KitchenAid KBMS1454SSS-3 Microwave Control Board Level Repair Help Request

[Nigel Goodwin]

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.

Except you're suggesting checking things that never fail, on the assumption that they may have failed in some completely unusual fashion, and in some amazing way caused the relay coils to fail? (the diodes are to protect the driver, NOT the relay - you don't even add diodes if there's no semiconductor driver to progrect).

Obviously it's no problem to check the diodes with the relays out, and it gives him something to do while he's waiting for new relays.

 

[Les Jones]

He could do a coil resistance check on the relay for the light with it still in the board to make sure he is measuring the resistance correctly. (After him saying the coil had NO resistance when he meant just the opposite.) If it is a normal DMM the voltage across the meter leads will be too low for the diode in parallel with the coil to effect the reading.

Les.

 

[Pommie]

This is why I like this site. Just a willingness to try to help.

Mike.

 

[MTJB]

He could do a coil resistance check on the relay for the light with it still in the board to make sure he is measuring the resistance correctly. (After him saying the coil had NO resistance when he meant just the opposite.) If it is a normal DMM the voltage across the meter leads will be too low for the diode in parallel with the coil to effect the reading.

Les.

I understand what he meant, although it may not be the proper, or right language to use when referring to resistance. Over the years, I have meet many techs that "flip the scale", and consider a short to mean 100% resistance 'to the flow of electrons'! Yes, checking the cavity light relay coil resistance, which appears to be working properly, would be a good way to confirm his determination of the other relays failure, and of his meter readings; as long as it is the same type of relay. Which it most likely is. The MW magnatron power relay many be the only one with a different coil winding resistance.

 

[wickline]

I am still waiting on the relays to arrive. Once I receive and install them I'll post on the outcome. I already have the transistors should it be necessary to replace those.

 

[wickline]

I received the relays about an hour ago and installed them on the board, put the board in the microwave, unplugged the magnetron to do a check run and both the fan and turntable operate. Looks like you solved the problem and at component rather than board cost! I can't thank the contributors to this thread enough for bearing with me as a neophyte!

Any last minute checks you would have me do prior to reconnecting the magnetron for a full test?

Thanks,
Joe

 

[Les Jones]

Excellent news. I thought it was so unlikely for two relays to fail at the same time with open circuit coils was that I was convinced that you must be doing something wrong with your resistance checks. I don't remember ever finding a relay with an open circuit coil. It is normally the contacts that give problems.
I don't think you need to check anything else. The relay coils failing open circuit would not damage the driver transistors. If they had failed short circuit they would have destroyed the driver transistors.

Les.

 

[Nigel Goodwin]

Excellent news. I thought it was so unlikely for two relays to fail at the same time with open circuit coils was that I was convinced that you must be doing something wrong with your resistance checks. I don't remember ever finding a relay with an open circuit coil.

There's certainly something strange - I've repaired huge numbers of microwaves, and relays don't ever fail - and two at the same time is incredibly unlikely.

I suppose there 'might' have been a manufacturing problem on that batch?, but it seems pretty unlikely they would have worked for very long at all if they were going to fail. It might be worth checking the supply voltage to the relays?, as it's a switch-mode PSU it has the potential for running too high (due to capacitor failures), and too high a voltage 'could' possible kill the relays - but this is really a stretch, as relays are very sturdy devices, and I suspect would need a BIG increase to blow them.

Still, working now, good job.

 

[wickline]

I'm puzzled as well as to how I have 2 relays fail simultaneously. While I did say the coils were "open" that was my perception when I had the DMM scale set to K ohms (because I was expecting to measure about 2300 ohms). When I left the scale on "auto" the resistance initially checks about 0.76 M ohms on one and 0.56 M ohms on the other and continuously climbs (on both) as if the coil windings are heating up. This is with the bad relays uninstalled. My guess is a breakdown of the protective coating on the coil windings. I checked coils of new relays prior to installation and they came in around 2300 ohms as expected.

I've closed up the unit, tested it with a container of water and all is good. I will return it to the owner later today. Again thanks loads for the help guys! I'll be back for more help the next time I have a control board issue I can't figure out.

 

[MTJB]

I'm puzzled as well as to how I have 2 relays fail simultaneously. While I did say the coils were "open" that was my perception when I had the DMM scale set to K ohms (because I was expecting to measure about 2300 ohms). When I left the scale on "auto" the resistance initially checks about 0.76 M ohms on one and 0.56 M ohms on the other and continuously climbs (on both) as if the coil windings are heating up. This is with the bad relays uninstalled. My guess is a breakdown of the protective coating on the coil windings. I checked coils of new relays prior to installation and they came in around 2300 ohms as expected.

I've closed up the unit, tested it with a container of water and all is good. I will return it to the owner later today. Again thanks loads for the help guys! I'll be back for more help the next time I have a control board issue I can't figure out.

Glad to hear the relays were the only problem. As I had previously mentioned, with my experience with repairing some HVAC control PCBs. The relay failures are most likely due to the heat or steam build up inside of the microwave oven. Or due to the poor quality of the mass produced relays. Especially if the cooling fan relay failed first, or if the microwave oven was operated inside of a close quarters cabinet, with poor air circulation; as many are. Or maybe a combination of both factors. Most control PCBs have very wide copper traces for the relay connections, which 'sink' a lot of the internal cabinet heat, (the same as with HVAC furnaces), into the very small relay wire terminals. Pop the old relay covers off to inspect the coil winding solder connections at their bottom plate terminals. Some are soldered, others appear as they may have been spot welded. The ones that appear to be spot welded, are the ones I've seen fail. The coils are made with very fine enamaled wire. They also could just be poor quality soldering, besides the heat produced in the coil winding itself during operation, and their close proximity mounting on the PCB.

Mike

 

[wickline]

Thanks Mike. This microwave was indeed installed in a closed cabinet and directly above an oven in the same cabinet space so I suspect you are correct on the underlying cause. That said however the installation expects it to be mounted as such with suitable clearances surrounding the unit. I didn't pop the covers off the old relays before tossing them so I can't really inspect them now as you had previously suggested and again above. Thank you for the help though! If the unit comes back again I'll suggest investigating a way to vent that cabinet.

 

[Nigel Goodwin]

Thanks Mike. This microwave was indeed installed in a closed cabinet and directly above an oven in the same cabinet space so I suspect you are correct on the underlying cause. That said however the installation expects it to be mounted as such with suitable clearances surrounding the unit. I didn't pop the covers off the old relays before tossing them so I can't really inspect them now as you had previously suggested and again above. Thank you for the help though! If the unit comes back again I'll suggest investigating a way to vent that cabinet.

No disrespect to MTJB, but his experience isn't with Microwave ovens, and none of what he suggested would cause the relays to fail. Even assuming that steam 'could' be a possible cause? (and I don't believe it could) there's not likely to be any steam where the electronics is (and steam is only in the oven cavity, and is vented by a fan put the top or back). The electronics area, where the magnetron is also mounted, is also well ventilated anyway by the cooling fan, that prevents the mag over heating.

When mounted in a cabinet with a proper fixing kit, the correct air flows are maintained as it's normally ducted out the front above the oven.