[andreamazzario]

I have a Thermador CM301 oven, and the microwave function stopped working. Actually, all the secondary functions operate correctly (door latch, fan, timer, etc.,) but the food does not get heated.
The oven is 5 years old, but the microwave function has only been used rarely, so I would be surprised if it would be a magnetron failure, but I am writing here to get some expert advice!
I verified that the switch connected to the door latch works properly (after the door latches, power reaches the Microwave PC board). But I do not get any power on the lead exiting the Microwave PC board, so I strongly suspect the Microwave PC board.
An additional hint that the problem could actually be the Microwave PC board is that when the microwave was in partial mode I could hear by the noise that the magnetron was going on/off, now I can no longer discern that.
Looking at the schematics of the CM301, it looks like the only circuit elements between the input and output of the Microwave PC board are a switch and a DIAC thyristor, so I have been tempted to just bypass the Microwave PC board for a test, but I am not sure that is safe to the downstream components of the oven, so I am holding off on that.
Bottom line, it seems to me that I need to replace the Microwave PC board (http://www.appliancepartspros.com/partsearch/model.aspx?model_id=232522) which is actually simple enough to do, but I would like some expert advice before ordering the replacement.
Thanks
Andrea

[Servicer]

Hello,
I just found this and you may have already fixed the problem, but just in case, I'll let you know how it works.

As you know, the power comes into the MW board on P2-1 and only transfers to p2-2 when the MW board receives the correct input voltage from the main relay board. These voltages can be read on the harness that feeds P1 and are as follows:
1-4 = 18-24VDC
3-4 = 5VDC

If these voltages are present and the MW board does not pass the voltage, the problem is the MW board. If the DC voltages are not present the problem is in the relay board.

Hope this helps,

KC

[andreamazzario]

Hi,
first of all, thank you very much for replying to my question. No, I have not yet solved my problem, I was always hoping to get a reply like yours!
I have performed the measurements you recommended, and I get some measurements that are NOT within your ranges for the P1 feeds:

Feeds 1-4
Should be: 18-24
Measured, MW Off: 24-15.3 starts at 24 then goes down to 15, but after a few seconds again up to 24, then down again. Not very stable, also if disconnected from the MW board.
Measured, MW On: 10.2

Feeds 3-4
Should be: 5
Measured, MW Off: 0
Measured, MW On: 13.6

How should I interpret these voltages? It seems that the Relay Board tries to apply them, but why are they out of spec? Is it because the relay board is not working properly, or is it because the MW board does have the proper loads, which cause the measured voltages to be out specs?
Also, when the power between 3-4 is applied I can hear a "click" from the MW board, which I assume is the K1 switch closing. Similarly, when the power between 3-4 is removed, another "click" from the MW board, probably K1 opening. But no power on P2-2 at any time.
Since I hear this click, I would assume that the Relay Board generates acceptable voltages, so the problem may be in the MW board? What do you suggest?

Than you again for your help, I am looking forward to your reply!
Andrea

[andreamazzario]

I just wanted to conclude this thread by letting anybody interested know that I replaced the MW board (part # AP2828582) and that solved the Microwave problem.