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Posted: 1/14/2019 9:20:11 PM EST
Having successfully repotted tubes in the past I intend on taking it to the next level.
The basics of potting are pretty straight forward, however the long term success of a tube repair lies in the details, if ignored a repot will last a couple of months or a couple of years at best.
Doing it right is worth it.

Let's start with the diagnosis:

There are three main categories of tube failures, the first and easiest to fix is a failure of the potting material, where a tube will usually short-circuit between the MCP and Phosphor Screen.
Tubes with this problem are identified by: The power-supply is drawing a healthy current between 20 and 40mA and the tube is flickering.
The fix here is a repot.

The second category is a PSU failure, either delivering a too low voltage or no power at all to the tube core.
Tubes with this problem are identified by: The tube doesn't light up at all and the current drawn is outside the normal range, either close to zero or way above 40mA indicating an internal short-circuit within the PSU.
The fix here is a repot and PSU replacement.

The third category is a tube core failure, where the tube has cosmetic/visual damage beyond usability, or a leak where the tube lost it's vacuum, or physical damage to the input/ output window.
Tubes within this category are identified by: visual image is unusable but remaining areas indicate a healthy PSU or the tube doesn't light up at all yet the PSU draws current within it's normal operational range of 20-40mA.
The only fix here is to salvage the PSU.

Once diagnosed, the tube is taken apart. Some people try to reuse the tube housing, but that makes this process really difficult. Because the only practical way to take a tube apart is by force.
Separated from the tube housing, you'll be holding the potted tube core and PSU. The next step is to remove the PSU from the tube core by desoldering the wires and prying off the PSU from the tube core.
The following step is also the most time consuming one, cleaning off the potting material. To be noted here, do NOT use any metal objects in this process. Scratching the ceramic surface of the tube with metal objects can cause the repot to fail due to metal trace particles left on the surface of the tube. A sandblaster is of great help for this step, just don't forget to tape off both optical ends of the tube.

Once clean, wash the tube in distilled water then rinse off with lab grade isopropyl.

With the tube core, PSU and tube housing ready to go. It's time to solder them back together and getting ready for the RTV potting. Make sure your RTV potting silicon is rated at 12kV/mm or higher.
Mix the RTV and place both the tube assembly and the RTV mixture in a vacuum chamber. This has two reasons, you degas the potting mixture, meaning getting the air out of the silicon ensure the material won't lose it's dielectric strength and secondly by also placing the tube in a vacuum before potting, you remove any remaining surface moisture on the tube core. (L3 wisely chose Tempe as it's location for their tube plant, the air is really dry there most of the year around)
Trapped moisture between tube and the RTV can easily lead to arcing.

It's now time to pot the tube, tape off the bottom of the tube, fill up the fitted tube with the degassed RTV using a syringe and place it back inside the vacuum, letting it cure for 24h.

Once cured, you will most likely need to re-adjust the feedback and ABC potentiometer. First adjust the ABC to a light level of your liking, then carefully probe the feedback potentiometer until you are happy with both the noise level at low and high light conditions. An optical tube test setup is extremely useful at this stage.

Done.

In case you call yourself the owner of a broken tube then your tube deserves a second chance. Pm me and let me breathe new life into it or hit me up with questions if you want to give it a go for yourself!
I am looking for all kinds of broken tubes and tube parts.
Link Posted: 1/14/2019 10:27:35 PM EST
If there is any way you could make a video of you doing this on your next one, me would love you long time....I find shit like this fascinating.
Link Posted: 1/14/2019 11:02:01 PM EST
Good summary. Also good to see more people repotting tubes. Probably can count the number of people who repot tubes on one hand, maybe have to start using another hand now.
Link Posted: 1/15/2019 11:59:49 AM EST
Originally Posted By MILSPEC556:
If there is any way you could make a video of you doing this on your next one, me would love you long time....I find shit like this fascinating.
View Quote
Hmm I'll look into that! If not, I'll take a bunch of pictures for sure.

Originally Posted By txdx:
Good summary. Also good to see more people repotting tubes. Probably can count the number of people who repot tubes on one hand, maybe have to start using another hand now.
View Quote
Thanks! Yes, there are not many out there.
Link Posted: 1/15/2019 1:13:52 PM EST
how toxic are the chemicals involved in the process? I've known a few users who said they were pretty toxic so they scrapped the idea.
Link Posted: 1/15/2019 1:34:55 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By chosos:
how toxic are the chemicals involved in the process? I've known a few users who said they were pretty toxic so they scrapped the idea.
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You do need to be careful in case you break a tube core, you don't want that phosphor getting anywhere.
Link Posted: 1/15/2019 10:06:35 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By chosos:
how toxic are the chemicals involved in the process? I've known a few users who said they were pretty toxic so they scrapped the idea.
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Mostly stuff like isopropyl alcohol, which are pretty safe, and most elastomer compounds are no more dangerous than common household glues and certainly don't stink as bad as many of the resins I've used in the past, but generally as long as you're using them in a well ventilated area, you should be fine. Basic precautions like wearing gloves when handling the elastomer, alcohol, etc, try not to spill it on you, wash it off if you do, etc. You can download the MSDS on something like Sylgard 170 if you want to know more.

http://www.farnell.com/datasheets/1521089.pdf

You can almost drink the stuff ( not recommended ) and it's usually only considered bad if you get it in your eyes, so it's safer than most of the stuff you'll find the the average kitchen.

