Air conditioner indoor unit PCB problem

[shamooooot]

Hello guys

I have a Chigo AC and it is giving E1 error which on the label (Communication between indoor and outdoor unit problem). I noticed a smell coming from the indoor unit and after opening it I noticed the PCB had a toasted resistor which is on the Signal/comunication line.

I replaced the two components (the resistor and the high voltage cap next to it (just in case)) and inspected the outdoor unit PCB which appears to be perfectly fine.

Ran it several times, at the initial attempts no overheat was noticed but after trying couple of times the new resistor overheated  again and all the parts in the resistor circuits appear to be fine.

Here's the photo of the newly installed components:

[Swake]

Is the resistor you write about the grey blob in the left-middle?
Do you have a schematic? Or at least the chip id's and their datasheets with hopefully a reference design or application note?
Did you do any other checks or measurements? Voltages and looking for shorts for example.

[ace1903]

Is that black wire going to that burnt resistor and to optocoupler only?
If that's the case I think that the problem is on the other board.
Probably due to failed capacitor supply voltage has excessive ripple and dissipates to much power in that resistor.
There is diode under resistor and that can be fried too effectively doubling power dissipation.
It is cheap to replace it and the optocoupler also.
Probably it will be difficult to take picture from outdoor unit PCB and to post here.
   

[shamooooot]

This is honestly very confusing, tracing the S line between both ends (at the optocouplers on both PCB's) everything tests fine!

I've noticed on the outdoor PCB there was accumulation of shellac specifically on the optocouplers (IC37 & IC38) and I replaced them, but nothing changed.

[NiHaoMike]

With the interconnect cable disconnected at both ends, check that none of the wires are shorted to each other or to ground. Ideally, use a hipot tester as faults may only appear at high voltage.

[ace1903]

Only logic I can think of is that that red capacitor forms auxiliary power supply.
Then two optocouplers are wired in "wired or" configuration  and two optocouplers are used as sense.
It is strange that red box that I think it is capacitor is marked as ZR1 on the board(as far as I can see).
Can you look at more detail what it is?
Could you please provide more close up picture of that part of the board?

[shamooooot]

Thank you for the reply guys.

I don't see any shorted wires on both PCB's..

The part is 14d681k-k which is Metal-Oxide Varistor.