Trying to repair microwave, after some assistance. - Page 1
Forum section: electronics / repair

author: LeWidget
author_rank: Regular Contributor
timestamp: March 06, 2022, 03:24:23 pm
content: Hi all,Long story short, I took down our microwave to give it a wipe over. It appears some water (small amount, but enough) got through a gap in the touch control panel, resulting in rapid clicking (of relay I believe) followed by a little smoke, though display LED still worked.Video #1https://youtu.be/TKIxzIIyI1cI have a thread on 'badcaps' forum, they've been helpful, but issue still unresolved. Thought I'd broaden my search and ask here too.Specs: SHARP R-990K Convectional microwave.Service manual attachedI noticed the areas under the Zener Diode (ZD2), Resistor (R2) & Resistor (R3) are scorched, but others on Badcaps says it can be normal with age. (see pictures)I desoldered the three larger black relays and noticed two of them had water/moisture (very minimal) on the bottom (but no scorching). I dried them then resoldered.I desoldered the two white relays and noticed one of them had a scorch/burn mark on one of the legs. (see pic). I cleaned and resoldered.Could it be possible that the relay became damaged?Specs according to service manual;1. (ZD2) Zener Diode, HZ16-12. (R2) Resistor, 470Ω 1/2w3. (R3) Resistor, 330Ω 1/2w4. (Q1) Transistor, 2SB1238multimeter readings (in circuit, no power)(R1) Resistor - 455Ω(R2) Resistor - 320Ω(ZD2)Zener Diode - (-) cath .688. (+) cath 1.329One user suggested"If you believe that it is a relay doing this then remove the wires from the bottom of the big transformer and hook them up to a incandescent light bulb and and turn on the microwave and and see if your light bulb stays on the whole time"I haven't tried this yet. I decided to plug it in again, see if the issue is still there after drying, and whether I could see exactly where the smoke and clicks were coming from.This is where things change...I plugged it all in again and powered it on (see video). This time, there was some sort of crackle/crunch sound, so I turned it off. Then on again and seemed to have gone this time, I waited several seconds for sounds/smoke, nothing (which is good).Now. I went to set the time (hence the beeps in the second video), BUT, pressing #5 displays #4, and pressing #0 displays #9. I started teardown again, I was going to remove the touch panel to see if there was some sort of short across the 4-5, and 9-0, but it's stuck down so I stopped. I don't see how any droplets could have gotten onto the panel, it's stuck on pretty well.Video #2https://youtu.be/LjiOcNa_TLkAny ideas on what to do from here ?

author: inse
author_rank: Super Contributor
timestamp: March 06, 2022, 10:10:01 pm
content: It's difficult to give an advice here, as with moisture you can have created a multitude of problems.I don't think the relay has been damaged by moisture as it looks like it is a sealed type.Maybe it has seen some wear by the anomal activation frequency.I would suggest to disconnect the MOT and the heater for initial testing once the control panel has been verified dry.Heat it with a hair dryer from behind and see if there is condensation on the display window (just an idea in case you cannot open it)And remove any residue on the PCB as it might develop a conductive path.Then you should be able to verify the correct activation by listening to the relay clicking according your selection.The 'smoke traces' at the zener diode most probably have developed during normal operation as this may get warm.Dont forget rule #1: check supply voltages.

author: LeWidget
author_rank: Regular Contributor
timestamp: March 07, 2022, 07:37:17 am
content: Hi Inse, thanks for the feedback,Quote from: inse on March 06, 2022, 10:10:01 pmUnderstandable. When I inspected, there was very little on the brown side of the PCB. I have a feeling its shorted something, but could only see water on the black relays, no where else, and the only signs of damage are either the heat damage near the diodes/resistors, which you and others mention is likely due to normal operation, the the burn on the white relay (which I'm convinced is the source of the smoke).As for the touch pad, I have no idea. I suppose it could have been a fault from an unrelated issue some time ago as I see no possible way anything would have gotten in there (recently).Quote from: inse on March 06, 2022, 10:10:01 pmWhat is MOT? Convec motor? After disconnecting, should I connect the PCB back into the microwave with all power inputs and go from there ?I ran the heat gun over the board for a little (board got warm), but didn't see any condensation on the display screen I inspected the board and can't see any moisture/water. I also gave it a little scrub with some IPA.Quote from: inse on March 06, 2022, 10:10:01 pmLooks like this will have to be the next step as I can't think of anything else to do (besides remove the adhesive touch panel to access the touch pcb).

author: inse
author_rank: Super Contributor
timestamp: March 07, 2022, 09:56:57 am
content: MOT = microwave oven transformer

author: LeWidget
author_rank: Regular Contributor
timestamp: March 11, 2022, 11:49:53 am
content: Just an update: It seems Microwave is good now. Not sure what the deal was with the key pad, perhaps a connection issue with the ribbon cable. I put it back together and buttons seem fine. I preheat the convec oven and it heated up. SO far so good.Thanks for the feedback inse.