Various Repairs, Progress, Eye-Candy

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Liam David
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Various Repairs, Progress, Eye-Candy

Post by Liam David »

Post still under construction

This is a mishmash post of various things I've been working on over the last little while as I prepare to assemble a new cube fusor. Maybe you'll find some things interesting. I sure didn't know how the inside of a cold cathode gauge or gate valve looked.

RGA Repair

My old (1980s) Spectra Vacscan Plus RGA system began to die early this year. It had always suffered from reduced sensitivity but it was plenty good enough to tell me that hydrocarbons were contaminating my chamber. Then, the signal disappeared. No peaks or total pressure measurement. The slow fading over time was consistent with oxide buildup on the ionizer assembly, confirmed by a rep of MKS. Examining the ionizer assembly indeed revealed a significant buildup of oxides and thin films.

PXL_20220525_184418980.jpg

I attempted to mechanically remove the worst offenders with scotchbrite and fine sandpaper, and a test under vacuum appeared to be successful... until a minute later the signal died. Skipping the many hours spent on diagnosing the mechanical and electrical systems, I found that there were two issues. The oxides did indeed reduce the sensitivity, causing the steady reduction over time, but it somewhat hid the dying transimpedance amplifier. It's an obsolete part, but thankfully Ebay exists.

PXL_20220615_191616377.jpg

I used 35% hydrochloric acid to completely remove the oxides and films from the ionizer assembly. While I was at it, I completely disassembled the RGA to clean it and look for further issues. None were found and the analyzer works again, matching my vacuum gauge within ~10%.

Partially disassembled, showing precision-aligned quadrupole rods.
Partially disassembled, showing precision-aligned quadrupole rods.

RF + voltage sweep to the quadrupole rods.
RF + voltage sweep to the quadrupole rods.

RF frequency
RF frequency

PXL_20220608_200059832.jpg
Almost like new...
Almost like new...


Cold Cathode Gauge Cleaning

These gauges need cleaning every now and then, and mine was years overdue. It's a Pfeiffer PKR 261.

Lots of oxide buildup
Lots of oxide buildup

Gauge parts. Pirani element is brass
Gauge parts. Pirani element is brass

The internals are remarkably simple and two gauges (cold cathode and pirani) are operated with only two wires! The 261 is metal sealed, so I had to add a thin washer to the single-use spring C-seal compression assembly to get a seal again. I used fine sandpaper to bring the housing and anode to a bright finish and cleaned everything as detailed in a recent post before reassembly. It works well down to <5e-9 torr.


Gate Valve Cleaning and Repair

My old primary/throttle valve was a butterfly valve, and just about every single one in existence leaks through the shaft seal (unless it's a UHV bellows type), especially when it's actuated. That was unacceptable for UHV, so I purchased a quite used and dirty MDC UHV manual gate valve. As it arrived, it was difficult to actuate and would get stuck unless held in a specific orientation. It was also quite dirty and somewhat rusty. I opened it up, only to find that it uses a non-standard copper crush seal. Following extensive sandpapering, cleaning, a custom laser-cut copper gasket from OshCut, and a new gate o-ring, I was able to get everything working smoothly again. The leak rate through the gate (with one side at ATM) is ~1e-6 torr l/s, likely from a small radial scratch in the recessed o-ring groove. The custom gasket, once cleaned, polished, and very thoroughly crushed, holds to UHV.

The whole actuation assembly.
The whole actuation assembly.

O-Ring Troubles

This is a minor thing, but the viton o-rings I recently ordered from LDS Vacuum arrived in a state completely unsuitable for vacuum. They were covered in powder, grease, and who knows what else. They did not use to be like this and used to arrive in almost vacuum-ready conditions.

New on left, used but clean(ish) on right.
New on left, used but clean(ish) on right.


NIM Bin Repair

The bin arrived functional but died within a few hours. The culprit turned out to be a shorted high-temperature aluminum electrolytic capacitor.

Shorted cap, replaced both for good measure.
Shorted cap, replaced both for good measure.

PXL_20220615_192117764.jpg


The rack contains two HV supplies, one preamp+amp unit, and one amplifier. One system is for gamma spec and the other for neutron detectors.



<some error with wrong file uploading>



Misc

Vacuum system ready for baking.
Vacuum system ready for baking.

Rough pump with oil trap and mist filter.
Rough pump with oil trap and mist filter.

Rewired control + DAQ box.
Rewired control + DAQ box.

Foil, foil everywhere...
Foil, foil everywhere...

Rough pump cleaning
Rough pump cleaning
Last edited by Liam David on Wed Jun 15, 2022 8:43 pm, edited 2 times in total.
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Richard Hull
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Re: Various Repairs, Progress, Eye-Candy

Post by Richard Hull »

It is always nice to take a working system and go through it several times over the years and clean and neaten things up a bit to improved serviceability and ease of use. Often, this allows for easy update in future. It can take a good bit of time and effort, but it always pays dividends in the end. Great photo essay on your system. Clean and clear images. well done.

It is always nice to see the guts of a great vacuum system.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.
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Dennis P Brown
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Re: Various Repairs, Progress, Eye-Candy

Post by Dennis P Brown »

Excellent post packed with good information and photo's.
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