Bosch HMC80251UC/01 Internal Fuse Blowing

[mala]

Model Number

HMC80251UC/01

Brand

Bosch

Age

1-5 years

I have a HMC80251UC/01 with a dead display. I removed it from the cabinet and checked the fuses. One of the two 250V/20A slow blow fuses was dead. I replaced it with a new 250V/20A slow blow fuse but as soon as soon as I connect the microwave to power the fuse blows again. So something else in the circuit seems to make it blow. Any help/pointers is appreciated.

 

[mala]

This is a hardwired microwave so I purchased a 230V connector. I plugged this into a dedicated 230V outlet with the breaker in the breaker box set to off. As soon as I turned the breaker on I saw a bright flash and the fuse was gone. So yes, the fuse blew on "plugging it in". I didn't touch the microwave at all. So it seems something is shortening the circuit without doing anything.

 

[Wx4usa]

After looking at the schematic, I would recommend checking the switches. It appears your switches are hot before the control board on your model and that could create a short circuit path through the primary and monitor switch even when the microwave isn’t set to run if your primary switch is bad. I would get the switch kit 12038730. I attached a picture of the marked up schematic.

 

[mala]

@Wx4usa I got the 2 switches and exchanged them. Now the fuse doesn't blow anymore. However, the display still doesn't come on. It's not just dim like some other people experience but it's completely gone.

Btw, I don't usually promote companies but I ordered this part on Tuesday afternoon after the shipping cut-off time and got it on Thursday morning. It was packed very well, too. I believe they are up north and I am on the west coast... Maybe they have a distribution center in our area but still, super fast. They also have a chat to easily communicate with them. Awesome!! https://partsdr.com/

 

[Wx4usa]

Any functions? Internal lights?
It appears that the control board gets power after the magnetron thermal fuse. You can check continuity two places and see if there is continuity. With power unplugged check for continuity between the L1 lug on the power cord and the primary interlock switch. Also check continuity between L1 on the power plug and CN1 Pin 1. Schematic says these are red wires. See attached image. If you have continuity, you should have power at the board. You can further validate by checking for 120v between cn1 pin 1 and GND. If you have 120 at the board, the board has failed. If no power or continuity from these 3 tests, it is likely the magnetron thermal fuse has failed open Or the fuse on L1 is bad.

 

[mala]

Nothing. I see 240VAC on the board but 0VDC on the capacitor. Fuses are working.

 

[Wx4usa]

Cn1 pin 1 on the board to ground should be 120v. Stay away from the capacitor, diode and magnetron, it’s very dangerous. 240v at the red thing you circled is normal.

 

[mala]

Ah, I have 0V on pin 1, pin 5 and pin 7. So it seems the magnetron thermal cut-out is off. I am not sure where that part is located and what the PN is. The leaflet doesn't say anything about it (it just mentions the magnetron itself).

 

[Wx4usa]

Can you confirm L1 lug on the power cord to cn1 pin 1 has continuity AND L1 on power plug to the primary interlock switch has continuity? If NO continuity, yes that confirms the thermal cutoff has failed open.

Here are the parts diagrams: Bosch Microwave HMC80251UC/01 Repair Parts

And here is the magnetron thermal cutoff and and image showing where it is located. It is located ON the magnetron: Regulator-Temperature 00631508

Make sure to discharge the capacitor and unplug the microwave before accessing: This is very dangerous.

Here is an image of what it looks like…
And also a photo of removing the magnetron.

 

[mala]

Interesting. I see 1MOhm between L1 and cn1 pin1/3/7. I see pretty much 0 Ohm between L1 and either of the two connectors of the magnetron thermal cutoff. So I guess that part is fine. I do see the 1MOhm between the magnetron thermal cutoff and cn1 pin 1/3/7. That's strange because according to the schematics there should be a direct wire connection between the magnetron thermal cut-out and cn1 pin 1.
Btw, thanks for the note about the capacitor, etc. I always unplug the microwave and shorten the capacitor except for the one time when I wanted to check if there is any DC voltage on the capacitor (there wasn't).

 

[Wx4usa]

Yes the schematic shows a direct connection between those. You may have an issue in the filter board. It may be worth pulling it out and inspecting the back side solder traces and look for bad solder joints. Unplug its connections and you will have 3 pass through connections with continuity. L1 in and L1 out. L2 in and L2 out. N in and N out. If those 3 check ok, then perhaps a bad wire or bad connector. Process of elimination. Sounds like L1 issue

 

[mala]

Well, I have continuity from L1 and through both door locks. I also have 120V from Cn1 pin 1 to ground. I have two displays and both of them stay blank. Now the board where L1 and L2 and N come in is interesting. There are two resistors on that board. The one on the right side measures 1MOhm as expected based on the color code. The one on the left measures close to 0 Ohm. The yellow part seems to be a capacitor (100nF 275V). The capacitor looks fine (not blown or anything like that) but who knows. It still doesn't explain why the screen stays blank since it does get 120V.

 

[Wx4usa]

As for the filter board, the one with the fuses and power input…. You just want to check for continuity three ways, between the three blue dots. If you have continuity, we can rule it out. It is simply a low pass filter that protects the home from getting stray RF in the power line to other home devices. It’s also where the fuses are. So leaving the fuses in, you should have continuity in all three scenarios.

 

[Wx4usa]

120v from CN1 Pin 1 to ground is good. So the main board is getting power. Without poking around on the board and doing testing, I believe the board is bad. That main board supplies power to the UI and display.

 

[mala]

It looks like I am not getting any power at the connector I marked up. Of maybe I just didn't get to the metal part. I am just somewhat reluctant to get a 3rd control board as it costs over $300. Kind of crazy after we paid that much for this microwave...

 

[Wx4usa]

How are you measuring it? What is your meter set on? Thats usually just a driver voltage. I’m afraid I wont be much help. The schematic I have shows 3 wires on CN4. I’m so sorry.

 

[mala]

I just have it set to measuring VDC. It's set to the volt range rather than mV.

 

[Wx4usa]

I wish I could help. It appears the board is bad. Without being present to test it at the board level, I am at a loss to help. I’m so sorry.

 

[KhunThai]

Hi Wx4usa, I have a similar issue issue with blowing fuse.
Bosch HMC80252UC/01 microwave
2 yrs old
While using the microwave it went to low power mode then lost all power.
I replaced the green switch as it was broken.
I replaced the blown fuse.
I replaced the temp sensor on the magnetron.
The wire connector on the convection element at the back of the unit was overheating as the insulator was fried and a part of the female connector. Cleaned and installed connector.
I did get power to the microwave, ran it for a few seconds and shut it off. When I opened the microwave door it blew the fuse and killed the green switch on the door. I have inspected all connections for shorting, no black or brown marks on connections. I am thinking there is a short somewhere. I did follow the previous post.
You know your stuff so any help would be greatly appreciated. Thank you.

 

[Wx4usa]

Hi there. If you will, start a new thread and I’ll jump in there and see what I can do to help.