Damaged vintage variac

[ajgriff]

The attached is lifted from a Variac (Lyons/Zenith) manual.

Alan

[Electronpusher0]

But don't use carbontetrachloride as it is carcenogenic.

Some years ago I visited Saudi to train some technicians for our agent and needed to clean flux off a pcb. I was given Carbontetrachoride. When I pointed out it was carcenogenic and he should be using something like IPA I was told that this was unobtainable as it was alcohol and all alcohol was banned!

Peter

[ajgriff]

I'd assumed that neither crocus paper nor carbon tetrachloride would be readily available. I used already worn P1600 carbide paper and meths on mine. Seems to have survived ok.

Alan

[TrevorG3VLF]

If this is a Variac then the track is likely to be plated with noble metals such as hard gold alloy. Therefore be very careful not to rub it out.
If you do go through to the copper then it should still work but wear and corrosion will be worse.
White spirit would be a better solvent than IPA for petroleum contamination. Carbon tet was used to clean clothes with oily stains.

If the knob will come off, I would leave the set scews and drill and tap for another set screw.

[Tractorfan]

Hi,
Just as an aside, I have an ancient Variac which kept seizing up if it wasn't used for a while and kept in the cellar.
It turned out that the shaft is made from something akin to Bakelite. It was a snug fit into the brass top and bottom frames, and used to swell ever so slightly. I learned from this forum that bakelite and its ilk is 'organic' and hygroscopic. I've had a similar thing with old bayonet lamp adaptors stuck in the holders.
Easing the holes slightly with a rotating 'flapper wheel' has sorted it (for now).
Cheers, Pete.

[bluepilot]

Originally Posted by Electronpusher0

But don't use carbontetrachloride as it is carcenogenic.

I don't think it's so much carcinogenic as damaging to liver and kidneys the effect being exacerbated by drinking alcohol.
Many years ago when I was young and naive I worked in a lab in a basement. I had a colleague with an old motor bike. We had a few beers at lunch time and then the colleague spent the afternoon cleaning the grease from his motor bike in the lab using a gallon bottle of carbon tetrachloride. At the time I don't think the health risks were known. It was only afterwards that we heard of health warnings. Luckily it doesn't seem to have done us any harm.

[WaveyDipole]

Wow! Lots of useful info here!

Originally Posted by Electronpusher0

But don't use carbontetrachloride as it is carcenogenic.
...
Peter

Originally Posted by bluepilot

I don't think it's so much carcinogenic as damaging to liver and kidneys the effect being exacerbated by drinking alcohol.

I don't have any carbon tet, but clearly either way there appear to be risks to health so I will avoid.

Originally Posted by ajgriff

I'd assumed that neither crocus paper nor carbon tetrachloride would be readily available. I used already worn P1600 carbide paper and meths on mine. Seems to have survived ok.

Alan

Never come across crocus paper before but will see if I can find some or otherwise some P1600.

Originally Posted by TrevorG3VLF

If this is a Variac then the track is likely to be plated with noble metals such as hard gold alloy. Therefore be very careful not to rub it out.
If you do go through to the copper then it should still work but wear and corrosion will be worse.
White spirit would be a better solvent than IPA for petroleum contamination.

I have now tried both IPA and white spirit but the results remain the same. I think whatever is left is oxidisation/corrosion rather than dried deposit so unless there is some other way, then some kind of abrasive will be required. I will stick to the very fine ones that have already been mentioned. Might also give Brasso a try?

Originally Posted by TrevorG3VLF

If the knob will come off, I would leave the set scews and drill and tap for another set screw.

That's a good idea but sadly it won't come off. I could probably tap the shaft down a bit to make the tips of the grub screws bite, but I suspect that wouldn't last long.

Originally Posted by Tractorfan

Hi,
Just as an aside, I have an ancient Variac which kept seizing up if it wasn't used for a while and kept in the cellar.
It turned out that the shaft is made from something akin to Bakelite. It was a snug fit into the brass top and bottom frames, and used to swell ever so slightly. I learned from this forum that bakelite and its ilk is 'organic' and hygroscopic. I've had a similar thing with old bayonet lamp adaptors stuck in the holders.
Easing the holes slightly with a rotating 'flapper wheel' has sorted it (for now).
Cheers, Pete.

