Microwave will not heat

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Working on a microwave that the display lights up and accepts inputs but when you press start the microwave does not heat, I have done the resistance checks on the magnatron and it checks good. would appreciate any help. thanks
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
There is not a whole lot to a microwave, usually there is one of three things that cause it not to work, the high voltage capacitor, the rectifier or the thermal high limit. Usually it's the rectifier or the thermal high limit, under voltage conditions usually causes one of these to blow.
 

John120/240

Senior Member
Location
Olathe, Kansas
Depending on the microwave, is it cost effective to diagnose & fix it vs just buy a new one ?

If it is a counter top unit, just buy a new one. For a over the range model or a built in model,

in my experience these repairs are best left to the appliance guy.

I imagine the cheapest part to fix whatever the problem is to be $50 minium. Now add your labor

to the equation. You still have a microwave that is __ years old that you just invested $$$ in.

REPLACE the damn thing.
 

broadgage

Senior Member
Location
London, England
Microwave ovens have a safety interlock that prevents operation with the door open.
This sometimes "fails safe" that is it prohibits operation even with the door shut.
With the appliance unplugged, check for any damage to the door or any switches operated by the door.
Sometimes dirt or grease jams these.
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
Microwave ovens have a safety interlock that prevents operation with the door open.
This sometimes "fails safe" that is it prohibits operation even with the door shut.
With the appliance unplugged, check for any damage to the door or any switches operated by the door.
Sometimes dirt or grease jams these.

Yes, sometimes those get broken by slamming the door or a misaligned catch, but if I remember correctly, if that is broken, nothing works when the start button is pressed, I just have assumed that the light and fan is running, but it is not heating. If that is not the case, then it probably is the door switch.
 

kwired

Electron manager
Location
NE Nebraska
Yes, sometimes those get broken by slamming the door or a misaligned catch, but if I remember correctly, if that is broken, nothing works when the start button is pressed, I just have assumed that the light and fan is running, but it is not heating. If that is not the case, then it probably is the door switch.

That is what I understood too, everything appears to be working - just no heating. If door switch is not closed, all the units I have ever seen doesn't do anything at all when the door is open. You may be able to enter things on touchpad but can not "start" until the door is closed.
 

480sparky

Senior Member
Location
Iowegia
Someone accidently set the Power setting to 0 or 1.

I'd buy a new micro for $50 before I spent money to hire an electrician to fix it.
 

Electric-Light

Senior Member
Magnetrons are quite expensive new.

My old microwave self-destructed itself. I took it apart and kept the high voltage transformer for experiments. I had no use for magnetron, but before I chucked it, I put it on eBay just to see if anyone buys. Someone bid it up to $15 + shipping. I'm guessing there are some expensive over-the-range of built-in models that use the same type.

From the buyer end, there's a certain satisfaction in needing a part, then typing in the part or typing in the equivalent part # and have it come up right in front of you at fractions of new price.

"just chuck it"...is the reason we're losing skills to troubleshoot.

Look at the schematic. Usually, microwaves are stupidly simple. It's simply a vacuum tube fed through a single stage voltage doubler. The capacitor size limits the power. Most microwaves have a pair of switch. NO and NC and the latter being the suicide switch. When the door is closed, NC opens first, then NO is closed. If NO is closed first and microwave is activated, it will short across the power line.

http://031d26d.namesecurehost.com/mwd/mv1526_sm.pdf

Mine had a door timing mishap. It blew the fuse, but the short circuit also vaporized the traces on the main board that connects terminals to relay and welded the relay contacts.

On some microwave, you can bypass the suicide switch and its perfectly safe since you have NO still in place, but some newer models requires proof. It must see the NC is closed, then opened, or computer won't allow power up.

Microwave 'engine' still worked fine when directly wired, but it blew the most expensive component (the main board), so I sold the magnetron and glass plate, then scrapped it.

If you're getting voltage to PRIMARY side of high voltage transformer, there's something wrong with the microwave generation system. if you're not, its the controls.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
There are usually at least 2 fuses in the oven, one on the incoming line power and one on the upper portion of the cavity as a thermal protection device. There may be another one on the magnetron itself. Find and check anything that looks like it MIGHT be a fuse.

Aside from the glorified timer and safety controller that is the PCB, which you already think is working, there is a simple circuit:
Power goes through a triac (controlled by the PCB) to control the power level, through a transformer and a capacitor to charge it up to about 3kV, then into the magnetron. You could have a bad triac (highest likelihood after the fuses), an open coil on the transformer (low likelihood) a bad capacitor (low likelihood) or a bad magnetron (lowest possible likelihood).

