Fixing a Remotely Controlled HVAC Unit Circuit Board (Essentially a Thermostat?) - Page 1
Forum section: electronics / repair

author: Curmudgeon10
author_rank: Contributor
timestamp: December 31, 2022, 07:35:52 pm
content: This might be a longshot…I have a ductless ("minisplit") HVAC unit in my shop. It is controlled via a hand held remote. The receiver for these remote commands is a small circuit board in the front cover of the unit with two sensor lamps. While cleaning the filters behind this cover a few weeks ago (cover raised up) the cover slipped from its mounting lugs and fell on the floor. In so doing, five small wires were pulled from the terminal block on the circuit board as the cover crashed down. These wires originate in the guts of the wall unit and it’s not clear I can access them there to determine their order. The order in which they must be reinserted in the terminal block on the circuit board in the cover is the current mystery.I can post pictures of everything if I can figure out how to do it on this forum. So far none of my tries have been successful --- I keep getting the message that the images are too big, but they are all about 2Mb.I'm competent enough to measure voltage and resistance --- and if instructed how, how to repin those wires in what is an awfully small block. The point now is to see if there is anyone out there with some experience in this area that might help me, through further posts and pictures, get this unit back in operation. A replacement has been quoted for $5K! Oh, BTW – none of the HVAC tradespeople I contacted wants anything to do with this project. They just want to sell me a new one.

author: james_s
author_rank: Super Contributor
timestamp: December 31, 2022, 08:08:12 pm
content: It would help if you posted the model number of your specific unit, somebody may have the same or similar unit. Does the owners or installation manual show any photos of the unit with the cover off? If you're getting errors about images being too large, use a program like Irfanview to resize them before uploading.

author: fzabkar
author_rank: Super Contributor
timestamp: December 31, 2022, 08:56:15 pm
content: Imgur is an alternative image hosting site.

author: Curmudgeon10
author_rank: Contributor
timestamp: January 01, 2023, 03:35:15 pm
content: Quote from: james_s on December 31, 2022, 08:08:12 pmI'll try answering the question about the unit with a photo. File Properties says the image is 3.86MB. As backup, in case thie .jpg doesn't load, the unit model number is Chigo QC1H18A/QCOH18A.First, to head off some other suggestions that might be offered, please be advised this is an old unit, probably dating from 2003. It was made in China, but the tradename and its products are still active in the United States. I have attempted to contact the manufacturer --- no replies after several attempts --- AND I've reached out to several stateside distributors of Chigo products, with no luck. I've even corresponded with some overseas distributors in South Africa, and one in Australia. Yes, there are two very brief schematics in the owner's manual, but neither deals with this part of the circuitry.I resized all images before posting to well below 5Mb.There are no photos in the owners manual with the cover "removed," as it is never intended for the cover to be removed during normal operations. I suppose it would be possible to do so by first removing the circuit board installed in the cover that receives the remote control signals, which would leave the wire connections to that board intact, just not sure why one would want to do that. Thanks for your interest.

author: Curmudgeon10
author_rank: Contributor
timestamp: January 01, 2023, 03:39:26 pm
content: I see my latest attempt at loading a photo was successful. I'm going to load some more. First, two pics showing the front and back of the circuit board. The "front" is the surface facing the user when they operate the remote.

author: Curmudgeon10
author_rank: Contributor
timestamp: January 01, 2023, 03:43:09 pm
content: The final photo is of the wires that have become disconnected. I'm pretty sure the red wire is power.

author: themadhippy
author_rank: Super Contributor
timestamp: January 01, 2023, 03:54:20 pm
content: with nothing more to go on id be tempted to go black brown red orange yellow

author: sokoloff
author_rank: Super Contributor
timestamp: January 01, 2023, 04:30:41 pm
content: With the wires disconnected, measure the DC voltages across pairs of them. I'd start with the red probe on the red, orange, and yellow (in turn) and the black probe on the black (first) and brown (second). The goal is to find a pair (hopefully exactly one) that has about +5V from one to the other.The negative of those will connect to the pin farthest from the CN4 silkscreen designation.The one that carries power will connect to the pin closest to the CN4 silkscreen.The next two farthest from the CN4 will be drive signals for the two LEDs (coming from the control unit to that satellite PCB), green being in the 4th position, and yellow in the 3rd position. (This might help you figure out which is which if you know what LED is lit [if any] when the unit is initially powered on.)The last [2nd from the CN4 designation] will be the IR signal back from this board to the main controller.The two LED drives probably have a current limiting resistor (that the positive supply will not have). If you have some spare discrete LEDs, you could connect LEDs across the various lines that show positive supply to the positive pin and put the negative pin to the negative wire and see which wire has the LEDs burning too brightly (and quickly burning out): that one will be power, going to the one closest to CN4. If the other one or two light the LED but don't burn it out, the worst that will happen is you'll get the LEDs reversed and have to fix that later.

