Repairing A Variac

iately of course.

e AC input had pulled the turn of the winding off, soldering took care of t hat. There was a little catch right at that point but it wasn't bad. Months , maybe about a year ago that "catch" got worse and I reworked it, tighteni ng up the winding and making sure it was smooth. It was fine.

rger to feed something a tad over its usual output, which was working fine. And no overload at all, the drain was about 2.5 amps at 16 volts or so, so that was well within spec for the variac. Later I turned it down because i t was time to test the UUT with lower voltage. (this is an intermittent ove rheating problem in the DC/DC convertor of an AC/DC version of the Tek 422)

ng between the windings.

nd this thing. Is there a chemical solution to this problem ? (pun intended )

I don't use it enough to buy another one but when I need variable AC I nee d variable AC. I might have to add some boost/buck transformers to my newly kludged bohungus isolation transformer. It even has a fan.

t I would need them to be able to handle the current of the main one, right ? Otherwise an overload could be a catastrophe, and I have had enough of t hose in my life.

l, then filing it down for the wiper to contact and ??? Maybe. I don't know if I can find such chemicals, or if they even exist. And how would I keep the windings on place once they're stripped ?

oncerned with ground faults because I can just always run it off isolation. It will almost fit n that cabinet but not with the knob in front, and I do n't want it on top. But then it might be ready for the dump anyway.

Variacs are never ready for the dump. If you can get all traces of the carbon out from the gap, you're most of th e way there. If not, and I expect you won't, I'd cut the wire and unwind ha lf a turn each side. Put a turn of new wire on & solder its ends to the old ends where they're not in the way. With 2 half turns out the way it should be easier to remove any carbon.

NT

Used its equivalent - it works.

Peter Wieck Melrose Park, PA

Does it have the correct width of brush? it must not be able to contact more than 2 turns at any time. The proper ones look suspiciously unmanly for the job.

Peter, does that have a suitable breakdown voltage for 130 - 150VAC? I didn't bother sending the manufacturer my email address so they would send me the docs to verify. However the ad says it is suitable for LEDs and circuit boards, doesn't mention higher voltage applications.

For the OP be really sure to clean between the windings where the arcing occurred, you want to remove any sharp edges or points on the wire or any carbon residue. There are dialectic varnishes designed for higher voltages such as MG Chemicals Corona Dope:

John :-#)#

iately of course.

e AC input had pulled the turn of the winding off, soldering took care of t hat. There was a little catch right at that point but it wasn't bad. Months , maybe about a year ago that "catch" got worse and I reworked it, tighteni ng up the winding and making sure it was smooth. It was fine.

rger to feed something a tad over its usual output, which was working fine. And no overload at all, the drain was about 2.5 amps at 16 volts or so, so that was well within spec for the variac. Later I turned it down because i t was time to test the UUT with lower voltage. (this is an intermittent ove rheating problem in the DC/DC convertor of an AC/DC version of the Tek 422)

ng between the windings.

nd this thing. Is there a chemical solution to this problem ? (pun intended )

I don't use it enough to buy another one but when I need variable AC I nee d variable AC. I might have to add some boost/buck transformers to my newly kludged bohungus isolation transformer. It even has a fan.

t I would need them to be able to handle the current of the main one, right ? Otherwise an overload could be a catastrophe, and I have had enough of t hose in my life.

l, then filing it down for the wiper to contact and ??? Maybe. I don't know if I can find such chemicals, or if they even exist. And how would I keep the windings on place once they're stripped ?

oncerned with ground faults because I can just always run it off isolation. It will almost fit n that cabinet but not with the knob in front, and I do n't want it on top. But then it might be ready for the dump anyway.

Remember the Mitsubishi deflection yokes that used to arc over on the PTVs? Back in the old days, I'd just kind of gently pick apart the area where t he arcing occurred, clean as best I could, and dose it with several layers of thin cyanoacrylate. That stuff penetrates almost like Kroil. Of course , those windings were much thinner and easier to move around. Not advice r eally, just reminiscing.

removing every little trace of carbon is vital IME

NT

John:

I must admit I am not sure. I used its functional equivalent on a Hot Tub c ontrol board switching 240 VAC heater and 120 VAC pump circuits without any problems - but for the heater on one just failed this weekend. Not the boa rd, the heater itself. I suspect that the material you linked might be mor e suitable given its brush applicator and specific prescribed uses.

e-4226

The 'thing' about AC arcing is that once an arc is struck, the plasma gener ated helps sustain it. Hard to strike, but once struck not hard to maintain if there is a source for the plasma. Just a few weeks ago, a large Norway maple tree behind us started rubbing against a 13,200 primary. Those arcs m ade 6-8 inches until sufficient of the tree was eroded that the distance ev entually was too much. PECO took nearly a week to get to it as the heavy sn ow and wind that caused the condition in the first place did far more damag e elsewhere. 90 houses around us were without power for 4 days. We were exc eedingly lucky not to.