David.
Link Posted: 1/15/2019 10:15:00 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By Alex_B:

You do need to be careful in case you break a tube core, you don't want that phosphor getting anywhere.
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Hi Alex,

Good write up. I'll add that, for home brews, if you set up your jigs correctly it is possible to pour the elastomer with excellent results at atmospheric pressure once degassed, also you need to refer to boiling temps vs vacuum levels if you want to dewater the parts by vacuum without low-temp baking, and that requires better vacuum equipment than most people have ready access too. For DIY at home, you can degas the elastomer by 1/4 filling a 100mm syringe with the mix, after mixing, and then pulling on the syringe to create a vacuum several times while the end is stoppered. Doing it a few times will also cause shock to the bubbles which will help smaller bubbles form larger ones. Letting the stuff cure in a container and then cutting it open prior to doing a run on tubes allows for microscopic evaluation on bubble levels so you can fine tune the process.

Field potting methods are quite suited to amateur repairs without requiring complex setups.

Generally I find the biggest impediment to new attempts at repotting is finding a ready supply of boots to fit everything into prior to potting.

David
Link Posted: 1/16/2019 2:14:01 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By cj7hawk:

Hi Alex,

Good write up. I'll add that, for home brews, if you set up your jigs correctly it is possible to pour the elastomer with excellent results at atmospheric pressure once degassed, also you need to refer to boiling temps vs vacuum levels if you want to dewater the parts by vacuum without low-temp baking, and that requires better vacuum equipment than most people have ready access too. For DIY at home, you can degas the elastomer by 1/4 filling a 100mm syringe with the mix, after mixing, and then pulling on the syringe to create a vacuum several times while the end is stoppered. Doing it a few times will also cause shock to the bubbles which will help smaller bubbles form larger ones. Letting the stuff cure in a container and then cutting it open prior to doing a run on tubes allows for microscopic evaluation on bubble levels so you can fine tune the process.

Field potting methods are quite suited to amateur repairs without requiring complex setups.

Generally I find the biggest impediment to new attempts at repotting is finding a ready supply of boots to fit everything into prior to potting.

David
View Quote
Hi David,

Great idea with the syringe!

Agree on the boots, I am considering investing in a plastic injection mold for this reason, as this will give by far the best results.
Link Posted: 1/16/2019 6:24:08 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By Alex_B:

Hi David,

Great idea with the syringe!

Agree on the boots, I am considering investing in a plastic injection mold for this reason, as this will give by far the best results.
View Quote
Can one 3d print boots?

Also, I can put you in contact with a very affordable molder and toolmaker, if you ever want to get into that.
Link Posted: 1/17/2019 8:24:52 AM EST
Yes, you can 3D print boots quite effectively. The walls are a little thick of FDM printers, but they come out OK and still work.
Link Posted: 1/17/2019 10:15:17 AM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By cj7hawk:
Yes, you can 3D print boots quite effectively. The walls are a little thick of FDM printers, but they come out OK and still work.
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Nice. It seems like it would be cheaper for low volume to print boots. And easier to make some of various dimensions.
Link Posted: 1/17/2019 10:31:52 AM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By fmj_shooter:

Nice. It seems like it would be cheaper for low volume to print boots. And easier to make some of various dimensions.
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The problem is ITAR. Sharing stuff like this should be practical;


(MX10130 boot)

Except stuff like this is often ITAR controlled and there are even other laws about sharing this kind of stuff now... So pretty much if you want something like this, you either have to design and print it yourself, or control the files, or buy from someone who makes them. A long time ago I put up a generic MX10160 boot - a very early example - and I think it's still the only one in the public domain, and while it works, it is pretty terrible. It would be ideal if stuff like this could be spread around to help.

It's worth keeping an eye on what happened to Defence Distributed.
https://defdist.org/

Hopefully one day ITAR will be amended to allow things like this to enter the public domain.
Link Posted: 1/17/2019 11:38:52 AM EST
fortunately, boots are fairly easy to draw in cad, so 3d printing them is viable.
Link Posted: 1/17/2019 11:51:56 AM EST
Tag
Link Posted: 1/17/2019 12:54:32 PM EST
Originally Posted By cj7hawk:

The problem is ITAR. Sharing stuff like this should be practical;

http://aunv.blackice.com.au/userfiles/david-boot2.jpg
(MX10130 boot)

Except stuff like this is often ITAR controlled and there are even other laws about sharing this kind of stuff now... So pretty much if you want something like this, you either have to design and print it yourself, or control the files, or buy from someone who makes them. A long time ago I put up a generic MX10160 boot - a very early example - and I think it's still the only one in the public domain, and while it works, it is pretty terrible. It would be ideal if stuff like this could be spread around to help.

It's worth keeping an eye on what happened to Defence Distributed.
https://defdist.org/

Hopefully one day ITAR will be amended to allow things like this to enter the public domain.
View Quote
Originally Posted By chosos:
fortunately, boots are fairly easy to draw in cad, so 3d printing them is viable.
View Quote
Yeah. I've got solidworks, calipers, micrometers, and optical comparator, a cheap 3d printer at home, and a $60K printer at work. I laugh at ITAR

Admitted ITAR is serious business (licensed 07/SOT and accessory producer here). And it does hamper productivity of honest people.
Link Posted: 2/11/2019 6:46:41 PM EST
Here are some pictures of a tube I am working on right now. I took the tube out of the tube housing, carefully desoldered the external contacts (good for reuse in the new housing later), desoldered the PSU from the core and cleaned the components.

Tip: Always take a picture of the PSU wires soldered to the tube before desoldering them!

Next step: Test each individual component, tube core and PSU.



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