Samurai Appliance Repair Man

[Samurai Appliance Repair Man]

Even on VAC, the reading between Neutral and Ground should have been 0 VAC. Double check that reading with your new meter and make sure of your test point locations.

[Samurai Appliance Repair Man]

Unlikely you damaged the board because its internal thermal overload probably did its job and opened the power supply to the motor windings.

Cord was installed properly but found 120 from neutral to ground.

If this reading is correct, something is miswired. Should be 0 VAC between Ground and Neutral. Ground could be floating (not connected). Did you make this measurement with a loading meter (LoZ) or on VAC?

[Samurai Appliance Repair Man]

If L2 and Neutral were reversed, then the motor was running on L1 to L2– 240VAC. That would explain the winding noise. If you let it run for a while like that, you may have damaged the motor.

[Samurai Appliance Repair Man]

If the load opens, no electron flow.

Correct

It would lose neutral and no longer have current.

Careful– If the load itself opens (breaks, whatever) it will still have Line and Neutral but because of the physical open inside the load itself, electrons will not move in the circuit.

I was confused why we were not considering the thermostat as a potential issue

No reason, just wasn’t the problem scenario we picked for this case study.

But here’s the key: If the thermostat was stuck open, say, (bake contacts stuck open) you would still troubleshoot the same way– starting at the Load of Interest (inop ignitor) and working your way back in the circuit until you found the missing voltage.

Just remember that troubleshooting always starts at loads, not switches. That’s why step 3 of the Tango is selecting your Load of Interest. Ideally, it starts in step 1 with the LOI being named in your problem statement.

[Samurai Appliance Repair Man]

Luckily, this schematic says it.

Another trick with tech sheets is to read everything on them. There’s usually not much but what is there may be just the thing we need to know to figure something out.

This one sort of confused me, I’m not sure why.

That’s normal and also why this tech sheet was chosen for this case study– to get you used to working from different tech sheet layouts and schematic “styles.”

One thing that confuses many techs is following the black jumper wires for the indicator lights and, more importantly, to understand the circuit algorithm. The jumper effectively creates a logical OR condition for when the hot surface light comes on. It will turn on when EITHER the element is turned on at the infinite switch OR if the temperature is still above 150F.

Keep asking questions if you’re confused. We’re happy to help “unconfuse” you. That’s why we’re here!

[Samurai Appliance Repair Man]

I read in a different response that the entire surface element will need replaced because the thermal limiter (is this another term for the bimetal portion?) is part of the element. I’m not sure I would have known that,

You should– it was covered in the video in Module 2 Unit 7.

The thermal limiter controls power to the ribbon element. But there’s a separate bimetal attached to the element can with it’s own bimetal contacts just for the hot surface indicator light. This, along with the thermal limiter is part of the surface element. The way I know that is because the tech sheet for this case study explicitly says it (plus it’s standard technology used in electric cooktops):

Also from the same tech sheet:

but I do remember the box for the thermal limiter that is not present on the type that uses an RTD

RTDs are used in ovens for temperature sensing. So I’m not sure what you’re asking. Try again?

[Samurai Appliance Repair Man]

If the issue was that the supply voltage is dropping below 120 (to 110) and therefore the current is dropping below the minimum required to have the gas valve open – how do we detect or fix this? I

As with all AC loads– like the ignitor and valve heater– you begin with an amp measurement just to see where you’re at. If amps are low, then verify your voltage supply. You are correct that if volts are low, amps will be low, I=E/R. I and E are directly related. So if E goes down, so does I.

The Molex G-1 and G-2 – what is this? Can I always assume that a wire passes directly to the other side and continues its path?

Molex connectors are used all over the place in all appliances. It’s a plastic wire harness connector that connects to a mating connector on a board or somewhere else. It’s like a removable splice so the wires pass from one side of the connector to the other to continue the circuits that it’s connecting. So you read the wiring diagrams with the understanding that the connecting ports (or pins) of the molex pass straight through to the other side.

If the issue is the thermostat, would the ignitor not glow at all?

Define “issue”.

A circuit-based question would be, “if the thermostat is OPEN, would the ignitor glow at all?”

What does a load need to do its work? A power supply connected to a complete circuit.

The power supply for the load (the ignitor and bimetal heater in series with each other in this case) is L1 and Neutral. The load gets its Neutral through the bimetal heater and L1 through the thermostat. From N to bimetal heater to ignitor to L1 is one series circuit with two loads in series.

Pop Quiz: What happens to electron movement in this series circuit if either load opens?

Bonus question: What would happen to electron movement through the ignitor if molex pin G1-2 pulled away and separated from pin G2-2? (this sometimes actually happens in real life)

[Samurai Appliance Repair Man]

But in this case they jump the gap (understand that aspect), ignite the gas, hit the burner head, and go…. where?

The electrons are jumping the gap from the electrode TO THE CHASSIS (via the burner head and base). The spark module can detect this electron movement. It sounds amazing, but that’s what happens.

