BERNINA power supply board (Switch Mode) issue diagnosis and beyond (fixed) - Page 1
Forum section: electronics / repair

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 18, 2024, 04:27:59 am
content: Another project fixing an elusive issue of a power supply board. Photo of both sides attached (back side flipped).What's known:There are two sections of the circuity on the board, separated by a broad line on the frond side, which I believe are electrically isolated. AC mains comes in from the socket at the bottom right corner. There are three output sockets in the upper section of the board: P162 (green, 2 pin), P 161 (black, 2 pin), and P163 (red, 5 pin). The 4 pin green socket (P160) is input (into a couple of opto couplers), which we can ignore for now.The output voltage of the three sockets are:P162: 6V DC (pin 2)P161: 28V DC (pin 1)P163: 5V DC (pins 1/3), 28V DC (pin 4)Symptom: The voltage of 6V DC from P162 presents at some time, not other times.The circuit might be plain to the experienced who might be able to pinpoint the culprit at the first glance. All input is welcome, including the working of this board, topology of the transformers U146 (top left) and U145 (smaller, bottom right).What's been done:Caps on the affected rail C88(1000uF 50V) and on the one across the PWM controller IC (UC3844) VCC/GND (100uF 25V) are replaced with used similar ones (for test, no new replacement handy) but no cure.[Worse than that, probably due to error while probing, further damages occurred beyond that. Detail to follow.]Questions:1. C86 (1000uF 50V) and C87(100uF, 35V) are in parallel, side by side, on the 28V DC rail. Why there need to be two caps, and of different voltage ratings (not simply one 1100uF, 50V) – for filtering different frequencies of ripples (even so, why different V ratings)?2. From what I think is a fairly simple single layer board and the clear photos of both sided, side by side, is anyone able to tell the working principles of this SWPS? And likely weak spots probably common to this topology?3. More to come.

author: u666sa
author_rank: Frequent Contributor
timestamp: December 18, 2024, 06:06:48 am
content: List voltages of all pins for UC3844

author: inse
author_rank: Super Contributor
timestamp: December 18, 2024, 06:12:07 am
content: Bernina is a brand of sewing machines, afaik.The 6V is missing while the other voltages are present - did I get this right?There is not much circuitry to check in between the connector P162 and the transformer: test the diode FES8DT, the small inductor L142 and the transformer winding.Answer 1: Maybe they calculated that 1000uF was not sufficient, 1100uF not available, so they placed two caps.Voltage rating maybe due to price and availability.The smaller transformer/coil is connected to P159 which may (only assumption) be the motor connector. The motor is not mains isolated and has some speed control circuit via ST13005

author: floobydust
author_rank: Super Contributor
timestamp: December 18, 2024, 06:18:10 am
content: I notice it uses a discrete 4 channel opto-isolator made up of IR LED's and phototransistors. They would be sensistive to ambient light and need a cover. D110/T132, D107/T127, T129/D109, D108/T128. They remind me of Lite-On LTR301, LTR302.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 18, 2024, 08:12:11 am
content: Things have become worse than just that, thought this original issue still needs to be figured out and fixed.When probing, I suspect there was a short circuit between the supply UC3844 (VCC/GND) blowing R37 and probably D115 (possibly more but hopefully all can be replaced). Fuse is also blown. So unfortunately before these damaged components are replaced, I'm unable to power it on again and measure voltages.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 18, 2024, 08:12:44 am
content: Quote from: u666sa on December 18, 2024, 06:06:48 amDue to further damage, unfortunately I can’t do this at the moment.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 18, 2024, 08:16:05 am
content: Quote from: inse on December 18, 2024, 06:12:07 amThat's correct before the recent further damage.Quote from: inse on December 18, 2024, 06:12:07 amI think I tested some of these but didn't get a clue. Thanks for your answer.Quote from: inse on December 18, 2024, 06:12:07 amThat's exactly right. There is also a small daughter board connected to socket P190.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 18, 2024, 08:17:14 am
content: Quote from: floobydust on December 18, 2024, 06:18:10 amThat's exactly right. There is a black cover over the diodes and phototransistors.

author: inse
author_rank: Super Contributor
timestamp: December 18, 2024, 08:38:46 am
content: If the fuse is blown, probably T131 is shorted as well.If so, replace UC, diode , resistor and T131, check R35

