Electronics > Repair

Boonton 41-4B power sensor repair advice

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xrunner:

--- Quote from: edpalmer42 on October 25, 2023, 09:42:01 pm ---Do you have access to a VNA?

--- End quote ---

Yes I do.  :)


--- Quote ---It would be interesting to see what the impedance of your rebuilt probe looks like compared to an official one.

--- End quote ---

I wish I'd have documented it before it failed. However I have another Boonton sensor model 51010 (4A), I can check the response of that sensor. Or do you have one closer to the 4B that I can compare to?

You see the model 4220 meter I'm using. I got the meter for a steal from Fleabay a couple of years ago because I couldn't repair the abused Boonton Model 42A Microwattmeter the ham gave me with the sensor.


--- Quote ---I think that's the purpose of the offset washers that the resistor leads were clamped under.  By turning the washer you could tweak the termination.

Ed

--- End quote ---

Oh really that's interesting. I just thought the washers were a secure method of holding the wire. The offset of the washers provides a little spring to the area so if the screw ever got a bit loose the connection would still be tight. But hell if I know you are probably right.


edpalmer42:
I don't have anything that could make those kinds of measurements at the high frequencies that would be necessary.  For all I know, you might need to step up to one of the 18 GHz probes to see any effect from turning the washers.

Your cable looks a lot different from mine.  I've got two cables and both of them are almost perfectly round rather than wavy like yours.  One of mine has Amphenol connectors and the connectors on the other one are labelled 'KINGS1-1464'.  Unfortunately, it looks like that's the part number for the rubber strain relief.  Kings is a well-known manufacturer of high quality RF connectors.  Neither cable has any labelling on the cable itself.

xrunner:
Well that was fun.  :P

I got the 160 ohm SMD 1206 parts. I cobbled together a helping hands holding tweezers to hold each part at the right angle. I then soldered the two diodes and two 160 ohm resistors in place like the original parts were.

It sure looks like it's working - it zeros and calibrates great and no strange noisy tantrums.

I need to test it at more power levels and frequencies which I'll do in the next few days using my Agilent 8648A. Of course I can't compare it directly to the original sensor response now. But I do know during the periods when the original was working, it was only a few tenths of a dB off from the setting of the Agilent over it's power range.  I'll report those results here.

edpalmer42:
Looks good, xrunner, but I have to admit that I'm worried about those surface mount resistors.  They have no physical strength so a single thump could crack one or more of them.  It should be okay if you're careful, but you could also consider something like a blob of silicone to provide some support by 'glueing' the resistor to the main chassis.  I don't think that would affect the readings, but YMMV.

A radical thought just occurred to me.  Imagine taking a tiny piece of PC board with just enough copper traces to allow you to solder the resistor and a wire at each end.  Maybe a prototyping strip board.  The board would remove almost all the stress from the resistor.  It would be like converting a surface mount resistor into a leaded resistor.

xrunner:

--- Quote from: edpalmer42 on October 29, 2023, 02:49:30 am ---Looks good, xrunner, but I have to admit that I'm worried about those surface mount resistors.  They have no physical strength ...

--- End quote ---

Oh now ... it's not true that they have no physical strength. They have a physical "strength" x greater than zero and less than Max. We don't know what "x" is. How hard is it to crack one in two? I have no idea but they don't seem too frail. I've held them with small pliers and tweezers and they don't seem delicate for their size at all.

But were using subjective terms here, so I don't know. I mean, look how the original parts ended up. The red resistor (100 ohm) cracked off at the end on it's own, and I said the 160 ohm just crumbled when I barely touched them and they aren't SMD. That's how Boonton engineers decided to mount those parts. 30 years from now if they break I won't care because I'll be long gone and this meter will either be in a museum of ancient technology or a landfill (which is where it will end up if my family members get hold of it).  :-\

But I'm not worried really. This thing ain't going into the Gaza strip to check transmitters. Remember I made a custom mounting for the 100 ohm that use the copper sheet strips. I specifically made the strips to bend a bit using a little curved spring at one end.

Look at this. I was searching Fleabay for Boonton cables and found this item -

eBay auction: #186114963838

A 41-4B sensor and cable for $60. But one of the connector pins has nothing inside. How well do you think it works? The parts are listed as used which means they works correctly but it's not new. I messaged the seller but no reply yet ...

 :popcorn:
   

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