Pete, thanks. When I dismantled, I gave the top and bottom "bearings" a clean by removing the mostly loose white powery crud with abrasive paper which did help, but they are still very snug indeed. I haven't tried re-assembling yet, however, I don't appear to have got down to the bare metal yet so a deeper is still required. I don't have a flap wheel at present but I could probably source one from B&Q, or if I'm careful maybe I could use one of the abrasive wheels (barrel type) in the Dremel kit?

[ajgriff]

I'm not sure whether or not crocus paper/cloth is available these days but it was made using jeweller's rouge, in other words ferric oxide (rust). Ferric oxide is a very mild abrasive and Lyons/Zenith were obviously keen to stress that nothing more aggressive should be used for track cleaning. I used (very gently) P1600 carbide paper because I happened to have some which was already well worn. Silicon carbide is a very hard and sharp abrasive so if you decide to use carbide paper go for the finest grade you can and wear it down beforehand.

Alan

[Leon Crampin]

An ink eraser is often effective for removing oxidation from sliding contact surfaces such as sliprings and commutators. It may be worth a try, especially if the Variac track is plated. Then a clean with a stiff toothbrush and a wipe with a little meths on a rag should do the trick. Don't let the brush get wet.

Leon.

[MotorBikeLes]

I think the main danger with carbon tet is if you happen to be a smoker.
I think COCl2 (carbonyl chloride) is the toxic result.
Les.

[turretslug]

Finely divided hot carbon (i.e. glowing cigarette butt!) can be an effective catalyst/facilitator for many reactions- I'm sure there is a documented case where someone actually died as a result of carbonyl chloride (phosgene) poisoning resulting from something like using carbon tet whilst smoking. There was also a rumour in the company I worked for (from before I joined) that someone had become very ill from using Colclene TF (a long-banned CFC-rich cleaner) whilst smoking.

[WaveyDipole]

Originally Posted by Leon Crampin

An ink eraser is often effective for removing oxidation from sliding contact surfaces such as sliprings and commutators. It may be worth a try, especially if the Variac track is plated. Then a clean with a stiff toothbrush and a wipe with a little meths on a rag should do the trick. Don't let the brush get wet.

Leon.

Using an ink eraser sounds like a brilliant idea, except that I couldn't find one at home or in the shops locally. It seems that are all now made from that rubberised plastic stuff whatever it is rather than proper rubber.

In the circumstance and with TrevorG3VLF mentioning that there might be a noble metal coating present, I was very nervous about using anything abrasive and I am glad that I didn't. Not having anything remotely close to crocus paper to hand, I tried some metal polish wadding that I had on hand in the shed and was pleased to find that this worked a treat! The dark crud/oxide came off with moderate effort and did indeed reveal a silver metal coating on top of the copper. This does have some tiny scratches and slight pitting in places but overall, the track looks to be in reasonably good condition.

Having cleaned with the metal cleaner, I then wiped down and cleaned again with white spirit. I now get consistent resistances across the entire winding. Is it best to put a slight film of grease on the track during re-assembly?

I am just cleaning up the brass coil mountings after which I will be almost ready to start re-assembly. I just need to fix the small bit that broke off the insulator cylinder during dismantling.

[Techman]

I certainly wouldn't put any grease on it, the carbon contact should be enough.

[Leon Crampin]

Definitely no grease. Resistance measurements are not accurate in this application as there is a significant breakdown voltage associated with carbon brush contacts. Having cleaned the track, swing the brush from end to end about a dozen times with no applied voltage. Don't then clean the track again - you need a graphite deposit on the track.

Finally, test on a light load first, possibly monitoring voltage drop between the brush and track using a fine probe. A full load test should result in only a little brush heating.

Over-cleaning a well run in slip ring, track or commutator is a mistake.

I hope a full load test goes well. I have an identical Variac which runs cool at its rated 8A load. These were built to last, but damp storage will swell the phenolic core shaft and cause seizure.

Leon.

[Radio Wrangler]

Originally Posted by WaveyDipole

I don't have a flap wheel at present but I could probably source one from B&Q, or if I'm careful maybe I could use one of the abrasive wheels (barrel type) in the Dremel kit?

Ah, here's another trick for you.

You need a high-ish speed electric drill or dremel and some emery cloth (not emery PAPER)

Get a length of rod which will fit in the chuck and is smaller than the bore you intend to polish. Cut a slot with a hacksaw straight down the centre of the rod, starting on the far end, to a depth of maybe an inch. So your rod ends in a pair of half-cylinders with a sawn gap between them. Tear off a 1 inch strip of emery cloth and fit one end of it through the slot. Oriented so that when the drill spins, the emery cloth is left behind like a rotary whip, gritty side out.

