Author Topic: BERNINA power supply board (Switch Mode) issue diagnosis and beyond (fixed)  (Read 3946 times)

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Online max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #50 on: December 22, 2024, 08:06:49 am »
Stick this in your datasheet records:
https://www.ti.com/lit/ds/symlink/uc3844.pdf
Readling closely this very moment.  :popcorn:
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Online max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #51 on: December 22, 2024, 08:12:13 am »
Variable PSU and a current limiting resistor normally finds reverse breakdown voltage with little fuss.
Gotcha. This requires it to be taken off the board if I understand correctly.

Connectivity 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 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.  |O
 
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.  |O
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 !  :palm:
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.  :phew:
Key word is magnifying glass, and potentially physio therapy (which I found useful in one project). Gotcha.  :popcorn:
« Last Edit: December 22, 2024, 08:13:54 am by max.wwwang »
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Offline inse

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #52 on: December 22, 2024, 08:38:28 am »
Sure enuf clickity clack all day long.
I ended up hand reflowing every component on that PCB and never saw it again.  :phew:
Or the owner became tired of sending in the unit all the time…
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Online tautech

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #53 on: December 22, 2024, 08:45:27 am »
Sure enuf clickity clack all day long.
I ended up hand reflowing every component on that PCB and never saw it again.  :phew:
Or the owner became tired of sending in the unit all the time…
😉
:-DD

Nah I inquired a few times over the next few months and confirmed I'd won !  :phew:
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Offline inse

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #54 on: December 22, 2024, 08:45:54 am »

If 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.  |O
Don’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) broken
One secondary voltage missing: secondary (circuit) broken.
Don’t make it more complicated than it is…
« Last Edit: December 22, 2024, 08:47:27 am by inse »
 
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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #55 on: December 22, 2024, 08:50:32 am »
Variable PSU and a current limiting resistor normally finds reverse breakdown voltage with little fuss.
Gotcha. This requires it to be taken off the board if I understand correctly.

Connectivity 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 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.  |O
Can'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.
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Online max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #56 on: December 22, 2024, 09:15:20 am »
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.
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Online max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #57 on: December 24, 2024, 08:15:33 am »
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?
« Last Edit: December 24, 2024, 08:55:44 am by max.wwwang »
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Offline inse

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Re: BERNINA power supply board (Switch Mode) issue diagnosis and beyond
« Reply #58 on: December 24, 2024, 09:51:06 am »
1. comparators are usually open collector/drain output, so you can parallel them as you need
2. if you trust in your control circuit or software, you can skip a hardware interlock
3. yes, they are high impedance inputs

Reverse 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.
 
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Re: BERNINA power supply board (Switch Mode) issue diagnosis and beyond
« Reply #59 on: December 25, 2024, 03:52:56 am »
1. comparators are usually open collector/drain output, so you can parallel them as you need
2. if you trust in your control circuit or software, you can skip a hardware interlock
3. yes, they are high impedance inputs

Reverse 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.

Indeed 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).
« Last Edit: December 25, 2024, 08:45:00 am by max.wwwang »
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Offline inse

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Re: BERNINA power supply board (Switch Mode) issue diagnosis and beyond
« Reply #60 on: December 25, 2024, 05:38:41 am »
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.
« Last Edit: December 25, 2024, 05:41:34 am by inse »
 
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Online max.wwwangTopic starter

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Re: BERNINA power supply board (Switch Mode) issue diagnosis and beyond
« Reply #61 on: December 25, 2024, 08:26:00 am »
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.
Good 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.
« Last Edit: December 25, 2024, 08:36:39 am by max.wwwang »
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Offline inse

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Re: BERNINA power supply board (Switch Mode) issue diagnosis and beyond
« Reply #62 on: December 25, 2024, 08:50:37 am »
One OR the other transistor pulls down the output.
It’s NOR, to be precisely
« Last Edit: December 25, 2024, 08:52:24 am by inse »
 

Online max.wwwangTopic starter

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Re: BERNINA power supply board (Switch Mode) issue diagnosis and beyond
« Reply #63 on: December 25, 2024, 09:02:53 am »
One OR the other transistor pulls down the output.
It’s NOR, to be precisely

I 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).
« Last Edit: December 25, 2024, 09:10:05 am by max.wwwang »
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Offline inse

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Re: BERNINA power supply board (Switch Mode) issue diagnosis and beyond
« Reply #64 on: December 25, 2024, 12:41:50 pm »
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.
« Last Edit: December 25, 2024, 01:50:29 pm by inse »
 

Online max.wwwangTopic starter

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Re: BERNINA power supply board (Switch Mode) issue diagnosis and beyond
« Reply #65 on: December 25, 2024, 10:06:48 pm »
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.

I 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, but not vice 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).
« Last Edit: December 26, 2024, 07:45:31 am by max.wwwang »
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Offline MathWizard

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #66 on: December 26, 2024, 07:10:29 am »
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
Ok 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.
« Last Edit: December 26, 2024, 07:50:39 am by MathWizard »
 

Online max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #67 on: December 26, 2024, 07:21:08 am »
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
Ok 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.

Feedback 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.

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.
There 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?
« Last Edit: December 26, 2024, 07:46:20 am by max.wwwang »
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Offline MathWizard

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Re: BERNINA power supply board (Switch Mode) issue diagnosis and beyond
« Reply #68 on: December 26, 2024, 07:53:32 am »
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.
 

Online max.wwwangTopic starter

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Re: BERNINA power supply board (Switch Mode) issue diagnosis and beyond
« Reply #69 on: December 26, 2024, 08:11:26 am »
(It's puzzling what the function the relative independent part, top right around U3B, is.)

Figured 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/
« Last Edit: December 26, 2024, 08:19:26 am by max.wwwang »
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Online tautech

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Re: BERNINA power supply board (Switch Mode) issue diagnosis and beyond
« Reply #70 on: December 26, 2024, 10:08:46 am »
Now showing a China flag ?  :-//
I thought you were in Wellywood Max ?
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Online max.wwwangTopic starter

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Re: BERNINA power supply board (Switch Mode) issue diagnosis and beyond
« Reply #71 on: December 26, 2024, 06:55:03 pm »
Now showing a China flag ?  :-//
I thought you were in Wellywood Max ?
>:D >:D >:D
Have been and still in Wellywood.  :popcorn:
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Re: BERNINA power supply board (Switch Mode) issue diagnosis and beyond
« Reply #72 on: January 09, 2025, 08:01:15 am »
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.
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