KBMS1454SSS microwave not heating

[wickline]

I have the KBMS1454SSS-3 Microwave back in my shop that I posted a thread on a few years ago. The problem this time is that it is not heating. I'm attaching the tech data sheet for the unit. When I attempt to operate the unit I can hear the 4903 relay chattering so 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 reinstalled the relay and it behaves the same way. I'm stumped as to why the relay behaves differently while in circuit. It seems like it must not be getting adequate voltage but I'm not sure what to check. I do believe the humidity sensor is bad however (I show an open circuit when checking between pins 1 & 3 and also between pins 1 & 2. However, I don't believe that's the core problem causing the MW relay to chatter. I think it's function is to initiate a timer for remaining cooking time so it would affect cooking "doneness" but I would suspect manual cook time functions should still work normally. I would appreciate receiving direction on what to test. 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.

 

[Nigel Goodwin]

The humidity sensor may well read O/C anyway, so I wouldn't get too excited over that. However, looking through the manual it does give test resistances for it - and weird things do happen with duff sensors (I seem to remember Panasonic ovens switch-off after 28 seconds - or something like that - if one of the sensors is duff, and it's the exact same time period across many years of models).

For a chattering relay I would suspect a power supply problem, such as the reservoir capacitor on the PCB having dried up, so supplying AC to the relay coil rather than DC. Although having repaired microwave ovens for 40+ years I've never seen a duff reservoir capacitor, or seen a chattering relay?.

 

[wickline]

Thanks for your reply Nigel. Attached are pics of the board. Is the reservoir capacitor you're referring to the one labeled 2131 on the board? It's the largest one on the board so I'm thinking that's the one you're referring to of the 8 caps I see on the board.

 

[Nigel Goodwin]

Yes, and perhaps the other two large ones nearby as well - a scope would be handy to check for ripple on them. I've never seen a faulty one, but it's something which could cause the relay to 'chatter'.

 

[wickline]

I replaced the three capacitors, and it still behaves the same way with that relay clicking on and off rapidly. I don't have a scope so I'm unable to conduct that test. Any other thoughts on what to try? I disconnected the humidity sensor to eliminate it from the circuit, no change in outcome from doing that.

 

[wickline]

Nigel - I found the culprit. the secondary interlock switch was dicey. It would have continuity when the plunger was partially depressed but fully depressed it would lose continuity. The relay coil just wasn't being energized enough to keep it in a made position. I had checked the door switches prior to going after the board but missed that malfunction because of the intermittent nature. I thought I was just not making good contact with my meter probes once I saw continuity with the depressed plunger. As always thanks for your help with these!!

 

[Nigel Goodwin]

Nice one, well found