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Using a Test Cord to Diagnose a Microwave

Samurai Appliance Repair Man

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A powerful troubleshooting technique when working on microwave oven that won't heat is to split the problem in half to determine if the problem is coming form the low voltage side or the high voltage side. A test cord can be a very useful troubleshooting tool because it allows you to so just that. Sublime Master Chat_In_RI enlightens us on this technique:

Why can't you make a test cord to power the M/W?

No model number here, but a test cord to the primary side of x-fmr to by-pass door switches, relays, etc... and if your diode and cap check out OK, narrows it down to the mag if no heat.

Source: GE Double Oven Microwave not heating



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    • QualityMike
      Oscilloscope Primer
      any info i get from any source i weigh it by testing with my instrumentation whenever possible...  
    • Samurai Appliance Repair Man
      Oscilloscope Primer
      I'm not sure what you're saying here, but here's what I was saying:   This is true with acoustic waveforms but not voltage waveform! But you can't apply this to voltage waveforms because you'd be ignoring some fundamentally different physics. Just because a physical phenomenon like sound or voltage can be represented by waveforms does not mean they behave the same and you can't ignore the basic physics involved with each.  Voltage waveforms do not cancel each other out. When two voltage waveforms are 180 degrees out of phase with each other, the voltage potential difference DOUBLES because this is how electrons see it.  Yes, and if we're talking about the split-phase voltage from the secondary winding of a center-tapped transformer, the reference is that center tap and from either end of the secondary. In fact, if we're talking about voltages from the secondary of a center-tapped transformer, there simply aren't any other reference or measurement point that makes sense-- those are exactly the reference points to use.  Be careful with construction-oriented forums-- they are not a reliable or informed source of information on these topics. Lots of well-meaning seekers, some knowledgable information but mixed in with lots of gobbledy-gook. Can be very difficult to parse out true physics from misunderstandings.  A much more reliable and informed resource is All About Circuits - forums, tutorials, reference guides, articles, and a fantastic newsletter that I recommend you subscribe to.   
    • QualityMike
      Oscilloscope Primer
      thanks scott, its true if you use the center tap of the secondary as your reference, the two waves are mathematically out of phase, but you are using two opposite points for your positve instead of one. in my case i am using L1 as a reference. this makes more sense as far as adding 120+120=240. i found a good link  http://www.indiana.edu/~emusic/acoustics/phase.htm two identical waves cancel each other out, in this case with neutral as a reference, one has L1 as the posotive and the second wave is L2 as a positive. if these two points are tied together there is a short. these two mathematically out of phase waves do not add up to 240, they would add up to 0… IF both waves were on the same positive line.  the advantage/disadvantage of the full wave rectifier is twice the current potential at half the voltage. and as kinda explained in the 2nd video easier to control dc ripple. just putting a center tap on a transformer doesn't change the fact that it is a singe phase transformer. i think the critical piece is what point is one using as a neutral reference. a great forum topic on this is here: http://forums.jlconline.com/forums/forum/jlc-online-expert-forums/the-electric-shop/24357-2-phase-power  
    • Samurai Appliance Repair Man
      Oscilloscope Primer
      Great info on o-scope selection! Thanks for putting that together.  In the latter part of your video, you had a drawing with the that the center-tapped, split-phase voltages in phase with each other. The split-phase voltages (from end to center-tap) are really 180 degrees out of phase with each other. This is exactly why it's called split-phase voltage and it's just the physics of how transformers work. If you look at each phase on a dual trace scope, you'll see it. Transformers resist changes in current and the counter-EMF produced in the secondary is always opposed to the inducing EMF (from the primary). When the secondary is center tapped, you are effectively splitting the secondary into two windings. Since the center tap is shared between the two windings, the voltage picked off between the two half windings are 180 degrees out of phase with each other.  Here's a reference that may help explain it: http://engineering.electrical-equipment.org/electrical-distribution/centre-tapped-transformer.html If you were to look at each split-phase voltage on the secondary using a dual channel scope, you would see the inversion, as this video shows:   The phase-splitting (and inversion between the two phases) in the secondary of a center-tapped transformer is why exactly these transformers are used with fullwave rectifiers, as this video explains:         
    • QualityMike
      Oscilloscope Primer
      Thank walter, You blow something up? thats a streatch…  what is the model you got? 1.i use my scope primarily on anything contacts that will give me a non DC voltage. ac or data or liie the compressor inverter output, real odd looking… even though it has the dmm i rarely use it and if you can save a buck by not getting it, i would recommend that. 2.i have not seen any limitations in the field for using the handheld scope. one thing the bench scope has is the ability to fine tune voltage division. the handheld jumps from 1v per division to 2, to 5. not a big deal though. i dont see an advantage for going cheap then expensive, just following and understanding afew simple guidelines would be good. connecing a scope to ground is a good place have an alarm in your head to be careful and doble check that your connections are right. 3.ive used both in the past and the digital scope for our purposes outways the analog scope, (i have used mostly digital in the past because i favor it). with an analog scope you see the waveform in realtime, thats one of the downsides of the digital scope, but for our applications in the field i don't see a need for that. a slight delay (downside) but the ability to record and scroll through a pause is a big advantage for the digial scope. (im really gonna stick with talking about budget scopes, not the ones that are $10k, $100k and up) a good analogy is the old style analog voltmeter (the one with the needle that goes from left to right as the voltage gets higher for the melenials) the digital multimeter does not sample and record the voltage so if you test a compressor starter ac input at turn on you dont get the whole story of what the voltage is doing. you can see it in realtime on an analog meter. this could be an advantage theoretically in the field, but its not enough for me to carry one on my van. my new fieldpiece SC640 has an inrush current feature that i havent really needed to use quite yet, but im sure will come in handy. (this mimics analog) 4… you guys are so demanding  im glad to be of help to you guys, we all are contributing in our way, thats what makes this a community. i learn just as much from you as vice versa. and ty i saw your electronics set, NICE!