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CB and Ham Radio Techs Love Their Bird Wattmeters

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xrunner:
Right now I'm sidetracked because I have a weird issue with a Boonton 4220 power meter that was working fine last time I used it. I've had it for a while like over a year and I wanted to get it off the shelf and use it for this project to measure a transmitter power out, but it seemed like the sensor had gone bad because it wouldn't zero. The sensor came from a local ham who got it at a hamfest and I suspected it has been abused, so perhaps it decided to finally fail. I convinced myself that was the case so I ordered another compatible sensor and I could not get that sensor to zero either.

So was it the power meter ... or cable? I messed with everything for an hour and then for no reason I could see, both power sensors zero'ed and calibrated. OK well now I have a spare sensor. But then a while later the new sensor wouldn't zero again. I took off the outer shell of the new sensor, and presto it zero'ed again. But then why did the original sensor stop zeroing, and what does removing the shell fix?

Right now I have no idea what's going on but I'm determined to find out!  >:(

joeqsmith:
You have the HP.  Why did you purchase the Boonton?   Do you have the thermistor or diode sensors for it?   

Kosmic:

--- Quote from: xrunner on June 07, 2023, 12:32:12 am ---So was it the power meter ... or cable? I messed with everything for an hour and then for no reason I could see, both power sensors zero'ed and calibrated. OK well now I have a spare sensor. But then a while later the new sensor wouldn't zero again. I took off the outer shell of the new sensor, and presto it zero'ed again. But then why did the original sensor stop zeroing, and what does removing the shell fix?

--- End quote ---

Do you have the 14 points linearity data for the sensor ? (4220 Manual page 4-1, section 4-5). If not, this would explain why you are struggling to zero the sensor. Normally the meter and sensor are sold together and can't be separated. If you want to pair a new sensor with a meter you need the calibration data of the sensor.

You can also validate the good working state of the sensor by applying a RF signal and connecting a multimeter on the output. You should see a DC voltage in the millivolts range if you apply a signal around 0dBm. The sensor is really just a rectifier.

xrunner:
I have fixed the 4220. I had to adjust the chopper per the service manual, now it works perfectly (both sensors). It fell out of adjustment and was on the edge of operating correctly, hence the seemingly random results.

Kosmic, about 1.5 years ago we were talking about this in the TEA thread here -

https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/msg3943906/#msg3943906

Of course it would be better if I had the cal data, but amazingly it is very accurate without that (both sensors). Seems to be less than 1 dB error in my testing down to -60 dBm. Good enough for testing ham gear.

Joe, I will write more about how I got all the Boonton equipment later, can't at the moment.

Kosmic:
What's the page in the manuel where they describe how to adjust the chopper ?

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