GE electrically commutated motor repair help [SOLVED] - Page 1
Forum section: electronics / repair

author: Agentfoot
author_rank: Regular Contributor
timestamp: March 17, 2025, 08:15:31 pm
content: I have a GE ECM that is the inducer fan motor out of a carrier furnace that stopped working. Replacements are expensive so I would like to repair the one I have. Here is what I know or have done.Motor spins free and will turn if a quick pulse of power is applied.Outputs are 6N60 Mosfests that are not shorted or open.All diodes and resistors were checked and found to be good.Has Nichicon electrolytic capacitors that are not bulged, and capacitance tests fine.Any help is appreciated. Some pictures are attached.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: March 17, 2025, 11:29:21 pm
content: Hello Agentfoot,The motor of your ECM looks like a vanillashaded pole motor. The latter does not need any electronic board to run. Just AC voltage and no capacitior either, because the offset magnetic pole is created by the copper short(s) in the stator, calledFrager spire.Do you see any Frager spire on the motor, behind the white nylon cover ?How many connexions on the motor 2? 3? more ?Don't really understand the purpose of the board. Is it for varying the speed of the motor ?Did you check theHall sensorH1 ? Can you reed a reference or marking code on the part ?If it's a3 legs part with digital output,testing is quite easy : 6F22 battery block, 1 LED, 1k Ohm resistor and a small magnet.Unsolder the part and try to test it.Analog output hall sensorsalso exist but at this early stage of investigation, it's difficult to be conclusive about the need of measuring the magnetic field as an analog value.The aim of the Hall sensor is to tell the µC when the rotor is at a particular angle, so the current can be reversed in the H bridge (the 4 power MOSFETS).Try to locate a voltage regulator on the board. The PIC µC will need a stable voltage (3.3 VDC or 5 VDC). You can measure its power supply directly on its legs (use its datasheet to locate VDD and VSS).Take much care when measuring on the board :the circuit is NOT isolated from the mains, so everything is to be considered as live.

author: Agentfoot
author_rank: Regular Contributor
timestamp: March 18, 2025, 12:48:51 am
content: Quote from: timeandfrequency on March 17, 2025, 11:29:21 pmThe Motor has 2 connections. The board does control motor speed. I will have to check the Hall effect sensor. It has 3 legs but no markings. To be honest I totally missed it and thought it was just a plastic stand off. I did check all the tiny 3 legged ICs for shorts or opens and all tested good. I have yet to check voltages powered up. This may explain more.EC motors use permanent magnets and electrical windings to generate their magnetic fields, in a similar fashion to brushed DC motors. However, as their name suggests, they achieve commutation electronically rather than mechanically. This is only possible by incorporating on-board electronics within the housing of an electronically commutated EC motor.The on-board electronics include a rectifier that converts the AC supply to DC. An integrated controller then directs the right amount of current, in the right direction, at the right time, through each of the windings. This develops magnetic poles in the stator, which interact with the permanent magnets in the rotor.The position of each magnet is determined by using Hall effect sensors. The appropriate magnets are attracted, in sequence, to the magnetic poles in the stator. Simultaneously, the rest of the stator windings are charged with the reverse polarity. These attraction and repulsion forces combine to achieve rotation and produce the optimal torque. Since this is all done electronically, precise motor monitoring and control are possible.

author: wraper
author_rank: Unknown
timestamp: March 18, 2025, 12:53:53 am
content: Quote from: timeandfrequency on March 17, 2025, 11:29:21 pmIt looks like one on a first glance but it's not. One hint is that you don't see copper shading rings.

author: wraper
author_rank: Unknown
timestamp: March 18, 2025, 01:03:40 am
content: You should check if MCU receives low voltage power. Also it's not necessarily motor fault, as it needs to receive proper signal to work.

author: abrasumente
author_rank: Contributor
timestamp: March 18, 2025, 01:45:11 am
content: I can't speak to a lot of the components on the board as I'm just now getting into electronics, but am an HVAC tech.If you haven't already checked, look for micro fractures in the solder joints. With the crappy way the factory solders these, over time with vibration and expansion/contraction due to heat the joints will crack on the back side of the board, leading to intermittent faults and lots of cussing. They typically show up during winter, especially on cold nights (lots of runtime=board expansion due to heat).Sometimes a reflow can solve the problem. More often than not though the excessive heat from intermittent contact fries something else on the board.