author: Ian.M
author_rank: Super Contributor
timestamp: January 01, 2023, 04:46:59 pm
content: As a first step it would be worth trying to identify the ground wire by checking continuity to chassis ground (with the mains power off).Simple voltage measurements are likely to be ambiguous if the LEDs are driven from CMOS logic, as the outputs are typically rail to rail, and the IR signal wire may well have a pullup resistor to the +supply which will make it measure almost exactly the same voltage as the +supply wire. Repeating each measurement with a 10K resistor connected in parallel to the multimeter and noting the resulting voltage drop will rapidly show which wires are fed via resistors on the main board, so, by elimination, locate the power wire. Also the voltage drop will tell us the approximate resistor value on the main board, and therefore if that wire is probably safe to connect to the IR sensor.

author: Swake
author_rank: Frequent Contributor
timestamp: January 01, 2023, 06:21:49 pm
content: Found a pic with a similar IR / LED combo. Source:https://www.ptacsolutions.com/CHIGO-QUIETSIDE-TURBO-AIR-331400074-PRINTED-CIRCUIT-CONTROL-BOARD-With-2-LED-Lights-FOR-MODELS-QSHE-181-TAS18V-AND-OTHERS-Important-Confirm-number-of-LED-light-on-existing-before-ordering-_p_201232.html

author: Swake
author_rank: Frequent Contributor
timestamp: January 01, 2023, 06:33:47 pm
content: There are Chigo boards for sale On aliexpress. An example in this link, but there are more:https://www.aliexpress.com/i/1976370038.html?gatewayAdapt=glo2nldDifferent models probably, but an important constant: that connector is having the wires in the same order on both sides. This means that if you can find out the order on the 'unbroken' mainboard you have a solution.

author: sokoloff
author_rank: Super Contributor
timestamp: January 01, 2023, 06:38:30 pm
content: Impressive searching, Swake!

author: Ian.M
author_rank: Super Contributor
timestamp: January 01, 2023, 06:55:57 pm
content: The pin order at the main board end in Swake's photo goes yellow, red, ?, ?, ? so *if* its the same at each end, we can be fairly certain the red wire is *NOT* +Supply.

author: TheMG
author_rank: Frequent Contributor
timestamp: January 01, 2023, 07:45:22 pm
content: Looks pretty straightforward to figure out based on the components present on the board.One wire will be ground/common, to the negative side of that electrolytic capacitor.Another wire will be likely +5VDC, to the positive side of the capacitorAnother one will be data from the third pin of the infrared remote sensor which is not ground or VCC.That leaves only two wires, one for driving each of the LEDs.With a multimeter, you can identify the ground and 5V pretty easily and connect them to their respective terminal. Once you get those figured out, very likely the rest of the wires will go in the same color order as they are on the main board end. If not, then trial and error would work at that point (connect one at a time to the data pin of the IR sensor until the remote works).

author: fzabkar
author_rank: Super Contributor
timestamp: January 01, 2023, 08:36:38 pm
content: I would think that the IR sensor's output would be pulled up to Vcc. The LEDs appear to be grounded, so no pullups. However, that should still be useful because simply measuring the resistance between Vcc and each of the three signal pins should differentiate the LEDs from the IR sensor.

author: Curmudgeon10
author_rank: Contributor
timestamp: January 01, 2023, 09:20:51 pm
content: I want to say very much appreciate all the suggestions for troubleshooting! It will take me some time to digest. Practically speaking, I'd like to extend those wires via some temporary splices to I can troubleshoot on my workbench, several feet away. And how do I go about getting the bare wire ends back in that terminal block?

author: sokoloff
author_rank: Super Contributor
timestamp: January 01, 2023, 09:26:29 pm
content: If the existing connector is not obviously repairable, I would desolder it [remove it entirely] and either solder the wires directly to the PCB or replace it with a new connector of suitable pitch and that you can get the pins for.

author: Curmudgeon10
author_rank: Contributor
timestamp: January 02, 2023, 02:56:49 pm
content: The wiring to the connector measures 1mm in diameter. What gauge should I look for to splice to it?

author: Swake
author_rank: Frequent Contributor
timestamp: January 02, 2023, 03:49:32 pm
content: There is not much current going over those wires, so about any type or diameter of wire will be good enough.The length of the wire will always have an impact, keep it as short as possible.

