Heat Pump LCD controller problem

[ab86uk]

I've had this Air Source heat pump for nearly 3 years, though it was manufactured in 2010 (and I think sat in a garage somewhere waiting to be bought). It's a no brand cheapo chinese model (ESDIV-9).

I've had nothing but problems. After about 3 months the starter capacitor for the fan motor burst and died - easy fix. After about 9 months I ... ummmm.... let rain get into it when I didn't screw the cover panel on properly ... this shorted a transformer and killed that dead. I couldn't find a like for like replacement so I re-wound it and fixed it. (also replaced a suspect SMPS IC which got wet).

The latest problem ... the LCD controller no longer controls. Instead all the segments come on and after 20 seconds or so the power light flashes, I think it's trying to tell me it has no signal to the control board of the heat pump .... or it's in it's death throes. The heat pump itself is working fine, but I can't adjust any settings or get status info.

So I've taken a controller board out of the heat pump (which the LCD controller connects to via COMM2) and had a poke around. I don't fully understand what every part is doing. I've attached a couple photos of the board so you can follow along.

I've identified a bad optocoupler (PC2). When the LED side was powered with 10mA the transistor still had a resistance in the kOhms. It's a Sharp PC817 but I had a Lite-On LTV-816 which as far as I can tell is exactly the same .... but the LCD controller still doesn't work. 

I've done some further investigating and I believe the communication protocol is something like UART... I think that because the LCD is powered by and communicated with via the 2 lines in the COMM2 port. That must be power and ground, with the transmit and receive over that power line. There is also no indication of a clock signal being sent, so ... i dunno, draw your own conclusions. I don't have an oscilloscope by the way.

Anyway, from that I've decided that the optocoupler I replaced (PC2) is the receive line for the microcontroller on the control board. Following that trace back to the microcontroller, one of its neighbours follows back to the same area and into a series of 3 SMD transistors (Q4, Q3 and Q6). Q3 and Q6 are PNP S8550's and Q4 is an NPN S9013. Q3 seems to be shorted through the collector and emitter. Could this be the transmit line for the microcontroller to the LCD controller, modulating the power to the LCD controller?

Is this a normal arrangement? Surely it would have been easier to put in another couple of lines for separate comms and power to the LCD controller?!

I've ordered some replacement SMD transistors which should arrive in a day or so.

Any thoughts?

Alex

(P.S, very new to this forum but seen EEVblog on youtube many times)

[Zucca]

Hi ab86uk and welcome!

Well it´s time to buy a scope from TopLoser. Since you are in UK, there is this one guy who is a legend here who sell good stuff at very low prices...
Jump to the Buy/Sell section and keep your eyes open, you could also send him a PM just in case...

I means 70GPB for a 50MHz digital scope new is very cool, who knows what he has in storage right now.

Since that product is just a problem after the other, I think it is just a bad design. Before you invest more time and money in this rabbit hole, just think twice and maybe consider to toss everything and buy a proper one.

Now since here we looovve to fix stuff back to your issues: what is happening is very obscure, can you post some LCD module pictures too?

[ab86uk]

Hi Zucca,

Thanks for the scope advice. I'll look into those suggestions. I definitely think one would be worth the investment.

I agree, this heat pump is becoming more trouble than it's worth. Having said that, it was my mistake that caused one of the issues I've had. Maybe if it breaks again I'll draw a line under it and get a better one.

I'll post some pictures of the LCD controller when I get home tonight.

Thanks for your comments.

Alex

[ab86uk]

Hi.

Pics of the LCD controller attached.

The good news is the transistors arrived today. I removed 2 PNP transistors. I re-tested the NPN transistor in circuit while the  others were off and got good values so I've left that one as is.

One of the two PNP transistors still tested bad out of circuit but I replaced both with fresh ones since they were off.

Great news: It all works now! Job done. Fixed!

The soldering even looks neat and tidy despite being SMD components at the limit of my visual threshold! It did take me over an hour to solder those two things back on though.

[Zucca]

 

Congrats... it looks like the Comm line are AC which delivers the DC power to the LDC after a diode bridge DB201 and C1.
Wired, maybe so they can place the LCD Module very fair away from the furnace and still communicate with it. May be a differential communication protocol which is used as an AC power supply....