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 18, 2024, 10:59:19 pm
content: Quote from: inse on December 18, 2024, 08:38:46 amThe fuse (and other parts) was blown probably by accident when probing. These parts may need replacement (at least some of them). But I will have a close analysis of the circuit (after drawing up the schematic) before that.Back to the original question, and take a time machine to travel back to the point when the only problem was the intermittent presence of the 6V –In the upper section of the board, there appear to be three separate subcircuit for the 5V/6V/28V output, all through the transformer, which is driven by the lower section of the board.Is the problem certainly in the upper section, and specifically in the subcircuit for the 6V supply? In other words, is it possible to have a problem in the lower section but the 5V/28V both work well?More information: the 6V output is solely for the lights. So at that stage, the machine was working sweet as (the boards for the main business of the machine were working well), only with the lights sometimes on and sometimes off.

author: inse
author_rank: Super Contributor
timestamp: December 19, 2024, 05:38:44 am
content: If the other voltages work again, you only need to concentrate on the marked mesh.Are you sure the intermittent light is caused by the power supply, not the connector, switch, socket or lamp?What kind of lamp is it, 6V halogen?

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 19, 2024, 11:13:36 pm
content: Quote from: inse on December 19, 2024, 05:38:44 amThanks. Yes, my question was exactly, if all other output voltage rails work just fine, if we can be sure the problem must be in the upper section. Sounds like your answer is yes.Yes, I checked; It's not the problem of the bulb, wiring, or switch. When the light is not on, there is no positive voltage across the two pins of the socket. The bulb is traditional incandescent 6V.By the way, I'm close to finishing the circuit schematic – a bit messy atm as you might imagine. Hopefully the affected parts are limited. And anyway, I'm going to replace all of the electrolytical caps, except those which I cannot find replacement for.

author: inse
author_rank: Super Contributor
timestamp: December 20, 2024, 07:20:38 am
content: I wouldn‘t replace the capacitors unless there was failure evidence.In case it turns out, a failure in the transformer winding is causing the intermittent problem (what I suspect), maybe a workaround was the only option: replace the incandescent bulb by a 24V LED lamp and feed it from the 28V rail.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 20, 2024, 07:40:36 am
content: Quote from: inse on December 20, 2024, 07:20:38 amThat's a really credible and solid point. I hope I thought of this earlier. Now I can only hope I can get it back at least to its previous status, with an intermittent 6V output but the rest is working just fine.Facing the current state, now I have another specific question.I have now identified power (N-channel) MOSFET T131 (IRFBC30) as bad and needs replacement. Its Source pin is connected to ground via a 2W or 3W resistor (R38), which has band green/blue/grey or siler/gold. I'm not sure if it's 0.56 Ohm (silver) or 650 M Ohm (grey), because I'm not sure if the 3rd band is silver or grey.Given the radical difference, which one clearly does not make sense?My gut feeling is that, given the MOSFET as large current device, 650 M Ohm does not make sense. (It must be 0.56 Ohm.)

author: Sorama
author_rank: Frequent Contributor
timestamp: December 20, 2024, 07:54:28 am
content: Beter check first for dry solder joints.

author: inse
author_rank: Super Contributor
timestamp: December 20, 2024, 07:57:27 am
content: 0.56 Ohms for sure, it’s a current feedback shunt.Checking the solder joints is always a good idea though those look as shiny as you wouldn’t see them anymore today.

author: Sorama
author_rank: Frequent Contributor
timestamp: December 20, 2024, 08:22:56 am
content: I do see some suspicious rings on some solder joints.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 20, 2024, 09:00:38 am
content: Here's the schematic of the upper section. Excuse my perhaps non-standard (incorrect) use of symbols (however feel free to point out).

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 20, 2024, 09:12:21 am
content: Quote from: Sorama on December 20, 2024, 08:22:56 amThanks. Had a close l but didn't find any obvious problems. Which ones you find suspicious?

author: inse
author_rank: Super Contributor
timestamp: December 20, 2024, 09:24:41 am
content: Nicely drawn schematic, though boring.Btw, R52 is 160Ohms and R53 150Ohms.The circuit around the TL431 is providing the feedback for the voltage regulation of the 5V rail.The other voltages are determined by the turns ratio.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 20, 2024, 09:47:10 am
content: Quote from: inse on December 20, 2024, 09:24:41 amThanks. Corrected version.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 20, 2024, 10:26:39 am
content: More boring the other half, without the daughter board.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 20, 2024, 08:11:56 pm
content: Is someone able to figure out the topology of the bigger transformer?

author: inse
author_rank: Super Contributor
timestamp: December 20, 2024, 09:10:12 pm
content: It looks like a flyback converter, or what do you mean by topology?