author: Curmudgeon10
author_rank: Contributor
timestamp: January 02, 2023, 09:02:01 pm
content: It may take me a few days (lots of other things going on), but I'll report back on how it went. Again, many thanks to those who offered advice.

author: james_s
author_rank: Super Contributor
timestamp: January 02, 2023, 09:04:06 pm
content: A good source of wire like that is old computer cables. Pick up a 25 pin serial cable or a parallel printer cable at a thrift store and cut it up, you'll get several feet of wire in a rainbow of colors. Some cables have nicer insulation than others but the wire is generally similar.

author: Ian.M
author_rank: Super Contributor
timestamp: January 02, 2023, 09:12:30 pm
content: ... or hack up an Ethernet patch-lead. Patch-leads are stranded wire, but fixed Ethernet wiring is (or rather should be) solid core. You wont get all the nice colors, but it is an expedient source of minimally color coded hookup wire.

author: Curmudgeon10
author_rank: Contributor
timestamp: January 03, 2023, 01:47:20 am
content: One final question before I take this on: is there a risk of killing something permanently on the board (or even on the other end, where the wires dive into the wall unit itself) if I mess up a trial connection order?

author: sokoloff
author_rank: Super Contributor
timestamp: January 03, 2023, 02:11:57 am
content: There is. It’s small, but not zero. Sending a voltage in to a pin that’s not intended to receive it, or connecting two outputs together and having one try to drive low and the other high can cause problems. Or connecting a 5V supply directly across the LED could burn it out.Connecting boards together without knowing the wiring is not zero-risk.I think using reasonable care to figure out what’s ground and power goes a long way to reducing the risk.

author: Ian.M
author_rank: Super Contributor
timestamp: January 03, 2023, 02:15:04 am
content: Yes. If you mis-identify +Power and/or Ground, and connect them to the front board in the wrong place, you could blow the IR receiver, one of the LEDs, or possibly even something on the main board. There's also a much smaller risk that a mixup between one of the LED wires and the IR signal wire could damage the IR receiver, if its using low enough series resistor values for the LEDs.If there's a part no. on the underneath of the IR receiver, it could be replaced, and LEDs are cheap and easy to swap, so it wouldn't necessarily be game over, but would certainly cause delays and expense. If you damage the main board its anyone's guess how hard it could be to fix - anything from replacing a single small cheap part to a complete write off requiring a new board , if its badly designed and the power supply burns upMake the suggested measurements, and if you aren't 100% certain you've positively identified the power and ground wires don't experiment! If in any doubt, post your measurements here and wait for us to analyse them.The safest option would be to dig into the unit enough to get decent photos of both sides of the main board, which would let us figure out the wire colours and functions from what they connect to, only leaving the possibility the LED wires might need swapping. Sure its a pain in the butt, to get to it, but you are gambling with at least $50 of boards, and that's if you are willing to wait for them to come on the slow boat from China!

author: Curmudgeon10
author_rank: Contributor
timestamp: January 13, 2023, 04:03:03 pm
content: Here are the voltages. In the picture provided in this thread of a main board wired to a control board, the control board looks identical to mine, and an enlargement of the main board shows what I see as a yellow-orange-brown (last one a little iffy) order, left to right on the control board connector. Does that along with the voltages indicate a pretty good guess for the order?

author: Ian.M
author_rank: Super Contributor
timestamp: January 13, 2023, 04:32:48 pm
content: Nope. Those voltages indicate probable CMOS logic driving the LEDs, but they aren't internally consistent. The first line shows the same voltage on each of the other wires with respect to Yellow, so one would expect to see zero volts between any two of the other wires for the rest of the table, unlike what you've got. Therefore either you made several mistakes measuring them or they are changing as you do so. Re-read my reply #8, power off, and check if *any* of the wires have continuity to chassis, then power on and re-measure them all for voltage giving both the unloaded voltages and the loaded with 10K voltages, in the table and filling out both above and below the diagonal so we have two measurements loaded, and two unloaded for each pair so we can more easily spot mistakes or changing signals.Then we *may* have enough to determine +Power and Ground and to start figuring out the rest, but I've got a nasty suspicion that if it continues to be ambiguous you'll have to get the main board out to trace tracks.

author: Swake
author_rank: Frequent Contributor
timestamp: January 13, 2023, 04:57:21 pm
content: Reverse engineered the board.Certainties:- C2 is polarized, this likely indicates GND/Negative.- J1 connector has pin numbers, these are respected in the schematic.Uncertainties:Polarity of LEDs assumed based on polarity of C2. Might be inverse in reality.IR receiver is unknown. In the Kicad Library many if not all have the pin-out as in the schematic.

