Boonton 41-4B power sensor repair advice - Page 1
Forum section: electronics / repair

author: xrunner
author_rank: Super Contributor
timestamp: October 21, 2023, 01:55:01 pm
content: 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.

author: Kosmic
author_rank: Super Contributor
timestamp: October 21, 2023, 03:13:48 pm
content: OuchThose 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!

author: jonpaul
author_rank: Super Contributor
timestamp: October 21, 2023, 04:09:38 pm
content: 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

author: Kosmic
author_rank: Super Contributor
timestamp: October 21, 2023, 04:49:17 pm
content: Quote from: jonpaul on October 21, 2023, 04:09:38 pmThe inductance on Metal Film or Carbon Film is going to be too high I think. Carbon Composite would be a lot better.

author: edpalmer42
author_rank: Super Contributor
timestamp: October 21, 2023, 06:04:29 pm
content: 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

author: xrunner
author_rank: Super Contributor
timestamp: October 22, 2023, 12:12:33 am
content: Quote from: Kosmic on October 21, 2023, 03:13:48 pmYea really. Thanks for the schematic, wasn't able to find it earlier. Bad news follows ...Quote from: edpalmer42 on October 21, 2023, 06:04:29 pmI was going to attempt to but when I touched the end of it with my soldering iron the end of the resistor just fell off. In any case there was hardly any sliver plating left to solder to anyway. This is old parts I'm dealing with.First thanks to everyone for their inputs I was studying the options and before that I was going to scab in a cheap 50 ohm resistor in place of the broken red one, just to see if the sensor would behave at a low frequency - i.e. without the erratic way it was acting. But the party is over for now.When I heated up the place where the two red resistors and two diodes meet (center pin of the N connector), the solder indeed melted. However, even being careful all the parts moved a bit, and one of the tan colored 160 ohm resistors broke, even though it was at the other end of a diode. Wow really? It's that delicate?So that puts an end to things for now. I will put all the parts in a bag, and perhaps get back to it at a future date. Fortunately I have another Boonton sensor (41-4A) that works, even though it doesn't have the frequency rating of this one, it's OK for my needs.Any closing advice accepted of course. If you have suggestion as how to proceed at this point, please advise

author: edpalmer42
author_rank: Super Contributor
timestamp: October 22, 2023, 02:28:01 am
content: Many years ago, I bought a dead 41-4A sensor. The problem turned out to be a cracked capacitor. Once I had it working again, I decided to see what mods I could do since it was a spare sensor. I removed the original diodes and replaced them with a couple of random Schottky small-signal diodes to hopefully survive accidental overloads. I also increased the main isolation capacitor to improve the lower frequency response. My probe used a capacitor on the input side of the 100 ohm resistors so that DC wouldn't fry them.At 1 GHz, my modified probe is within about 0.2 dB of my R&S SME03 generator from 0 dBm down to about -50 dBm. The low frequency reading doesn't move from 1 MHz down to the generator's low end of 5 KHz as compared to its normal -3 dB frequency of 200 KHz. This iswithoutany calibration. I went looking for my notes, but I can't find them so I can't give you part numbers or values.Bottom line is that these things aren't rocket science. Remember that even the official probes have to have their calibration constants entered into the meter. As long as you aren't dealing with a requirement for absolute accuracy or crazy high frequencies, just have fun!Ed

author: xrunner
author_rank: Super Contributor
timestamp: October 22, 2023, 12:20:10 pm
content: Quote from: edpalmer42 on October 22, 2023, 02:28:01 amWell the Boonton schematic actually says -"When making your own probe, you may use ..."So OK, you inspired me to continue. I'll try to re-make the probe using the original diodes. I'll post whatever progress I make in this thread. I'll start by removing the diodes, 100, and 160 ohm resistors and clean up the area, and substitute new resistors. I'll have to order the 160 ohm parts.