author: Agentfoot
author_rank: Regular Contributor
timestamp: March 18, 2025, 02:11:37 am
content: Quote from: abrasumente on March 18, 2025, 01:45:11 amThis inducer motor did fail a few times and began working again. I figured I would find a burnt component, burnt trace, or cracked joint but nothing is obvious. I could be missing a crack due to the silicone like coating or multitude of tiny surface mount components. I may need to look into this better.

author: Agentfoot
author_rank: Regular Contributor
timestamp: March 18, 2025, 02:17:14 am
content: Quote from: wraper on March 18, 2025, 01:03:40 amMCU did have voltage 4.4-4.8VDC it was not too stable though.Just need 120VAC to 2 pins and around 10VDC to 2 other pins to power the fan at medium speed. This came from the furnace troubleshooting guide.

author: wraper
author_rank: Unknown
timestamp: March 18, 2025, 02:28:45 am
content: Then check if there is a signal after optocoupler on applying/removing those 10V.

author: bdunham7
author_rank: Super Contributor
timestamp: March 18, 2025, 02:49:57 am
content: Quote from: Agentfoot on March 17, 2025, 08:15:31 pmBy "spin free" do you mean it turns totally smoothly or does it "cog" (tend to jump to specific points)?And what happened here? Is that a crack? (see circle and arrow in picture)

author: xavier60
author_rank: Super Contributor
timestamp: March 18, 2025, 02:50:36 am
content: Quote from: Agentfoot on March 18, 2025, 02:17:14 amCapacitive dropper type 5V supply maybe. Trace out the circuitry of the large red capacitor,

author: floobydust
author_rank: Super Contributor
timestamp: March 18, 2025, 03:59:57 am
content: With these appliance boards, first thing to do is a visual inspection.I see a fractured (ring) solder joint on the connector J1, the pin which goes to the pad that someone has soldered. So I would touch up J1.Check the the film cap values reddish brown C06? and I'm not sure about C01- which is the dropper-capacitor. Maybe pull off some silicone blobs see what's under them.There should be a fuse somewhere.

author: Agentfoot
author_rank: Regular Contributor
timestamp: March 18, 2025, 10:49:59 am
content: Quote from: bdunham7 on March 18, 2025, 02:49:57 amIt spins free as in no binding, rubbing or failed bearings. The motor does have the magnetic pull in 3 spots when rotated.That is just silicone I cut to better see and access components. There was a hair on there too.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: March 18, 2025, 02:12:59 pm
content: Quote from: wraper on March 18, 2025, 12:53:53 amSure that they are not hidden behind the white plastic cover ?Because the board provides not more than an AC waveform, as does the mains. The sole difference is variable frequency.And the motor has only two connexions, which is normally not sufficient to create a rotating magnetic field.However one can now findBDLC motor driversfor (PC) fan control that need only one coil (and a magnetic rotor).Contrary to the rotor of a vanilla shaded pole motor, which is not magnetic, it's just a shorted squirrel cage.Quote from: floobydust on March 18, 2025, 03:59:57 amWhen following the tracks on the PCB, it seems that C01 is in // and the bordeaux color cap in series (relative to L-N line input). The latter should be the dropper capacitor.There's no fuse but a PTC (RT1) and a I cannot read its markings form aboveThe board likely has two power supply rails :- 162 VDC power rail to supply the H bridge- a low voltage rail (ballpark 3 to 5 VDC) to supply the µC, the Hall sensor and other glue analog/logic stuff like the optocoupler.Each of these rails has its own rectifying diodes and smoothing cap(s)Quote from: Agentfoot on March 18, 2025, 02:17:14 amA not stable power supply can spawn infinite mess. So the failure quest starts at this location.Replace unconditionally the small electrolytic cap on the left of the Hall sensor : it probably serves to smooth out the µC power supply.And there must be a linear regulator or a zener somewhere. ThePIC16F628µC accepts supplies from 2.0V to 5.5V.If no improvement after changing the above cap, replace unconditionally the bordeaux color capacitor.

