Inside the NISSEI RS-70 Digital SWR/Power Meter

[@M0UAW_IO83]

Thank you!!! Finally, someone who understands the difference between cal and alignment. I used to be a partner in a cal laboratory and I've met so many people online and off who just don't understand the difference.

[@mikesradiorepair]

I have two different models by MFJ (not MJF) and both are very accurate. One is the other model like you have that covers VHF and UHF frequencies and the other is the bench style that does HF, VHF and UHF. I connected both of them in line to my R&S CMA 180 which had just been calibrated a month before and both meters were within their specification in frequency range and power. As for batteries I don't think I have ever met anyone that used the USB to power it. The small ones are almost always used in the field for things like adjusting mobile antennas and HF wire antennas outdoors. Great little meters that fit in your pocket. I can verify Nissei makes them for MFJ. The first one I got was the VHF and UHF version like you have. When I popped the battery cover off to install a battery I took a peak inside and could see the Nissei name silkscreened on the PCB. Out of curiosity I just now removed the sticker and popped the back of my VHF / UHF handheld unit and it uses a different SWR bridge setup. It uses PCB traces instead of a piece of coax. In addition to the two adjustment VR's it also has two trimmer capacitors. One trimmer cap on the forward and one on the reverse pickup traces. They use brass sheet stock to form RF shields and they are grounded to the case. I would think attaching the coax shield on yours to the ground plane may solve your problem.

[@joesmith-je3tq]

I wouldn't expect the coupler for UHF to be the same as one only working to HF. Please pull the cover off your HF one and post a photo up on EEVBLOG or somewhere else. I would like to see if that braid is attached on yours.

[@OneBiOzZ]

Yeah its feeling like the engineer made a design choice to ground one side of that braid (or both given the pads being there) and someone came along later and tested the unit without soldering the braid, felt it worked fine at their test frequency and made the change without consulting the design engineer or properly testing it i had this happen to my designs a few times at an old job, mostly someone thinking they can save a few bucks on a critical opamp that i chose carefully for one with roughly equivalent first page specs

[@edkretchmer2167]

There is a service manual with alignment procedures for the Nissei DG-503 which might have a similar or same design. At least he display looks to be the same as on the RS-70 and maybe from the same era. The DG-503 does HF and VHF/UHF but it seems to have HF as a separate section. There are four pots instead of two and they are marked as "R1HFREV", "R2 HFFWD", "R3 UVREV," and "R4 UVFWD." Maybe the basic design is similar enough to get an idea. BTW, My RS-70 does not have the braid attached but I do not know the accuracy across the band. It agrees with my "ATS-10" tuner. I have mine open because the LCD contrast is almost invisible when looking straight on but dark and sharp when viewing from an extreme angle at the top. The bottom board is really wedged in there and I already broke the USB connector. Once its all the way out, I will see if there is a separate LCD Controller like on the DG-503 and check the datasheet for contrast adjustment.

[@joesmith-je3tq]

Thanks for posting. I've thought about making a video where I would attach the braid and attempt to align it and see what, if any improvements I see.

[@ligius3]

I appreciate your effort but still think you are giving too much air to people claiming nonsense (commenters). But the USB guy I think was spot on. Usually they cut at around 10% of the load, so smaller USB banks cut out at around 100mA and sometimes less, larger ones at around 200mA. I keep around a 1000mAh powerbank just for ESP8266 stuff, anything larger just restart after 10-20s.

[@feeatlastfeeatlast5283]

You are on to something here, Joe. Very interesting. I am surprised that you discovered this (grounding one side of the coax) but didn't identify it as a Faraday shield. I tried homebrewing these things twice and had the same results as you found - horrible frequency response such that it is unusable. If you look up the old power meters Doug DeMaw made for QST articles back around 1968 you will see a similar design, with just the center conductor of the coax being used. In fact he built a printed circuit board and etched a conductor on it, with the toroid core mounted in a cutout. BUT if you check out a Heathkit SWR/power meter, the one with the black meter face, (HM-102 ???) you will see a tiny, tiny core being used and mounted over a miniscule little piece of brass which extends up through the core and is grounded, which would simulate the shield of a piece of coax, grounded on one side and open on the other. The design used in this Nissei is similar to Drake, Collins and maybe many other wattmeters and I think called the Bruene design, named after a Collins engineer. It would be informative to check out a Collins wattmeter such as used in the 312B4 wattmeter that matches the S Line and see if Bruene used a Faraday shield. Good work. Thank you, 73 de Rick K2XT

[@feeatlastfeeatlast5283]

