Computerised sewing machine BERNINA Activa 145 repair

[max.wwwang]

Got this lovely puppy. Had good fun tearing it apart and appreciating its beautiful design and construction.

I want to kick myself because, in poking around and diagnosing, I made the problems significantly worse. When I got it, its display worked perfectly well and responding to all the buttons; only the motor did not run when pressing the pedal (and I've checked with an external PSU and can confirm that the motor is ok). I inadvertently shorted two tiny pins where the cable from the PSU comes in (with a 35V DC in between) causing a spark. Now the machine has no signs of life at all. 

Apart from some superficial mechanical damage of a "polyfuse" which is believed ok (discussed here), there are no other apparent visible damages that would indicate a quick starting point.

This youtube video is very very relevant, rare to find, and in the meantime, of high quality (though a slightly different model).

[max.wwwang]

Front side of the only board (apart from the modular PSU).

[max.wwwang]

The other side ...

Also captured in the photo is a delicate plastic thingy, which is supposed to clipped on the six SMT components above it -- three of these are believed to be LEDs (visible or IR or whatever but should not be laser because there is no laser risk warning on the outside of the machine), the other three must be optical sensing puppies (don't know its exact type yet). The six prongs, each with its end cut at a 45 deg slope serving as a mirror, serve as the pathways of the optical signals between the three pairs of senders/receivers, which couple with the three slotted discs mounted on the main axle. So this is the critical part of the sync mechanism.

[max.wwwang]

At the first step, a thorough examination of the PSU, which is a linear PS not the SM rubbish. Attached include the reverse engineered schematic.

Once shorted was between the pins #6 and 7, with about 35V DC across. Nothing seems to be broken. With this good understanding of the internal working of the PSU, I don't think that momentary short circuit would break anything on the board that this PSU serves? Any idea?
 
[One correction -- well, it's not completely no signs of life. The LCD display lights up, and the sewing light turns on. The three LEDs on the main board of the sync module also glow. The sensing LEDs(?) also work by sending highs or lows depending on whether the light passes through or is blocked. But nothing else. No display on LCD ...]

[max.wwwang]

I have carefully taken off the cover of the LCD unit and had a look, which is electronically connected to the traces on the board through two zebra rubber pieces. Underneath it is the lighting module. So I believe there is not another driver module for the LCD display, i.e. it's driven by the only MCU -- a Hitachi HD6473837F chip.

First questions I'm keen to hear your thoughts on:

1) What is likely the reason of the LCD module not showing anything (the voltages received form the PSU onto the board all seem correct)?
2) Does the PSU schematic make sense? I don't quite understand the reason for pin 7.

Thank you.

[max.wwwang]

Any one knows what this is in the photo (transistor or MOSFET) with the marking of "A4t" and "13"(vertical)?

Is it this?

[Update] I think it is. There are many such little chips with minimal markings. Fortunately datasheets all seem available online.

[max.wwwang]

What has been done so far (however the situation sees no improvement yet):

- Some tracing/reverse engineering, to understand how the parts of the circuit work.

- Checked the PSU module. All seems fine. All the output pins measure correct AC/DC voltages.

- All the analogue/digital power rails read ok. Digital Vcc +5V, stepper motor driver supply +35V or so (surprisingly coming from pin 7, not 8, i.e. through another diode after the bridge rectifier, not directly after the rectifier). Lamp supply 12V AC ok.

- Looked into the MCU, particularly the LCD driver module. OSC1/OSC2 pins measure correct pulse waveforms. The module has standby/sleep etc modes, so I guess the LCD won't show anything unless these modes are cleared. According to the datasheet, these modes are cleared by setting RES to low. It measures low, but the problem is it never gives a high. Perhaps this is a problem (because I think until RES returns to high, reset is not completed)?

RES comes from a 5-leg IC marked with "BF". Google tells me it's a "Bipolar Voltage Detector IC". It's fed by 5V on Vdd(5), GND(3), Vout(4) goes to RES, which seems its only input. I'm unable to see how RES could return to high. Does anyone know what the trick is here?

[max.wwwang]

Progress. Reading into the datasheet of the Bipolar Voltage Detector IC, it does work the way by first setting Vout to low then returning back to high, as expected. So when it's powered on for a while, and if Vdd is above the relevant threshold (Vdet+ delta Vdet), but Vout remains low, probably it's broken.

By forcing Vout (then RES) to high (+5V) with a jump wire, I can see segments of LCD showing up.

This at least confirmed that the MCU's LCD driver module (hopefully the whole MCU) is not faulty.

Highlighted in photo is this bad boy.

[max.wwwang]

Just when I see some light at the end of the tunnel, now a catastrophe. There is virtually no chance now can I bring it back to the way it was, because not only one 74HC00D chip is blown, the MCU (as the brain of the whole thing) is also gone. It won't work even with a new MCU chip because I will not have the code in it.

So I will have to go down the less satisfactory options of my 'multi-tiered' plan for the repair. What's common for all the 'tiers' is to have fun and learn things along the way. Other than that:

1) Ideally, bring it back to work the way it was (now virtually impossible); or
2) Repurpose the working parts of the machine if the first cannot be achieved.

Since all the motors and the mechanical parts are all good, it's possible to make my own control module (based on Arduino for example) to drive the stepper motors etc. to make the machine run.

Under this second option, there are a wide range of possibilities -- from making it barely run to turning it back into a fully functional sewing machine close, or even superior, to the original design (because I will be in full control of that can be done! Of course, anything on this side of the scale may remain only in imagination).

At the easiest end of this scale, it will be a plain straight stitch sewing machine, perhaps even without back-tack (which means it's practically useless).

Next to it, is to have the stich-length control, so also back-tack. This will be a practically useful sewing machine.

Even better, the sync related parts, both mechanical and electronic, may be brought back to action. One good thing is that those LEDs and optical sensors are still good. Due to its delicate design, I will still need to have the dead PCB in place to hold those LEDs and the plastic optical thingy in the right position relative to the main axle to make the synchroniser work. Anything starting from this point (i.e. requiring the synchroniser) will be challenging to me, even merely with zig-zag stitch ...

[max.wwwang]

Despite the possibilities of upcycling the remains of this machine, since the 74HC00D replacement chips have arrived, I attempted to replace this one to see if there was any chance of getting it back to the condition of when I got it.

Although miracle did not happen, I tried for my first time desoldering and soldering an SMD chip like this, and the result was more than satisfactory. One of the trace under the chip was blown away so I had to connect one of the legs to the through hole on the PCB (from the other side), which was also done neatly in my eyes.

It was much easier than I thought. I cut off the legs of the broken chip with a sharp knife then took the remaining legs away one by one under the solder iron. Clean the scene with desoldering braids, then solder the new chip back carefully. Not too bad, woohoo! 

[Shown at the bottom right corner of the photos.]