Thursday, January 7, 2016

Superlink teardown

Here is part 3 of the Cryocooler Saga! You can see Part 1 and Part 2 also.

So what does all that stuff in the box do? What's necessary to keep it running happily along and what can be cut out? Let's figure it out!

First off, what do we have under the hood? Here's a rough labeling of some of the important parts:

Click for big!
Wow, that's crammed in there. Nice!

In addition to the cryocooler that's the reason I bought this whole shebang, there's a power and control circuit board, a dewar containing the superconducting elements (connected to the cryocooler with a vacuum flange, presumably to maintain a tight seal on the cold side to minimize leakage), and a cooling fan that forces air through the cooling fins on the heat-rejection location on the cryocooler. There's also a jumble of RF wires that presumably connect to whatever it's supposed to be super-filtering. Those can be disconnected from the dewar by loosening each of the screw connections.

The cryocooler is held in position with two machine screws near the cooling fan, and the dewar is held in place with a pipe clamp. The cryocooler screws can be easily accessed by removing the cooling fan first, as shown below. First, I removed the dust filter in front of the fan. This makes visible the four screws holding the fan in place.

Once the fan is removed, the screws can be removed by putting an allen wrench (or a star drive, as I used) through the fan hole.

Next, I disconnected the back plate with all of the RF wires from the base of the unit by removing a few screws.

The two on the bare steel tab. Ditto on the other side.

Next, I disconnected the clamp holding together the vacuum flange connecting the cryocooler and the dewar near the heat rejection fins, and completely loosened the pipe clamp holding the dewar down.

This allowed me to remove the entire cryocooler-Dewar assembly, or more easily, separate the two and take them out separately. The cryocooler's cold head is covered in a thermal paste.

The dewar looks to be of an interesting construction - it has a piece that slides up the cold head past the flange, which appears to be welded on the end. My guess is that it's two pipes with a small gap between them in which there's a vacuum that further insulates the cold head from the heat rejection area. Maybe someone who knows better can correct me.

Before removing the cryocooler, you'll need to disconnect a few wires from the circuit board. Here's an annotated photo of what's going on there. I disconnected the rejection temperature sensor (bottom right) and the cryocooler power connector (bottom left) and loosened the cable ties holding the wires in place in order to remove the cryocooler. I also needed to disconnect the wire bundle to the dewar on the right in order to remove it.

Click for legible

As mentioned in the photo and a previous post, the only signals needed to run the cooler are the rejection temperature sensor and the cold-side temperature sensor from the dewar (which is the pin pair the fourth from the bottom on the wire harness). Some of the other pin pairs show a voltage difference while the board is running, but I can't figure out what they're doing, and it doesn't seem to throw any faults if they're disconnected.

Me trying to figure out what the pins do. Turns out, not much that I need worry about, maybe?

Next up: Spoofing that temperature signal so I can use my own cold side temperature sensor!

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