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 20, 2024, 11:09:53 pm
content: Quote from: inse on December 20, 2024, 09:10:12 pmI mean the internal wiring and polarity of the windings of the transformer U146.Obviously the schematic is a result of pcb tracing and reverse engineering (at the early stage). That means its layout probably obscures its logic and working, which I will revise as my understanding of the circuit evolves.As always, I tend to like to spend time to learn from a repair project, beyond just getting it back working (which is of course also keenly sought).

author: MathWizard
author_rank: Super Contributor
timestamp: December 20, 2024, 11:58:47 pm
content: On U146, pins 3 and 4 are high voltage power in, then after a winding pin2 goes to the MOSFET drain. And there's an auxiliary winding between p5 and p6 on GND. Some of the energy that gets stored in the main winding, get transfered into this little winding, and then typically around 10-15V is created and rectified by D115, and smoothed by 100uF/25V near by.I'm thinking this is Forward Converter topology, but I don't remember the details enough to explain it all, but when the average current in the primary is increasing, energy gets stored building up a magnetic field. And then when the averge current in the inductor starts decreasing, and the field starts decreasing, then you get an induced voltage on the output side and aux windings, and current flows up from ground, and out the top of the windings.

author: inse
author_rank: Super Contributor
timestamp: December 21, 2024, 04:14:36 am
content: Read the UC3844‘s datasheet, it explains what the chip can drive.The windings can be measured with a multimeter, the polarity only tested when desoldered (or without the transistor).Feed a suitable (50kHz) signal in the primary and check the phase angle of the secondary voltage.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 21, 2024, 05:13:08 am
content: Diode D111 is in a metal casing, with marking "V5 319" visible. What type of diode is it? Zener?

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 21, 2024, 05:38:56 am
content: Quote from: MathWizard on December 20, 2024, 11:58:47 pmCorrect, rectified hi V from mains.Quote from: MathWizard on December 20, 2024, 11:58:47 pmCorrect. So if pin 2 and pin 3 or 4 is one windings, then its on/off will be switched by the MOSFET, used to generate induced voltage on the secondaries.Quote from: MathWizard on December 20, 2024, 11:58:47 pmThis is where the difficulties come from. I've had a closer look at the poles of the transformer trying to deduce the internal wiring.I use numbers to indicate the sizes of the wires connected to the pins, the smaller the number, the thicker the wire. And I use "2x" to indicate there is two wires to one pin (of same size)On the primary side:Pin 2: 1Pin 3: 1Pin 4: 3 (2x)Pin 5: 2Pin 6: 2So pin 2 and 3 has to be of one winding, so are pin 5 and 6. What's puzzling is that how can pin3 be connected to both ends of the same winding?!On the secondary side:Pin 7: 1Pin 8: 1Pin 9: 2, 3Pin 10: 2Pin 11: 1, 2(? not very sure)Pin 12: 1This is more difficult: One possibility is that pin3 7/8 are one winding, so are pins 11/23, and pins 9/10. But again, where is the single size 3 wire for?! (Unless the size 2 wire on pin 11 is actually size 3, going to pin 9.)There is no way that one end of the winding of the primary side goes to the secondary side, that will defeat the isolation. And I checked with DMM. Absolutely both sides are isolated.It looks like this weird thing – attached.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 21, 2024, 05:43:02 am
content: Quote from: inse on December 21, 2024, 04:14:36 amYes, taking U146 off helps. But I'm not prepared to do that due to the likelihood of ruining the transformer and the board.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 21, 2024, 05:56:41 am
content: Lower section after a bit tidy-up and correction.

author: inse
author_rank: Super Contributor
timestamp: December 21, 2024, 07:01:25 am
content: There is no connection between P159-3 and pin 6 of the UC3844.Otherwise mains/dc could be switched to the output (only once).Better move the motor speed control away as it has no connection to the power supply block.Then there is room for the transformer on the right hand side

