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Sears Microwave

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JMW

Member
Went to make some popcorn and the microwave failed to heat. There was no arcing sound, nor burning smell. I tried it at 50% power, you could hear the power supply change tone as if the maggie were turning on, but still no heat.
Did a search and found some very useful tips. I opened it and found a technical tip sheet. I shorted the the Hi voltage circuit and didn't get any spark. I got to the point of checking the Hi voltage diode, and it checks open. Used a 9V battery and looked for a voltage drop as per the tech sheet. I have a 500 VDC Megger, but hesitate to use it. My question is what would cause the diode to open? Is this common? Is it worth repairing, it's only 2 1/2 yrs old? My line voltage is 128 or so depending on what I'm running.

Thanks in advance

Jim W
 

debe

Active Member
Yes you can check the HV diode with a Megger as it wont check with lower voltage ohm meters. You are lucky it wasnt working when you shorted out the HV!!!!! These Voltages KILL as they are around 1500Volts @ .5A. Also after switching off make sure the HV capacitor is discharged as they can hold a Lethal voltage. NEVER poke around the HV parts of a Microwave oven when operating. this voltage is capable of KILLING you. Yes the HV diodes do just fail. & are fairly cheep around $5.
 
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JMW

Member
Did I imply it was powered when I checked the HiV? If so, I lead you astray, it was off, and I used a screwdriver attached to a jumper lead attached to ground. Thanks for the info, now I just have to find one in PR
 

JMW

Member
Just got done with the megger. Hadn't used it for awhile, had to replace the leads, they had corroded to the extent I couldn't 0 the meter. With the megger the diode checks good. Followed all the test procedures. I no longer have the my 10KV probe, it didn't make it, didn't think I would need it any longer. Any ideas as to what to try next.
 

debe

Active Member
Does it have a HV fuse similar to the pic? If so check its not blown. If the Capacitor is ok then the only other part will be a faulty Magnetron.
 

debe

Active Member
The microwave & power supply part of this circuit is pretymutch the same in all microwave ovens. & Ive been repairing them for nearly 30Yrs now.
 

JMW

Member
debe: You're right that is almost identical to mine. On the Hi Voltage cap, I have a bleeder resistor as you have, however, I don't see it physically.
I'm thinking it is the maggie also. I guess the Hi Voltage transformer could be bad, but it ohmmeters well. I've static checked through it and all the interlocks are all correct.
Thanks to all for your help.
 

debe

Active Member
The bleed resistor these days is internal to the capacitor. Generaly if the Transformer tests ok with an ohm meter, then its probably ok.
 

johansen

Member
it does seem strange to me that you would hear the power supply change tone as if its under load.

generally, the reason you hear the tone change is because the 1000 watt load pulls the line voltage lower, or because the current going through the secondary changes the tone produced by the transformer.

in any case, the reason for the 2 second delay is because it takes that long for the filament to heat up, causing the magnatron to draw current from the -4Kv line.
otherwise it should be nearly open circuit without any filament current.

I suppose it is possible for the magnatron to fail, but still draw 300-1Kw from the -4Kv line, but it is unlikely.

so my advice is turn it on for a few minutes, and if it is pulling a load then something is going to heat up :)
 

JMW

Member
I'm back, had to get off the exercise mat, change out of shorts and sandals and travel to the big city. It was worth it. Picture of exercise mat attached.
I'm going to do some further tests today, as I no longer have a hi voltage probe and am thinking that is where the problem lies, I'm going to make a test circuit.
I'll be using 1 watt, flame proof Hi Voltage resistors. The expected Hi voltage is 4.1 KV. I'm getting 30 Meg, 10 Meg, and 1 Meg for a total of 41 meg giving me a current of .1 ma and a power level of .4 watts. I'll measure across the 1 meg and expect 100 VDC. OView attachment 61171f course I'll be powered down when I attach the leads via alligator clips. Anyone see a problem?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
The problem is probably the voltage rating of the resistors - they might not be rated for high enough voltage, what are their ratings?.

I would suggest using ten (or more) 1W resistors in series, to meet the voltage requirements.
 

JMW

Member
Did some further tests. Namely, I have 125 VAC on the primary of the HiV/Fil transformer. Checked the HiV cap with a megger, it reads 1 meg ohm, I'm assuming that is the bleeder resistor that is inside the 1.05 ufd capacitor. The secondary of the transformer is 100 ohms, just what the spec sheet states. As i hear no arcing, and the trannie is not getting hot, I must conclude the maggie is not emitting. As I can buy a new one for the price of a maggie, that seems to be the logical course.
Has any ham used the Hi Voltage trannie as a power trans for a home brew 40 meter rig?
As for the resistors, they were listed as Hi voltage, again how hi is up?
So I shall put this back together, and workout for an hr or so, unless someone has a great idea to try.

Thankyou one and all for the help.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Has any ham used the Hi Voltage trannie as a power trans for a home brew 40 meter rig?
As for the resistors, they were listed as Hi voltage, again how hi is up?
As you're probably aware, stating 'high voltage' is meaningless, you need to know what the specs. on them are.
 

JMW

Member
I priced a new microwave at 200 so a new maggie seemed the way to go. Ordered it from Ebay for 53 including shipping. Got it yesterday and installed it today. The popcorn is great.
I specifically asked for a maggie manufactured in an Asian country starting with K. The OEM was from C. The numbers were the same, but the maggie was rotated 90 degrees. It fit but, the cable was a bit tight.
Thanks to all for the help
 

unclejed613

Well-Known Member
Most Helpful Member
so the way to test the mag is to disconnect it from the transformer, and measure across the filament. it should read about an ohm or so (or even less), if it reads open it's burned out. either filament lead should read open to the mag case (which is the mag plate electrode). the antenna tip should read shorted to the mag case.

i've seen some mags read ok until power is applied, then it blows the fuse. if all of the other components (diode, cap xfmr)are ok, then the mag is gassy, and arcing internally when HV is applied.
 
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JMW

Member
That is true of the RADAR mags I've worked on. This one, the filament checked good, and no fuses cleared. It is as if the filament just stopped emitting. (I never did the hi voltage check). Also I never did a "burn in" .
My line voltage is at 125 during the day and sometimes goes to 132 at night. It appears as if the filament is instant heat, and the emitting substance just burned off the filament.
 

debe

Active Member
The Magnetron should not be fitted 90deg to the orig, if it is its the wrong one. At 90deg the cooling fan wont be blowing through the Magnetron & it will over heat & fail.
 

JMW

Member
By rotating it 90 CC the fins are exposed, the only difference is the filament connector is on the side.
 
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