author: Kosmic
author_rank: Super Contributor
timestamp: October 22, 2023, 05:23:53 pm
content: Well, everything is possible if your not necessarily interested in accuracy and higher GHz range.This guy made his own probe

author: TimFox
author_rank: Super Contributor
timestamp: October 22, 2023, 05:45:03 pm
content: Quote from: Kosmic on October 21, 2023, 04:49:17 pmThe inductance of low-resistance film resistors is often overestimated, but rarely specified.Carbon composition resistors suffer from internal capacitance problems at high resistance values.One should be able to use cermet or "thick film" resistors (with reasonable resistivity) that aren't helixed (spirals are planar geometric objects).

author: edpalmer42
author_rank: Super Contributor
timestamp: October 22, 2023, 07:04:45 pm
content: Here's a web page that talks about various RF probes.https://web.archive.org/web/20110303221109/http://members.shaw.ca/novotill/RfProbeSensorMvUw/index.htmNote that unlike most probes, ours have the terminating resistors exposed to DC that might be coming from the DUT. Even if they didn't burn out, thermal cycling could stress the solder joint and cause eventual failure. I thought that my probe had the resistors protected from DC, but it turns out that I was wrong. The same failure mode also exists with repeated exposure to high RF levels.I've also attached a French document that talks about repairing a Boonton probe.Ed

author: xrunner
author_rank: Super Contributor
timestamp: October 22, 2023, 07:35:05 pm
content: Quote from: Kosmic on October 22, 2023, 05:23:53 pmYea under 1 GHz is pretty much going to cover my needs.QuoteI did see that. Seems to work reasonably well.Quote from: edpalmer42 on October 22, 2023, 07:04:45 pmI'm sure the sensor I'm dealing with was abused, because I know the ham very well I got it from. He gave me a Boonton Model 42A and the sensor I'm trying to repair. He got them at a hamfest but didn't understand how to use it to test his handheld transceiver, which puts out either about 1 W or about 5 W. I'm pretty sure he zapped the sensor with of of those two power levels because he didn't understand he would need an attenuator, but for how long I'm not sure.QuoteOh nice thanks.By the way, does anyone know where I can get the connectors for the Boonton sensor cable (and preferably the correct cable wire also). I want to put new connectors on the cable or build a new one, because the one I have isn't in great shape. I can't manage to find the parts anywhere. I know here it comes ... they aren't available any longer.

author: edpalmer42
author_rank: Super Contributor
timestamp: October 22, 2023, 09:19:10 pm
content: Looks like the connectors are available on the standard auction site, but only at the cost of one or two body parts.If you have a strong stomach, search for < Amphenol microphone connector "2 pin" >. I think that's the right connector, but you should do some further checking.If you have to replace the cable itself, I think I read somewhere that it's very special. Due to the very low DC values that will be present at low RF levels, the cable has to be low microphonic, low triboelectric, shielded and yet physically rugged and flexible. IOW, pure magic!

author: Kosmic
author_rank: Super Contributor
timestamp: October 22, 2023, 09:42:22 pm
content: Quote from: xrunner on October 22, 2023, 07:35:05 pmThe connector is Amphenol 80-MC2M. They are not made anymore.As for the cable it look pretty standard to me (shielded with 2 conductors. see picture below). I made a new one using Canare L-2T2S Audio Cable and could not see any differences in term of performance.

author: xrunner
author_rank: Super Contributor
timestamp: October 22, 2023, 11:57:13 pm
content: Quote from: edpalmer42 on October 22, 2023, 09:19:10 pmAh thanks I found them on Fleabay - "AMPHENOL 80-MC2M 2 PIN OFF-SET MICROPHONE PLUG FOR COLLINS HEATHKIT BROWNING NOS"$39 to $50 for ONE? They can shove it where the sun don't shine!QuoteYou mean I can't use the roll of old Radio Shack speaker wire in my junk box?Quote from: Kosmic on October 22, 2023, 09:42:22 pmThat's what I was afraid of, and I'm also afraid of NOS prices.QuoteI could just unsolder the connector pins, clip back the cable a few inches past where it's been flexing for 40 years, and make it all back up again. I did see that brass ferrule that is crimped on the braid. I doubt I can use it again. I'll have to figure out to get a good ground from the shield to the connector housing. Do the two signal wires also have their own shield and are these shields tied to the main outside shield?