author: Agentfoot
author_rank: Regular Contributor
timestamp: March 19, 2025, 01:48:44 am
content: Was able to get a little more testing done today.Tested voltages across the electrolytic caps.C02 - 169.2VDCC03 - 165.5VDCC06 - 18.76VDCC07 - 17.09VDCH1 - Hall effect sensor if fed by the 17.09V supply. The Hall effect sensor did blink a LED hooked up to it when then fan was rotated and board powered. So the Hall effect appears good.U01- 2505 optocoupler was tested by raising and lowering DC input voltage to the board and monitoring the resistance across the emitter and collector. Resistance did change indicating it is good.Im new to testing both of these so if there are better ways let me know.So on to the PIC power supply issue. I found the supply to be at a steady 5VDC for a few seconds very intermittently. Usually voltage fluctuated anywhere from 3-4.8V. I traced this to be coming from a little surface mount diode that was a little off. (Picture below.) So I removed it. Tested good as diode. Prepped the pads and reinstalled. Now the voltage at the diode and PIC is 2V. Motor still does not run. So now I am unsure if 2V or 5V is the correct voltage since the PIC operates at 2-5.5V.PIC voltage was at the side of the diode with the arrow in the picture.More pictures of motor and components if anyone wanted or needed.

author: Josec
author_rank: Contributor
timestamp: March 19, 2025, 05:24:44 am
content: Change the red capacitor, is in series with a diode and generate the low voltage for the PIC, I bet the capacitance is far low than the nominal. I have similar problem.I don't know why those capacitors change value but believe me happens.

author: wraper
author_rank: Unknown
timestamp: March 19, 2025, 05:43:49 am
content: Quote from: Josec on March 19, 2025, 05:24:44 amThey have dielectric breakdown and self heal by evaporating metallization, those lose capacitance.

author: xavier60
author_rank: Super Contributor
timestamp: March 19, 2025, 06:00:14 am
content: But he has 17V. The print is difficult to trace. And now the PIC has 2V after disturbing a diode. Need to know what regulates 5V from the 17V.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: March 19, 2025, 12:37:06 pm
content: Hello Agentfoot,Thanks for the motor close picture. wrapper was right : no shaded ring at all and the rotor clearly shows the 3 magnets.First time I see such a BDLC motor in that kind of shape, furthermore with one unique coilBack to the boardQuote from: xavier60 on March 19, 2025, 06:00:14 amI agree with xavier60.I wound suggest you start drawing the schematic of your board by indicating the ID of the parts (silkscreen printing) and their value and voltage (if relevant), or marking code in case of SMD parts. Having a detailled schematic is far from overkill and provides thebest chances of success.As other members mentionned above, also start buying replacement parts (*) so that we can exclude the malfunction of some of those.Because just measuring on the board is not a guarantee of working partsIndeed, as we actually know that something is defective, the measured values are potentially wrong.Furtermore, as the schematic is unknown too, we can't guess what are the correct values we would like to see.Chicken and egg problem.Does the 'C06' marking correspond to the bordeaux color cap or does it refer to another part hidden under the white silastic blob ?I did not figure out that there's another part at this location.The picture IMG_3622.jpeg shows that it is an electrolytic capacitor. Value ? Voltage ?R06 seems to be connected to the bordeaux color cap, can you confirm ?In the affirmative, D05 might also be a zener diode (series resistor is therefore R06)The schematic we expect to see is closeto that.ID on schematic; ID on silkscreen printing (= your board)F1 does not exist but is replaced by RT1 (marking '08SP 010' which is anNTC 10 Ohms @25 Ohms: can you measure about 10 Ohms ?C2 is the bordeaux color cap 2,2 nF 400VR3 is probably R06 (the fact that on the board the capacitor comes first and the resistor is downwtream has no influence : it works too)R1 is just a bleeder resistor that seems to be missing on the board, but OK, not relevant.D1 and D2 must be small SMD diodes close to the bordeaux color cap (one one the right and the second above the cap) IDs are too difficult to read because of the shiny warnish.And the 'Load' is probably the µC and other glue parts.(*) procure and replace the following part :- bordeaux color cap :choose a part that suits regardingthe lead spacing. I selected 'Class X2' caps which better survive as dropper caps that vanilla mylar stuff.- both small electrolytic caps (value and voltage unknown)choose hereby selecting voltage rating, capacitance and maybe lead spacing