Still not likely worth the time to troubleshoot it as far as money goes, but the reason I know this is because I did it once. I found it interesting. Not lucrative, just interesting. I got mine working (dead triac), fixed it, then the cap blew 3 months later. My curiosity satisfied, I tossed it the 2nd time. But now after reading Electric-Light's post, I should have tried to sell the parts on FleaBay! Mine was a GE over the stove unit, probably expensive enough for someone to have looked for parts.

Dammit...
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
The majority of the ones I repaired (I had to moon light as an appliance repairman to keep my electrical contracting business afloat back them) was either a blown thermal fuse on top of the magnetron (by the way it does not look like a fuse, looks kinda like a high power diode) or the high voltage diode (rectifier). Both only have two wires on them, and usually the diagram inside the cover will show their location. Had only one bad magnetron, but that was due to a lightning strike, and pretty much everything in there was toast. Got lots of microwave repair business because I was the only repairman in town that would work on them. This was back when they were still relatively expensive. Knew nothing about them, but could read a schematic.
 

Electric-Light

Senior Member
The majority of the ones I repaired (I had to moon light as an appliance repairman to keep my electrical contracting business afloat back them) was either a blown thermal fuse on top of the magnetron (by the way it does not look like a fuse, looks kinda like a high power diode) or the high voltage diode (rectifier). Both only have two wires on them, and usually the diagram inside the cover will show their location. Had only one bad magnetron, but that was due to a lightning strike, and pretty much everything in there was toast. Got lots of microwave repair business because I was the only repairman in town that would work on them. This was back when they were still relatively expensive. Knew nothing about them, but could read a schematic.

Diode is pretty much universally compatible among all 120v microwave in household range power output too.

There are some microwaves that use a triac, but they're usually for heavy duty units where the frequent use of power level setting is anticipated. 50% power doesn't really mean its adjusting the output. It's basically like 10 seconds on, 10 seconds off, and continue for the whole cooking time. As you can guess, this is a torture on mechanical relay, but its not a big issue for consumer units, because most people use full power most of the time where the relay only operate one cycle per use.

Powering into safety switch gets costly. It's usually down stream of relay, which is soldered on PCB, so what often happen is that PCB traces get vaporized and relay contacts get welded. So, you have to do replace the relay on board and fix the traces, or replace the main board (product line specific. You either have a spare used part or the microwave is a total loss, because service main board costs more than a basic microwave from Walmart)
 

dhalleron

Senior Member
Location
Louisville, KY
I fixed my first microwave oven sometime about 1989. I knew it was the diode because it was broke in half. I was doing electric work at an appliance repair shop at the time so I showed it to a technician. He got me a $10 part off the shelf and I fixed it that evening.

A few years ago my over the stove microwave quit working. I took it apart looking for the broken diode but didn't see it.

At least I was able to fix 1 microwave oven in my life.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
My understanding has always been that the high voltage capacitor on a microwave can store HV for a pretty long time even after the microwave is unplugged.

And the capacitors were easily large enough to deliver a fatal amount of current.

That would make microwaves very dangerous appliances to work on for anyone who isn't aware of this danger.

Does anyone know if this is still true, or do today's microwaves have something to bleed the charge off the cap when its unplugged??
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
My understanding has always been that the high voltage capacitor on a microwave can store HV for a pretty long time even after the microwave is unplugged.

And the capacitors were easily large enough to deliver a fatal amount of current.

That would make microwaves very dangerous appliances to work on for anyone who isn't aware of this danger.

Does anyone know if this is still true, or do today's microwaves have something to bleed the charge off the cap when its unplugged??

It's been a long time since I've worked on a microwave, but if I remember correctly they had a bleed resistor on the cap, but I don't remember for sure.
 

G._S._Ohm

Senior Member
Location
DC area
My understanding has always been that the high voltage capacitor on a microwave can store HV for a pretty long time even after the microwave is unplugged.

And the capacitors were easily large enough to deliver a fatal amount of current.

That would make microwaves very dangerous appliances to work on for anyone who isn't aware of this danger.

Does anyone know if this is still true, or do today's microwaves have something to bleed the charge off the cap when its unplugged??
1 J and more can be fatal and the joules stored in a cap are (1/2) C (V^2) with C in farads. Yet, these defib machines can deliver up to 200 joules.

Grainger sells a 15k, 2W resistor to bleed down HVAC caps, which occasionally may have a few hundred volts DC stored on them.
Some resistors can only stand 500v so put two or three in series from Radio Shack if you want to make one of these.
 
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