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Author Topic: Bird Wattmeter/Dummy Load shorted shunt coil  (Read 1765 times)
K4JJL
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« on: October 25, 2006, 09:18:19 AM »

I have a Bird Wattmeter/Dummy Load combo.  It uses interchangeable thermocouples to generate DC to run the meter.  In order to keep the DC from going back up the coax into the transmitter, it has a shunt.  The coil in the shunt has gone from 1.5 Ohms to a dead short.  Anybody know what the value in mH the coil should be?  It's pretty old (circa 1955) and I doubt I'm going to find another shunt to replace it, so I was going to replace the coil in the shunt.

Thanks,

Jared
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K4JJL
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« Reply #1 on: October 25, 2006, 09:46:47 AM »

Just wanted to post more info.

Rated frequency range: 20MHz-1000MHz
Rated power: 1 kw

The meter itself is a DC mV meter.  I think max scale is 100 mV or so, so the shunt wouldn't be carrying massive voltage on it, just blocking RF from being shorted.
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WB2WIK
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« Reply #2 on: October 25, 2006, 10:27:01 AM »

Are you sure they are "thermocouples?"  A thermocouple used to measure power is pretty common, but it would be mounted right on the load resistor and with the oil cooling most of the Bird loads have, the time constant would be very long...I'd think it would take a long time to reach thermal equilibrium and get any sort of meaningful reading.

Bird wattmeters normally use a diode detector, not a thermocouple.  The detectors are interchangeable, frequency selective elements covering a frequency range and a power range.  If your load/meter has something different, it would be interesting to know what that is.

Meters don't respond to voltage, they respond to current.  If the meter is labeled "100mV" that means it has a resistor in series with it, to convert it from a microammeter to a millivoltmeter.  That's also a bit unusual, as all the Bird stuff I have uses a 30uA FS meter and the shunts are in the detector elements.

What's the model number of the unit you have?  Maybe there's a manual on line we can look up...

WB2WIK/6
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K4JJL
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« Reply #3 on: October 25, 2006, 11:52:24 AM »

I've got a manual, but it doesn't mention the mH rating of the shunt coil.  All it says is that if the resistance between the center pin of the shunt and the shield is < 1.5 Ohms, the shunt is bad and replace it.  On my shunt, the resistance is .2 Ohms (equivalent to touching the meter leads together).

The way you hook it up is this:

transmitter --> shunt --> thermocouple --> dummy load

The meter taps onto the coax between the thermocouple and the dummy load.  The thermocouple makes a small DC voltage that the meter reads.  The shunt is there to keep the DC from backing up the coax into the transmitter.

This is like no other Bird wattmeter made.  You change the thermocouples to set the scale of the meter, but it's not a directional wattmeter (like the Bird 43).  The thermocouple also does not go inside the dummy load.  It's an N-connector that goes in series with the coax.
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K6AER
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« Reply #4 on: October 25, 2006, 12:31:42 PM »

OK… has any one called Bird Electronic Corp. service department? I know this might seem silly but I would first call their service department at: techsupport@bird-technologies.com or call 1-866-695-4569.

I’m not saying this is the case in this post but I don’t know why hams have such an aversion to going direct to the source such as the manufactures for technical questions. Manufacturers in general go to great extremes to developed technical support groups with history as well as technical support. It took only 5 seconds to find the contact information on Google.

I also agree with Steve, I have never seen a thermocouple in a bird dummy load. Given the load is a constant 50 ohms over the specified frequency range, the dummy load input “T” could be nothing more than a voltage detector with a current limiting resistor feeding the standard 30uA meter movement.

Also a thermocouple could never generate enough voltage to bother a transmitter output. Nor would prudent engineering practices connect a thermocouple to a RF line. I suspect the pickup is nothing more than a voltage`detector connected to the standard Bird meter movement.
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WB2WIK
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« Reply #5 on: October 25, 2006, 01:04:53 PM »

I agree, this doesn't seem like it could be a thermocouple.

A thermocouple provides a current based on temperature rise.  Thus, it must be thermally bonded to the load itself.  10W applied to a 1000W oil cooled load creates almost no thermal rise, and if there is any measurable rise it might take an hour to occur.

I suspect also this is really a diode detector...
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K4JJL
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« Reply #6 on: October 25, 2006, 01:06:04 PM »

Do you honestly think that Bird would have any knowledge of products manufactured over 50 years ago?  If you truly do, I'll call them.