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 21, 2024, 07:59:27 am
content: Quote from: inse on December 21, 2024, 07:01:25 amThank you for your eagle-eye spotting. Revised again. It is now much neater, so will make more sense. I've grouped the daughter board connector pins and motor connector pins together, which are somehow separate and independent (only somehow because, if the schematic is correct, P190-5 plays a part in the power supply).[Correction: P190-5 does not play a part in the power supply; it only controls U145, and through which controls the motor.]Questions:1. When both T125 and T126 are open, there will be a short circuit across the high V DC input. This makes me wonder if there is error here. (Double checked; this path exists, so short circuit can only avoid by design that T125 and T125 will never both turned on.)2. I recall I probably put the DMM at the Ohms setting when measuring the VCC voltage to GND (physically pin 7 of UC3844 and left hand side of R19), which appeared to cause a short circuit. R37 (1k) was violent blown. Further investigation suggests that R35 (220 Ohms) and R38 (0.56 Ohm) suffered damage, and T131 is blown. It also appears T125 is gone (and of course the fuse). The rectifier bridge is intact (thanks to the fuse). What the explanation of such damage? (All known affected parts are highlighted in dotted boxes.)

author: inse
author_rank: Super Contributor
timestamp: December 21, 2024, 08:15:50 am
content: T125 and T126 must not be engaged at the same time.T125 does the PWM for speed control, T126 is probably a motor brake shorting the motor for fast stop.If T131 is gone and takes R38 with him, the poor UC3844 is attacked by Mains/DC from two sides via R35 and R37 - no chance…

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 21, 2024, 08:21:50 am
content: Quote from: inse on December 21, 2024, 08:15:50 amThat's the only possible explanation.Quote from: inse on December 21, 2024, 08:15:50 amThat seems to make sense – when T125 is off, motor's power is off. Turning T126 on now will short P159-1/3 with R20 (0.82 Ohm), this will brake the motor actively through the IMF before it stops. Beautiful!

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 21, 2024, 08:29:52 am
content: Can you analyse the working around transformer U145?

author: inse
author_rank: Super Contributor
timestamp: December 21, 2024, 08:39:11 am
content: U145 generates the drive signal for the highside switch T125 as it needs to be above the Mains/DC level.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 21, 2024, 08:45:51 am
content: Quote from: inse on December 21, 2024, 08:39:11 amYes. So correction is made to my post above – P190-5 does not play a part in the power supply; it only feeds in U145, which controls the motor. This part is separate from power supply for the upper section.Though I'm still very keen to understand how exactly U145 controls the motor.Revised again based on improved understanding.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 21, 2024, 09:43:27 am
content: Correction: T125 is probably ok. Previous thought it was likely gone was because when measuring across B/E with the DMM diode mode, both ways seemed shorted. Now I understand this is the correct behaviour because these two poles are across not only R22 (88 Ohms) but also – and more importantly – the secondary winding of U145. This will make T125 shorted between B/E both ways.Reorganising the schematic makes it further away from the centre of disaster. It looks like if I replace these parts, (hopefully) at least I can get back to where I was. To be safe, I will also order replacement UC3844.This also means that, the motor is driven through U145 by input from P190-5, which much be a pulsing signal, and it's always only when the potential of the RHS of R22 is higher than LHS. This will turn on T125 and supply power (DC) to the motor. This is never a stable or sustained status, and will be an alternating on/off process. The nature of this pulse will determine the speed of the motor.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 21, 2024, 10:50:48 am
content: Some questions asked previously remain unsolved, such as those related to the wiring of the U146 windings. One more question –How is the input DC voltage on pin 3 of I150 (MC78L12ACP) achieved (presumably around 25V)?

author: inse
author_rank: Super Contributor
timestamp: December 21, 2024, 12:29:42 pm
content: There is an auxiliary winding between U146-5 and -6 that is generating the supply voltage on the primary side, just like the secondary windings except that it‘s tied to primary GND.There is an example circuit of a flyback converter in the Onsemi datasheet of UC3844 which shows a lot of the features of your patient.Except its using the aux winding for both supply and feedback, not an optocoupler.Or look at this app note if you really want to dive deeper:https://ww1.microchip.com/downloads/en/Appnotes/AN18-APID.pdf

