Maytag CMV1000ADW Microwave Control Board Repair Attempt

[wickline]

I'm no electronics expert but rather a DIY hack trying to learn and fix a problem I caused on a microwave. I've made some progress but certainly not success as it appears I have a short or other failure on the board causing the microwave to malfunction.

Background is that the under cabinet light bulbs were missing and I failed to unplug the microwave before installing new bulbs (wrong length bulbs, tried to remove the socket) and inadvertently caused a short circuit. The microwave appeared to be functioning properly other than lack of bulbs before this incident.

I took the control board out and could see that the short circuit had burned a couple of traces on the pcb which I have repaired with jumper splices. I also tested all the capacitors (EC1 thru EC6) with an ESR meter and replaced all that measured bad. I used a 12V battery to test relays 2, 3, 4, 5, & 6 (p/n OJ-SS-124LM) and verified that all were functioning (fired & continuity checked). Relay 7 would not fire at 12V so I removed it from the board and was able to get it to actuate with an 18V power supply (haven't reinstalled it since). I also checked coil resistance on relays 2 thru 6 and all appear to be within spec tolerances (spec sheet attached).

Prior to removing relay 7 I reinstalled the board in the microwave the under cabinet lights, internal cabinet light, and exhaust fan all come on but it is now not possible to turn any of these off from the keypad (hence the short circuit or component failure I'm trying to locate). Exhaust fan and light are also on regardless of whether the door is open or closed (door latch has several microswitches that should affect on/off condition of internal cabinet light, magnetron, etc.). I have verified that all microswitches for the door latch are working properly.

I will upload a copy of the wiring diagram that comes with the unit but I don't have a schematic for the board. I'll also attach pictures of the board. When removing Relay 7 I noticed part of the covering of diode D15 had flaked off. I have no idea what rating any of the diodes are, should they need replacement I'd need help determining what spec part to use.

Thanks in advance for any assistance offered - if at the end I am unable to fix it there's no big loss. The microwave is about 20 years old, my primary interest is the learning experience of troubleshooting a board.

 

[unclejed613]

have you checked the thermal switches for continuity?

 

[cowboybob]

Welcome to ETO, wickline!

Knowing no more than we do, and since you now have a device that is unresponsive to keypad input, my guess is that microprocessor is fried.

 

[wickline]

unclejed613 - the thermal switch between the yellow and pair of white wires reads open while those between red/red and between red black have continuity.

cowboybob - thanks for the welcome! It is my fear that you're right. If so, then lesson over...

 

[rjenkinsgb]

It looks like one area of the board has been getting very hot, it's rather well scorched - between & above the CPU and the blue connector.

I'd test / replace the components in that area as a first move.

 

[wickline]

rjenkingsgb - I agree it is a highly suspect area. I tested (and replaced where faulty) relays and capacitors because I could determine specifications for those. I would need someone to tell me what the ratings are for each of those transistors, axial components (capacitors, diode, Zener diodes, and resistors). For the axial components I have difficulty determining which color is which and likewise which end to start reading the value from. If you or someone else is able to assist with that (see attached pic) my education would continue...

 

[wickline]

I believe I actually found the schematic and parts list for this board in a service manual (see upload of those pages)!! Anyone up for helping me troubleshoot the underlying cause of the fault with this? I could definitely use the help as I am a novice at best. I'm not really sure where to start next. The symptoms are that interior cabinet light, turn table, fans, and under cabinet lights all come on as soon as the unit is plugged in and the display shows 1:00. These symptoms continue whether the doors is open or closed. I have verified all door switches and TCO's in the cabinet are good. I have also verified the keypad is good by doing a pin to pin. The cancel button has no effect on shutting oven components off but all pin to pin checks are good.

Since my last post I did pull components from the area of the board that obviously overheated and tested each - all passed and were reinstalled. I did replace the three axial capacitors in that area.

 

[wickline]

I finally finished this repair. The end of the story was that there were about 4 bad KRC106M transistors on the board wherein the internal bias resistors were blown. Once this was isolated as a problem I replaced them with equivalent FJN3314 transistors purchased on eBay (thanks to member ronsimpson for the help identifying this part number). After these arrived and were installed on the board the microwave works fine. It was a fun learning experience and as an aside I have a functioning microwave for much less invested than the cost of a replacement control board. Thanks to all who contributed to this success!!