replacement ion + thermocouple gauge controller

In an older post I showed my original hot filament ionization gauge controller and the abuse it took during shipping of my vacuum chamber setup. Despite the fact that the only likely damage preventing it from achieving operational status was most of the front panel switches and some broken LED leads, I decided to relegate it to spare parts duty, and instead purchased the same model with some different options for $60 shipped.

Despite it looking more modern than my original, it's actually from the same period. This one just had the -1 dark grey finish option specified. The most notable difference with this one is the inclusion of two additional needle gauges to the left. I mentioned in the previous ion gauge post that some of these controllers include circuitry to read thermocouple-type vacuum sensors, and in the case of this unit, it can read two of them, though I only have the need for one. So this unit covers all the bases I need for high vacuum measurements, and the rest of the range can be covered by a mechanical diaphragm gauge. There are a few options that my old unit has that this one doesn't: some process control related stuff. For example, I could set some pressure trigger points that would activate relays for the purpose of automating some tasks. Nothing I particularly care about. Because of that, this unit is missing some relays, but otherwise has all the necessary trim pots. However, the front panel doesn't have the requisite holes to get access to them. Jeez...

Not everything has been rosy with this unit, and I haven't even powered it on yet. When I first removed it from the box, it was further packaged in a few layers of Saran wrap, but I heard something rattling around inside. After peeling away the polyethylene plastic, I found that neither the top nor bottom lids were screwed on. Well, I was going to open it up anyway, so I guess that's.... convenient?

The thing that was rattling around, but had already been corrected before taking these photos was that the single relay on the right side PCB fell off its base somehow despite it having been secured with a tie wrap that was too tight to slip over for reuse (so what you see is a new one). Also, the ribbon cable towards the bottom that connects the two boards popped off. Easy fix, and luckily my old unit had a plastic retaining clip that keeps the connector secured. Lastly, my original unit had a small shield that covered the sensitive electrometer circuitry in the upper right hand corner. Strangely, I don't see any markings/scratches on the PCB around the hole where it should have been attached from any fastener hardware. It might never have had it. Still, I have a shield so I think I'll use it. Other than that, I have the necessary fasteners for both lids, and the missing fuse holder cap.

I did just notice that unlike this unit, my original one didn't have solder masks on the PCBs. So despite the fact that the manual I have very much covers this unit, I'd say that this one is definitely newer.

The one other thing I'm aware of that I need to address is the thermocouple sensor interface which is via the the left hand card edge connector at the rear. I found one on Digi-Key that I think is suitable, but I'll wait to purchase it until I have a few more things to add to the list. Then I'll need to come up with a design for a 3D printed housing to securely attach it to the controller.

feeling overpowered

Finally took a bit of a better look at the 3 channel filament power supply from my vacuum chamber purchase, and I realized that it contained 3 variable autotransformers rather than my initial claim of wirewound potentiometer. In retrospect, it makes a lot more sense.

From what I could find online, SEM filaments tend to operate from roughly between 3 and 4 volts at 5-ish amps. As we can see from the second image, the isolation/step down transformer puts out 24v. I didn't check the circuit routing in the filament holder, but based off the way the two leads were coming off of it, I suspect that the filaments are wired in series, which makes sense in light of the isolation transformer output.

This is not going to be suitable for my needs, so I'll need to figure something else out. I could potentially use a computer PSU and add an additional variable voltage regulating circuit off the 5V output, but I'd also need to convert it so that the outputs are floating instead of being tied to ground as is typical. Since the only spare computer PSU I have available apparently has a bunch of electrolytics spewing their guts, it might not cost much more to buy a decent suitable bench power supply off eBay.

At least I'll have a few center tap 24V transformers, variable autotransformers, and 5A AC panel meters in my parts bin for possible future use.

mystery flange

I've been disassembling the vacuum chamber system to transport it home more practically. I didn't exactly want to remove the ion gauge, but I was reasonably worried that I would break it if I didn't. After I got home I took a better look at it and saw a mating flange style I wasn't aware of. Several hours worth of google searches didn't reveal anything, and so I still have no idea what kind of flange is on this.

On the gauge is a male-type plug with a small step that has what appears to be a brass-like gasket. It did not easily come off, and I'm not sure I should even try to remove it, therefore I don't know if it has any kind of Conflat style knife edge. On the chamber side is a female socket. It doesn't have a knife edge, but it's possible the the first step is ramped, and it has a pretty sharp edge, but it's hard to tell if that's actually the case. 

Anybody know what kind of flange this is?

sourcing electrons

In my last post I mentioned that I received two packages in the mail. What was in the other?

Tungsten SEM filaments!

I can find a bunch of references to the Cambridge S4-10 (or S-410), but very little about the unit itself. Ah, well. It's not particularly important. While I'm not building a SEM, my planned experiments will require an electron source, so these filaments are a good option. And since I purchased this unused lot of 20 for $15 shipped, it's economical too. I think this supply will outlast my needs.

quick recap

Two packages arrived at my doorstep today. The first, with Digi-Key logos on the box were the electrolytic capacitors I ordered for my TCP 270 turbomolecular pump power supply.

A veritable mix of Panasonic, Vishay, and United Chemi-Con

While it's probably overkill to to have chosen 105°C parts due to the very well ventilated power supply case, the increased price over ones rated for 85°C is negligible for a one off project like this. More annoying is price and limited selection of axial caps. If I was just trying to get the job done and all I had were radials on hand, then I would have happily stuffed those in. 

Before

Who needs solder mask? That might help prevent traces from peeling off when reworking the board, and we wouldn't want that! The annular rings seemed unnecessarily small, and of course the holes were unplated. There were a few ring to lead spots I had to bridge because they got ruined during component removal.   

After

I had to avoid setting one of the larger caps that replaced an old maroon part (center right) flush with the board due to a ruined trace on the top, so that I could actually get an iron on it. Honestly, I don't know how they could have had confidence that there would have been an acceptable solder joint between the cap lead and the trace in the original assembly.  

I somewhat uprated the large cap. Originally it was 4700uF, which I would have left as-is, but it was rated -10/+50% anyway, therefore I opted for a +/-20% 6800uF because it was easier to find at a reasonable price. The original cap had 4 leads, whereas the replacement has 2. Two of the holes had adequate spacing (though slightly too small for the leads themselves), but the unpopulated relay socket seen to the right of the cap got in the way. So instead I had to choose two different holes which were spaced slightly too far apart, and the hole on the negative side wasn't actually connected to any trace. So I had to wallow out the holes a bit to get it to fit. Also, a small strip of copper tape and solder connected the negative lead to the negative trace. It was a bit of a pain in the ass to work on because there's wires soldered to the board on all sides, and the only way to increase accessibility would have been to disassemble the unit more than I wanted to.

I slapped it back together. Hopefully there are no unpleasant surprises when I finally get a chance to use it.