When PECO did come, they worked from our back yard, and I got 'the dope' on arcing from their engineer on-site.

Peter Wieck Melrose Park, PA

A Jacob's Ladder is an excellent example of how an AC or DC arc plasma, once started, can more than double its initial width as the plasma climbs up the wires...

I had one in my parents home where I had a fat 5KV Neon sign Xformer - which made nice fat arcs, but that was way back in the 60s...

John :-#)#

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                      John's Jukes Ltd. 
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How does it do that? The voltage from one winding to the next is pretty small.

don't worry about carbon tracks, if you ppur this stuff over the part that's arcing, the carbon will be encapsulated.

Give it 2-3 days to completely dry and get hard.

It if still arcs, throw it in the trash.

--
"I am a river to my people." 
Jeff-1.0 
WA6FWi 
http:foxsmercantile.com

Thanks folks, I'll look into these options and try it.

I don't know how it's possible to arc between windings with only

117/220VAC. My guess(tm) is that there's arcing between the contact brush and the windings caused by a layer of filth on the windings or contact brush.

No way am I going to pay $18 for 2 oz of Q-Dope. Q-Dope is polystyrene disolved in MEK. Make your own from packing peanuts and MEK:

Maybe some anti-corona goo will provide some better arcing protection:

Variac cleaning videos: etc...

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

On Mon, 30 Apr 2018 18:43:11 -0700, Jeff Liebermann wrote: (...)

This kinda sounds like the arcing problem as described. The arcing is from the winding to the carbon brush wiper:

Variac Repair using DeOxit:

However, I'm not sure that's such a great idea. It looks like he managed to burn up the Variac:

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

om the winding to the carbon brush wiper: "

No, the wiper was nowhere near it. It is either between the windings or fro m the windings to the core.

From what I see there are probably 100 turns, if not there must be at least 50. That means only a couple volts between each turn. This is not hopeful.

Since it all seems to be at the boost end, maybe I can eliminate that part and just have something that will cut the line voltage. It beats nothing.

There is another possibility, I could put the rectifiers back in the "isola tion transformer" cabinet and use the DC supplies which are + - 90 volts to power a class D amp. I know they can get hairy to design like for high fid elity audio but for these lower frequencies it should not be all that diffi cult. Or I could tap off an existing class D amp before the output filter ( except for certain Crowns et. al.)and use that to drive bigger transistors, set up a feedback attenuator to match the level and make a bigger output f ilter. It would be easier than designing from scratch. It would also automa tically be isolated.

But honestly that is a pipe dream. Maybe I should just think about eliminat ing that part of the winding and hope the rest is alright. Seems like that would be the hottest part of the winding and would fail first, no ? For the amount of time I use it...

Soon, I'll take the cover off and have a closer look at this arcing. But I am sure it is not to the wiper.

from the winding to the carbon brush wiper: "

rom the windings to the core.

st 50. That means only a couple volts between each turn. This is not hopefu l.

t and just have something that will cut the line voltage. It beats nothing.

lation transformer" cabinet and use the DC supplies which are + - 90 volts to power a class D amp. I know they can get hairy to design like for high f idelity audio but for these lower frequencies it should not be all that dif ficult. Or I could tap off an existing class D amp before the output filter (except for certain Crowns et. al.)and use that to drive bigger transistor s, set up a feedback attenuator to match the level and make a bigger output filter. It would be easier than designing from scratch. It would also auto matically be isolated.

ating that part of the winding and hope the rest is alright. Seems like tha t would be the hottest part of the winding and would fail first, no ? For t he amount of time I use it...

I am sure it is not to the wiper.

if it's a small section of winding, how hard is it to rewind that bit?

I trust the variac is grounded, so this may be an arc to the case...or failing insulation on the power cord. How OLD is this variac anyway?

John :-#(#

--
(Please post followups or tech inquiries to the USENET newsgroup) 
                      John's Jukes Ltd. 
MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3 
          (604)872-5757 (Pinballs, Jukes, Video Games) 
                      www.flippers.com 
        "Old pinballers never die, they just flip out."

Since it's on the "hgh" (boost) of the variac, He's got between 120=135 vac to gound at that point.

And knowing Jurb, it's probably something he fished out of a dumpster 30 years ago. He's already admitted to "fixing" it twice already. I doubt he did a good job either time.

--
"I am a river to my people." 
Jeff-1.0 
WA6FWi 
http:foxsmercantile.com

Variacs are all (or almost all) many decades old.

NT

Not all are decades old:

John ;-#)#

--
(Please post followups or tech inquiries to the USENET newsgroup) 
                      John's Jukes Ltd. 
MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3 
          (604)872-5757 (Pinballs, Jukes, Video Games) 
                      www.flippers.com 
        "Old pinballers never die, they just flip out."

Not off them are fished out of a dumpster either.

--
"I am a river to my people." 
Jeff-1.0 
WA6FWi 
http:foxsmercantile.com