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koØm
11-01-2010, 12:29 PM
In my 39 year experience of troubleshooting electrical and mechanical devices down to component level and repairing them, I have suddenly run into a spate of issues that have me stuck in neutral. Normally, when issues pop up, I just go get a cup of coffee or, leave it alone for a day or so and then get back to it with good results; not so this time.

To sum it up, suddenly while "Qualifying" a device for delivery, the equipment on my work bench seemed to systematically have developed problems:

1. A mono-band amplifier using two of the 813 type of tubes died while I was using it as a driver.

2. My Yaesu FT-101 E radio developed receiver problems; it went "deaf".

3. My Bird 43-P wattmeter went erratic and non-responsive.

4. I smoked one Radio Shack digital VOM because the input maximum was 400 ma, oops!

5. My Heath Cantenna/Non-radiating load changed in value.

I have spent more time fixing the test equipment so that I can demonstrate that this device works and to establish a price and warranty expectation than working on the item. My readings keep changing. The resistance on my Dummy Load is 50 ohms cold and, Zero ohms after 30 seconds at 600 watts; then after cooling some, the value goes up to 230 ohms and drifts down until it gets to 50 ohms. This sent me tearing into the Bird wattmeter before I realized the load was going crazy.

I observed this a number of times over a 2 day period (at first, I thought the replacement RS meter was gimp) because, Heath kit Canntennas cannot go bad, right? I have owned this Dummy Load for 30+ years and it still has the same oil in it from when I first filled it.

I spent the better part of 3 hours yesterday disassembling and re-assembling the 50 ohm Dummy Load; twice, once to inspect it and, the second time to replace the 50 ohm element, the SO-239, and to remove the meter circuit that is native to this device. The only thing that I did not change was, the Mineral Oil in the can.

Question: After 30 years of “Tube w/handles QRO” what would be the failure mode for the Heath Non-Radiating Load and, how are the properties of the Mineral Oil changed by 30 years of QRO Radio Frequency energy?

.

WØTKX
11-01-2010, 01:05 PM
Good question... did you try stirring the oil? Are there tiny cracks in the resistor(s) in there, that may have changed things a lot... as the heat might not be dissipated evenly along the resistor? Opening the can from time to time will contaminate the oil, especially the older transformer stuff. If the resistors and such are still good, I'd buy a new can and get some mineral oil... vet supply places sell it cheap.

http://www.k3hkr.com/1KWdummy.html

I have a brand new MFJ here, and it works well, tho I might do the giant 5 gallon bucket immersion trick. I saw this done as a kid, my Elmer was messing around with full power 100% duty cycle for RTTY and put one in a large asphalt sealant metal can... and was able run for almost 15 minutes before the R value started to change. Just top off the bigger can below the level of the smaller cans top. :yes:

P.S. Doing Google-Fu, I found this gem. The message had me at "Hello".

:rofl:


KA5PIU
06-08-2007, 04:26 PM

Hello.

Paint buckets are now coated for the water based latex paints.
One can go to a Sherwin-Willams paint store and buy an unlined can, needed for some automotive coatings.
Paint cans also come in grades, everything from the consumer grade to the thicker industrial grade that is fire rated.
The lid size is the same.
If one has real concern over the heat of the dummy load, one trick is to put the 1 gallon bucket into a 5 gallon bucket that has mineral oil in it.
Drill a hole in the bung and fit a grommit and you are all set.
Do not fill the 5 gallon bucket up, just to below the top of the 1 gallon bucket, the UHF connectors may not be compatible with the oil.
I decided to go whole hog and built my paint bucket antenna as a 5 gallon job to begin with.
I needed to, as the transmitter is rated to 10 kilowatts.
Even than, it is just a few seconds at a time.

NQ6U
11-01-2010, 02:50 PM
You know, that seemed completely believable--right up to the 10KW part.

kb2vxa
11-01-2010, 03:36 PM
I have my doubts about those old Heath dummy loads having been made with carbon composition resistors. They have been known to change value going way out of specs permanently due to heating. Even NOS has its little surprises, metal film is what's used today for stability. Rebuilding it may not be worth the time and cost of parts, you may want to check this out. http://www.surplussales.com/RF/RFDummy-3.html

NQ6U
11-01-2010, 03:41 PM
http://www.surplussales.com/Images/RF/DummyLoads-Terminations/rf-5750_.jpg

10KW water-cooled. That should be enough even for Rudy's station.

WØTKX
11-01-2010, 03:46 PM
Yup, I've seen small cracks in the old ones, from abuse. Same site, this part number... maybe. (RNI) 00050-30A

If the oil gives you a factor of 100, the 30 watt resistor should handle even an Alpha (not for export) driven hard.

N8YX
11-02-2010, 06:29 AM
Collins, B&W and a couple other firms made nice air-cooled dummy loads - some with power ratings of 25+ kW.

A couple years back I "inherited" a 1kW fan-cooled unit (with onboard wattmeter) from an SK's widow. It works so well as a piece of test equipment that I gave my Cantenna away to a prospective ham.

Drake made a 1kW dummy load - the DL1500. I have four of those in the shack, each dedicated to a specific tuner/coax switch. These units have provisions for fan cooling; one of these days I'll mount some 24v ball-bearing TubeAxial fans on them and obtain power from one of the station's Astrons. Given the short duty cycle for the loads, a lower RPM/lower noise setup will supply adequate air flow for cooling.

WØTKX
11-02-2010, 10:23 AM
I saw that old post as well. :stickpoke:

N8YX
11-02-2010, 12:52 PM
I saw that old post as well. :stickpoke:
What wut?

WB2WIK
11-02-2010, 01:32 PM
I have my doubts about those old Heath dummy loads having been made with carbon composition resistors. They have been known to change value going way out of specs permanently due to heating. Even NOS has its little surprises, metal film is what's used today for stability. Rebuilding it may not be worth the time and cost of parts, you may want to check this out. http://www.surplussales.com/RF/RFDummy-3.html

Heath Cantenna loads didn't have carbon composition resistors in them. They used a single cylindrical Glowbar resistor, 50 Ohms at 150W. With the oil, this increases to 1kW for 60 seconds or something. But there was never more than one single load resistor in a Cantenna, and they can fail -- always from overheating. A Cantenna can't safely take the power of a "tetrode with handles" for more than a few seconds.

KC2UGV
11-02-2010, 02:30 PM
I'm pretty sure the load resistor on your dummy load is going bad. The changes with temp are pretty indicative of that.

KA5PIU
11-08-2010, 11:16 AM
You know, that seemed completely believable--right up to the 10KW part.

Hello.

This is pulsed radar service.
I was playing with the idea of the old world war 2 ten meter radars.
The transmitter is an old broadcast transmitter for shortwave service.
If I had the antenna and power for it I could easily pump out at least 20 kw. ;)
Quite a few of the old shipboard transmitters are rated at 10+ kw AM and CW.
The old AM marine band is around 2 MC, MHz today. ;)
With the use of old marine radios, crystal VHF as well as AM HF, you can get on 160 and 2 meters cheap.
Add in a crystal CB and you have 10 meters covered.
Do it right and you can do it all for under the cost of a talkie.