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Monday, April 1, 2019

Toshiba Ultrasound Machine Part 2: Ultrasound Probe Teardown

      You might have seen Part 1 of my Toshiba Ultrasound machine  post, where I did a teardown of a Sonolayer SAL-32B  and also explained the basic physics that goes into turning sound into images.

    One thing I couldn't really do was to also take apart the probe. That's because it would have been a destructive teardown and I still wanted to play around with the thing.
  But now, the teardown gods have  blessed me with two other Toshiba probes, one of which is very similar (i.e. same connector but the mating is a bit off) therefore, I'm going to cut one open and actaully see what's inside and how the arrays are mounted.

   Key note here.... this is 80's, early 90's tech we're talking about, so some technical aspects may have drastically changed in the new generation probes. (Yes, this is a hint to anyone with access to a modern probe to also perhaps post some pics of the insides of one)

Figure 1. The probe in all its glory

   I actually got two probes, this and a cavity probe. So yes, no  expense was spared when hosing the probes down with the germicide. Now, it's time to get the tools out.

Fig.2 Probe connector

  The casing around the connector is pretty easy to take off. Just a few screws...

Fig.3  Just like Fig.2 except in more pieces

  There seems to be some colour coding to the wires... white blue, red and the ground and shield. Will maybe have to see later in the probe head how they're connected.

Fig. 4 Probe connector wiring. Also, the 4 black wires seem
 to perhaps code some settings for the
 ultrasound machine? Probe type or other atributes?

  Now time to take care of the other end. The plastic itself is a bit weird, in that it's fairly soft and elastic. Once inside, you can aleady see that everything is potted :|
 Technology, am I right...

Fig.5 Probe head. Cry havoc and let slip the
 flat-blade screewdriver. 

  Of course, the potting makes sense. One has to attenuate any signal also emitted from the back of the piezo element.

 Fig. 6 Probe head elements

   As in the drawing  above,  you can see the "Acoustic Insulation", in this case the potting compound in the probe head, filling up all the space behind the piezo elements. It's dense and rubbery and kind of a PITA to get through. Exactly what you'd want from an RF and sound dampening material.

Fig. 7 Probe head cracked open. Black stuff is the actual potting compound. 
The black stuff on the outer plastic is conductive

  And of course, the name might be "ultrasound" but we're still talking low-frequency, low amplitude RF here. 1-10 MHz, so of course there's also copper shielding around the whole probe head. 
  The potting compound isnside the probe is not conductive, however, on the inside of the plastic casing there is a thin deposit of conductive material...carbon maybe?

Fig. 8.  Just in case nobody believes me it's conductive...

   On the front of the probe there seems to be a plastic casing or bezel, together with even more copper shielding underneath.
  As a side note, the copper shield is pretty thich stuff. It kinda feels like 35um copper clad thickness. And no, no blood was drawn for the teardown gods.

Fig. 9 Probe head...head. Plastic bezel and some more 
copper shielding visible beneath the bezel.

Fig. 10 Probe head: Black shiny thing is the back of the  piezo element 
backing material

Once the bezel is out of the way, the potted body and the actual probe head come apart, connected only through some  flex PCB material. 

The black glossy stuff you see in the pic above is actually glass (just normal, perhaps tempered  or borosilicate glass). This would be the "Backing Material" from Fig.6

Fig. 11 Probe head, front view. Flex strip, piezo elements 
(brown thing) and rubber face (white thing)

    The actual piezo elements, in their full glory (that would be the retangular brown thing). So, the wires come in on a PCB flex board then that goes out to every piezo element. The white thin plastic on the front (the one partly cut off) is some kind of rubber or silicone, with probably some stuff in it. It felt grainy when cutting it with a blade.

Fig. 12 Probe head side view

   So there's a bit more going on in the probe head than I initially thought. The piezo elements are backed by a rectangular piece of rubbery epoxy with  probably tungsten carbbide powder and then, behind that, the piece of glass from Fig. 10.

Fig.  13 Flex PCB connection to piezo elements (Top. left and right) 
and Piezo elements themselves (Bottom)

   Here you can see how the connections are made to the individual piezo elements. Each wire from the probe connector (Fig.2) comes in onto a small PCB (Fig. 14) with 330R resistors connected to Ground and then go further to the flex strip which then connects to 4 piezo elements (Fig. 13, Top, left and right) At least that's my assumption.
  The return from the piezo elements seems to be common and of course split into two groups, just like the PCB with the 330R resistors.

Fig. 14 Main body of the probe head and PCB with 330R resistors

   I don't know if this is to better achieve beam forming or it's because of how the actual hardware part works. This will require more investigation on the machine itself to find out. Also, because of the potting, I couldn't really figure out the meaning of the blue, white and red wires.
  And no, I didn't manage to count all the traces on the flex PCB. After loosing track 2 or 3 times, I just gave up on this.

If someone more knoledgeable about this can share some info, he's welcome to do so in the comments, below. Thanks.

   I've already started reverse engineering some things on the machine side of things and got some scope shots, but I need to probe around more to get a feel for what's really going on in there. 
  Also, because of ground loops, and not knowing how the architecture of the thing is layed out, I'm hesitant to poke my probe willy-nilly, so until I'll design a 1:1 differential probe, things won't progress much.

     That's it fow now. As always, you can find all the pics from the teardown in the albun located here.


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