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Author Topic: Kenwood TR7850 power output puzzler.  (Read 299 times)
KF4CZV
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Posts: 142




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« on: December 29, 2007, 03:44:48 PM »

I was testing out a nice Kenwood 7850 2 meter mobile I recently acquired. According to the specs it's supposed to put out 5w on Low and 25w on High. Imagine my surprise when my LDG TW2 talking wattmeter tells me it's putting out 45.3w on Low and (almost falling out of my chair) 179w on High! Is it even possible to modify this radio to output that much power? My LDG is about a year old and has been accurate up to now. Also, on the Low setting I was hitting repeaters 50+ miles away so I'm inclined to believe the meter. What sort of mod should I be looking for when I open it up? Is it safe to leave it at these power levels? Thanks!

Terry
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AC5UP
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« Reply #1 on: December 29, 2007, 04:01:54 PM »

I have a hunch your power meter is being optimistic... If there was a mod for this you'd find non-stock finals (and I believe the originals were flat ceramic pill jobbies with cloverleaf tabs so there wasn't room for anything larger) and a beefed up power cable and fuse.

Assuming you really are seeing 170+ watts on high power that would be something like 35-40 amps of draw on your power supply. Is your DC supply that stout? Typically a rig like that will draw < 10 amps at full tilt TX.
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KF4CZV
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Posts: 142




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« Reply #2 on: December 29, 2007, 05:14:34 PM »

DOH! Boy is my face red! Mystery solved. I forgot to set the LDG meter to the proper band before testing. It was set on 50mhz. Once I reset it and tested the Kenwood again I got 12.5w on Low and 52w on High. Still pretty good for an old rig. Sorry to bother you guys! Getting old is heck.

Terry
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AC5UP
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« Reply #3 on: December 29, 2007, 06:04:20 PM »

A couple of years ago I picked up an HP 427A voltmeter (FET "VTVM") on eBay then acquired the matching 5300 True RMS AC voltmeter from an SK. Forty years ago that was one high-dollar pair of bench buddies and they're as clean and good as new.

A few months ago I reconfigured the workbench and made a space for them. First exercise for the 427A was to check a resistor before using it. Tested bad. Tried another. Tested bad. Went through four resistors before I realized they were all bad by about the same amount.

Yeah, I was trying to read Ohms on the Volts scale.

We all have our moments. Wink
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KB9CRY
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« Reply #4 on: December 29, 2007, 06:05:55 PM »

A "talking wattmeter".   Wow!
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AC5UP
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« Reply #5 on: December 29, 2007, 06:06:09 PM »

Correction: 3400A RMS Voltmeter.

See what I mean?
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WA3SKN
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« Reply #6 on: December 30, 2007, 09:33:19 AM »

Power meters actually measure volts.  If you are not looking into a 50 ohm resistive load, readings will vary!
Glad you found the problem!

-Mike.
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DA2KI
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Posts: 87




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« Reply #7 on: December 31, 2007, 09:01:44 AM »

I trust these output power measurements were made into a 50-ohm dummy load, and the radio was powered by a well-regulated DC power supply set to 13.8 Volts?
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KF4CZV
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Posts: 142




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« Reply #8 on: December 31, 2007, 09:44:56 AM »

They were indeed!

Terry
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