author: Agentfoot
author_rank: Regular Contributor
timestamp: March 19, 2025, 03:46:20 pm
content: Quote from: timeandfrequency on March 19, 2025, 12:37:06 pmThank you for your help.I will try to draw some sort of schematic.I will attach a picture that shows most of the circuit powering the PIC until then.C06 is a 220uF 25V electrolytic.The bordeaux color cap is C04.R06 is connected to the cathode of D03. The anode of D03 is connected to C04 the bordeaux cap.R06 is also directly connected to the positive of C06.RT1 measures 14.1 OhmsC04, C06, and C07 have been removed and replacements will be ordered. I also tested these with my fluke and not so accurate Chinese component tester. Below are the results.C04 - fluke 2.13uF - component tester 2122nF ESR=0.20C06 - fluke 228uF - component tester 214.7uF ESR=0.38 Vloss=1.9%C07 - fluke 105.2uF - component tester 100.7uF ESR=0.55 Vloss=1.5%

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: March 19, 2025, 07:25:04 pm
content: Great, thanks for the mesured values and description of the connexions between parts.Any readable marking code on D05 ?Even if you measured the parts with top notch TE, there's still a bias :- electrolytic caps may show leakage, and a capacitance meter is unable to measure it. Leakage has to be measured at nominal voltage.- same for CO4 : voltage on the part when the board is running is probably more than 100 VAC while the measurement is done with less then one volt. This may hide really weird failure modes that are pretty difficult to analyse.The board seems to be about 15 years old. So recapping is just the right thing to do.

author: Agentfoot
author_rank: Regular Contributor
timestamp: March 19, 2025, 08:01:03 pm
content: Quote from: timeandfrequency on March 19, 2025, 07:25:04 pmD05 is marked with what looks like BK $72. Will attach a picture. The dot between the 7 and 2 in the picture is just a piece of the silicon coating.Your guess on age is close. The furnace that this came out of was installed in 2007.

author: wraper
author_rank: Unknown
timestamp: March 19, 2025, 08:06:07 pm
content: Quote from: Agentfoot on March 19, 2025, 08:01:03 pmIt's SMAJ14A TVS from Taiwan Semiconductor.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: March 19, 2025, 08:36:33 pm
content: Quote from: wraper on March 19, 2025, 08:06:07 pmInteresting : theSMAJ14Abreakdown voltage is min 15.60 VDC and max 17.20 VDCOn the board, you measured 17.09 VDC, which is just in the parts' tolerance span.Here's a riddle: why does the TVS actually trigger?This would mean that the voltage at D05 is too high.Usually, when you select a 14 VDC TVS, it's for a 12 or 15 VDC supply rail.

author: wraper
author_rank: Unknown
timestamp: March 19, 2025, 10:56:13 pm
content: Breakdown voltage is when it starts passing certain current, 1mA in this case. It's not exactly the same as Zener voltage which is specified at higher current.QuoteIt may be used as a Zener diode.QuoteYou certainly should not use 14V TVS for 15V rail if you do not want failures happening over the time. Even if you have barely any margin, very likely TVS will do more harm than good.

author: Agentfoot
author_rank: Regular Contributor
timestamp: March 20, 2025, 02:01:05 pm
content: Thanks for the part identification. I struggle identifying small surface mount parts. I had a feeling what I saw as "$" was a company logo.

author: Agentfoot
author_rank: Regular Contributor
timestamp: March 20, 2025, 02:10:02 pm
content: Here is my ugly attempt at a schematic. It is not complete but I think it shows what's needed. Let me know if more is needed or better pictures of something is needed.D03, D04, D07, D09, D10, D12, D13, D14, and 2 others not numbered are onsemi RS1J.DO5 this is a SMAJ14AD16 marked WR W9. Which is part of the low voltage turn on signal circuit.Then a small one marked A2 That has no diode number marked on the board.Also D01 and D02 are 1N5406The component I had starred (3 legged surface mount IC marked 2X) is a onsemi MMBT4401L, SMMBT4401L NPN Switching Transistor

