Meaco Arete One / Deye Dehumidifier not working

[CG2]

I've got a dehumidifier that has stopped working. Plug it in, press the power switch, and it beeps 5 times. Nothing showing in the display (which is just two 7 segment displays) apart from the two decimal points flashing. The model is a MeacoDry Arete One 18L, which seems to be a rebadged Deye (all the boards are marked Deye). Here's what I've got so far (details in case anyone else in future should need them).

Starting with the power board. Mains power comes in at the red connector top left. The compressor is powered by the orange connector to the right of that and is switched by the relay just below. There seems to be no feedback circuitry. The large connector to the right of that controls the fan. Three of the pins are designed for running the fan at different speeds, High, Medium and Low, and each of those are relay controlled (the relays below the blue capacitor). Only two are used on this model, the High and Low. Once again there is no feedback circuitry. The white 5-pin connector to the right of the blue capacitor labelled "S Motor" is unused on this model. I suspect it controls a swinging vent on higher end models. The red connector to the right of that goes to a temperature sensor on the coils. Holding the sensor changes the resistance. The white connector to the right of that leads to a reed switch that detects whether the water tank is full. That switch tests fine. When the tank is empty it reads open, waving a magnet nearby changes that to short. The large connector on the very right goes to the display board. The red connector below that is not used on this model. The black connector below that is an auxiliary power connector for an optional extra. Meaco sell a small pump that sits in the water tank. When the tank full sensor is triggered, that connector is activated for a short period of time (and it's purely time limited) so the pump can empty some of the water in the tank to wherever you have run the pump outlet. I don't have that pump. The rear of the board is covered with a thick conformal coating and no components seem damaged.

Moving to the display board, there is little here to go wrong. The front has four buttons that all test OK, plus a few leds. The rear has a TM1638 7 segment display driver. There are two connectors. The large one goes to the power board, while the small one leads to a daughter board holding a humidity sensor and a temperature sensor. Traces from this connector lead straight to the larget connector.

Sensor board. Extremely minimal. Four wires lead to two individual sensors. Two to a temperature sensor and two to a humidity sensor. The temp sensor seems OK. Resistance around 5k, changes when touched with a finger. The humidity sensor is where I suspect the problem is, but I've no idea how it works nor how to test it. Measuring the resistance while breathing on it shows no change. Is it broken or just not a resistive sensor? Searching for similar looking sensors brings up nothing useful. The results are all swamped by the three or more wire sensors that do some sort of processing on board and report a calculated result. This is just a two wire sensor.

But I can't see what else it could be beyond that. There is surprisingly little feedback to the processor. There is a stable 5 volts being produced. The 5 beeps on start-up suggests that the microcontroller is working. Does anyone recognise that humidity sensor, or at least have some suggestions on how to bypass it? Turning the unit on with the humidity and temperature sensor board disconnected gives the same 5 beeps then nothing. Any suggestions gratefully received.

Edit: Oh, the vagaries of search engines. Googling "Meaco 5 beeps" returned nothing helpful. "Meaco five beeps" leads straight to a document on the manufacturer's homepage that tells me it is due to the water tank being full. It's not, and the reed switch is fine, so time to start checking SM resistors.Time for my pointiest probes.

[CG2]

Too late to edit, but it looks like that was a red herring. I'm getting a rock solid 5 volts across the pins where the water level connector plugs in. Leaving the sensor unplugged gives the 5 beeps at start up. Shorting the pins together to mimic the switch being triggered gets rid of that, but the dehumidifier still doesn't start up. Now there's still nothing on the display but no beeps to give a clue as to what is happening. I'm puzzled.

[R_J]

Do you have any a/c voltage going to the compressor? Can you tell if relay K1 clicks? Also start by resoldering the connections on the plugs, CN13 looks to be bad
The humidity sensor could be like a CJ-HR31

[CG2]

I don't think it's ever getting to the point of triggering the relay. I'm not hearing any clicks. According to the manual:

When the dehumidifier is turned on the reading on the display will show the current humidity level in the environment the dehumidifier is being used in. Press the smart humidity mode button once to see the target humidity level – the display will change for 5 seconds and then change back to the current humidity level.

But my display is blank. Pressing any of the buttons just lights up one of the decimal points on the 7-segment display. I don't think I've ever seen a segment light up.

The temperature sensor is driven by a solid 5v dc. It shows a resistance of 4.5k which drops to 4.2K when I touch it. That all seems fine.

