What is this? A 5 kw Dummy Load or what?

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Fran3
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What is this? A 5 kw Dummy Load or what?

Post by Fran3 » Mon Oct 05, 2015 2:31 pm

This is on the back of a 5kw AM station and i was told it was a dummy load.

But I would have expected a bunch of purely restive devices so the two inductors and two black things (caps?) confuse me... and, I'm not sure what the other things are unless they are some type of restive elements.

If it is a dummy load I would guess you must tune the r/c network to the proper frequency and connect the other elements in some sort of series and/pr parallel configuration to get it all to look like a 50 ohm restive load.

I would appreciate any comments, suggestions, how-to's or whatever.

And if this was an off the shelf item or its manufacturer is known then that would be helpful as well.

thanks for any help.
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RodeoJack
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Re: What is this? A 5 kw Dummy Load or what?

Post by RodeoJack » Mon Oct 05, 2015 3:04 pm

Could be a dummy load.

On the other hand, If the station is/was daytime-only and had an older transmitter that couldn't get below 500 - 1,000 watts or so, that box could have been intended to swamp the transmitter's output down to whatever the pre-sunrise power was.

We had an engineer on the other side of our state who built a lot of those things. Most were made with ceramic resistors, mounted on an aluminum sheet, but I recently saw one that was made up of a bunch of those heater elements you could screw into a light socket.

TPT
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Re: What is this? A 5 kw Dummy Load or what?

Post by TPT » Mon Oct 05, 2015 3:12 pm

Kind of agree with Rodeo Jack. Normally wouldn't need any tuning for the typical dummy load made up of a bunch of power resistors.

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Deep Thought
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Re: What is this? A 5 kw Dummy Load or what?

Post by Deep Thought » Mon Oct 05, 2015 3:30 pm

Probably a dummy load with an L network to match it to 50 j0 (or whatever) at the station frequency, which wasn't that unusual back when this was built. I can't see what is under the red fiberglass panel but the power reduction units usually tapped off one of the resistors to head out to the tower, and I don't see anything like that on this one.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

Fran3
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Re: What is this? A 5 kw Dummy Load or what?

Post by Fran3 » Mon Oct 05, 2015 4:52 pm

You know... maybe those are something like heater elements... and the network for tuning it to purely restive 50 ohms.

If they are something like filaments or heater elements I wonder how you would determine the expected resistance/impedance under load? Or would the elements have the same characteristics at ambient as under load?

We will take it down tomorrow and see what is under the fiberglass base plate... and then see if we can figure out how to rewire it and put it back in service.

And while we are doing this measure the dormant resistance of the elements.

Thanks for the help.

RodeoJack
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Re: What is this? A 5 kw Dummy Load or what?

Post by RodeoJack » Mon Oct 05, 2015 5:19 pm

Here's something I think may be similar. In its daytimer days, the station ran a Bauer 707, and had to reduce its 250 low power setting down to its 39 watt pre-sunrise. (Disclaimer... Just got this as a client station. No... I didn't wire it. Pretty sure the engineer who built it didn't leave it that way).
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ChuckG
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Re: What is this? A 5 kw Dummy Load or what?

Post by ChuckG » Mon Oct 05, 2015 7:41 pm

I'm not seeing an output either, only an input.
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Jim Sofonia
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Re: What is this? A 5 kw Dummy Load or what?

Post by Jim Sofonia » Tue Oct 06, 2015 11:37 am

I have a Wilkinson DL5-A 5 KW dummy load 1980 vintage. It has a number of 200 ohm wire wound power resistors in a series/parallel network that equals 50 ohms. I always wondered about the reactance of the wire wound resistors and there is no tuning network. The resistors are mounted in pairs with one inverted to the other, so I thought maybe they cancel each other's reactance out. This is now out of service and I would sell it to anyone needing one.

RodeoJack
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Re: What is this? A 5 kw Dummy Load or what?

Post by RodeoJack » Tue Oct 06, 2015 1:48 pm

ChuckG wrote:I'm not seeing an output either, only an input.
I'm going back there in the next week or two. I'll get some more pictures and pay more attention to it then.

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Re: What is this? A 5 kw Dummy Load or what?

Post by RGORJANCE » Tue Oct 06, 2015 3:09 pm

That looks like a Harris dummy load. Look for a part number that resembles 994-3968-001. This number is for is a 5kw dummy load. If the number is different, it may be either a 50 ohm, or a 70 ohm dummy. Another area to check is the coils. Look for a 10 digit part number resembling 931-6138-026. An additional coil number that is for this 931 number is 24FC1655. that translates to 24uhy, fixed ribbon coil, 20 amp rating.

Fossil

Fran3
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Re: What is this? A 5 kw Dummy Load or what?

Post by Fran3 » Tue Oct 06, 2015 4:25 pm

Thanks. This is a good lead. We are going to take it down tomorrow and give it a close inspection. I will also contact Harris Broadcast/Imagine Communications.

grich
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Re: What is this? A 5 kw Dummy Load or what?

Post by grich » Wed Oct 07, 2015 4:47 pm

Fran3 wrote:Thanks. This is a good lead. We are going to take it down tomorrow and give it a close inspection. I will also contact Harris Broadcast/Imagine Communications.
You'll need to contact GatesAir. They are the transmitter/RF side of the old Harris. Imagine is the video encoding side.

/dev/null
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Re: What is this? A 5 kw Dummy Load or what?

Post by /dev/null » Fri Jan 05, 2018 5:55 pm

Yep, looks like a PSA reject load to me. Aeons ago in college, there was a 5kW. DA with a 14.2 W. PSA. We got a bunch of non-inductive carborundum resistors,, dunked them in a 55 gal. drum full of mineral oil, and shunted them across the tx. line. Crude as homemade sin :wink:

W2XJ
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Re: What is this? A 5 kw Dummy Load or what?

Post by W2XJ » Fri Jan 05, 2018 8:01 pm

I agree with those who say it’s a dummy. It very much looks homebrew and not that well done. I have worked with better home made loads. As was already stated, there are some kind of industrial resistive devices and a matching network. Usually not that much of a matching network is required so that means the resistors are probably not totally non-reactive.

I inhertited something similar rated at 10 KW. I used it for a backup 10 KW TX and had a ‘real’ Altronics water load for the 50 KW main. This worked out well because we could make both transmitters see 50 j0 in their respective loads. I had another one at a 50 KW site with 2 50 KW transmitter. The problem was that when either transmitter was on the dummy, it radiated enough to cause interference and the resulting complaints.

There are neater ways to do this. Among them, baseboard heaters come to mind. That said, retune it for your frequency and it will be ‘good enough’

COMMENG
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Re: What is this? A 5 kw Dummy Load or what?

Post by COMMENG » Mon Jan 08, 2018 10:51 am

The "resistive" strip appears to have come from an older Gates BC-1T or similar vintage vacuum tube transmitter.

Each resistor is most likely rated at 412 ohms, 150 Watts or so.

The other other stuff appears to be added on to get to J0.

COMMENG

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