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: March 20, 2025, 09:17:07 pm
content: Quote from: Agentfoot on March 20, 2025, 02:01:05 pmYes wrapper identifiedTaiwan Semiconductor.But small SMD parts most of the time have no logo.

author: Agentfoot
author_rank: Regular Contributor
timestamp: March 20, 2025, 11:40:07 pm
content: Quote from: timeandfrequency on March 20, 2025, 09:17:07 pmI thought I replied to wrapper with the thanks. Did I accidentally reply to you with the thanks? If so I am sorry.Were you and wrapper indicating there is problem with D05? Or something else causing the problem at D05?Is my schematic of any help? I was afraid it would be harder to follow that the traces on the PCB.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: March 21, 2025, 10:11:32 am
content: Is is not important at all who discovered a part of the clue and you made no mistake.I'll provide later some information that using TVS as zeners is not usual at all (and not recommended either).I'll start looking at your schematic in the coming days.PIC µCVDD : pin 14VSS : pin 5Can you add the connexion to pin 5 of the µC on your schematic ?

author: Agentfoot
author_rank: Regular Contributor
timestamp: March 24, 2025, 02:29:59 am
content: Quote from: timeandfrequency on March 21, 2025, 10:11:32 amAdded VSS pin 5

author: xavier60
author_rank: Super Contributor
timestamp: March 24, 2025, 02:59:33 am
content: Most if not all of the "?LBSM" parts are resistors.SMD diode marked "A2" is likely a 5.1V Zener diode. There should be a VDD bypass capacitor near the PIC.To avoid damage to the PIC, don't power the board with A2 diode removed.Faulty Zener diodes usually measure leakage resistance in both directions on a typical DMM.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: March 24, 2025, 09:42:47 am
content: xavier60 assumption is correct : marking code 'A2' corresponds to a5.1 V Zener diode.Can you complete the schematic upstream of pins 1 & 2 of the optocoupler ?Is the optocouplers ID U03 ?Does it go towards a connector ? Is there a series resistor ?Once you received and installed the replacement parts, can you mesure the following voltages :- at the 'A2' Zener diode : '+' (of meter) connected to the cathode, '-' to the anode- at the microcontroller : '+' (of meter) connected to pin 14 (VDD), '-' to pin 5 (VSS)Later, we'll need to check the circuit around the optocoupler, the transistor (MMBT4401L) and the pin 11 of the µC.This will probably require to energize the optocoupler.Will provide additional advice on that topic once the schematic is updated.

author: ealex
author_rank: Frequent Contributor
timestamp: March 25, 2025, 03:32:59 pm
content: HelloCheck this app note for a description on how it should work:https://ww1.microchip.com/downloads/en/Appnotes/00001779A.pdfI've repaired something similar used in a hair drier.The motor is connected in the middle of a H bridge and the mcu selects between 2 comutation patterns based on the data from the hall sensor - that's detecting the position of the rotor.One transistor is full on, and the diagonal one is PWM-ed to reach the target speed and motor current.You can test this thing at low voltage: one power supply for the MCU - should be 5V and a higher one for the H bridge. The motor I had started spinning at ~ 24V.The mcu should generate something right away - but there might be some under voltage lock-out or some enable signal

author: Agentfoot
author_rank: Regular Contributor
timestamp: March 25, 2025, 08:24:55 pm
content: Here is an updated schematic. This should include everything but the H bridge output section. I will get that drawn as well soon. Also attached is the PIC pinout.The small SM diode marked A2 being a 5.1V Zener does make sense. I did measure 5V at the diode and PIC when the measurement was steady. It looked to me like this diode got hot and partially desoldered itself. I figured a bad solder joint was causing the voltage fluctuations. Which is why I removed cleaned and re-soldered it. Why I only have 2V at the diode now I do not know. I suppose some of the silicone conformal coating may have got in a joint when I re-soldered but I did clean and remove what I could. Also when this diode was removed it was tested. Standard forward bias voltage drop and open circuit in reverse.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: March 25, 2025, 08:42:21 pm
content: Nice update.So now you have only 2V acroos the 'A2' zener diode, meaning the µC is unable to work currently.Q35_A) Can you measure the voltage across the 47 Ohms resistor ('-' of the meter = VSS) ,located just below the 'A2' zener on the schematic ?Q35_B) An then measure also the voltage between ¨Pin 3 and 2 of the MMBT4401L transistor on the right side of 'Pin 11 PIC' ('-' of meter = pin 2 of the transistor)Q35_C) Is the resistor on the left of pin3 of the same transistor about 22 kOhms ?