author: sokoloff
author_rank: Super Contributor
timestamp: January 13, 2023, 05:01:18 pm
content: I saw similar physical topology, but assumed OUT was direct to pin 2 and Vss was (for reasons unknown to me) powering the cap through the resistor.(Neither makes perfect sense, but reversing OUT and VSS made more sense to me.)

author: Swake
author_rank: Frequent Contributor
timestamp: January 13, 2023, 05:50:14 pm
content: From the measurements in combination with the schematic black is likely GND and on pin5.Please do what Ian.M suggests: power down the unit, measure resistance between black and chassis and report for next step.If you are lucky the resistance is near zero and we have found the ground wire.(as you are at it also measure resistance between chassis and the 4 other wires, it might help out too)

author: james_s
author_rank: Super Contributor
timestamp: January 13, 2023, 06:00:08 pm
content: It seems to be that the easiest thing to do is just remove the cover to expose the other board the cable goes to and then note the order of the wires there. It's pretty certain that the order will be the same on both ends of the cable.

author: Curmudgeon10
author_rank: Contributor
timestamp: January 14, 2023, 11:06:55 am
content: I'm removing that cover today, one way or the other.

author: Ian.M
author_rank: Super Contributor
timestamp: January 14, 2023, 12:31:04 pm
content: Good. You should then be able to confirm the theory that the wire order is the same at both ends. From photos of other Chigo main boards, its almost certain you will find a 7805 regulator IC (TO-220 package, with a small, probably black heatsink). Its tab is certain to be the 0V rail of the circuit, even if its not grounded to earth ground, so continuity testing from there to your wires will positively identify the 0V/GND wire. With the regulator part number side up, tab towards you, its output is its leftmost pin, so you can confirm the +5V supply wire by continuity testing. If those two wires are the end pins of the connector, the order probably is the same at both ends, but as we don't want to blow the IR sensor, you should also perform the previously suggested unloaded and loaded (10K) voltage tests, with respect to the known 0V/GND

author: Curmudgeon10
author_rank: Contributor
timestamp: January 14, 2023, 04:23:58 pm
content: Closing in on a solution?Just got the cover off, and was able to extract the Main board just enough to confirm that the wire order on that connector (from bottom of connector as it sits vertically on the board) is Yellow-Red-Orange-Black-Brown.

author: Swake
author_rank: Frequent Contributor
timestamp: January 14, 2023, 04:41:57 pm
content: ok, good.Can you remove the connector from the main board and plug it in the IR sensor board? If it fits because it is the same connector, I think you then have the order of the colors because I would expect the cable to be 'straight' else it would have been very problematic during assembly stage.

author: Curmudgeon10
author_rank: Contributor
timestamp: January 14, 2023, 07:25:00 pm
content: Unfortunately, I can't do that. The connector on the main board is soldered to the board (as was the one on the sensor board), and the wires were fixed to the connector via tiny pins. In fact, on the sensor board, since I can locate no one in my town who could actually "repin" the wires to it, I had the connector removed and some individual wires soldered directly to the board.

author: james_s
author_rank: Super Contributor
timestamp: January 14, 2023, 07:44:00 pm
content: I think it is extremely unlikely that the pinout is different on each end. I don't think I have ever seen a piece of equipment where a cable of that sort wasn't straight through.

author: Swake
author_rank: Frequent Contributor
timestamp: January 14, 2023, 07:51:46 pm
content: On the connector on the sensor board there are pin numbers. I guess the one on the main board has some too.connect 1 on 1, 2 on 2 and so on.

author: Curmudgeon10
author_rank: Contributor
timestamp: January 14, 2023, 08:34:13 pm
content: I have a competing theory from another forum where some are proposing a red-yellow-orange-brown-black order. Here is their analysis:"First, 5 volts is a standard voltage for electronic controls.Well, just looking at the printed circuit traces, I see a couple of things.The thicker traces on red and black suggest these are Positive and Negative (often called Common/GND when you're dealing in electronics, but it's definitely NOT AC power safety ground).The conventional color choice for + is Red...... and the preference for - is Black ... and the polarity of the capacitor strongly suggests that was done here.The mystery orange/brown wires both do the same thing. They go to a "device of some kind", probably a switch or light, and the other side of the device is black (- or GND)? That would be very conventional.And here is the takeaway from that: if you exhanged brown and orange, most likely all that would happen is the lights or switches would be exchanged; e.g. it would light up ERROR and not ON instead of the other way 'round like it's supposed to.The 3-prong thingie in the middle looks an awful lot like an infrared sensor and the schematic seems to match up.I think the schematic is this. (https://electronics.stackexchange.com/questions/649949/what-is-this-component-and-did-i-make-this-schematic-correctly) "I did measure resistance from each wire to chassis ground and came up with all open circuits. The multi meter did record reistance between several points on the sensor board, so I know its operating correctly.

