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Kitchenaid KBMS1454SSS-3 Microwave control board troubleshoot & repair

jwicklin

Jan 30, 2024
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I have the KBMS1454SSS-3 Microwave that has a relay that clicks rapidly when I hit the start button to begin a cooking cycle. I have replaced the 4903 M/W relay on the board as well as 3 of the capacitors however the problem continues to exist. I tested each capacitor that I removed from the board with an EMR meter and all tested good. Since I hear the 4903 relay chattering I removed it from the board and used my 18V makita battery to bench test it. It appears to work fine on the bench - apply the voltage and it fires the relay, no chatter. I replaced the relay and the new relay behaves the same way. I'm stumped as to the cause of this relay chatter. It seems like it must not be getting adequate voltage but I'm not sure what to check. I would appreciate receiving direction on what to look for. I'm a handyman, not an electronics technician. I've repaired microwaves in the past usually amounting to swapping out magnetrons but occasionally replacing board relays or capacitors. There is an 8 pin transformer on the board with the number 467123810 stamped on it but it doesn't have any visible damage to it nor can I identify what the replacement part would be if I were to pursue replacing it. Attached are pictures of the board along with the tech data sheet for the unit.
 

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H2814D

Nov 4, 2017
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Which capacitors did you change, and why did you change three that tested good? What about the rest of them? Have you checked them? And are you using an "ESR Meter" on them while they are still on the board? You said "EMR Meter." Did you mean ESR? There are two similar medium sized caps near the green wire that appear to be a bit misshapen, but it could only be the way the photo was taken. Have you checked those two?
 

jwicklin

Jan 30, 2024
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I changed the three largest capacitors on the board (including the two next to the ground wire you mention) at the recommendation of someone on another forum as a possible solution to the same stated problem. As I took each capacitor off the board I tested it with an Atlas LCR40 passive component analyzer. All 3 tested good but I went ahead and installed new capacitors anyway. The photo was taken before I had replaced these capacitors. There are 5 other small capacitors on the board labeled 2134, 2921, 2250, 2251, & 2464. I have not removed nor tested any of these caps.
 

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H2814D

Nov 4, 2017
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One of the most common problems with a lot of circuit boards come from failing/failed capacitors. If it was me, I would check the rest of those that are on the board before doing anything else. Note, however, that a cap can often show a within tolerance farad reading on a tester, like the one you are using, but after time, they develop a higher internal resistance, which is tested and verified by an ESR Meter (Equivalent Series Resistance< Look that up). You probably don't have an ESR Meter. So you may find the other caps read OK on your tester, that may not be. And those that are clearly out of tolerance will always need to be replaced.

So test the rest of the caps, either by taking them off the board, or by pulling one leg off the board and see what you come up with.

One other thing. Since we don't know the rated voltage going to the relays you are testing with the 18vdc battery, I wouldn't suggest you use that high of a voltage to test anything on that board. It may be way too high for it. For now, check the rest of the caps, and let us know. Be careful with the soldering. It ruins a lot of boards.
 

kellys_eye

Jun 25, 2010
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I'm stumped as to the cause of this relay chatter.
Inevitably as a result of overload somewhere. Disconnect the connections to the off-board items (magnetron etc) and see if the clicking stops. If it does then you need a new magnetron.
 

jwicklin

Jan 30, 2024
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H2814D: I actually do have an ESR meter as well as the analyzer. I rechecked the capacitors that were originally on the board and all passed on the ESR meter as well (images attached). All of these have been replaced. I have not as yet attempted pulling the 5 really small caps off the board to test them.

Kellys_eye: I am working through disconnecting various offboard components - so far no luck with getting the relay to stop behaving the same way. I started with the magnetron since the relay in question is the microwave relay. I had already replaced the magnetron when I took it out of service with a new magnetron. I will let you know if disconnecting loads off the board causes the relay to stop chattering.

Bluejets: It's a [imath]1300 - a new magnetron was[/imath]55. I wish it had only needed a new magnetron. It's already been replaced at the installation location but I'm trying to resurrect this one to have a spare on hand.