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 21, 2024, 07:48:29 pm
content: Quote from: inse on December 21, 2024, 12:29:42 pmYes! And the app note is very helpful after a quick look. Will read it closer. This discussion has been very enlightening. Thank you very much. I wish you a super merry Christmas and a very fulfilling new year!

author: inse
author_rank: Super Contributor
timestamp: December 21, 2024, 08:07:21 pm
content: My pleasure and thanks in return

author: tautech
author_rank: Super Contributor
timestamp: December 21, 2024, 08:39:20 pm
content: Quote from: max.wwwang on December 21, 2024, 07:48:29 pmSo very often use of a SMPS controller IC very closely mirrors App notes and/or datasheets.They should be your first port of call when troubleshooting SMPS.Typical failure points are the controller IC Vcc cap (high ESR and/or diminished uF capacity), dropper resistors from HVDC and sometimes a flyback diode.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 21, 2024, 11:30:29 pm
content: Quote from: tautech on December 21, 2024, 08:39:20 pmThank you sir. Very true, the circuits using the chips are typically very similar, or even identical to the examples in the corresponding app notes. This is demonstrated even in my limited experience.I will take note of your nice summary of the common failures of these things. Probably you said the same in one of my previous repair projects asking for help here. And I vividly remember in one previous project, another member, seeing through the problem of the circuit I was working on, was almost screaming out --- just replace that silly cap!And it turned out he was exactly right.My problem is, probably due to my relatively low frequency of exposing to these projects, things learned at one point easily fades away from memory. This is something I feel very frustrated about.

author: tautech
author_rank: Super Contributor
timestamp: December 22, 2024, 12:22:26 am
content: Quote from: max.wwwang on December 21, 2024, 11:30:29 pmWhile I don't do many repairs these days I do take the time to inspect broken/damaged appliances and try to diagnose them and you soon get a feel for which parts of common SMPS are under stress and it stays with you constantly haunting......Much can be diagnosed simply and quickly;Won't go or in a tick mode:Is UVLO exceeded ? > check dropper resistors which are necessary for SMPS to start.Is the IC Vcc cap sound, I normally pull them anyways and tests generally suggest replacement is necessary.Flyback diode of course....IIRC yours has a zener which is another thing to check....IMO often the wrong ESR cap is used as they should be Low ESR to absorb charge from the flyback which is pulsing @ 15+kHz.The LV output caps are normally pretty easy, either deformed and/or running hot.....which if you are brave can be checked with a finger.A good amount of datasheet study and the experience gained has served me well.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 22, 2024, 03:19:46 am
content: Quote from: tautech on December 22, 2024, 12:22:26 amIt has more than one Zeners. What's the best way to check them apart from DMM diode mode? There does not seem to be a way to determine with DMM their voltage spec.What's your gut feeling about its original symptom of only not having stable 6V output (but the rest was all good)?

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 22, 2024, 04:48:00 am
content: Combined schematic tidied further based on the app notes, and it is probably close to figuring out the winding wiring, with one exception that a closed loop winding at the primary side does not make sense to me.Parts ordered from the mighty Ali. Fingers crossed.

author: tautech
author_rank: Super Contributor
timestamp: December 22, 2024, 07:45:15 am
content: Quote from: max.wwwang on December 22, 2024, 03:19:46 amVariable PSU and a current limiting resistor normally finds reverse breakdown voltage with little fuss.QuoteConnectivity issue somewhere or something intermittent with the diode or its snubber.There's really not much to check in a standalone winding on the secondary side.If nothing is obvious get out the magnifying glass.A little story about a MIG welder I repaired years ago after it had taken a fall.....Seemed simple enuf in that a small PCB mount transformer that provided control supply was only mounted by its through hole pins and in the fall the transformer had yanked on one and broken the fine copper to the primary.Poor construction not having a zip tie around it so to not rely on just the pins.Anyways, seemed an easy fix as RS had the exact transformer so it was briefly tested and sent back to the customer.A week later it was back and on further inspection a few cracked solder joints were found and repaired on the SMD part of the PCB.All was well one would think but another week later it was back !After spending ages looking for cracks none were found then a EE buddy suggested I flex the PCB and listen for clicking.Sure enuf clickity clack all day long.I ended up hand reflowing every component on that PCB and never saw it again.

author: tautech
author_rank: Super Contributor
timestamp: December 22, 2024, 07:50:37 am
content: Stick this in your datasheet records:https://www.ti.com/lit/ds/symlink/uc3844.pdf

