Author Topic: BERNINA power supply board (Switch Mode) issue diagnosis and beyond (fixed)  (Read 3956 times)

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Offline max.wwwangTopic starter

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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.
« Last Edit: January 09, 2025, 08:01:38 am by max.wwwang »
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Offline u666sa

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #1 on: December 18, 2024, 06:06:48 am »
List voltages of all pins for UC3844
 

Online inse

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #2 on: December 18, 2024, 06:12:07 am »
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
 

Offline floobydust

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #3 on: December 18, 2024, 06:18:10 am »
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.
 
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Offline max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #4 on: December 18, 2024, 08:12:11 am »
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.
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Offline max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #5 on: December 18, 2024, 08:12:44 am »
List voltages of all pins for UC3844

Due to further damage, unfortunately I can’t  do this at the moment.
« Last Edit: December 18, 2024, 06:08:03 pm by max.wwwang »
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Offline max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #6 on: December 18, 2024, 08:16:05 am »
Bernina is a brand of sewing machines, afaik.
The 6V is missing while the other voltages are present - did I get this right?
That's correct before the recent further damage.

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.
I think I tested some of these but didn't get a clue. Thanks for your answer.

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
That's exactly right. There is also a small daughter board connected to socket P190.
« Last Edit: December 18, 2024, 06:08:59 pm by max.wwwang »
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Offline max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #7 on: December 18, 2024, 08:17:14 am »
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.
That's exactly right. There is a black cover over the diodes and phototransistors.
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Online inse

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #8 on: December 18, 2024, 08:38:46 am »
If the fuse is blown, probably T131 is shorted as well.
If so, replace UC, diode , resistor and T131, check R35
 

Offline max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #9 on: December 18, 2024, 10:59:19 pm »
If the fuse is blown, probably T131 is shorted as well.
If so, replace UC, diode , resistor and T131, check R35

The 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.
« Last Edit: December 18, 2024, 11:07:51 pm by max.wwwang »
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Online inse

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #10 on: December 19, 2024, 05:38:44 am »
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?
« Last Edit: December 19, 2024, 05:41:08 am by inse »
 

Offline max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #11 on: December 19, 2024, 11:13:36 pm »
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?
Thanks. 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.
« Last Edit: December 19, 2024, 11:47:04 pm by max.wwwang »
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Online inse

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #12 on: December 20, 2024, 07:20:38 am »
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.
« Last Edit: December 20, 2024, 07:58:22 am by inse »
 

Offline max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #13 on: December 20, 2024, 07:40:36 am »
In case it turns out, a failure in the transformer winding is causing the intermittent problem (what I suspect), maybe a circumvention was the only option: replace the incandescent bulb by a 24V LED lamp and feed it from the 28V rail.

That'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.)
« Last Edit: December 20, 2024, 07:44:34 am by max.wwwang »
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Online Sorama

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #14 on: December 20, 2024, 07:54:28 am »
Beter check first for dry solder joints.
 

Online inse

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #15 on: December 20, 2024, 07:57:27 am »
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.
« Last Edit: December 20, 2024, 08:17:19 am by inse »
 
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Online Sorama

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #16 on: December 20, 2024, 08:22:56 am »
I do see some suspicious rings on some solder joints.
 

Offline max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #17 on: December 20, 2024, 09:00:38 am »
Here's the schematic of the upper section. Excuse my perhaps non-standard (incorrect) use of symbols (however feel free to point out).
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Offline max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #18 on: December 20, 2024, 09:12:21 am »
I do see some suspicious rings on some solder joints.
Thanks. Had a close l but didn't find any obvious problems. Which ones you find suspicious?
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Online inse

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #19 on: December 20, 2024, 09:24:41 am »
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.
« Last Edit: December 20, 2024, 09:28:25 am by inse »
 

Offline max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #20 on: December 20, 2024, 09:47:10 am »
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.
Thanks. Corrected version.
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Offline max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #21 on: December 20, 2024, 10:26:39 am »
More boring the other half, without the daughter board.
« Last Edit: December 20, 2024, 10:29:45 am by max.wwwang »
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Offline max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #22 on: December 20, 2024, 08:11:56 pm »
Is someone able to figure out the topology of the bigger transformer?
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Online inse

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #23 on: December 20, 2024, 09:10:12 pm »
It looks like a flyback converter, or what do you mean by topology?
 

Offline max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #24 on: December 20, 2024, 11:09:53 pm »
It looks like a flyback converter, or what do you mean by topology?
I 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).
« Last Edit: December 20, 2024, 11:32:56 pm by max.wwwang »
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