author: james_s
author_rank: Super Contributor
timestamp: January 14, 2023, 08:36:40 pm
content: I would not expect chassis ground and DC ground to be the same thing. The other poster that mentioned the 3 terminal regulator had a good point, the tab is almost certainly tied to DC ground.

author: Curmudgeon10
author_rank: Contributor
timestamp: January 15, 2023, 03:16:04 pm
content: As I reported in another post, I did measure resistance but apparently I only used main power ground, not DC ground, so the measurements just showed open circuits. Not sure how to find the "DC Ground."I have exposed the main board, and can pull it out a bit, but it is (I guess) tied in to the wire bundle that exists the rear of the board through the wall unit casing, and then on to the outside unit.

author: sokoloff
author_rank: Super Contributor
timestamp: January 15, 2023, 03:25:09 pm
content: The DC ground is probably the black or brown wire. It is the negative side of the linear regulator. That’s probably tied to the negative side of an electrolytic capacitor in the power supply area.

author: Ian.M
author_rank: Super Contributor
timestamp: January 15, 2023, 03:41:54 pm
content: If you show us photos of the main board in the unit, we can probably point out a DC ground (0V) to you so you can continuity test from there to the wire ends.

author: Curmudgeon10
author_rank: Contributor
timestamp: January 15, 2023, 04:36:02 pm
content: This is the best I can do with a picture of the main board. I can only draw it from the housing about an inch or so.

author: Ian.M
author_rank: Super Contributor
timestamp: January 15, 2023, 05:09:29 pm
content: That's not a lot to work with as we cant get to the regulator. However there are two accessible points that have a high probability of being grounds - the negative leg*of an electrolytic capacitor next to the off-board signal connectors, and the metal frame of the tactile switch next to them. If there is continuity between those two points the probability becomes a near certainty.In all cases, test for continuity from the capacitor leg to the wire ends, and from the switch frame to the wire ends, and tell us what you find.* For a through hole Electrolytic capacitor, the negative leg is the one next to the contrasting stripe on the printed wrapper round its can. Other types of polarised capacitor have different marking conventions.

author: Curmudgeon10
author_rank: Contributor
timestamp: January 15, 2023, 07:03:31 pm
content: OK, thanks. Will do. Results in a little bit.

author: james_s
author_rank: Super Contributor
timestamp: January 15, 2023, 07:05:58 pm
content: Why not just wire it straight through and try it out? It's very unlikely that it won't just work at that point and if something is wired wrong it is unlikely to do any catastrophic damage, the only vulnerable part on that PCB is the IR receiver and those are cheap and readily available.

author: Curmudgeon10
author_rank: Contributor
timestamp: January 15, 2023, 07:33:06 pm
content: Quote from: james_s on January 15, 2023, 07:05:58 pmI'm only being super cautious because I was advised earlier to be super cautious.I measured the resistance between one leg of the capacitor (my probe could only reach one leg due to interference from other components), and the wires, as well as the metal frame of the switch and the wires. All such measurements showed values except the brown wire, which went to zero.That would seem to confirm that the brown wire is DC ground. If the fattest trace on the sensor PCB board is ground, then brown would go to that pin (on the bottom), and the other wires, ascending, would be Black, Orange, Red Yellow per the order found on the main board connector, discussed earlier.Its starting to sound like I could solder this thing up and hope that it works. Any other tests or precautions recommended?

author: Ian.M
author_rank: Super Contributor
timestamp: January 15, 2023, 07:43:00 pm
content: Check the yellow wire is +5V with respect to the confirmed Brown 0V/Gnd, and it doesn't droop when you connect a resistive load (any resistor within a factor of three of 1K should do) to verify that's the +Supply. If it passes that test, then IMHO you can connect them in the 'straight through' order with confidence that it wont cause damage, and with good odds it will 'just work'.

author: Curmudgeon10
author_rank: Contributor
timestamp: January 16, 2023, 08:07:35 pm
content: The Minisplit Lives!! Had to throw together a temporary work surface so I could solder the wires 4 ft above the floor, and I'm not going to be conducting any tutorials any time soon on soldering techniques for tiny wires, but everything works. All that remains is to reinstall the various skins/covers that were removed to determine the wiring order.Many thanks for the professionals who hung in here on this thread and helped me figure this out.