author: xavier60
author_rank: Super Contributor
timestamp: March 25, 2025, 11:12:25 pm
content: The "14.56M" resistor should be the dropper for the 5.1V Zener but the value should be much lower, a few K maybe.I hope that the 5.1V Zener didnt really overheat. Doesnt make sense and possible reasons would be bad. Does it still test well?

author: johansen
author_rank: Super Contributor
timestamp: March 26, 2025, 12:01:54 am
content: i'm a little suspicious of the accuracy of that schematic.but anyway, i would power up the pic from an external supply and measure its current draw... it may be burned up.

author: Agentfoot
author_rank: Regular Contributor
timestamp: March 26, 2025, 12:17:37 am
content: Quote from: timeandfrequency on March 25, 2025, 08:42:21 pmI will say these voltages were hard to get due to fluctuations.The drop across the A2 Zener liked to hold about 2.2V but was anywhere from 2-4.8VThe drop across the 47 Ohm held around 0.014 but ranged between 0.09-0.30VMMBT4401L pin 3 to 2 was 2.3V ranging from 2-4.8Q35_C) Is the resistor on the left of pin3 of the same transistor about 22 kOhms ? Not Sure what you are asking hereI did update 3 resistor values. These are unmarked and measured in circuit. It seems something probably the capacitors is affecting the measurements. I can pull and test out of circuit if needed. They are small surface mount resistors.

author: Agentfoot
author_rank: Regular Contributor
timestamp: March 26, 2025, 12:19:32 am
content: Quote from: xavier60 on March 25, 2025, 11:12:25 pmI updated some values. The A2 diode had voltage drop in forward open circuit in reverse.

author: Agentfoot
author_rank: Regular Contributor
timestamp: March 26, 2025, 12:24:31 am
content: Quote from: johansen on March 26, 2025, 12:01:54 amIt should be a little suspicious. I am no professional and do not reverse engineer circuit boards all the time.If something looks off let me know and I can check it. Im willing to bet it is more correct than you think it is. It is defiantly ugly and messy I will give you that.

author: xavier60
author_rank: Super Contributor
timestamp: March 26, 2025, 12:48:12 am
content: 43K still seems too high, the supply to it is still 17V?If you have the parts, you could replace or bridge the 43K with something lower like 10K to see if the 5V rail comes up.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: March 26, 2025, 09:51:30 am
content: As xavier60 underscored, the 43k resistor does not bring enough current to supply the µC.The value of that resistor has likey shifted, so we'll need to go on stepwise to find its actual value experimentally.The limit values cas be calculated as follows.Let's assume we keep about 17 VDC across D05 (SMAJ14A) and CO7 (this has to be checked at each iteration).Let's also assume we have no load downstream the zener diode (that's just for the calculation, do not cut any track).Nominal Zener current to regulate firmly is given at 5 mA, as stated by the datasheet.So the '43K' resistor should have not more than Rmax = (17-5) / 5 mA = 2.4 kOhmsMaximum Zener dissipation is 100 mW, so Iz max = 100 mW / 5V = 20 mAWe take these 20 mA and calculate the minimal value of the '43k' resistor , which is Rmin = (17-5)/ 20 mA = 600 Ohms.To avoid attaining the zener maximum dissipation, we'll at first limit the min value to 1 k.Lower values (down to 680 Ohms) will only be used if behaviour is not successful.Converted to E12 values this leads us to :R43k max = 2,2 k OhmsR43k min = 1 k Ohms (or 680 Ohms at almost 100 mW Zener dissipation)How to proceed ?At each step, mesure the voltages across :- the SMAJ14A TVS,- the new '43k' resistor,- the 'A2' zener,- the supply of the µC (pin 5 / pin 14)Start with R43k at 10k as suggested by xavier60 and then go down with the values of resistors you haveat home.As an alternative, you may use a 10 k ajustable resistor in series with 1k or 680 Ohms fixed resistor.If the µC is still alive, the 5,1 VDC for its supply should show up around 3.3 k ± 1.5 k