If you’re asking HOW the module detects this current in the chassis, that’s the FM. And really, we don’t care because the spark module is treated like a “black box” that does strange, mysterious stuff that we don’t need to know. We just need to know how to troubleshoot it. And to do that, you need to know what are it’s required inputs and what are the specified outputs. If we know that, which is what you learned in the course, then you know all you need to know in order to troubleshoot accurately and correctly.

When it comes to circuit boards and special modules, like spark modules, we troubleshoot them by looking at inputs and outputs. We don’t really care how it processes those inputs, only that it produces the correct outputs. That is called “black box troubleshooting.”

[Samurai Appliance Repair Man]

The current that is detected via “FM,” where do these electrons go?

They go to chassis, the same electrical location of the reignition module ground screw. The burner head and the ground lug on the reignition module are EEPs. The FM module in the reignitor detects these amps. We’re talking MICRO amps, like 0.000001 amps. That’s the FM.

This spark doesn’t result in any shock risk?

Yes, in the same way that a static electricity imparts a shock. You’ll feel it but probably won’t die because the current is not sustained– it is interrupted and pulsing. Electricity kills by continuously pushing electrons through your body, burning up tissues and organs as it goes.

[Samurai Appliance Repair Man]

Current/spark jumps to the burner head. Does the current actually return to the spark module’s ground (the spot on the bottom when screwed in) for the flame sensing?

Correct– the spark jumps the 3 mm gap from electrode to burner head. The burner head is supposed to be in direct contact with the burner base and therefore chassis. The 14,000 volt spark is caused by electrons screaming through the air and oxidizing oxygen molecules which produces the visible spark.

Remember that “current” is just a name we give to a directed stream of electrons. Well, that spark IS is a directed stream of electrons. The ground lug on the spark module detects this current using what we call “FM technology” (FM – F*cking Magic).

I said previously that the burner head is supposed to be in good metal-on-metal contact with the burner base. In the real world, sometimes pots of possum stew boil over and leave food gookus underneath the burner head. This gookus acts as an insulator that blocks the spark electrons from entering chassis where it can be detected by the spark module. So a good quick check on a re-ignition problem where it keeps sparking after flame is established is to check the burner head and base for cleanliness and ensure good metal on metal contact.

2. In unit 9, you write that the neutral is “unswitched.” What does this mean?

Meaning that if you look at the Neutral line on the schematic from source to load of interest, there shall be no switches in that line. Just hardwired Neutral from source to load.

But can you elaborate on what pull off looks like? Does this just mean pull out one of the pins? Does the whole connector come off?

It will vary by the appliance and how the manufacturer wired the loads. Sometimes it’s a molex harness connector. Sometimes it’s individual wires connected by single terminals. There are multiple ways to skin the cat. As long as you know what you’re trying to do electrically, then the mechanics of disconnecting is obvious and straightforward.

[Samurai Appliance Repair Man]

You got it!

This business with the computer board making software decisions about supplying voltage to a load or not is common today with computer-controlled appliances.

Keep in mind that computer-controlled appliances really ARE computers. Instead of powering loads like a monitor, keyboard or printer like our desktop computers, they power loads like heating elements, compressors, fans, LED displays, solenoid valves, etc. So this concept of software-controlled power supplies is an important one to keep in mind when troubleshooting modern appliances.

Even if the manufacturer does not explicitly tell you about software-controlled power supplies, it’s usually there in some form and implemented through things like POST and sensing lines. Just keep this in mind so you don’t get head faked by this.

Classic example is Samsung refrigerators. If one of the evaporator fans get stuck (say by ice) and the board is not getting a speed signal back from the fan motor, it will kill power to the affected fan motor for 15 minutes and try again. It will repeat this three times and then throw an error code. The only way I know this is because Samsung explicitly tells us this in the Fast Track.

[Samurai Appliance Repair Man]

… but the Amana only had resistance specs. In this situation would you just work with the resistance specs if that is the only spec you have…

Exactly. We work with the specs we’re given, not the specs we wish we had.

An exception to this is implicit specs.

Example: You’re given a wattage spec on a heating element. As an MST student who has learned Ohm’s Law, you know that amps are an excellent surrogate for watts. So you fire up the element and measure amps. If you know volts and amps, you can calculate watts using everyone’s favorite formula: P=I*E. You plug in the volts (E) and amps (I) that you measured and get watts. Then compare this calculated wattage to the specified wattage to see if you’re in spec.

Not so easy in the case of thermistors because they are in what’s called a “voltage divider circuit.”

So it’s not a nice, straightforward Ohm’s Law calculation.

[Samurai Appliance Repair Man]

Pop quiz 1 = 240 (subtracting a negative is the same as addition)

Correct!

Pop quiz 2 – You would not measure a difference. The 120 volts coming from L2 would be present at both points, so the difference would be zero.

Correct again!

Bonus – I think it was called half splitting, but I remember the technique from the core. I think it would be good for me to review though.

Correct yet again!

Sorry to keep you, feel free to move on.

It’s fine, that’s why these forums are here!

