microwave no heat

Thread Starter

shawdou

Joined Feb 8, 2021
14
My microwave has light, fan is on, turntable is turning, but no power goes to hv transformer. I disconnecting and reconnecting cable from control panel to hv transformer gives me the same results. Can I assume the control panel is bad? Any suggestions are appreciated
 

MrChips

Joined Oct 2, 2009
23,515
The first thing is to tell is the make and model of the oven.
The next thing is to find the wiring diagram.
There are a number of safety and interlock mechanisms. You can go through it and check each interlock with a continuity test.
 

Thread Starter

shawdou

Joined Feb 8, 2021
14
I bought a new HV diode, replaced old one, still no power. I checked HV transformer two input legs, no voltage shown. Simply no power goes into HV transformer.
 

Tonyr1084

Joined Sep 24, 2015
5,969
Before you buy and replace anything more - fully diagnose the problem. Then only fix what needs fixing.

There are a number of reasons why the MOT (Microwave Oven Transformer) will not power up. It's not the diode or the HV cap. If you're not seeing any voltage GET to the MOT then it's not the MOT. Whatever is controlling power to the MOT is the issue. Not problem, issue. It could be the control board isn't turning on the relay that applies power to the MOT. It could be a safety switch that prevents the MOT from running when the door is open. You're just going to have to isolate the problem then deal with it. Otherwise you're going to end up with a new oven for your old oven frame.
 

Tonyr1084

Joined Sep 24, 2015
5,969
I've downloaded the picture of your schematic. It's not clear enough to tell what some components are. From what I CAN make out - the secondary switch power relay is controlled by the control board. If it doesn't close (turn on) then the MOT isn't going to get any power. The switch above the PCB also tells the control board when it's safe to turn the MOT on. There appears to be yet another relay on connector 1. Another switch just above the MOT appears as if it would short the power. I just don't understand why someone would put a switch across mains power. A clearer picture would be useful if you can manage.
 

Tonyr1084

Joined Sep 24, 2015
5,969
Studying your print further, check for 110VAC from the neutral line (white at the filter board (top of the drawing)), the "N" point; check from there to the secondary switch power relay (RY2) (if that's what it says). If you don't have power there then your problem is not the control board. The obvious failure would likely be the fuse. Since other things work then the fuse must be good. It's a simple and safe assumption. From the power inlet PCB (assuming it's on its own board) power goes to the control board PCB on pins 3 and 1. 3 connects to line voltage, 1 connects to neutral. It's possible the control board may be faulty. I have one that won't turn the exhaust fan on at all. Some day I'll get after that, but for now lets stick with your problem. RY2 must switch on in order for the MOT to turn on. You said you don't have power at the MOT so I'm assuming RY2 is either not being commanded to close or the contacts in the relay may have burned out. 10 years of service is certainly getting your money's worth out of the oven. It's just that darn switch with yellow wires just above the MOT that has me baffled. Maybe someone else understands it - I don't.

But that's the things I'd check for first - before buying or replacing anything else.
 

Tonyr1084

Joined Sep 24, 2015
5,969
I checked HV transformer two input legs, no voltage shown. Simply no power goes into HV transformer.
Measure the voltage across the White and Black wires on the Transformer primary, and work back to the relay .
Already checked for power at the MOT. I just hope the oven door was closed before doing that kind of test. OR the magnetron (part that produces microwaves) is unplugged before testing power to the MOT.
 

Tonyr1084

Joined Sep 24, 2015
5,969
Agreed.
Controller board not activating the relay would be my bet. Test all the interlocks first.
I would disconnect the MOT and replace that with a simple test lamp. If the lamp lights then the circuitry is working. If not - at least the MOT and Magnetron are not operating, presenting a radiation hazard. Safety is the first and foremost concern when messing with high voltage and potential microwave radiation energy. Using the same test light you can touch one lead (safely) to the relay and turn the oven on. If the light comes on - all's good. If not, touch the test light to the other terminal of the relay. If it comes on regardless of the oven setting - then the relay is not closing. It might not close due to bad contacts - or it might not be commanded to close by the control board.

The control board won't turn the oven on if the door is open. (or removed) All interlocks must be reading the door closed. If the door is not closed, or if an interlock switch is not functioning then the oven will not come on.

I had a water dispenser on the fridge that would not dispense. The culprit was the microswitch. Replaced the switch and the dispenser worked. It could be a switch has gone bad. After all, 10 years, lets not forget that's a lot of use. AND more than one switch can go bad. Only, if one goes bad, the oven will likely stop working before the second could go bad. But it still remains a possibility. In fact, the switch I replaced on my fridge came from a scrapped microwave oven.
 

Thread Starter

shawdou

Joined Feb 8, 2021
14
I've downloaded the picture of your schematic. It's not clear enough to tell what some components are. From what I CAN make out - the secondary switch power relay is controlled by the control board. If it doesn't close (turn on) then the MOT isn't going to get any power. The switch above the PCB also tells the control board when it's safe to turn the MOT on. There appears to be yet another relay on connector 1. Another switch just above the MOT appears as if it would short the power. I just don't understand why someone would put a switch across mains power. A clearer picture would be useful if you can manage.
I agree with you. When microwave is on, light is on, fan is on, turntable is on, I measures power relay output, zero resistance, which means power relay is closed. then I measured MOT input two legs, zero voltage between. The switch above MOT shall be open while microwave is on. I checked the switch which is good, but two legs have interesting color. I used a sand paper making them shining again. I will put them back tonight and try.
 
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