Wolf 24MWC Microwave Convection Oven (Help Analyzing a Circuit) - Page 1
Forum section: electronics / repair

author: edy
author_rank: Super Contributor
timestamp: October 28, 2022, 09:21:42 pm
content: I've been having some issues with my microwave convection oven (Wolf Subzero 24MWC) and managed to repair it but I'm not sure what I did and need help figuring out what happened as it may occur again! I've attached the service manual:https://service.subzero.com/KnowledgeBase/DownloadExternal?id=137&an=14050001(for MC24)Please look at the wiring diagram on the last page as I believe it's diagnosable there.Basically, the appliance powers on and control board works fine. You can start a cooking function (microwave, convection bake, etc) and it turns on the light and fan and the turntable inside turns, just like normal. It looks like it's functioning except one thing... no heat! There is no voltage going to either the heating element (for convection) or the transformer (for magnetron microwaving). I tested all continuity, voltages, relays on the circuit board turn on/off when they are supposed to, thermal cutouts all good, etc.The relays (RY3, RY2 and RY1) on the main control board all work fine as I tested continuity across each relay when I tried different cooking functions. So the control board is properly activating those circuits. The RY1 is getting good voltage as I have a working turntable motor, lightbulb and fan... so BLK (Common) to RED/PNK (N.O.) at RY1 shows 120VAC difference. However, when I tested RY3 and RY2 commons (BRN) (which are actually connected to each other at the junction nearby) to the BLK Common (of RY1) I get very low voltage, meaning it's not getting voltage at RY3/RY2 Commons.If you trace the RED and GRY of the NOISE FILTER/FUSE from the main power input (bottom right corner) you'll see that the RED wire goes through all the thermal cutoffs, and winds up being the COMMON BLK at RY1. However, the GRY of the NOISE FILTER/FUSE goes through a bunch of door switches and eventually makes it through to the COMMON BRN at RY3 and RY2. So I would expect when my doors are all shut that we would see 120VAC across from the BRN at RY3 /RY2 and BLK at RY1, but was only getting about 6VAC, which explains why it didn't matter that the relays were closed, I would not have enough voltage to drive the heating element for convection or magnetron.I figured that the issue had to do with the door switches/interlocks and 3 of them are "normal open" (primary interlock switch, door sensing switch and third door switch) and 1 of them is "normal open" (monitor switch). I fooled around with those and then ended up getting 120VAC at the BRN contacts at RY3 and RY2! I'll keep trying to analyze the circuit but I'm curious to pinpoint the exact way it killed voltage to RY3/RY2 and how (if it is true) it did it. Note that when the door is closed, ALL 3 of the switches close the circuit but the "Monitor Switch" opens (which seems to help turn off the light). It is just confusing.

author: edy
author_rank: Super Contributor
timestamp: October 30, 2022, 03:46:41 am
content: I've attached the wiring diagram as a PDF in case there is trouble getting it from the link above. Thanks for any help figuring this out.This link in the previous post is for 24MC, slightly different from the link here below for 24MWC:https://service.subzero.com/KnowledgeBase/DownloadExternal?id=144&an=14050001Either way I think the wiring diagram (attached PDF) gives the answer to the symptoms I described.

author: amyk
author_rank: Super Contributor
timestamp: October 30, 2022, 04:15:15 am
content: Microwaves usually have both a normally open and normally closed interlock for redundancy. The normally closed one usually shorts out the supply and blows a fuse or circuit breaker if it isn't actuated.

author: edy
author_rank: Super Contributor
timestamp: October 30, 2022, 04:48:13 am
content: Thanks. The behaviour experienced was that everything was operational except no voltage going to heating element (RY3) when using a convection oven, or no voltage going to the transformer (RY2) to power magnetron when using microwave. Otherwise you would never suspect anything wrong, it ran like normal, bulb, turntable, fan, everything working like you would expect. My wife put something to bake and noticed after a few minutes that it wasn’t getting hot… that’s how we found out something was wrong.I’m trying to figure out what in the wiring diagram circuit would basically allow everything to work except just kill voltage to either the heater coil (when running a convection bake) or kill voltage to microwave transformer (when microwaving). Something that would otherwise have no other effect on the appliance.As I mentioned, it now works again after I took it apart and jiggled all the connections as I was checking them, testing continuities and voltages, had disassembled the front panel with the door switches and mechanically had things apart. Then at some point it worked again as I was doing some more testing for voltages. I don’t know what I did, I’m trying based on that wire diagram to pinpoint it. I suspect it was something with the door because a lot of safety “kill” depends on all 4 of those switches sensing door closure. Which one of those if not in the proper state (assuming other 3 are) to indicate door closure would inactivate the voltage to the RY3/RY2 yet have no effect on anything else? Or could be a combination of them, or am I not looking in the right direction even?