Thanks to all for your ideas on where to look. I spend time on it as my schedule permits.
 

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Bluejets

Oct 5, 2014
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It's already been replaced at the installation location but I'm trying to resurrect this one to have a spare on hand.
Not the best pastime playing with microwave ovens or anything microwave with b'all knowledge or qualifications.
Unless you want to end up glowing in the dark or sprouting extra extremities etc.
 

H2814D

Nov 4, 2017
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H2814D: I actually do have an ESR meter as well as the analyzer. I rechecked the capacitors that were originally on the board and all passed on the ESR meter as well (images attached). All of these have been replaced. I have not as yet attempted pulling the 5 really small caps off the board to test them.

Kellys_eye: I am working through disconnecting various offboard components - so far no luck with getting the relay to stop behaving the same way. I started with the magnetron since the relay in question is the microwave relay. I had already replaced the magnetron when I took it out of service with a new magnetron. I will let you know if disconnecting loads off the board causes the relay to stop chattering.

Bluejets: It's a [imath]1300 - a new magnetron was[/imath]55. I wish it had only needed a new magnetron. It's already been replaced at the installation location but I'm trying to resurrect this one to have a spare on hand.

Thanks to all for your ideas on where to look. I spend time on it as my schedule permits.
Ok. I didn't know you had the ESR Meter. That makes things easier. You don't have to remove the cap from the board to check it with that device. You don't even have to remove one leg from the board, with the ESR Meter. Just keep proper polarity when checking the caps from the soldered side of the board. You would still need to remove one leg to get a proper farad reading, though.

Let us know what you find.
 

kellys_eye

Jun 25, 2010
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The technical document says the MW transformer section is replaced by a inverter. Clearly the inverter is faulty. Show the inverter board (both sides).

Alternatively, disconnect the power TO the inverter board and test to see if the unit will power up. If so then the inverter board is dud or the magnetron is.
 

jwicklin

Jan 30, 2024
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I found it - turns out it was the limit switch (labelled secondary interlock on the previously uploaded schematic). I had eliminated it as a cause earlier because I read continuity on the NO leg. Turns out under more careful examination that continuity was only present at partial depression of plunger, when fully depressed it lost continuity again. I jumped to thinking the problem was on the control board because of the very noticeable relay chatter. Thanks kellys_eye for the suggestion to look off the board again. The relay coil just wasn't getting enough energy to keep it triggered. Thanks too H2814D on info on using the ESR meter - I didn't see your post about not having to remove the caps with it. I didn't know that, I will definitely use that piece of info going forwards.

Bluejets - don't try this at home!!! But if you must - get yourself a 20K ohm ceramic resistor first!
 

jwicklin

Jan 30, 2024
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Oh kellys_eye that marking out on the schematic isn't valid for my unit - it still has the coil & diode - no inverter. I lifted that schematic from a service manual that covers lots of different model numbers.
 

H2814D

Nov 4, 2017
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I found it - turns out it was the limit switch (labelled secondary interlock on the previously uploaded schematic). I had eliminated it as a cause earlier because I read continuity on the NO leg. Turns out under more careful examination that continuity was only present at partial depression of plunger, when fully depressed it lost continuity again. I jumped to thinking the problem was on the control board because of the very noticeable relay chatter. Thanks kellys_eye for the suggestion to look off the board again. The relay coil just wasn't getting enough energy to keep it triggered. Thanks too H2814D on info on using the ESR meter - I didn't see your post about not having to remove the caps with it. I didn't know that, I will definitely use that piece of info going forwards.

Bluejets - don't try this at home!!! But if you must - get yourself a 20K ohm ceramic resistor first!
Thanks for returning to let us know about your success, and not giving up. Well done.
 

Bluejets

Oct 5, 2014
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Bluejets - don't try this at home!!! But if you must - get yourself a 20K ohm ceramic resistor first!
No thanks..... your brain is apparently already a-up if you think a 2 bob resistor will deter any rf damage effect.
 
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