I DO have the manual for the dummy load.  I'm not making this stuff up or guessing at what the components are.  In the manual is a schematic and a theory of operation.  The meter came with 8 THERMOCOUPLES (exactly what the manual calls them), 4 for general use and 4 for calibration.  Each thermocouple is rated differently to change the scale of the meter.

Each thermocouple package is basically a low-value resistor (different values for different power levels) that generates small amounts of heat as power is applied.  The thermocouple contained within detects this heat and applies DC to the coax.  This DC is then detected by the meter showing you the power in watts.

If you need a more detailed explanation than that, I'll have to get out the manual.

So, I repeat my question:  what value in mH would be required in a shunt to keep the DC from going back up the coax to the transmitter?
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K6AER
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« Reply #7 on: October 25, 2006, 01:55:38 PM »

Bird is the one point of contact where owners will most likely bring questions of equipment repairs on a regular base’s, no matter what the age, for Bird equipment.

I find it incredulous that you would take the word of a fourm which has never scene your dummy load over the experience of trained technician and engineers at Bird. Call them, what have you to lose. Should no one have an answer to your question go to the ARRL Hand Book and look at the chapter on inductance and filters and calculate the inductance necessary for your circuit impedance with a cutoff well below the bottom range quoted as the loads operating range of 20 MHz.
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WB2WIK
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« Reply #8 on: October 25, 2006, 02:23:58 PM »

Of course I would call Bird.

I called them about a military load I purchased without documentation at a Swap Meet, and it was labeled only with a military designation for a model number, a serial number, and manufacturing date in 1959.

They looked it up and provided me with an operating manual (photocopy) for it, in 1991.

I am amazed they used thermocouples for a "1 kW" load.   The thermocouples must be decoupled a LOT, like 20-30 dB, and then the TX power delivered to a real load while the thermocouple measures rise in a tiny little load that is driven by highly decoupled power.  That would be the only thing that makes sense.

While you're making a measurement, if you grab the thermocouple device in your hand, does it change the power reading on the meter?  It should....

WB2WIK/6
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K4JJL
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« Reply #9 on: October 25, 2006, 02:45:00 PM »

I called a friend and had him look up the info on FedWeb.  It's a TS-118A/AP and the shunt is an MX-1784/U.  He's trying to find the specs on the shunt right now.  He said that the shunt is listed in FedWeb for $360 (ouch!) so repairing it is a must.  In the mean time, I emailed Bird about the shunt to see if they could pull up some specs on it.  Waiting to see what they say...

Oh and to answer your question about putting my had on the thermocouple, it still reads the same.  At least right now it does.  I could dump 500 watts in it right now and it still reads 0 (because it's broken) Smiley

I think the shorted shunt is what is killing my readings.

If you want to see what it looks like, I found a copy of the manual I have on BAMA.  http://ftp://bama.sbc.edu/downloads/miltest/ts118/
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W6EM
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« Reply #10 on: October 25, 2006, 02:49:11 PM »

This sounds like the old surplus Bird TS-118A/P that I used to have.  I didn't have the "shunt" so I could only use it for a dummy load.  As I remember it had about 5 or 6 different color-coded adaptors for different ranges.

Sounds like you might be better off just using the terminating resistor for a dummy load and sticking a 43 in line ahead of it.

Lee
W6EM
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K7AAT
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« Reply #11 on: October 25, 2006, 07:36:24 PM »


  Too bad your manual image is a .djv file..... a format I suspect most of us can not readily open.  I'd have liked to have seen it.


   Ed   K7AAT
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N6AJR
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« Reply #12 on: October 25, 2006, 07:54:47 PM »

there is a .djv available on the web for download, free, I think it is at BAMA
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K4JJL
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« Reply #13 on: October 26, 2006, 07:50:43 AM »

You can get a plugin for your browser to see DJVU images.    It's available at http://www.lizardtech.com/download/dl_download.php?detail=doc_djvu_plugin&platform=win

They have Windows, MacOS and Linux versions available.  It works and looks similar to Acrobat, but with crummy viewing controls.

And to N6AJR, if you read previous posts, I posted a link to the .djv file containing this manual.
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K4JJL
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« Reply #14 on: October 27, 2006, 11:09:02 AM »

It's been 2 days and no response from Bird.  Any recommendations in the shunt coil rating in mH?
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