Calling All Microwave Oven Techs.

[cnc-me]

Have an old Hotpoint microwave that's acting up.
Still works and heats stuff up just fine, but a little before it shuts off, it makes
a loud buzzing noise and sometimes will throw the breaker.

Any ideas, have not tore it apart yet, thought I would get some advice here first. I think it might be a relay/output contactor.

I fixed this unit around 5 years ago when the split circuit outlet that it was plugged into Mal-functioned and caused the microwave to see 240 volts.
Replaced some MOV's and fuses.
This is an American made unit that's got more room, because it doesn't have
the silly turntable that most of the newer ones have. So not to keen on just
pitching it. Thanks in advance.

 

[matt151617]

Sounds like a bad short inside, no dobut something fried inside of it beyond a fuse when it got blasted with 240. I understand you want to keep it, but toss it before it starts a fire and get a new one for $40.

 

[nanofrog]

Definitely a short somewhere, but given it's been fine 5yrs after the initial repair, I'm less inclined to think it's a result of being exposed to 240V.

Is the timer control mechanical or digital?

Wondering if there might be a short in that due to broken plastic in a mechanical unit allowing a short to occur when it's reached the point it's supposed to open (as another possibility to a bad relay/contactor).

If it's digital and that's the cause, you may be able to fix it if you're good at electronics and it's not constructed from unobtainium components.

 

[laser3kw]

intermittent harmonic modulation of the magic smoke oscillator.
it's a known problem with that model...

 

[clarkebd]

Seriously chuck it. A nice new one is $199. Ours just caught on fire. It was popping and sizzling a bit but still heating fine. Then just last night I smell burning something - and peak into the vent at the top and I see FLAMES! ****! Unplugged and got them put out. Not worth the risk - I should of trashed it earlier.

 

[coal_man]

Just a reminder....

Stay out of the capacitor. I've read news stories of people getting electrocuted when discharging one.

coal_man

 

[laser3kw]

Just a reminder....

Stay out of the capacitor. I've read news stories of people getting electrocuted when discharging one.

coal_man

I use to discharge the HV cap on tv's with a screw driver and a ground strap. They hit hard enough to take the tip off the screw driver. Just grind a new edge and go again.
I hate to see what a doubler setup would do.

 

[cnc-me]

Definitely a short somewhere, but given it's been fine 5yrs after the initial repair, I'm less inclined to think it's a result of being exposed to 240V.

Is the timer control mechanical or digital?

Wondering if there might be a short in that due to broken plastic in a mechanical unit allowing a short to occur when it's reached the point it's supposed to open (as another possibility to a bad relay/contactor).

If it's digital and that's the cause, you may be able to fix it if you're good at electronics and it's not constructed from unobtainium components.

It's a digital control with the usual keypad and LED display.
I'm at least going to look, before possibly chucking it.
Thinking some parts inside a relay have let go, and when she goes to break
(open) that's when it has a problem.

Going to rip it apart this weekend.

 

[nanofrog]

when she goes to break
(open) that's when it has a problem.

This is what I'm thinking too.

But wasn't sure if it was a relay or the timer switch had it been an older mechanical model (had an Amana Radar Range that had a mechanical dial for timer control). Built like a tank, and I wish I hadn't left it behind in a move some years ago (no choice ).

 

[G_P]

I use to discharge the HV cap on tv's with a screw driver and a ground strap. They hit hard enough to take the tip off the screw driver. Just grind a new edge and go again.
I hate to see what a doubler setup would do.

They hit hard enough to knock you on the floor if your hand slips and you touch the shank of the screwdriver while shorting the cap.
Apparently socks on concrete allows for a decent enough ground to really hurt:banghead:

Sent via carrier pigeon.

 

[Milton Shaw]

When I worked for GE a microwave was the only appliance the consumer help line would not tell you how to fix, because they are so dangerous with extremely high voltage stored in the capacitor. If it is tripping the breaker you probably have a short in the high voltage side of the transformer or mag tube. It's time to replace it. Parts would cost more than a new microwave, which would come with a warranty and additional warranty on mag tube. Don't risk your life trying to go into it.

 

[cnc-me]

*******Update************
Got the parts today to fix the ol' micro.
It was a bad triac (shorted).
Also replaced a couple of resistors, and new 15 amp fuse,
now that its not blowing them out anymore.
I'm happy to say that it works like a champ with no issues at all.
Run some power tests, and it appears to be putting out the correct wattage.
Sure beats tossing it out, and cheaper too, being the parts only cost five bucks.

