also from what i've read there is a low voltage winding on the transformer that supplies power for the magnetron filament ?
I haven't had time to test the circuit live yet as i'm busy with other things, hoping to take another look at it this weekend.
Thanks everyone for responding.

 

Most microwave ovens have at least two safety interlock switches to verify that the door is fully closed. AND they all have at least one fuse that may be well hidden. In addition there is the control device relative to the timer, which may be a relay or a triac semiconductor. IN ADDITION, all of them that I have investigated ALSO have a switch that closes when the door is open, to short circuit and fail the fuse.

 

Fuses tested fine, the 137 ohms was the resistance reading across the HV winding . From what i've read , 50 to 150 ohms is normal for the HV winding? All safety switches, door locks etc and overloads all tested fine with the multimeter. I've tested between all windings and the transformer body and everything appears ok no shorts. But a have not tested it live yet, i do not have the equipment to test the HV side of the transformer , is there a way to do that safely ? My multimeter has a 2KV max setting but i don't want to damage my meter.
Also from what i've read there is a low voltage winding on the transformer that supplies power for the magnetron filament ?
I haven't had time to test the circuit live yet as i'm busy with other things, hoping to take another look at it this weekend.
Thanks everyone for responding.

Sure.
Take a step down transformer, say 120/24V.
The output will be 5 times lowered

 

As I do not know that model of microwave cooker here are some suggestions that MIGHT help. If it has a turntable and/or a cooling fan then do they start when when you operate what is required for it to start heating ? There is a good chance that these items are powered by the contacts that supply power to the main transformer primary.
If it does not use a relay or triac to switch the power to the transformer primary. (I.E There is just a mechanical on/off switch to control it.) then measure measure the the resistance between the mains input wires. If so then I would expect the DC resistance between them to drop to quite a low valuew hen the switch is opperated to start heating.
Try measuring the current taken by the device and note if it increases when heating is started.
Measure the primary resistance of the transformer directly on it's terminals. It MAY have a themal fuse embeded in the windings which has failed.
A safe way to check the voltage to the primary would be to connect one end of a length of two core cable to the transformer primary connections and the other end to a mains voltage lamp.
Les.

 

I have not ever come across a microwave cooker with a failed high voltage transformer. The one fatal electrical fault was in a SEARS brand M.W.O. where a non-replacable TRIAC-TYPE device failed open. The actual magnetron tube can be damaged by long operation with an empty oven. And they do eventually suffer reduced power output.
So unplug it first, then look the circuit diagram, trace out the power flow, and use a meter to verify that the door switches all still make contact with the door closed. The cheap plastic parts are prone to wear out and not quite make contact. MOST of the failures are mechanical.

 

As I do not know that model of microwave cooker here are some suggestions that MIGHT help. If it has a turntable and/or a cooling fan then do they start when when you operate what is required for it to start heating ? There is a good chance that these items are powered by the contacts that supply power to the main transformer primary.
If it does not use a relay or triac to switch the power to the transformer primary. (I.E There is just a mechanical on/off switch to control it.) then measure measure the the resistance between the mains input wires. If so then I would expect the DC resistance between them to drop to quite a low valuew hen the switch is opperated to start heating.
Try measuring the current taken by the device and note if it increases when heating is started.
Measure the primary resistance of the transformer directly on it's terminals. It MAY have a themal fuse embeded in the windings which has failed.
A safe way to check the voltage to the primary would be to connect one end of a length of two core cable to the transformer primary connections and the other end to a mains voltage lamp.
Les.

Ok, i got some time today to revisit that troublesome microwave and do a little more testing. When a powered it up and pressed start, the cooling fan came on and there was 120 volts measured across the primary winding of the HV transformer confirming that the line side was working fine . I also checked the filament voltage on the Magnetron , it was 2.2 volts which appears to be somewhat normal ? I was expecting around 3 or 4 volts or so ?
So i decided to do a test run , opened the door, put in a cup half full of cold water, closed the door and pressed the start button, there was a quick flash as the high voltage fuse blew but the unit kept running, so i turned it off , unplugged it and replaced the HV fuse. Also replaced the HV diode and HV capacitor ( they were an exact match) just to be sure they were not the problem. When a pressed the start button again there was another flash as both the HV and line voltage fuses blew at the same time. Unplugged the unit and took another resistance reading across the HV winding , its now reading 129 ohms , originally it was 137 ohms ? That's as far as i got with it today , so it appears there's a problem somewhere in the HV windings ? That somewhat explains when my brother said he thought he heard a arcing sound coming from it once ?
Will remove the HV transformer from the unit to get a better look at it when i get another chance .

 

I have not seen that sort of failure in an MWO In that case it could be a transformer failure.
I have never heard or seen a "VIDA" brand appliance, so it may be time to get a replacement.

 

If the fault is the tranformer or the magnetron I think it will cost more for the parts than the cost of a new mivrowave oven.
After looking at the schematic of a typical microwave oven I think a short circuit high voltage capacitor or diode could be the cause of the problem. (I had not realised that the magnetron was supplied with pulsed DC at twice the voltage of the transformer socondary winding.)
Les.

 

I have not seen that sort of failure in an MWO In that case it could be a transformer failure.
I have never heard or seen a "VIDA" brand appliance, so it may be time to get a replacement.

I bought another MWO was still hoping to repair my old one but it appears a new transformer is quite expensive

 

If the fault is the tranformer or the magnetron I think it will cost more for the parts than the cost of a new mivrowave oven.
After looking at the schematic of a typical microwave oven I think a short circuit high voltage capacitor or diode could be the cause of the problem. (I had not realised that the magnetron was supplied with pulsed DC at twice the voltage of the transformer socondary winding.)
Les.

I agree Les, a new hv transformer will be expensive, I took a quick search and it's somewhere around $180 for that microwave, the cost of the unit itself was $169 on sale.