author: Kosmic
author_rank: Super Contributor
timestamp: October 23, 2023, 01:53:57 am
content: Quote from: xrunner on October 22, 2023, 11:57:13 pmI retract what I said earlier about the cable. It appear to be a little bit more complex than what I thought initially. The 2 conductors are also shielded and there's some kind of black isolator in between the second shield and the conductor wire.

author: alm
author_rank: Super Contributor
timestamp: October 23, 2023, 12:14:49 pm
content: Low noise cables sometimes use a graphite coating between layers to reduce friction and hence triboelectric effects. This is not an insulator and if anything slightly conductive. It serves as lubraction. For example this is discussed inthis thread on the Keithley / Tek forumorthis picture.

author: TimFox
author_rank: Super Contributor
timestamp: October 23, 2023, 01:58:43 pm
content: Another approach in low-noise cables was using a layer of conductive (carbon-loaded) plastic for the same purpose.

author: Kosmic
author_rank: Super Contributor
timestamp: October 23, 2023, 02:00:54 pm
content: Quote from: alm on October 23, 2023, 12:14:49 pmI'm all aware of the carbon layer used sometimes to reduce triboelectric effect.Nevertheless, In this case, the black coating is non-conductive and seem to isolate the center conductor from the second shield.

author: edpalmer42
author_rank: Super Contributor
timestamp: October 23, 2023, 05:46:34 pm
content: Quote from: Kosmic on October 23, 2023, 02:00:54 pmBut if you were looking for a replacement cable, one with such a carbon layer might be helpful, both for shielding and reducing triboelectric effects. A while ago I repaired an XLR-equipped microphone cable with just such a layer. That might be a starting point for a cable search - rugged, flexible, shielded, low triboelectric. Hmmmm.Ed

author: Kosmic
author_rank: Super Contributor
timestamp: October 23, 2023, 06:17:17 pm
content: Like I said, I used Canare L-2T2S in the past. Was working fine and did not notice any differences, even at -60 dBm.I remember looking around and never found a double shielded cable like that.

author: xrunner
author_rank: Super Contributor
timestamp: October 24, 2023, 02:16:46 pm
content: I removed all the components from the center pin. I took the opportunity to remove the input hardware and clean them up.When the solder is melted on the top on the center pin, all the components touching it slide around. This because the chip capacitor acts as an out-of-control force pushing in various directions as it will making it nearly impossible to work on individual components in that area. In order to hold things down, I made up a little assembly to press the chip capacitor down while the solder is melted, so I can solder two diodes and two SMD resistors in accurate positions.I have a little bit of material coming that I want to use in concert with the SMD resistors, so I'll get to that soon.

author: xrunner
author_rank: Super Contributor
timestamp: October 25, 2023, 01:22:22 pm
content: Here's my plan. I got some 0.1 mm thick copper sheet and cut off a strip with a width equal to a 1206 SMD part (0.06 "). I'm making a connection point with "wings" for the 1206 100 ohm resistors that doesn't require soldering directly on the pedestal. Trying to get all the components soldered in the right place without disturbing other components already there is too difficult with the parts I'm using. With this I can tack the SMD resistors on to the wings without melting the center area.So I first soldered the copper wing with chip capacitor on top while being pinned down. I checked the capacitance afterwards to make sure it was correct. Now the center area does not again have to be completely melted. I can tack on the SMD resistors on the wings and then tack the diodes on top of the chip capacitor quickly, without the whole area melting.Anyway that's my solution, others may come up with different ideas based on the physical construction of the components being used.

author: xrunner
author_rank: Super Contributor
timestamp: October 25, 2023, 09:37:41 pm
content: Today I installed the two 100 ohm 1206 size resistors using more copper strips. They have a little bend in them so the assembly has a just a little flex in it.