Here was the main cause of trouble, a 30 year old triac.
About the same thing as used in light dimmers, only bigger. (15 amp 800 volt)
They don't make this exact part anymore, had to substitute one with the same ratings.
Oh I also found some paperwork on the last time I fixed this, was 10 years ago, almost
to the day.

 

[James_B]

'Tis good news. When I was instructing on RADAR, I had a nice little "show and tell" piece courtesy of one of our "technical staff" that attempted to repair his own microwave oven.

He was an old hand with high power radio transmitters, and decided that the magnetron (makes all the microwave energy) is just a funny vacuum tube, and decided to use a Fluke digital multimeter to measure the filament voltage at the magnetron.

He knew just enough to be a danger to himself. Unlike most vacuum tubes found in transmitters and the like, magnetrons have a grounded anode and the filament winding is also the cathode. Both sides of the filament are at a voltage potential of about -3.5kv.

The Fluke was deemed to be unrepairable when that 3.5kv had its evil way with the meter's electronics. He's just lucky that he's still around to tell the story.

You develop a healthy respect for magnetrons when you work with the higher powered versions in RADAR transmitters, and know a few horror stories of people being killed because they didn't bolt the magnetron body down well enough, and the case became live ... and the technician became dead.

 

[cnc-me]

'Tis good news. When I was instructing on RADAR, I had a nice little "show and tell" piece courtesy of one of our "technical staff" that attempted to repair his own microwave oven.

He was an old hand with high power radio transmitters, and decided that the magnetron (makes all the microwave energy) is just a funny vacuum tube, and decided to use a Fluke digital multimeter to measure the filament voltage at the magnetron.

He knew just enough to be a danger to himself. Unlike most vacuum tubes found in transmitters and the like, magnetrons have a grounded anode and the filament winding is also the cathode. Both sides of the filament are at a voltage potential of about -3.5kv.

The Fluke was deemed to be unrepairable when that 3.5kv had its evil way with the meter's electronics. He's just lucky that he's still around to tell the story.

You develop a healthy respect for magnetrons when you work with the higher powered versions in RADAR transmitters, and know a few horror stories of people being killed because they didn't bolt the magnetron body down well enough, and the case became live ... and the technician became dead.

Well aware of what you said, especially when I seen the HV diode with the green wire bolted to the chassis. You defiantly want these things grounded
with the proper three blade cord-set.
Have worked on quite a few things with capacitors, just shorted it out with a
screwdriver. Probably not the best practice, but thought it would be ok for one this small. I kept my Simpson meter far, far away from the high voltage section.
Bet he had that Fluke set on a low voltage range, a lot of the old vacuum tubes used a low voltage for the filaments. Seems like a lot of microwave ovens use 3.3 Volts, didn't measure this one because of what you said earlier.

 

[James_B]

Yup. Microwaves have something like a ~3v potential difference between the two filament pins. The "lucky technician" was expecting something like the traditional 6.3v AC vacuum tube filament drive, and had the meter set accordingly. He was just lucky that he wasn't holding one meter probe when he touched the other probe to one filament.

 

[gayler]

Yup. Microwaves have something like a ~3v potential difference between the two filament pins. The "lucky technician" was expecting something like the traditional 6.3v AC vacuum tube filament drive, and had the meter set accordingly. He was just lucky that he wasn't holding one meter probe when he touched the other probe to one filament.

Yep I was taught to clip the ground lead to the chassis and keep one hand in you pocket when testing live tube amps!

 

[James_B]

The one hand in the pocket thing is pretty universal amongst knowledgeable technical staff that work in big power electronics. It becomes a survival instinct.

We had a visit to one of our RADAR sections in the early 80s by a brand new high level manager, and the visitor savaged the section head for the "slovenly attitude" of his staff because they seemed to do everything with one hand in their pockets ... then the reason for the "one hand in pocket" thing was explained to him.

When I was on a training course as a student, and was dealing with equipment that plugged into a wall outlet, the rest of the class (all non-technical operators) were surprised that before I did anything inside the equipment, I'd unplug the cord and put the plug end in my pocket ... every time without fail. I've seen too many people bitten on equipment by non-thinking "helpers" that want to shave a few seconds from the job.