In this case, the element is bad. So I can’t measure L2 on both sides of the element. This is what I mean, whether or not it is correct….

This is correct. If the element is open, it’s electrically the same as the two wires being disconnected from the element and measuring at the disconnected wires.

The element is open, whether it is the element or the connections etc., current can’t pass through it.

That’s right.

Scenario 1: the board has killed power to P1-1 because it does not sense the bake element, but it has not killed power to P2-6 because L2 is present for the upper right convection element (we assume). As long as P1-1 and the connecting wires are still intact, would I still measure 120V, because this is the difference between the V being supplied by L2 wrt L1 (the open bake element is not allowing current from L2 to reach the other side)?

No. If P1-1 is electrically open (because the board is not closing the relay to supply L1, for example), then you’re measuring the voltage difference between L2 and Open. This is a situation where, if you’re not using LoZ or a loading meter, you may measure some ghost voltage but you are not measuring the difference between L1 and L2.

Here’s a video demonstrating ghost voltage in a wall oven that looks eerily similar to the Acme wall oven in this case study.

https://youtu.be/vBiRjgMIuSE

Scenario 2: the board hasn’t killed power, and I would read 240V regardless of the open status of the element because the difference between these two points, due to being out of phase, is 240V.

Correct!

[Samurai Appliance Repair Man]

Or I guess I might see 120volts? Because L2 would still be active (bc the convection element is working)

Remember that when we talk about “voltage” in a circuit, it’s shorthand for saying, “the voltage difference between two points.” That’s why it takes two leads to make a voltage measurement.

In a 240 VAC circuit, you have L1 and L2. L1 and L2 to Neutral is 120 VAC. L1 to L2 is 240 VAC. But if each Line is 120 VAC wrt Neutral, then how can you have 240 VAC from Line to Line if it’s supposed to be a difference between two points? Don’t they simply add the 120 VAC together to get the 240 VAC?

Nope. Still a difference, meaning L1 MINUS L2. (Or L2 MINUS L1, works both ways). It works because L1 and L2 are 180 degrees out of phase with each other so, wrt Neutral, they are two distinct phases of voltage. In a split phase power supply, like most residences in North America, the two phases are 180 degrees apart. Meaning that when L1 is +120 VAC, at that same exact moment in time, L2 is -120 VAC.

Pop Quiz: What’s +120 VAC – (-120 VAC)?

What all this is getting to is realizing that, in a 240 VAC load, like the bake element in this case study, if one Line is missing, you will still only read 0 VAC across the element even though the other Line is present.

And how can this be? Because he IS the Kwisatz Haderach!

Sorry, I had a Dune moment.

Pop Quiz: Suppose L1 is missing but L2 is present (vice versa is same outcome). You place your meter probes on each element terminal and energize the circuit. What voltage difference will you measure?

Bonus Question: What troubleshooting move might you deploy to determine which Line is missing?

Voltage looks like p1-1 to p2-6.

Correctillia!

[Samurai Appliance Repair Man]

1. I followed the case study and understood. But why aren’t we checking voltage before ohms? Is this because we figured it was the bad element based on the error code and logic? I know live tests are preferred, I figured we would want to check that the board was providing L1 and L2 appropriately before relying on an ohms test.

Excellent question! You said the answer: we’re getting an error code for the upper bake element. That means the board is sensing the presence of the element during its POST (power on self test). Since the schematic/wiring diagram does not show a sensing line (a switched line back to the board), it is probably doing this via current sensing. You need volts to make amps.

Also, since the board is throwing this code, the usual algortihm is to kill supply voltage to the coded component. So measuring voltage may not be an option.

But still, checking the supply voltage is a solid troubleshooting step.

Pop Quiz: Where would you place your meter probes to see if the upper oven bake element is getting its supply voltage?

2. The upper right convection oven element, we assume, is functioning properly because there is no error code. Does the bake element get L2 through this convection element? Most of the schematics are not drawn like this (this being a wiring diagram). I mean, based on the drawing, it is impossible for the upper bake oven to get L2 if the convection oven is not getting L2. Is this correct?

Look carefully at the schematic section with the upper bake element.

See how the line for the bake element tags off the connection to the convection element? That means L2 for the bake element “daisy chains” and goes on to the bake element. The bake element and the convection element are in parallel, not series. You will encounter loads in series in appliance repair but this is not one of those times.

Also, manufacturers play loose with the terms “schematic” and “wiring diagram”. This diagram is more like a pictoral schematic. The simple distinction between a schematic and a wiring diagram is this: if you can easily trace the power supply circuit for your LOI on a diagram, it’s a schematic, not matter what the manufacturer calls it.

3. Can I circuit breaker on without the heat on for the oven, and disconnect only one of the connectors to measure ohms? The question seemed to imply I need to either kill power or disconnect both sides. Can I just disconnect one of these and measure from the wire end to the other connector?

Yes. As long as the circuit for the LOI is dead AND at least of the leads to the LOI is physically disconnected (either at the load or at the EEPs for the load at the board), you’re good to go.

Good questions, keep ’em coming!

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