author: edpalmer42
author_rank: Super Contributor
timestamp: October 25, 2023, 09:42:01 pm
content: Do you have access to a VNA?It would be interesting to see what the impedance of your rebuilt probe looks like compared to an official one. 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

author: xrunner
author_rank: Super Contributor
timestamp: October 25, 2023, 10:03:39 pm
content: Quote from: edpalmer42 on October 25, 2023, 09:42:01 pmYes I do.QuoteI 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.QuoteOh 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.

author: edpalmer42
author_rank: Super Contributor
timestamp: October 26, 2023, 01:27:41 am
content: 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.

author: xrunner
author_rank: Super Contributor
timestamp: October 28, 2023, 10:53:04 pm
content: Well that was fun.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.

author: edpalmer42
author_rank: Super Contributor
timestamp: October 29, 2023, 02:49:30 am
content: 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.

author: xrunner
author_rank: Super Contributor
timestamp: October 29, 2023, 12:00:04 pm
content: Quote from: edpalmer42 on October 29, 2023, 02:49:30 amOh now ... it's not true that they havenophysical 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: #186114963838A 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 ...

author: edpalmer42
author_rank: Super Contributor
timestamp: October 29, 2023, 05:51:18 pm
content: Quote from: xrunner on October 29, 2023, 12:00:04 pmThat could be fine. It likely means that when it was soldered at the factory, the solder didn't run down far enough to plug the end of the pin. I'm not aware of any requirement that the wire itself has to go right to the end of the pin. If I was looking for a cable I'd grab it and inspect it on receipt. Worst case, you're just resoldering the end of the cable.Ed

author: xrunner
author_rank: Super Contributor
timestamp: October 29, 2023, 11:30:45 pm
content: Here's the initial checkout of the repaired sensor. It's working about the same as it was before (whenever it decided to work for a bit). The Agilent 8648A has a stated amplitude accuracy if +/- 1 dB, although I know from many different comparisons of power meters over the years it's more accurate than that, more like tenths of a dB.On the +10 dBm setting I do know the output is always a little less accurate than the lower ranges (but still within specs) so the Boonton is reading correct there also.PWR 1 MHz 100 MHz 1 GHz+10 +9.7 +9.7 +9.50 0.0 0.0 0.2-10 -10.1 -9.9 -10.1-20 -20.1 -20.1 -20.2-30 -30.2 -30.1 -30.2-40 -40.1 -40.1 -40.3-50 -50.0 -50.0 -50.2-60 -60.2 -59.9 -60.4Oh I forgot to test the input impedance, I'll do that next.

author: ch_scr
author_rank: Frequent Contributor
timestamp: October 30, 2023, 08:16:33 am
content: Quote from: xrunner on October 29, 2023, 12:00:04 pmFrom my experience, what breaks (or ratherrips off) is usually the metallized termination on the ends, not the ceramic carrier itself. The lever on the metallized end is quite large if you have the part soldered down to something solid with it's end sticking up.In a carefull home shop you should be fine with it, but if one were to knock this around daily I'd guess one of them might break from the shock loads, from the looks I'd mostly be worried about the ones going to the diodes, with their rigid mounting on both ends. Would be interesting to mount this to a shaker table to see whatactuallybreaksI suspect mini- or microMELF components to be more rigid (regarding their terminations especially) than flat SMD's and that might make them more suited for such a repair.With all that said, good job on the repair nonetheless!

