Cantenna questions

[N0BST]

Got an old Heathkit Cantenna and the thing is starting to leak along the bottom seam.  Actually it's more seepage than a leak so I'm not losing a whole lot at a time, but I'd like to plug it before the oil level gets to where it no longer covers the resistor.  First- how do I know if the oil has PCBs or not?  If it is, should I pour it into a jug and haul it down to the nearest hazmat site or reuse it?  I do have some silicon oil I could replace it with but I was hoping to save that for some broadcast iron I have. 

I've heard PCB oil smells a bit like used motor oil.  It does have a distinctive smell but that analogy wouldn't have come to mind.  It's a yellow color.  I've also got a smaller, generic dummy load that uses a darker yellow oil and I suppose I should determine if that's got PCBs as well.

Second question is how would you recommend sealing the bottom seam?  I know I could solder it but am concerned the heat would kill the paint along the bottom on the outside.  The seepage is right under the schematic on the can.

Lastly, the rubber gasket on the little vent on top is pretty hard and doesn't seal well.  Has anyone had any luck replacing it, and if so, what did you use?

Thanks for any help you guys can render.

Scott Todd

[K6IC]

Hi Scott,

Well,  the easiest remedy for the can is to go to a paint store,  and buy a new metal one-gallon paint can,  and swap out the oil and lid.

Personally,  I do not believe that even if the oil was real transformen oil,  that it would be very toxic in its current fluid form.

Believe that Heath never did supply oil for these loads,  so,   the oil is prob just mineral oil from the Drug store.  Regarding the vent,  I have nothing to offer.  73  GL,  Vic

[K5UJ]

K6IC took the words right out of my mouth.  for venting I'd just leave the hole uncovered and don't worry about it.  Just keep the can vertical.  I have an old Bird oil filled DL with a vent plug on top.  The cover for it is a threaded disk with a hex in it and it screws into the vent hole for when the DL is moved around.  when it is sitting still and connected to feedline the vent hole is uncovered.  You just put it out of the way and leave it alone and let it do its DL thing. 

If you are not comfortable with oil and do not need more power handling than what you get with a Cantenna, then keep a lookout for Carborundum resistors and mounting clips on eBay and at hamfests.  If you find a set of four 200 Ohm 200 W. each resistors, (they are around 12 to 18 inches long and maybe 1 inch diam.) you can mount them parallel in a frame or box cage (one end insulated from the box) and construct a 50 ohm DL.  I did that for my test bench.  If you leave them exposed, you can put them in front of a fan and they'll handle fully modulated 500 w. with air moving over them easy.    Mine starts to show some reactance above 40 m. but I really need it for the low bands so it is okay.  This costs a lot less than those manufactured dry DLs.  I've seen broadcast DLs get parted out in this way with sets of 4 to 6 resistors for anywhere from $20 to $50.   I went to a hamfest a few months ago where someone had one all assembled with metal blocks holding the resistors so all you had to do was set it in front of a fan and you were done.  They wanted $100 and I went back later and the price dropped to $75.  If I had not had two DLs already I would have bought it in a second.  I couldn't believe no one wanted it. 

[WQ9E]

I don't think the vent seal is critical but you can probably buy vent parts from MFJ since they sell most parts for their products and their dummy load is very similar to the cantenna.

If you really want to go all out I saw a modified Cantenna with a "coolant recover tank" on the side.  The owner had neatly soldered a little copper tube from where the vent had been led into a small glass jar attached to the side.  In theory it would operate much like the overflow/recovery tank from an auto cooling system.  Overkill and not needed but I was impressed with the neat work (but not $75 worth which was the firm selling price).

[K1JJ]

I think the venting requirement will depend greatly on how much sustained power you run into the load.  Certainly, if the oil gets hot enuff and gas "steam" starts forming with no vent, it will blow the lid right off the can.  Could turn into a real mess in the shack.

I remember running some long 1KW tests into a Heath Cantenna back in the 70's and that vent was whistling like a tea pot..   Even if you drill a 1/4" hole into the lid, leave it open for tests and push a bolt into it when unused, that would be a reasonable idea.

T

[KA0HCP]

I would save the lid and get rid of the lower parts.  Buy a new paint can and poke a hole in the top.  Put the can in a bucket to catch any over flow.

-There is no way I know of to distinguish PCB laden oils.  95 years after Edison invented them they are still toxic.  If you can't find a place to take the oil, put some cat litter in a plastic bag and use that to absorb it.

[W1ATR]

Yes, don't block the vent. I have a couple of those can loads here I use and they both stink when I get them good and hot. The vent plug is a little thing that looks like a small nail with a rubber washer on the outside and a light spring on the inside. Probably wouldn't be too hard to duplicate it with a pen spring and a trim nail poked through a small rubber washer. As qaa said, def keep it in a bucket.

Heath sold these empty and unless it was bought by a purest that put real transformer oil in it, then more than likely it'll have mineral oil. Whichever oil you replace it with, it has to be a very light viscosity oil so there is proper convection cooling when the load gets hot. I tried SAE20 motor oil once and the load burned up in short order. My preference for trans/dummy load/cap oil is Shell Diala AX which I get from Graingers. Not too cheap at 20bucks or so a gallon, but it's good sh!t and can be used in everything we mess with. I drained and refilled a bunch of these old GE 'Pyranol' 5mmf 10kvdc 75lb caps with it and they were fine.

[Opcom]

I could swear I saw a Heathkit that included a gallon of 'transformer oil' in the box. What does the manual say.

Does not the old PCB oil have a 'perfume' odor?

[KA0HCP]

I'm told you can buy mineral oil by the gallon cheaper at a farm supply place, in the veterinary section.

