KitchenAid KBMS1454SSS-3 Microwave Control Board Level Repair Help Request

Les Jones said:

I also agree that this is the way forward. (An alternative way to do what I suggested in post #2.) Nigel's practical experience with microwave ovens was a a good reason to think it was unlikely to be a board fault but it is now looking like in this case it is a faulty board.

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As I said originally, it's a fairly common fault - and I've never seen a board failure cause it (with one minor exception - had a board once with part of the neutral track blown off, I never knew why, but repairing it cured the problem).

It's really annoying, as I can't remember what used to cause it (although it was usually different on different models), and I'm fairly sure it was either interlock switches, fuses, or sensors (humidity, temperature etc.)

There are some inputs going it to the board (door, NFS and LFS) one of those been incorrect 'may' be telling the microcontroller not to feed the motors.

Each one of those 4 switching transistors above the fan & turntable relays, trigger each of the relays, and the last digit of each part ID# is the same as the last digit for each relay. The center terminal of the transistors, appear to be the bases for the transistors, which all connect up to the logic outputs of the main micro CPU chip. You could also probe each of these base terminals with a VOM red wire probe, (black probe to ground), to check for a small (0.7 volt) voltage change, to be sure if the CPU is sending a proper signal to each driver transistor for the corresponding relay. Transistor 7901=fan, 7906=turntable.

A stalled or frozen motor could very well open a PCB trace, or burn up a set of relay contacts, the main reason I suggested applying 110v directly to each motor to be sure that is not the case here. He could simply jumper the 110v relay contacts to test the motors to be sure.

I don't know if the fan and motor stopped at the same time or not. When it came into my possession both were inoperable. I too wonder about having two failures simultaneously and if there is some other root cause that would lead to this.

I particularly like Nigel's suggestion of disconnecting the magnetron from the circuit while troubleshooting - that hadn't occurred to me and I had been laying the lid crossways over that part of the unit while running 5 second tests to limit exposure to microwaves and prevent overheating. His approach is much better. I also like the test he prescribed of going upstream to the transistors - I have not done that yet because before I received that post I had pulled the board from the unit again and de-soldered the two relays. I'm not getting any resistance measurement across the relay coils so I have replacement relays on order. They should be here about mid-week. If they don't resolve the issue I will proceed with the measurement at the transistor as Nigel prescribed.

Relative to questions about the fan and turntable motor - I noticed the turntable motor wasn't operable when I first received the unit and replaced it - so the one in there now is new OEM equipment. The new one operated intermittently but has since stopped. The old one tested open across the terminals. I pulled the fan out shortly after Nigel identified that as a cause of the overheating magnetron. As previously stated a resistance check came in at about 14 ohms. Not stated was that the blower wheel spins easily so it's not binding from grease build up as was asked about.

Turntable motors do fail - usually due to overloading and stripping the gears inside, or due to liquid spills inside the oven running down the turntable shaft. Fairly obviously, it's essentially just a clock motor.

Attached is a pic of the upstream transistor markings and what I believe is the data sheet for that transistor.

I've done a few HVAC furnace PCB control board repairs to the component level in the past, and if the boards were not usually fried due to some kind of electrical surge, they usually would either have some bad solder joints around heavy current carrying components like the relays and or power transistors, or the very cheap relays they use would have burnt contacts, melted or distorted plastic cases or covers where the contacts would no longer function, or have open internal coil winding connections at their terminal posts. That is why I mainly suspect the relays as the problem. I am a previous Sears Consumer Electronics service technician, and have also repaired microwaves. Here's hopefully the new relays will fix your problem. Most techs would usually replace the whole PCB, at added expense!

When you use the term "no resistance " do you mean zero ohms ( like when the probes are shorted together) or infinite resistance. (Like the meter reads over range the way it does with the probes not connected to anything) ?
It seems very unusual for two relay coils to go open circuit or short circuit.

Les.

And yes, Nigel's suggestion of disabling the Magnatron, would be standard excepted practices when working on any microwave, and should be apparent by internal safety labeling, once the cover is removed! You also need to be sure you are checking the proper set of terminals on those relays. By my experience, usually the coil windings, would have round pin like terminals, and the 110v contacts would usually have heavier flat or round terminals by the type of relays I see on the PCB.

Les Jones said:

When you use the term "no resistance " do you mean zero ohms ( like when the probes are shorted together) or infinite resistance. (Like the meter reads over range the way it does with the probes not connected to anything) ?
It seems very unusual for two relay coils to go open circuit or short circuit.

Les.

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VERY, VERY unusual - it's pretty rare for faults on the boards in microwave ovens - main sort of problems are the membrane keypads failing. And having repaired microwave ovens for 30+ years I've never come across an O/C relay.

Please also note, that by your attached documentation concerning the relays, these are 24v relays, and will need to be tested with a 24 volt supply, and NOT 12 volts as you had asked. You may possibly hear them click with 12 volts applied to the coil windings, but the contacts may not fully close with only 12 volts applied to the windings. If you already have the relays removed from the PCB, be sure to check if there are any spike transient protection diodes wired directly across the relay coil windings, (either mounted on the PCB, or internal to the relay), that may be shorted and NOT letting the relays fire. The 4 glass diodes between the MW relay and the TT & Fan relays, (6901, 6902, 6903, 6906), appear to be wired directly across the relay coil windings for each relay, and should be checked for a short or leakage, which would not allow a relay to fire. 6901 & 6906 would be the ones to check first!