You're not trying to make a hard cylinder of emery, the flexible whip nature makes it follow the bore you insert it into, the abrasive is centrifuged outwards into contact with the bore and it does gentle honing.

It also follows round shapes naturally and doesn't try to dig in in places like a hand-held rotary stone does.

Used to use long rods when decokeing cylinder heads to get into those hard to reach places.

David

[WaveyDipole]

Originally Posted by Leon Crampin

Definitely no grease. Resistance measurements are not accurate in this application as there is a significant breakdown voltage associated with carbon brush contacts. Having cleaned the track, swing the brush from end to end about a dozen times with no applied voltage. Don't then clean the track again - you need a graphite deposit on the track.

Finally, test on a light load first, possibly monitoring voltage drop between the brush and track using a fine probe. A full load test should result in only a little brush heating.

Leon, thanks. I will follow that advice. I did wonder whether there was any merit in smearing a little pencil graphite on the track first?

Originally Posted by Radio Wrangler

Ah, here's another trick for you.

You need a high-ish speed electric drill or dremel and some emery cloth (not emery PAPER)

Get a length of rod which will fit in the chuck and is smaller than the bore you intend to polish. Cut a slot with a hacksaw straight down the centre of the rod, starting on the far end, to a depth of maybe an inch. So your rod ends in a pair of half-cylinders with a sawn gap between them. Tear off a 1 inch strip of emery cloth and fit one end of it through the slot. Oriented so that when the drill spins, the emery cloth is left behind like a rotary whip, gritty side out.

You're not trying to make a hard cylinder of emery, the flexible whip nature makes it follow the bore you insert it into, the abrasive is centrifuged outwards into contact with the bore and it does gentle honing.

It also follows round shapes naturally and doesn't try to dig in in places like a hand-held rotary stone does.

Used to use long rods when decokeing cylinder heads to get into those hard to reach places.

David

David, thanks as well. I will avoid the rotary stone bit in the dremel kit then although there is also a soft wire brush. I will see if I can find some emery cloth at B&Q though.

[Refugee]

Originally Posted by WaveyDipole

Leon, thanks. I will follow that advice. I did wonder whether there was any merit in smearing a little pencil graphite on the track first?

David, thanks as well. I will avoid the rotary stone bit in the dremel kit then although there is also a soft wire brush. I will see if I can find some emery cloth at B&Q though.

The brush is in effect a very hard pencil anyway so there is no need to rub the track with a pencil.
In normal use in a dry workshop the most it will ever need is going over with a dry brush every 10 years or so.
Anything oil based will make it get clogged up with dust.
Once the shaft turns reasonably easily there should be no need to open out the shaft hole any more. It should stay free if it is used often enough.

[Leon Crampin]

I would avoid pencil derived graphite as it is mixed with binders such as clay which can be abrasive. Further, its resistivity and adherent properties when lodged between the turns are not characterised. Just run the specified brush over the track - Zenith will have done the development work.

From my work in the automotive sector, I have observed that brush selection is as much an art as a science, and a Variac must be critical in this, as current density is high and the characterisation of directional resistivity is important. The actual manufacture of carbon brushes is also an art, as Morganite found when they moved from Battersea to S Wales. As many of the old hands refused to move they had some real problems.

Such is the value of expertise.

Leon.

[WaveyDipole]

Originally Posted by Refugee

Anything oil based will make it get clogged up with dust.

That's a fair point. I guess any such clogging could possibly develop unpredictable conducting characteristics (similar to what was mentioned by Leon in the next post) so it makes sense that anything oil based is best avoided.

Originally Posted by Refugee

Once the shaft turns reasonably easily there should be no need to open out the shaft hole any more. It should stay free if it is used often enough.

I certainly hope so!

Originally Posted by Leon Crampin

I would avoid pencil derived graphite as it is mixed with binders such as clay which can be abrasive. Further, its resistivity and adherent properties when lodged between the turns are not characterised. Just run the specified brush over the track - Zenith will have done the development work.

Ok, noted. When I get around to re-assembling I will just rotate the brush back and forth a few times as suggested. Thanks.

[Radio Wrangler]

The bending of the bottom frame may have misaligned the bush in it. Along with the close fit, that may be enough to stop the shaft from rotating. Fixing the bend might be enough to restore normal operation

David