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 22, 2024, 08:06:49 am
content: Quote from: tautech on December 22, 2024, 07:50:37 amReadling closely this very moment.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 22, 2024, 08:12:13 am
content: Quote from: tautech on December 22, 2024, 07:45:15 amGotcha. This requires it to be taken off the board if I understand correctly.Quote from: tautech on December 22, 2024, 07:45:15 amIf the secondary side is all good, i.e. problem is on the primary side, is it possible to have all other voltage rails working like a dream with only one rail scrap? I just don't understand how this is possible.Quote from: tautech on December 22, 2024, 07:45:15 amKey word is magnifying glass, and potentially physio therapy (which I found useful in one project). Gotcha.

author: inse
author_rank: Super Contributor
timestamp: December 22, 2024, 08:38:28 am
content: Quote from: tautech on December 22, 2024, 07:45:15 amOr the owner became tired of sending in the unit all the time…😉

author: tautech
author_rank: Super Contributor
timestamp: December 22, 2024, 08:45:27 am
content: Quote from: inse on December 22, 2024, 08:38:28 amNah I inquired a few times over the next few months and confirmed I'd won !

author: inse
author_rank: Super Contributor
timestamp: December 22, 2024, 08:45:54 am
content: Quote from: max.wwwang on December 22, 2024, 08:12:13 amDon’t waste any thought on the primary side, the flyback transformer is just like a normal iron core transformer:no secondary voltage at all: primary (circuit) brokenOne secondary voltage missing: secondary (circuit) broken.Don’t make it more complicated than it is…

author: tautech
author_rank: Super Contributor
timestamp: December 22, 2024, 08:50:32 am
content: Quote from: max.wwwang on December 22, 2024, 08:12:13 amCan't be.Referring to your schematic the 6V rail is a stand alone dumb rail with no feedback to the primary control side.It is possible the secondary is open/faulty and as intermittent one might wonder.....Dry joint would be my first suspicion.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 22, 2024, 09:15:20 am
content: I have not seen any different view on the location (primary vs secondary) of the problem. All I need to do is first get it back to where it was then reflow all of the joints related to the 6V rail.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 24, 2024, 08:15:33 am
content: Managed to trace and draw up the schematic of the daughter board, which is connected through the P190 socket (10-pin). Due to my lack of grasp of its working, the layout is certainly not the best. Insights are welcome regarding its working (or error spotted).Clearly, this is not part of the power supply – only supplied by it.Questions:1. Two comparators' output pins (U2-13/14) are bound together. This doe not make sense to me. How does it work (unless their outputs are bound to sync, but in that case what's the point of having two identical outputs)? I double checked the board for multiple times and am sure they are like so on the board.2. We know that on the main board, T125 and T126 should never be on at the same time (otherwise the rectified HVDC will be shorted). This interlock mechanism does not seem to exist on the main board. Is it on the daughter board? If so, how is this achieved?3. A general question about comparator – I learned from Dave's video that in op-amp circuit analysis, it can assumed that the current through the input pins is zero. Does this rule apply to comparators as well?

author: inse
author_rank: Super Contributor
timestamp: December 24, 2024, 09:51:06 am
content: 1. comparators are usually open collector/drain output, so you can parallel them as you need2. if you trust in your control circuit or software, you can skip a hardware interlock3. yes, they are high impedance inputsReverse engineering with only partial schematics and without access to the hardware is very tiring.If you want to gain experience, for example older HP service manuals for test gear are a valuable resource, the functional blocks are explained in detail.Reverse engineering without any aids is then next level.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 25, 2024, 03:52:56 am
content: Quote from: inse on December 24, 2024, 09:51:06 amIndeed it's tiring with only part of the schematics and without access to working hardware. Thanks for taking the trouble to look and answer my questions.1. I still don't understand. IIUC, with VCC=12V (and GND), the output of a comparator is either 12V or 0V. So if one output is 12V but another gives 0V, there will be a short between these two outputs? Of course this may be avoided by software. The problem I have still is, what use can this possibly have?2. Very true. In this case, the only pins that are logic interface with the main part of the machine is P190-1/2/3, of which 3 is output and the other two (1/2) input. It's two-way communication. Still trying to figure out how this works in principle with the rest of the machine only as a black box.3. Thanks. This way I can figure out the voltages at the net points on a resistor ladder (lower part) +12V - 27k/27k/27k/1k//1k - GND (where '//' may be set by op-amp U1A when the motor is on, through P190-10), which I believe somehow serves as motor status indicators in various parts of the circuit.Yes, old gear like HP and Tecktronx etc. have amazingly detailed service documentation, which is a superb source for learning and to gain experience. Will not lose the opportunity when I get my dirty hand on one, particularly for repair.Updated the PSU schematic combined (without the daughter board).