The supply for the humidity sensor is more complicated. It is a square wave of +5V / -5V at a frequency of about 1KHz. That in turn switches on for a second then off for a second. The fact that such a complicated wave is being produced suggests again that the processor is working. Here are some pictures for completion:

I can't get any resistance readings using a multimeter, although it seems that that should be possible. I can't find a datasheet for the CJ-HR31. I've found one for the similar HR202L (attached) which suggest that resistance should be somewhere around 5-20k, so maybe this sensor is damaged. The HR202L is rated to be run at 1.5V ac at 500-2000Hz. A website selling the CJ-HR31 states the same 1.5V ac, 500-2000Hz parameters so it could be that this is the source of the problem. The manufacturers are instead switching a 5v supply on and off regularly and hoping that this doesn't damage the sensor. But then according to the manual:there should be an error code in the display but it continues working anyway:

E2 Error Code - The humidity sensor is broken. The unit still dehumidifiers but in continuous mode only.

I'm working through Aliexpress listings trying to find a sensor that is marked with a "3" like mine, but I might just buy a few different ones anyway and see what happens.

Thanks for the pointer.

[R_J]

The khz signal is applied to the sensor and humidity will change its impedance, it is not a dc resistive element

Yours is a SHINYEI resistive type humidity sensor CL-M53R or C15-M53R (with case)

[redwire]

Reading 4.5kΩ is way low, like it got wet or is dead.
I would just swap it out with a resistor like 100kΩ to spoof 50% RH.

[R_J]

The temperature sensor is driven by a solid 5v dc. It shows a resistance of 4.5k which drops to 4.2K when I touch it. That all seems fine.

4.5KΩ is what he measured on the thermistor for temperature not the humidity sensor

[redwire]

I would guess a sensor is out of bounds for the control board to go into lockout.

The capacitor rework soldering looks not great though, OP I would check that. CD2 soldering looks terrible, CD7, CD8 redo. You need more flux and heat in your technique, decent solder. Check solder ball at pin 12 U3, I would poke that out.
I'd try spoof a valid RH with a resistor. Or lift a leg and see if that changes its behaviour, open circuit is very dry.

[CG2]

My first thought was bad caps on the power supply, so I pulled the CD6 and CD7 to check ESR, but they were fine. I then pulled CD2 as that looked the most likely to be in a hot spot, but again it was fine.Didn't have the flux i8mmediately to hand when I put them back, but they worked OK, so reworking went on the list of things to do later.

I've put in a 12K resistor instead of the humidity sensor, but still nothing so that may well be OK.

Next thought was maybe the TM1638 display driver chip wasn't working. There must be a reason the display is not showing anything and none of the LEDs ever light. But pressing the buttons causes some beeps. Well pressing the power button after plugging it in gives a beep. Not the button down beep, button up no beep that suggests a direct connection between button and beeper, but a short programmed beep. And the data path for that is button pressed triggers TM1638 which sends a digital signal down the DIO wire to the processor which then in turn switches on the beeper for a short period. So the TM1638 is probably OK. Scoping that and the CLK line show clear 5V digital signals running at about 4KHz.

There's not much else on the bacl of the display board. There's a bit of mysterious circuitry in the top left of the photo. There's space for a big chip which they didn't put in, but they seem to have put all the associated caps and resistors in anyway.

The 12V line runs a bit high at 13.7V but it's only used for the beeper and that works fine. It's hard to identify the chips on the main board due to the thick conformal coat, but Q3 on the power board next to connector CN15 looks to be the 12V regulator. It seems to power the beeper, the optional pump and unused connector CN8. It also provides power to the relays which switch the compressor and fan on. The relays all click when tested with a bench PSU, so I don't think this is the problem. It certainy wouldn't explain why I'm getting nothing showing on the display.

The 5V is fine so no problems on that side.

I can't read any markings on the microprocessor, but it's doing enough to suggest that it is running.

The big chip to the side of that is a ULN2003CB Darlington transistor array. Three lines from that switch the relays that control the compressor and fan, and the other four go to the unused connector CN8, probably to control a stepper motor remotely moving an adjustable flap on other models. Nothing here would stop the display lighting.

The one chip I can't identify is the 8 pin chip in the bottom right.

