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Author Topic: Anybody recognize this dummy load?  (Read 2451 times)
KJ6ZOL
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Posts: 359




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« on: March 05, 2013, 11:35:40 PM »

When I first got my General, I was given some equipment from a kind Elmer. I said that I was planning on building my own dummy load and he handed me this dummy load:

http://i865.photobucket.com/albums/ab213/loquisimo/dl1.jpg
http://i865.photobucket.com/albums/ab213/loquisimo/dl2.jpg

I asked him what the specs were and he didn't know. Looking at it, it has zero markings telling who made it or anything about what its specs might be. It has some sort of mounting bracket; what it was meant to be bolted to, I have no idea. The resistance is lower than that of the G5RV antenna I'm using. Does anybody recognize this thing? It works for what I need it to do, albeit not very well. I just don't have the cash for a MFJ dummy, nor to complete my oil filled homemade dummy.
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G3RZP
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« Reply #1 on: March 06, 2013, 02:41:48 AM »

I vaguely remember seeing adverts for them. Have a trawl through 73 Magazine from about 1966  to say 1980, and through Ham Radio magazine up to 1980.
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W5CPT
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Posts: 557




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« Reply #2 on: March 06, 2013, 07:55:58 AM »

I don't recognize it, but you can find out what it is capable of very easily.  First connect it to your transmitter through an SWR meter.  Start at your frequency of interest and at low power. A Dummy Load should present a 1:1 SWR (or nearly so). If it does, move the frequency up and down to see if it flat across the range. It should be, as most DLs are built to be purely resistive with no or little reactance and it is reactance that makes a DL change impedance with frequency change.   Gauging the size by the SO-239 I would guess the power capability to be in the 100 – 300 watt range.  Remember that is for short periods of time.  Most commercial DLs have a de-rating curve chart printed right on them.

Alternately, if you have or can borrow an MFJ-259, you can very quickly check it over a very large range. 

Clint - W5CPT -
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K8AXW
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« Reply #3 on: March 06, 2013, 08:13:25 AM »

ZOL:  Not enough info.  When you took the photos you didn't have anything close to it that we can compare it to.

If you put an ohmmeter across the SO-239, what is the resistance?

Can you get it open?

If you can get it open, what's inside?

Is it full of oil or is it air cooled?

If it's a dry resistor, you can estimate the power handling capability by measuring the physical size of the resistor(s) and research resistors of that size.



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K1ZJH
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« Reply #4 on: March 06, 2013, 08:25:23 AM »

I'd bet it is comprised of discrete, paralleled resistors, judging by its age.

What is the DC resistance of the load--use your ohmmeter. It should read close to 52 ohms.

My personal opinion is that it is probably good for 100 watts MAX with a low duty cycle.

Pete
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AA4PB
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« Reply #5 on: March 06, 2013, 08:43:34 AM »

Perhaps a Gentec 525. If so, it's 50 Ohms at 125W.

Here's a picture: http://www.pbase.com/drcaronproplab/image/148850875
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KJ6ZOL
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« Reply #6 on: March 06, 2013, 10:47:40 AM »

ZOL:  Not enough info.  When you took the photos you didn't have anything close to it that we can compare it to.

If you put an ohmmeter across the SO-239, what is the resistance?

Can you get it open?

If you can get it open, what's inside?

Is it full of oil or is it air cooled?

If it's a dry resistor, you can estimate the power handling capability by measuring the physical size of the resistor(s) and research resistors of that size.





It's sealed, I can't get it open. Therefore, I can't tell if it's oil filled or not, but there's no sloshing sound when I shake it, so I bet it's air cooled. I haven't taken measurements of it yet.
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KJ6ZOL
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« Reply #7 on: March 06, 2013, 10:50:58 AM »

Perhaps a Gentec 525. If so, it's 50 Ohms at 125W.

Here's a picture: http://www.pbase.com/drcaronproplab/image/148850875


I think you're right, 'PB. How old is it, I would guess 1960s or early 70s by that pic.
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KJ6ZOL
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« Reply #8 on: March 06, 2013, 11:06:08 AM »

Ok, my MM says that it measures 61 ohms. So the resistance would be GREATER than my antenna. I have a Radiowavz G5RV Jr. fed into a Ten Tec 228 tuner, so my antenna is likely exactly 50 ohms, while this load is 61. The ohms of the DL is likely due to its age. I will likely have to get another DL eventually, but I can't afford it right now.
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K8AXW
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« Reply #9 on: March 06, 2013, 05:45:51 PM »

ZOL:  Not to insult your intelligence.... understand?  But I suggest you measure a precision 50 ohm resistor or one close to that resistance to make sure your MM is accurate.  Sometimes inexpensive MMs will read off when measuring low resistances.

But, in all probability, you're correct.  The old resistors have changed value through the years.
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KJ6ZOL
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« Reply #10 on: March 06, 2013, 08:53:13 PM »

ZOL:  Not to insult your intelligence.... understand?  But I suggest you measure a precision 50 ohm resistor or one close to that resistance to make sure your MM is accurate.  Sometimes inexpensive MMs will read off when measuring low resistances.

But, in all probability, you're correct.  The old resistors have changed value through the years.

My MM is no cheapie. It's a Lodestar autoranging MM. Granted, it's Chinese, but it seems to be fairly well built. I had a Lodestar soldering iron tip thermometer for a while and liked it, so I sought out their MM's. They produce equipment primarily for the Chinese domestic market.
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KJ6ZOL
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Posts: 359




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« Reply #11 on: March 06, 2013, 09:02:47 PM »

Perhaps a Gentec 525. If so, it's 50 Ohms at 125W.

Here's a picture: http://www.pbase.com/drcaronproplab/image/148850875


I googled it and couldn't find a definitive date. However, I did find that it's rather famous among fans of the classic Halloween comedy movie Ghostbusters-apparently one was part of the "equipment" for the Ectomobile, the converted Cadillac hearse that the Ghostbusters used to save New York. Mine is missing the metal flange around the SO connector that gave mfg info.
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G3RZP
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« Reply #12 on: March 07, 2013, 02:54:57 AM »

If its 61 ohms, it's an SWR of 1.22:1. Not enough to worry about for most purposes.
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KE3WD
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« Reply #13 on: March 07, 2013, 06:14:09 AM »

ZOL:  Not enough info.  When you took the photos you didn't have anything close to it that we can compare it to.




I think that the SO-239 on top of the thing yields a very good sizing gauge...
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