author: inse
author_rank: Super Contributor
timestamp: December 25, 2024, 05:38:41 am
content: Have a close look at the comparators (or LM311) datasheet which shows the internal structure.The output is not push-pull like an OPAMP, it‘s open collector.Also in your schematic, every(*) output has a pull-up resistor, so if you parallel them, either one can pull that node down.It is of the same use as an OR gate in a logic circuit.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 25, 2024, 08:26:00 am
content: Quote from: inse on December 25, 2024, 05:38:41 amGood point. Must be some sort of logic gate equivalent.It's LM339, and yes open collector according to its equivalent circuit. This however should mean binding two outputs is equivalent to an AND gate, not OR gate. See attached. Please correct me if I'm wrong.

author: inse
author_rank: Super Contributor
timestamp: December 25, 2024, 08:50:37 am
content: One OR the other transistor pulls down the output.It’s NOR, to be precisely

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 25, 2024, 09:02:53 am
content: Quote from: inse on December 25, 2024, 08:50:37 amI don't get that. In my eyes, any output of LO pulls down to LO. Output is HI only if all (if more than two tied together) is HI.Daughterboard schematic revised. It starts making some sense (though still a long way to go).Question now is that why U1A (op-amp) does not have feedback loop. How does that work, will any tiny difference between its inputs blow the transistor on its output? (I need to verify if the schematic is correct).

author: inse
author_rank: Super Contributor
timestamp: December 25, 2024, 12:41:50 pm
content: If we break it down to the output stage as in your equivalent circuit, it’s definitely NOR.As long as we agree that 0=0V and 1=Vcc (or at least >0V).The truth table is shown for example here:https://www.build-electronic-circuits.com/nor-gate/The Transistor cannot be blown as the base current is limited by the emitter resistor (1k) and whatever circuit is behind it.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 25, 2024, 10:06:48 pm
content: Quote from: inse on December 25, 2024, 12:41:50 pmI realise we are actually agreeing with the same end result only relative to different inputs. What I’m saying is, if without the knowledge of the comparator’s internal circuit, the outputs of two tied together can be treated as these two outputs passing through an invisible (and non-existing) logic AND gate.Your view point is from the signals on the bases of the two transistors (each through a resistor). Relative to these two ‘inputs’ (let’s call them B1 and B2), then the combined outputs of the two transistors is certainly NOR(B1,B2). Note that B1/B2 are only internal business of the comparators.We are saying the same thing, because —O1=NOT(B1), O2=NOT(B2)AND(O1,O2)=NOT(OR(NOT(O1),NOT(O2)))=NOT(OR(B1,B2))=NOR(B1,B2).On the transistor after the op amp, all after P190-2 is shown in the shaded box nearby, an LED going to GND, which is the only channel sending signal back to the main business centre of the machine. On what condition will this transistor turn on and off then?My analysis is here (based on the behaviour of ideal op-amps):1. When T125 (on the PSU board) is off, i.e. the motor is off (T126 is either on or off), P190-10 (connected to U1A-3) is 'subordinate' and will mirror whatever voltage on U1A-2, which will be 'driving' and is determined by the R ladder (along the dash line), and is 0.1V.2. When T125 is on, i.e. the motor is on (T126 must be off), P190-10 (U1A-3) will be forced to what is determined by the HVDC voltage (assuming 320V) and the R ladder (R16/R17/R18), which gives 2.1V (marked as "3.8V" in the schematic). U1A-2 is also flexible and is able to mirror this voltage (from 0.1V to 2.1V) by increase of current through the last 1k R on the R ladder. This transit will present a momentarily high level on U1A-1, which turns on the transistor and in turn the LED D109 (signalling to the business centre that the motor is being turned on). As soon as the levels on U1A-2/3 are (quickly )balanced, LED D109 is off.3. When motor continues to be on, the levels on the steps of the R ladder (along the dash line) are at different corresponding values, which will have an effect (as inputs) on the corresponding comparators. (The logic is complex, and more is to be figured out.)Similar to the case of two comparators' outputs tied together, but slightly different, is where U3-1 and U3-2 are connected through a diode. In this case, the latter can pull the former down, butnotvice versa. In other words, whenever U3-1 is LO, U3-2 will be LO regardless of the level of U3-2 when not tied together. But the level of U3-2 will not affect U3-1 at all. (It's puzzling what the function the relative independent part, top right around U3B, is.)My feeling about the overall working of this daughterboard circuit is like this (starting from the idle status – motor is not running). The motor on instruction comes in through phototransistor T128, on the PSU board (copied to the bottom left corner), turning on the 3F transistor, starting the charging of the C1 (which is part of the RC oscillator). Through complex log, ultimately, square wave is output through P190-6 (bottom right). This will turn on (and off) transistor T125 through transformer U146. The motor speed is able to be sensed outside of the PSU board, which can be used to adjust the intensity of light opposite of T128 (D108).

