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Restoring a Variac



 
 
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  #1  
Old June 19th 18, 04:20 AM posted to rec.audio.tubes
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Default Restoring a Variac

On Friday, January 31, 2014 at 4:47:00 PM UTC-6, hugeshows wrote:
> Hello all,
>
> Just got done doing the basic restoration of a vintage Radio General Variac. This is a really sweet 20A version, and it's in very good condition. Apart from sitting unused for too long, there isn't much wrong with it. When these things sit unused for a long time, it's a really good idea to open it up and see how badly oxidized and dirty the contacts have become. Heat is the enemy of a Variac, and clean contacts reduce heat.
>
> First, we need to remove the shaft and center wiper armature, and on this model that is accomplished via this Jesus clip here. You can call it a snap ring if you want, the first time one flies across the room and disappears, you'll call it a Jesus clip too. Use the correct tool to remove it. Don't muck about with a screwdriver and stab yourself or the Variac.
>
> https://www.dropbox.com/s/q4whhi2g95...Jesus_Clip.jpg
>
> There are lots of set screws that look like they need to come off to remove the wiper assembly. They don't. On this type of Variac, it's just the clip.
>
> Remove the assembly carefully. There are a stack of copper and steel bushings on the shaft that must go back on in the same order and direction. Keep them on the shaft so there is no guesswork:
>
> https://www.dropbox.com/s/df1da6qyvg...r_bushings.jpg
>
> Once you've done that, we can see the condition of things. Here is a birds-eye view of my Variac:
>
> https://www.dropbox.com/s/kaas9pl83v...iac_Opened.jpg
>
> So we can see that the coils are in really great shape, they're just dirty and oxidized from disuse. We also see that the center contact is pretty oxidized as well:
>
> https://www.dropbox.com/s/yt7hh6dapc...er_Contact.jpg
>
> Additionally, the center contact was loose to the terminal board. This is VERY bad, bound to create heat, sparks, intermittent connection, etc..
>
> https://www.dropbox.com/s/av8ljl15lg...inal_Loose.jpg
>
> It's just a simple phillips head screw that tightens this contact to the terminal. But before we tighten it, let's take it off and clean the contact points. With a wire brush or fine sandpaper, you want to clean this part he
>
> https://www.dropbox.com/s/lshbegy960...rminal_end.jpg
>
> That is where it meets the terminal, and you want it nice and clean there.. Obviously, you also want to clean it here as well, where it contacts the copper rings:
>
> https://www.dropbox.com/s/w2oqkm31tf...ct_Cleaned.jpg
>
> Using isopropanol, I cleaned hardened, dirty grease from the coil surfaces, from the ring contact, and all the copper bushings on the shaft which I carefully removed and replaced in order. Now on to the coil. Here's what it looks like half-cleaned, and then fully cleaned:
>
> https://www.dropbox.com/s/ahrqbe8kl3...Half_clean.jpg
>
> https://www.dropbox.com/s/43wwumhkwd...Clean_Coil.jpg
>
> Quite a difference, yes? While I used a wire brush to clean the center contact, DO NOT use a wire brush to clean the coil. The brush will scrape away between the windings and remove some of the lacquer insulation. That could ruin your Variac. Instead, I use a contact burnishing file. It take a very long time to do it that way, but you guarantee that you are not damaging the windings, nor leaving behind sandpaper residue. After you're done with all the filing, wipe down the coil contact surface with alcohol, and then a very light coating of deoxit. Don't drench it on here, you're just wanting to put a little bit of a seal on all that shiny metal.
>
> Inspect the brushes:
>
> https://www.dropbox.com/s/4gekbpj0ib...ac_Brushes.jpg
>
> These look quite good, there is still some extra length on them, and they aren't contaminated with grease or oil.
>
> One VERY important thing to remember on this style of Variac is that the entire armature assembly is LIVE. The center shaft is aluminum with a coating of insulating plastic. The center shaft conducts electricity from end to end. Check this with your meter. Therefore, the set screws that mount the shaft to the armature will cause the entire shaft to be energized if they manage to crack through the insulation. It is very important to make sure the shaft is in good shape, and that the knob is the correct type! Always use a plastic knob NEVER a metal one. If you see a Variac with no knob, approach it as if the center shaft were live.
>
> Once the copper rings have dried out from their alcohol bath, re-assemble them on the shaft and add a couple squirts of deoxit to protect them. You can also use electrical grease too, it will last longer. But I chose deoxit because it picks up less dirt, at the cost of having to reapply every now and then.
>
> The rest of installation is the reverse of removal.
>
> Cheers!
>
> -forkinthesocket




Greetings.. Not sure if anyone is here or not, Just stumbled across this while trying to solve a problem I have with this V20 variac I just got from a friend. The whole assembly is seized up in the frame. The outer shaft cylinder is too tight in the mounting frame and will not spin free at all.. I took the whole unit apart and put the base with the shaft cylinder in the cold to hopefully free it up, and still had to use a heavy mallet to remove it. This is supposed to spin freely, but I don't see how it would. There is no corrosion or dirt.. everything is nice and clean. any ideas?
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  #2  
Old June 19th 18, 01:13 PM posted to rec.audio.tubes
Peter Wieck[_2_]
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Posts: 26
Default Restoring a Variac

On Monday, June 18, 2018 at 11:20:38 PM UTC-4, wrote:
>
> Greetings.. Not sure if anyone is here or not, Just stumbled across this while trying to solve a problem I have with this V20 variac I just got from a friend. The whole assembly is seized up in the frame. The outer shaft cylinder is too tight in the mounting frame and will not spin free at all... I took the whole unit apart and put the base with the shaft cylinder in the cold to hopefully free it up, and still had to use a heavy mallet to remove it. This is supposed to spin freely, but I don't see how it would. There is no corrosion or dirt.. everything is nice and clean. any ideas?


If the frame is made of pot-metal, that is likely the issue. Pot-metal will swell, distort and generally deteriorate over time.

https://www.radiolaguy.com/info/PotMetal.htm

A solution (not "the" solution) might be to drill out the frame where it is binding and install some sort of new bearing. But, sadly, if this is the issue, it will continue. Please let us know if this is NOT the issue, so we might consider other options.

Peter Wieck
Melrose Park, PA

  #3  
Old June 20th 18, 02:11 AM posted to rec.audio.tubes
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Posts: 2
Default Restoring a Variac

I assumed it was aluminum, but looking at it again I think you are right. It has been in a garage in Houston so... lot’s of heat and humidity. I’ll try putting it through the drill press with a sanding wheel or something..
 




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