I am not certain of the physics involved here so let me say that my understanding needs to be verified and I haven't found it discussed anywhere. But, new designs of wattmeters utilize a different technique. They detect a sample of the voltage on the xmsn line and also the current. With these 2 measurements and the phase difference between them anything can be calculated with a microprocessor - R, X, Z, phase, power, etc. The LP100A is an example. The detector used in the NISSEI is sensitive to current flowing in the line (I think). To detect voltage on the line you would measure between the center conductor and ground. In practice a toroid core transformer would be used, connected across the SO239 connector. Now, why use a Faraday shield on the current toroid? I think the answer is, to prevent it from being sensitive to voltage which confuses the readings. It would be sensitive to both current and voltage. As I say, this needs to be verified; I am not stating it as fact. Rick K2XT

[@dixsusu]

Nissei makes these RS-70 Digital SWR/Power Meter for MFJ as well . Just rebrands .

[@BC_Outside]

I have one and it won't take any reading whatsoever even when hooked up to my receiver and antenna. Everything sits at 0

[@joesmith-je3tq]

Wouldn't you expect that? I mean, most receivers don't transmit...

[@BC_Outside]

@@joesmith-je3tq lol a *transceiver

[@joesmith-je3tq]

Assuming your radio is putting out enough power to meet what ever their minimum is, have you tied to fix yours. I showed the schematic for the coupler/detector stage. If you do pull it apart, post some photos of the internals.

[@matsbengtsson3756]

Joe, W2AEW have a video regarding the directional coupler and one side of the shield should be grounded. Here is the video : kzbin.info/www/bejne/mKqpYnmCmceIn6M About 7-8 minutes in is where he shows the shield grounding.

[@joesmith-je3tq]

There are many sources that show the braid being terminated, which includes the couplers I put together for my own Watt meter. What I not sure of is how the RS-70 meters that work (meet the suppliers stated specs) were constructed. Since making this video, I have only located one photo showing the internals and that meter also had the braid floating. I have no idea if that meter worked or not.

[@joesmith-je3tq]

Here is one I found on ARRL. Page 1, has a section on the directional coupler. 2nd paragraph talks about the shield. Again, fairly common but I am not sure why Nissei neglected to add it, nor if it was actually required. Their may be something else going on with the meter. www.arrl.org/files/file/Product%20Notes/2012%20Handbook/KAUNE.pdf

[@LawpickingLocksmith]

Nearly comes with its own matchbox. But if you ground the braid its sensitivity will be very low. We want you to rip out the board!!

[@joesmith-je3tq]

I suspect most have seen an LCD, switches, power supply and a microcontroller before. Cookie cutter stuff. Heart is the coupler.

[@cathyjohnson7199]

YOU NEED TO learn to read. It clearly says the power switch will turn the light circuit on and when tx power is applied it lights. it is not designed for full time lighting. the off setting is explanatory.

[@joesmith-je3tq]

It states: For back-light, please switch "LIGHT" to "ON". No function in 30 seconds. The backlit will automatically off. Once PTT launched, the backlit will be back. Your Chinese is better than mine. Of course the that's far more important than that fact it doesn't read power of VSWR even close to spec.

[@cathyjohnson7199]

I'd say it's all important. I never commented on the unit accuracy. It is deplorable. But at least the documentation seems adequate for a Chinese product.@@joesmith-je3tq

[@joesmith-je3tq]

@@cathyjohnson7199 The box states its a Digital SWR/Power Meter. That's the primary tasks it is supposed to perform. The backlight and any other features are secondary. There was a part2 and I have been asked about a part3 where I address the potential problem I identified and attempt to align it.

[@cathyjohnson7199]

@@joesmith-je3tq Nice vid! I watched the latest/greatest attempt to align/discover more and it was very informative. I'll be popping the covers off mine to look for a braid and add it in. I got my meter for peanuts so if I can make it somewhat accurate I'll be happy. It's only used mobile to alert me if something drastic happens, like my UVH6 fell off. LOL. no really, I don't care if the meter is 20% askew for what I want it to do.

[@cathyjohnson7199]

@@joesmith-je3tq Joe, also I found my manual and it does say 1-5w the readings are for reference only, to be fair. And 5+ w is 10%. Think of this. My $1100 HF mobile install suddenly goes bad, pinched/cut coax, water intrusion, squirrels. My SWR goes high enough it might damage the rig if run long enough. Chances of spotting it on rig display. 50/50. Chances of seeing it on the cheap Nissei. 100%. Sometimes accuracy is not the end all.