author: xrunner
author_rank: Super Contributor
timestamp: October 30, 2023, 06:18:18 pm
content: Quote from: ch_scr on October 30, 2023, 08:16:33 amThanks for your comments.Here's a microscope pic of several items for comparison.From top to bottom -SMD 1206 zero ohmoriginal 100 ohm Boonton partSame as aboveoriginal 160 ohm Boonton partThe 1206 part is very close to the same length as the resistors, which is the reason I chose that dimension SMD.The original problem was that one of the 100 ohm parts (red part) was cracking loose right at the solder joint. Why this happened we can't say for sure. However, these parts are decades old and I'd bet money they haven't been touched or repaired since they left the factory. Only the original damaged part was going to be removed and replaced. However, they were so delicate that they just fell apart at the ends (see red parts in pic). Even the metallic coating came off.Not to stop there, a 160 ohm part (tan resistor) had one end of it crack off. Basically the sensor was at it's end-of-life. After what - 3 decades, it needed to be re-built or tossed. What caused the degradation in reliability? Was it banged on a table numerous times or were the parts the wrong choice from the beginning (for long-term reliability). I know it was in the hands of Ham radio operators for some time, so draw your own conclusions there.The point is, the original parts Boonton selected, and the original way Boonton engineers told the assembly line workers to construct the center area of the sensor (the soldering) didn't fare all that well in the long run. What I've done is either going to last for a little while (subjective time) or a long while (subjective time). I don't abuse equipment so I'm guessing a long while (years) but we shall see. I'll test it at least once per year and report if if fails.

author: Kosmic
author_rank: Super Contributor
timestamp: October 30, 2023, 11:57:05 pm
content: Quote from: xrunner on October 29, 2023, 12:00:04 pmThe pins are hollow, and as Ed said, it's not required for the solder to go through up to the tip. As long as the wire is soldered correctly to the pin.I got some Boonton cables here and some of them are exactly like that.

author: xrunner
author_rank: Super Contributor
timestamp: October 31, 2023, 12:05:21 am
content: Quote from: Kosmic on October 30, 2023, 11:57:05 pmI've always seen pins like that with holes at the end soldered at the end and the excess wire cut off afterwards. That's always the way I've done it too, but hey, I haven't seen it all in in this field and never will.

author: Kosmic
author_rank: Super Contributor
timestamp: October 31, 2023, 12:14:35 am
content: Probably depend how it's soldered. Heatkit was recommending to have a extra long wire and cut the excess.

author: edpalmer42
author_rank: Super Contributor
timestamp: October 31, 2023, 01:41:18 am
content: The problem with trying to solder those pins from the end is that it's almost impossible (for me, anyway) to finish the job without getting solder on theoutsideof the pin. If that happens, the pin won't fit in the socket so then you have to remove the excess solder and if it takes too long, the plastic holding the pin softens and the pin moves and itstillwon't fit in the socket and now the plug is ruined and I have to cut it off and start everything from scratch!!!

author: xrunner
author_rank: Super Contributor
timestamp: October 31, 2023, 01:43:51 am
content: OK now I have to ask it ...If you don't solder it at the end where you canclearlysee the quality of the work, then how do you know it's a quality solder job on theinsideof the pin down in there.

author: edpalmer42
author_rank: Super Contributor
timestamp: October 31, 2023, 08:25:46 am
content: There are lots of situations where it's difficult or impossible to visually inspect a soldered joint. You do the best you can with the specifics that you've got, the limitations imposed, and the quality demands that are present.You might dump the soldered joint and go for a crimped joint, if you can find such a connector.You might get a roll of super small solder that you can fit through the hole in the end.You might cut a small piece of solder and shove it into the open end of the pin, then heavily tin the wire, heat the pin, shove the wire in.You might search for a connector where the pins are removable so you can solder the wire externally without worrying about melting the plastic.You might inject a drop of liquid flux into the pin to help ensure that the solder wets the surface.In all of the above, you're working blind. Being able to see the wire at the end of the pin tells you little about whether you got a good joint or not.We have to deal with:- old oxidized connectors vs. new, clean ones- old solder which, oddly, sometimes doesn't want to melt- unknown heat resistance of the plastic surrounding the pinsThe solution to the problem is critically dependent on whether you're doing a repair or building new or, as in our case, dealing with an almost unobtainium connector. Remember that the connectors available on the auction site might be NOS that have been sitting around for 20 years. Solder might stick to the inside of those pins with no trouble or it might be almost impossible to get it to stick. One brand might work fine while another brand might be a disaster.This is what you have to deal with when you're working on equipment that's old enough to vote!Ed

author: xrunner
author_rank: Super Contributor
timestamp: October 31, 2023, 12:06:46 pm
content: Quote from: edpalmer42 on October 31, 2023, 08:25:46 amWell let's move on, but seeing the solder work even at the end of a pin like that is better than not seeing it. Seeing something is better than seeing nothing. That's been my avowed policy for quite a number of years.Anyways ... here's the input impedance plots from both Boonton sensors I have. The top plot is a Boonton 41-4A which to my knowledge is original and not repaired. The bottom plot is the repaired 41-4B. Smith chart and S11. Draw your conclusions as you will.