author: Agentfoot
author_rank: Regular Contributor
timestamp: March 26, 2025, 03:29:52 pm
content: Well I removed the R43K resistor to try and get a stable measurement out of circuit. But the resistor flew off my bench when I was attempting to take the measurements. So It is now lost forever.I should have the resistors on hand to test replacements. So I will try that.Maybe I am not understanding this circuit correctly. But if R43K is causing the voltage fluctuations wouldn't is value have to be constantly changing?I took a video of the voltage fluctuations across the zener but then I realized I can't attach it here. So I will attach a hand drawn simulated oscilloscope picture. I just want to make sure everyone understands what I mean by voltage fluctuations.Also I will add the input voltages and other voltages on the board are stable In case anyone is wondering.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: March 26, 2025, 03:55:48 pm
content: Thanks for the drawing.Currently the zener and the µC do not have enough current to do their jobs.So what we see can be the consequence of a µC trying to boot but as the supply is too weak, its voltage drops. Then the watchdog or the brown-out reset triggers, leading to multiple restarts.

author: Agentfoot
author_rank: Regular Contributor
timestamp: March 26, 2025, 04:19:09 pm
content: Thanks for the explanation. Now I get it.

author: Agentfoot
author_rank: Regular Contributor
timestamp: March 26, 2025, 07:25:00 pm
content: The R43K was the failure point.A 1K replacement brought the voltage across the zener to a steady 5.112V.With a 2K voltage was still a little low at 5.072V.Mounted the board back to the motor and the motor came to life. The motor also works with the old capacitors.Is there anything that could have caused the R43K failure that I should look for?

author: xavier60
author_rank: Super Contributor
timestamp: March 26, 2025, 09:33:23 pm
content: Likely too much thermal dissipation for its size. To improve reliability, either replace it with a higher wattage part or combination of. Or a conventional 400mW axial resistor.Although I don't think it applies here, another killer of SMD resistors is Sulphur corrosion. I'm building a stock of Sulphur resistant resistors for use in mission critical products.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: March 26, 2025, 10:20:18 pm
content: Great achivement !You may now edit the title of this thread by adding the word that everyone wants to see in the repair section of the forum : ''.And distribute a few 'Say Thanks'Thinking about xavier60s' statement, Sulfur corrosion is indeed a potential cause of the SMD resistor failure on the board.Is the blower motor located in a natural gas furnace ?Natural gas is odorless, but to help detect it by humains, someMethanethiolis added.This chemical is also used in the natural gas industry as an odorant, as it mixes well with methane.See where the Sulfur might come from ?I would suggest to protect the board with some fresh varnish.You may usethis productbut not sure if it's easy to procure locally in the states.

author: Agentfoot
author_rank: Regular Contributor
timestamp: March 26, 2025, 11:02:28 pm
content: xavier60 and timeandfrequencyThank you for your continued support with this problem. With your help I was able to fix the motor, and was able to learn a lot too.timeandfrequencyYes it is the inducer motor for a natural gas furnace. Its job is to bring in fresh air and blow out spent combustion gases.I was just starting to think about how to re-coat this board. Lots of little areas need done but most of the original coating is intact. It currently has a silicone coating so I will likely have to re-coat with another silicone product to ensure adhesion.

author: Agentfoot
author_rank: Regular Contributor
timestamp: March 26, 2025, 11:10:37 pm
content: Just adding schematic and board picture with failed component marked.

author: wraper
author_rank: Unknown
timestamp: March 26, 2025, 11:17:08 pm
content: QuoteMost likely it's either heat or mechanical stress.

author: Agentfoot
author_rank: Regular Contributor
timestamp: March 27, 2025, 12:27:43 am
content: wraperThank you for your ongoing input on this subject also.