Right now I'm stumped

[redwire]

Q3+Q2 switches power to the pump motor connector CN15, not a voltage regulator. This PSU is likely regulated 5V (by Z1/opto U2) and another winding provides around 12V for the relays and lights. I'm assuming R69 is the regulated 5V rail.
U5, the SOIC-8 at the corner. 4 pins tied together to GND... my guess is I2C EEPROM with the options config etc. stored in it. This control board is used in many models. It might have lost it's EEPROM?

Otherwise I would look at a list of inputs the board has and see if they looking to make sense. Analog sensors for humidity and temperature you checked, tank full switch - is there anything else? Stuck pushbuttons?
Or the board had feedback on its outputs and is seeing something fishy.

What is CN16 for?

[CG2]

CN16 isn't used on this model.

But I've worked it out, and it's a pretty obscure fault. It was related to the display, though not as I expected. I buzzed out all the connections from the TM1638 just trying to understand it better. There was a short between two pins which both went to the 7 segment display. I took the display off and the short went away. I powered the board up on my workbench with the temp sensors connected, the water level shorted and a 12k resistor in place of the humidity sensor. Pressed the power button and a couplr LEDs lit up and the relays clicked on. I'm surprised that a short there stopped everything else, but there it is. Now to go and read up on 10 pin, 2 digit, 7 segment displays. I hope there is an industry standard pinout.

[redwire]

I've heard of some of the chinese LED displays having problems with the internal bus. I guess it would look like a stuck/shorted pushbutton to the MCU.
2-digit common pinout is this, confirm common anode or cathode, pic is CC.

[CG2]

The display is marked MLS-4201ASC1-B7 5-DY 2242. Searching for that gives no results. There is a 4201AS 2 digit 7 segment display which is the right size (0.4 in) but it is red rather than white. It is CC and comes in two types. Type I has pins 5 and 10 as the cathodes, type II pins 4 and 5. There is no suggestion of how they are marked to differentiate them.

Trying to test the broken one with a multimeter wasn't helpful. Using a bench supply suggests that it is common cathode, with the cathodes being pins 3 and 8. I've ordered some of the cheapest likely candidates from Aliexpress on the grounds that this is probably what the manufacturer did. It works quite happily without a display, but I don't really want to have to memorise button press sequences and hope I got it right.

[dizzycap]

The display is marked MLS-4201ASC1-B7 5-DY 2242. Searching for that gives no results. There is a 4201AS 2 digit 7 segment display which is the right size (0.4 in) but it is red rather than white. It is CC and comes in two types. Type I has pins 5 and 10 as the cathodes, type II pins 4 and 5. There is no suggestion of how they are marked to differentiate them.

Trying to test the broken one with a multimeter wasn't helpful. Using a bench supply suggests that it is common cathode, with the cathodes being pins 3 and 8. I've ordered some of the cheapest likely candidates from Aliexpress on the grounds that this is probably what the manufacturer did. It works quite happily without a display, but I don't really want to have to memorise button press sequences and hope I got it right.

Did you fix the problem? I have the same Meaco and it beeps 5 times, I have removed my display and thee unit powers ok. My seems to be Type I, on my scope there is no signals on 5 and 10 pins, where did you order the display?
Thanks

[CG2]

I ordered the cheapest white 2 digit CC display I saw on Ali. I tried to be sensible and go through datasheets, but half the sellers didn't have any, and those that did didn't seem to match up to my dead display. I decided that I was overthinking things, did what the manufacturers probably did, and went for the cheapest. I put one in, and it's been working fine ever since.

The five beeps are because you've taken the water tank out and the unit can detect it's missing. That got me at first too, then I realised it was a good sign and meant that the MCU was still working. It's astonishing that an engineering decision to save a few cents by multiplexing buttons and LEDs with the help of a couple of diodes can lead to a trivial fault like this killing the whole machine.

[dizzycap]

I ordered the cheapest white 2 digit CC display I saw on Ali. I tried to be sensible and go through datasheets, but half the sellers didn't have any, and those that did didn't seem to match up to my dead display. I decided that I was overthinking things, did what the manufacturers probably did, and went for the cheapest. I put one in, and it's been working fine ever since.

The five beeps are because you've taken the water tank out and the unit can detect it's missing. That got me at first too, then I realised it was a good sign and meant that the MCU was still working. It's astonishing that an engineering decision to save a few cents by multiplexing buttons and LEDs with the help of a couple of diodes can lead to a trivial fault like this killing the whole machine.

Thanks for the reply, ive ordered the display from ali, hopefully it does the trick. yea its crazy they would cheap out on these. Ah the 5 beeps got it now, im still learning all this stuff its fascinating