[Mike/W8BAC]

I bought one new in 1977 and built the kit from scratch. The instructions told you to go to your local drug store and buy 4 quarts of mineral oil and add that to a point just over the resistor. I have a feeling mineral oil was and still is easier to find than transformer oil. Your load most likely contains mineral oil.

The vent seal isn't a seal at all. Just a stopper of a sort. If the stopper isn't cracked or broken it is just fine. Just top up the mineral oil and your good to go. If you want to know the oil level measurement for the original build, I'll dig out my manual.

The new paint can post at the top is the best idea. Ace hardware or one of the big box hardware stores have them by the case. Fixing the old can is a non starter. I like the kitty litter idea, get rid of the 20+ year old mineral oil and make the dummy load new again. By the way the RCA jack opposite the SO-239 is a great place to wire in an old millivolt meter so you can monitor tuneup of your transmitter.

Mike

[WV Hoopie]

Dug out my manual for the HN-31.
"Pour transformer oil into the pail until it reaches a level about 3/4 in from the top. It is mandatory for proper oil circulation that the oil level be 1/4 in above the shield tube or the resistor element when the pail lid is installed. The recommended transformer oil may be obtained from most any bulk oil plant. If transformer oil is not available, mineral oil may be used, but DO NOT use any type of motor oil. The vaporizing temperature of motor oil is too low and would cause excess vapor."

I used mineral oil and was warned about the PCB issue. This was some time in the late 70's, I think.

Craig,

[W2PFY]

I don't know what brand of dummy load I have but it's called

BIG DUMMY right on the can and it was supplied with the necessary oil. I think it was mineral oil but I'm not sure.

Are there any other BIG DUMMY'S out there?

 

[WA1GFZ]

HMMMM a cantenna with the slim-a-tron mod.

[Ed/KB1HYS]

I built a cantenna out of the big resistor that was inside an MFJ tuner that I smoked (circuit board fried and the coil supports had actually melted).  1-gallon paint can, some heavy copper pipe hanging strap and a uhf bulkhead connector.  I filled it with USP mineral oil sold from the drug store.  That stuff was pretty thick, so I did dilute it with about a pint of mineral spirits, the type that says 'low odor' or some such. Sealed the can with RTV.   Tested it with the dual 4-400 rig (and vice versa) it did get warm to the touch, but not bad after a 3 minute 'key down' caw-mawn.  I don't figure on testing anything for more than 3-5 minutes straight through anyhow.

[WBear2GCR]

Ed,


I am not expert on this, but you might want to alter that setup asap. The mineral spirits are flammable, and under the right circumstances you might have the equivalent of a bomb. If you run it hard enough (hard to know where the threshold is) you will increase the pressure in the can, eventually the top will blow off, releasing hot gasses, which could ignite. BOOM.

(you might not intentionally run it too long, but "stuff happens"...)

Better to dump that and go straight Mineral Oil or else call around to oil wholesalers and/or go begging at the local electric company depo (they do not use PCBs any longer).

                                       _-_-bear

[WB6NVH]

I presume you would like to save the original can because of the graphics on it?  To do that, what may work is to pour out and save the oil, then wash the can out with some paint thinner or equivalent until dry.  Then use some auto gas tank epoxy sealer (auto supply store) around the bottom crimp seam of the can on the inside, and then refill when dry.  Note that some sealers are more of a putty and won't work very well.

[WD8BIL]

Terry, the Big Dummy was a Dentron Product. White label with orange n black printing??
Non PCB transformer oil was sent with it.

[KD6VXI]

FWIW,

So Cal Edison is replacing transformers where I live...  The reasons are twofold:  Obama was nice enough to pay for efficient xformers, AND they are doing an ice retrofit....  Three Xmas's ago, the entire community lost power for 2 days after two substations blew up due to severe icing...

Anywho, when they where replacing the 3 xformers on the pole at my driveway, I inquired about offering to dispose of the old xformers for them ...  No dice.  Then I asked what was in the new ones due to environmental concerns, and the guy told me they had canola in them.

Haven't been able to prove or disprove that, anyone here care to offer an opinion on whether or not it would work?

Might make VERY cheap cantenna and xformer oil, bulk purchased at Costco...

--Shane
KD6VXI

[N0WEK]

My horseback guess on the Canola oil would be that they are using some sort of stabilizer in it to prevent it from getting rancid.

[N0BST]

I presume you would like to save the original can because of the graphics on it?  To do that, what may work is to pour out and save the oil, then wash the can out with some paint thinner or equivalent until dry.  Then use some auto gas tank epoxy sealer (auto supply store) around the bottom crimp seam of the can on the inside, and then refill when dry.  Note that some sealers are more of a putty and won't work very well.

That was what I was hoping for.  Thanks for the suggestion.  I would be willing to go for a new can if someone has the graphics to put on it.

Scott Todd

[W1RKW]

When I was testing the 813 rig and dumping 600 watts my cantenna heated up very hot in 10 minutes time.  It made me nervous. I dumped it for a dry DL capable of 2KW.

If you have a cantenna don't place it on concrete, put a piece of plastic or wood underneath it.. Like most paint cans they'll corrode just sitting on concrete.

[W3SLK]

My horseback guess on the Canola oil would be that they are using some sort of stabilizer in it to prevent it from getting rancid

I wouldn't use any organic oil like that since it is subject to contamination. I use a silicone oil that we use for our ozone generators. I've dumped 500 watts into it for a good portion (better than 30 seconds). The Cantenna I thought was only good for a KW for ~seconds(?) Mineral oil will work well along with any of the modern transformer oils.