Electronics > Repair
Boonton 41-4B power sensor repair advice
xrunner:
Need advice to repair a Boonton 41-4B power sensor. Pics attached (yes the year is wrong I was setting the clock on the microscope and made a mistake).
I've had issues with this power sensor for the past year, in that it's VERY erratic. It might be measuring the internal cal signal for example (0.0 dBm) and if you barely touch the sensor the reading goes whacko (technical term). I have a different sensor and nothing like that happens while using it.
I've inspected the inside before, but I never found anything wrong. This time I decided to go further and start using a microscope. I think I found the issue - a failed wire connection to what appears to be a 100 ohm precision resistor (it measure about 100.2 ohms. There are two of them that are the same part). These are the two red components in the pics. Since they are essentially in parallel coming off the center rf input, it must be designed to provide a 50 ohm impedance to the source.
What can I use as a substitute 100 ohm resistance (I would replace both of them)? I have SMD 1206 / 100 ohm parts on-hand that would be about perfect for the size needed. It appears from initial research that SMD resistors are good into the GHz range.
Please advise - Thanks.
Kosmic:
Ouch :(
Those 2 100\$\Omega\$ resistor in parallel form the 50\$\Omega\$ termination on the input of the probe. It's going to be really hard to replace them with something equivalent to exceed 1GHz with acceptable S11.
The problem with SMD components is going to be fixing them to the aluminum chassis. Personally, I would try to find some good (low inductance) through hole 100\$\Omega\$ resistors.
In general 2 x 100\$\Omega\$ is better than 1 x 50\$\Omega\$.
Good luck!
jonpaul:
Use a selected 100.0 Ohm 1% 1/8 or 1/4W axial MF or CF.
Avoid spyril cut resistors.
Even a carbon comp 100 Ohm 1/8 W can work if selected.
At 1 GHz the axial small watts are OK.
Jon
Kosmic:
--- Quote from: jonpaul on October 21, 2023, 04:09:38 pm ---Use a selected 100.0 Ohm 1% 1/8 or 1/4W axial MF or CF.
Avoid spyril cut resistors.
Even a carbon comp 100 Ohm 1/8 W can work if selected.
At 1 GHz the axial small watts are OK.
Jon
--- End quote ---
The inductance on Metal Film or Carbon Film is going to be too high I think. Carbon Composite would be a lot better.
edpalmer42:
Have you tried to fix the resistor? Put the lead back in place and touch the connection with your soldering iron. If the central point measures 50 ohms to ground, you're done.
Ed
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