I meant open - infinite resistance on the relay coil check. However I had scale set to x1 on a 5 digit digital display. Testing off board now on "auto" scale I get readings of 1.1 Mohms and 1,3 Mohms. I do note that as I leave alligator clips from the meter attached to the coil legs the resistance climbs (or falls depending on polarity of connections). The spec sheet gives a coil resistance of 2304 ohms so I reasoned that these relays are bad.

As to be sure I'm on the relay coil I'm testing across the pins that are in line with each other (not offset) because that is what the pictorial in the data sheet shows as the coil location. Other than that I don't see any markings on the relay to indicate which pin is negative/positive side of the coil, I don't know how to determine if there are spike transient protection diodes internal to the relay.

The data sheet relay contact wiring diagram does not show any internal diodes, but I have run across them before if the supplier possibly out-sourced the manufacture of their relays in todays world. Then I went back to look at the PCB trace wiring and posted to check the diodes mentioned. Be sure to lift, or unsolder one leg of the diodes to fully check the diodes for opens, shorts, or minor leakage; which also may have caused the relay coil windings to open; although unlikely. The diodes are mainly there to protect the switching transistor circuitry from back EMF from the relay coil windings.

So 6121 and 6126 diodes?

Did you happen to find both relay coil windings open? If you actually want to go to the next level of troubleshooting, (and since you found the coil winding to be open), you could always carefully pop the cover off of the bad relay, and inspect the coil wiring connections at the terminal posts for unsoldering. Which I have seen before also, due to the heat that can develop inside a MW oven!

No. The GLASS style diodes between the relays. 6901 & 6906. Reread my previous post.

You mainly want to check to be sure that the diodes conduct in one direction, and not the other, when the VOM leads are reversed, to be sure the diodes are not open if using a digital meter. I will usually use an analog VOM meter, set to the highest resistance scale to check for leakage. Be sure to NOT touch anything while checking the diodes for leakage, as skin resistance will effect any readings. Hence the reason to also unsolder one leg to totally remove the device from any circuitry. Other than the meter probes. Also, to clarify; no need to completely remove the diodes to check them for leakage. Just unsolder and lift one leg, and check them from the top component side of the PCB, so as not to loose their correct polarity, or orientation in the circuit.

MTJB said:

No. The GLASS style diodes between the relays. 6901 & 6906. Reread my previous post.

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The diodes couldn't be faulty anyway - and I don't think they ever fail? (unless you fit the wrong diodes).

If the diode was O/C the relay would still work, but the driver transistor would probably go S/C, leaving the relay permanently ON.

If the diode was S/C when the transistor turned ON it would short directly across the 24V supply, and would most likely kill the transistor as well, and almost certainly with clear visible signs.

It would also be pretty unlikely if two completely separate diodes both failed at the same time!

Depending upon the level of leakage, (and NOT a full blown short), if it were to occur in one of those diodes, (which I have experienced in logic board and other electronic repair), that could definitely pull the 24v supply to the relay, down below the 18 volts required to fully close the relay contacts; since the diodes are wired directly across the relay coil terminals. Without necessarily destroying the driver transistor, although it may become overheated; and eventually fail. I am just trying to cover all of the bases here, as it appears he HAS found, so far, at least one of the relay coil windings to be open. I am just giving him further things to check while he has the relays removed, and awaiting replacement parts, as there may have been a surge to this unit, that opened the relay coil windings. Without an explaination. Also considering that the control PCB has a SMPS. Otherwise, he may replace the relays, and then still find that the unit does not work properly.

wickline said:

I meant open - infinite resistance on the relay coil check. However I had scale set to x1 on a 5 digit digital display. Testing off board now on "auto" scale I get readings of 1.1 Mohms and 1,3 Mohms. I do note that as I leave alligator clips from the meter attached to the coil legs the resistance climbs (or falls depending on polarity of connections). The spec sheet gives a coil resistance of 2304 ohms so I reasoned that these relays are bad.

As to be sure I'm on the relay coil I'm testing across the pins that are in line with each other (not offset) because that is what the pictorial in the data sheet shows as the coil location. Other than that I don't see any markings on the relay to indicate which pin is negative/positive side of the coil, I don't know how to determine if there are spike transient protection diodes internal to the relay.

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Yes, you do have the correct relay coil terminals, as the 110v contacts are offset. The rise and fall of that resistance value you are seeing, may be due to your skin resistance if you are in contact, or touching any of the metering connections; (since it would appear the relay coil windings are open, or of a very high resistance), as I mention later in my post concerning checking the relay coil winding back EMF transient spike protect diodes for leakage. And although I do believe you will NOT find them defective, it would not hurt to double check them, since you did find the relay coil windings open; and since they are there to specifically short out the back EMF voltage coming off of the relay coil windings, as their magnetic field collapses when the relay is de-energized.