author: factory
author_rank: Super Contributor
timestamp: September 22, 2024, 01:52:18 pm
content: Another Boonton 41-4B power sensor, this one is a little older than the one repaired in this thread, it came with a 1970s 42AD microwattmeter.I'm going to need to do some surgery to this one, as there is a short across the input capacitor, this is the one between the 100Ω resistors & the two diodes, to me it looks as if someone made some repairs and accidentally shorted it with solder. It would be interesting to see if anyone else has an older 41-4B & some pictures of the insides for comparison.The sleeving for one of the output leads needs replacing too, it's worn through where it goes into the connector section, this was intermittently shorting on the casing.David

author: xrunner
author_rank: Super Contributor
timestamp: September 22, 2024, 11:15:01 pm
content: This will be a good contribution to the thread. I'll be watching with great interest.

author: Uunoctium
author_rank: Frequent Contributor
timestamp: March 20, 2025, 03:54:05 am
content: If this is the ultimate thread for reviving Boonton sensor heads, may I contribute a few remarks as well?For troubleshooting and repairing my Marconi 2031 – which had level jumps in the output attenuator – I was able to get a 4210 at a very reasonable price, though without a cable or sensor. Overestimating my abilities, I believed I could build such a sensor myself using some guides from radio amateurs in various forums and private websites. For the circuit, I followed the 4B schematic from Boonton’s service information. Several diode types from Infineon (BATxx) and HP/Agilent/Avago were recommended as highly suitable. I ended up using the rather expensive Agilent 8052-2835.Result: useless – usable only up to 150 MHz, above which Z0 tends toward a short.To bring these failed attempts to a conclusion, I recently acquired a defective 51011 (equivalent to 4011, 4B) sensor head for just €20. I installed the same diodes as before – with equally catastrophic results.Now, I must confess to a bit of sacrilege, but I will report on it nonetheless. Since I lacked a cable and was unwilling to pay $99 + shipping + customs for a brand-new Amphenol connector from the '40s at a distributor, I had to find a more affordable solution. The sensor head has an internal partition glued in place, blocking access to any screws or locknuts. This left the GX16 series as the only viable option since it has the same threading. Yes, this means the instrument is no longer compatible with the original cable and sensors, but this can be reversed at any time.Result: seems to have no significant impact – unproblematic.For the connection cable, I used a professional extra-thin studio low-noise AES/EBU 110-ohm cable.Result: disastrous! When I connected an oscilloscope to TP9 in addition to a DVM for the chopper adjustment in the 4210, every touch or movement of the cable caused severe level jumps.Solution: Gotham cable with an additional conductive insulation layer between conductors and shield. Effects are still visible, but estimated to be only 10–20% of the previous issue.https://www.gothamcable.com/en/gothamcables/balancedanalog/10421gac2v1Diodes: DDC2353-220 Skyworks!https://www.rf-microwave.com/en/skyworks/ddc2353-220/zero-bias-schottky-diode-2v-100ma-0-2pf-hi-rel/ddc2353/It is also advisable to read Document 2 and the associated comment.During installation, just as the thread starter experienced, one of the 160-ohm resistors crumbled apart, so I replaced both with 0603 series resistors.First quick-and-dirty test last night: up to 2700 MHz (the limit of the 2031) and, without considering the correction values from the sensor’s lookup table (!), readings across dozens of frequencies ranged from -55 to +9.9 dBm, staying within tolerance with deviations of ±1 to 1.5 dBm – usable!