Kermi IRT 10 Towel Dryer Infrared Thermostat

[h0m3w0rk]

Hi everyone,

I have a Kermi infrared wireless room thermostat that simply does not function as it should.

The display turns on sporadically or remains completely off. When the display is on, I can use the buttons (rubber domes) and navigate through the menus. Sometimes the display shows plausible text but does not respond to button presses. In other cases, the device is entirely inactive.

After detailed measurements, I found 5V almost everywhere across the board, which indicates a stable power supply. However, the mentioned issues persist.

Based on the device's behavior, I suspect a power supply issue but cannot pinpoint it precisely. I am seeking help.

Steps taken so far:

• Power supply check: I tested the device using my lab power supply.
• Observation during power cycling: If I quickly turn the device off and back on (after the voltage has dropped to 0V), the lab power supply sometimes indicates a short circuit momentarily. It is unclear if the power supply is overly sensitive or if there is a real problem.
• Capacitors replaced: I replaced the electrolytic capacitor on the board, but this did not improve the situation.
• Both Quarzes are offering stable frequencies Q1 and Q2, Q3 and Q4 are for the DCF Transmitter

Attached is an image of the board.

The following chips can be seen on it:

• Power supply: MAX857
• Microcontroller: TMP87PH47U
• EEPROM: 24LC01B
• For wireless functionality: U4224B

The cables leaving the board connect to another board where the display and associated chips are located. These include:

• HD44780U
• 2x MSM5259
• Two unidentified ICs without markings.

A 4-channel SDS804x HD is available for analysis.


I appreciate any constructive help!

[CapLeaker]

Is the voltage to the LCD gone too?
I would be poking around with the oscilloscope. Also see if heating or cooling (can of air upside down) makes a difference.

I had a 8 bay AAA up to D and 9V charger. It did the same thing this is doing. Turn it on and it works for a bit, then clunks out. A while later comes back on, then back off… wash rinse, repeat. I checked about everything I could think of and it turned out to be the micro controller ic. As my controller was an STM32 based, I just gave it the boot. The way it behaves, sounds suspiciously being an IC to me.

[CapLeaker]

Either it is some IC or something thermal.

[h0m3w0rk]

Hello, thank you for your concern. Unfortunately, it turned out that the issue is with the microcontroller. Using the oscilloscope, it was clear that the power supply was stable, but the IC repeatedly failed to drive the crystal oscillator q2 which is directly connected to the IC
I conducted multi-channel measurements with the oscilloscope.

Thank alot!