The AM Forum

I have a unique problem with the line voltage meter in one of my homebrew transmitters.

It is a moving vane type of a.c. voltmeter with a multiplying resistor in series, to measure the 110v line voltage.  The multiplying resistor consists of a coil of fine cloth-covered wire wound on a wooden spool.

The old resistor became intermittent, and I never could locate the spot that was causing the failure, so I rewound the entire resistor, using wire salvaged from another discarded wirewound resistor of similar construction. I am  not sure of the alloy, but it is probably not pure copper, because it seems to have a high resistance for a given length.  I was able, by trial and error, trim the wire and get the meter calibrated to exactly match the reading with my DVM, and the calibration seems stable with temperature.

The problem is, now something in the meter is outgassing, and a liquid that appears to be water, but could be some kind of oily substance, condenses on the inside of the glass meter face, and after a few days, totally obscures the scale.  This happens only when voltage is actually applied to the meter, so I suspect the outgassing is from the rebuilt resistor, generated by heating, since it didn't do this before.  At first I thought maybe all the crud would soon evaporate and the problem would eventually go away, but it never did, so I now suspect the resistor is collecting moisture from the air, and when voltage is applied, the heating effect is driving it back out.

I disconnected the meter out of fear that the crud could also be accumulating on the mechanical movement and eventually gum it up.

I have noticed that in older radios the i.f. coils were always impregnated with some kind of wax like substance, and I have thought of heating the resistor to dry it out and then dipping the whole thing into molten wax to seal out the moisture.

Does anyone know what kind of wax was used for this purpose?  I am reluctant to use plain old candle wax and would prefer something made for the job, because who knows what kind of impurities they might put into candle wax.  I can't find any discarded rf coils in my junk collection to extract wax from.  I wonder if anyone sells anything new, made just for this specific purpose.  I did a little web searching but so far have come up with nothing.

I do seem to recall something about old-timers back in the 20's using a mixture of beeswax and paraffin on coils, but I don't remember the portions of the mix.

Why not just coat it with RTV or something like that?

Darrell, WA5VGO

RTV might compound the problem, it cures by outgassing.

I was going to recommend beeswax, but Don already brought up the problem with it, what's the mixture???

Maybe Walt W2DU could answer that one for us???

--Shane

It needs to be something that impregnates the cloth insulation and wooden form.  The original resistor had some kind of waxy coating, but not enough could be reclaimed for re-use.

For some reason it comes to my mind 2 to 1 on the mixture maybe even 3 to 1, reason, you'd want the wax to set but not completely to a hard Solid.

When I did CB work I'd use the white candles from church when I could get them to seal up the adjustments and those candles had a certain amount Paraffin in the mix with other chemicals, they weren't Pure Beeswax Solid. Those white candles worked great.

Interesting metering problem though.

73
Jack.

Don,

dunno exactly if you are going for a "period" look or not. but maybe some of that new high temp Silcone based (1200 deg F) "paint" that they sell would work well in this application? probably would require multiple coats...

       _-_-bear

Use pure beeswax, Don.  Not 100% sure where to get it, though.

from some bees?  ::)

Beekeepers and honey sales places.  Self explanatory.

Chocolate Confectioners:  They use it for custom molds.

--Shane

You can use Apiezon W wax as it outgasses very little. I used it in school for vacuum seals. It was sometimes mixed with beeswax for easier removal, I forget the ratio. But I would first bake up the resistance for 24 hours and then impregnate it, so that any moisture is gone. You may wish to avoid silicone glue as it releases an acid during curing.

Another possibility is that the wire you have used has something wrong with it or is the wrong application.

Everyone save the old wax/paper caps when you recap older equipment and send them to Don so he can melt "em down! Hey, that's recycling!

I hadn't thought of that.  I have a box full of those, and rarely throw away bad ones because sometimes I hollow them out and re-gut them with modern caps when restoring vintage equipment.  That should be a  good source, and it seem to be about the same stuff. A good reason to go through and test a bunch of my old wax caps and create a 'bad' and a 'good' pile.

I have a jar of wax taken from some old surplus potted passive L-C filters.  The trouble is that when it cools it turns into a brick-hard solid.  I prefer something that stays pliable like the stuff on the original resistor did. 

Whatever is outgassing, there has to be a finite supply stored in the coil, in the cloth insulation maybe, if it's anything other than moisture accumulated from humidity in the air. Maybe I'll bake the coil at 150° or so for 24 hours and then dip it in molten wax salvaged from old paper caps.

Sometimes familiar looking but unknown substances designed for other purposes, such as candle wax or cooking ingredients contain additives that may be corrosive, attack insulation, etc.  That's why I am reluctant to use it.

What about shrink tubing? Im assuming that little heat is involved.

Or just take out the multiplier and run it on a Variac until it decides to behave.

Carl
KM1H

How about running the multiplier on the outside of the case? At least until it decides to behave.

73 N8QPC