author: MathWizard
author_rank: Super Contributor
timestamp: December 26, 2024, 07:10:29 am
content: Quote from: inse on December 21, 2024, 12:29:42 pmOk they're using an AUX winding for feedback too. Yeah I forget SMPS can use windings for control, and not just opto's, or even high DC resistance networks between sides too I guess.A lot of the smaller switching transformers I've seen over the years, in things like DVD players and TV's, are just general parts, with extra pins for windings that are missing. Sometimes they would still have a winding, and just short both ends to GND. In some other model, they probably use that winding. But for some reason in this model, it's still economical just to use the same trans. with extra copper/etc, but short it.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 26, 2024, 07:21:08 am
content: Quote from: MathWizard on December 26, 2024, 07:10:29 amFeedback is still through the secondary side, with octo-coupler. There is auxiliary winding on the primary, which is more commonly on the secondary side and isolated, so is 'hot' (but is not a problem in this application, because it ultimately serves the motor, which is expected to be 'hot').This circuit is very interesting, and I'm deeply intrigued.Quote from: MathWizard on December 26, 2024, 07:10:29 amThere appears, as introduced above, to be a winding (primary side) both ends of which are tied together. I just wonder if that can possibly be done without blowing the transformer?

author: MathWizard
author_rank: Super Contributor
timestamp: December 26, 2024, 07:53:32 am
content: Maybe I'm mixing that up, since an open winding, would have no current, and a closed loop being cut by magnetic flux would have max current.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 26, 2024, 08:11:26 am
content: Quote from: max.wwwang on December 25, 2024, 10:06:48 pmFigured it out – the subcircuit around U3B works as an oscillator and square wave (between +12V and 0V) generator, output on U3-1 (i.e. U3B-1). Its frequency will be dependent on the values of the resistors and the (only) capacitor. This will shape the final output on U3-14 (P190-6), in a way that whenever it is LO, the final output will be HI (it's HI cycles will not have any effect).The relevant part reproduced in the attached.Such application is explained here:https://www.electronics-tutorial.net/analog-integrated-circuits/op-amp-comparators/comparator-as-a-function-generator/

author: tautech
author_rank: Super Contributor
timestamp: December 26, 2024, 10:08:46 am
content: Now showing a China flag ?I thought you were in Wellywood Max ?

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 26, 2024, 06:55:03 pm
content: Quote from: tautech on December 26, 2024, 10:08:46 amHave been and still in Wellywood.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: January 09, 2025, 08:01:15 am
content: Ordered parts all arrived sooner than expected. Replaced apparently blown R35, R37, and R38, as well as MOSFET T131 (measured as gone). Tried to power up the board without replacing the UC3844. No luck – Vcc had 1.2V or so. Then proceeded with a new 3844. It immediately went back to where it was, i.e. power supply working except for the 6V rail.When I was about to execute my reflow-all-related plan, one joint popped out as clearly suspious. Not sure if I just missed it before (unlikely?) or it became more visible after the time of handling and sitting idle. Anyway, redid that point, everthing is now sweet as.Thanks for all the help. It's been very enlightening (and occasionally refreshing my memory). Also had several reads of the IC datasheet, which helped a lot in understanding its working. Yet more reads are required.