TSP #58 - Teardown, Repair & Analysis of a Rohde & Schwarz FSH3 3.0GHz Portable Spectrum Analyzer

In this episode Shahriar examines a faulty R&S FSH3 100kHz - 3.0GHz Portable Spectrum Analyzer. This exceptionally dirty unit does not power on and is missing a power supply and battery. After verifying the correct operation of the battery charger circuit and keypad the problem is traces to a damaged SOT-23-6 MOSFET device which is part of a fly-back DC-DC converter. A replacement MOSFET with a lower current rating proves to be a failure as the device fails on startup. A higher rated MOSFET resolves the problem and device begins to operate correctly.
The block diagram of the system as well as a detailed block diagram of the RF section is presented and correlated with the teardown of the RF section. The super-heterodyne down-conversion circuits and components are examined.
The Signal Path
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[@AureliusR]

Of course we're still watching! I miss the 2-3 hours videos! Trust me, we love them!

[@hannonm]

***** Yes they are the best!

[@PapasDino]

Your videos, long or short, are always watched in their entirety! Pop some popcorn, sit back and learn something new every time! Congrats on another great resurrection.

[@SirBunghole]

One of the best channels for test equipment repair videos, especially the high-bandwidth equipment. Keep your format just like this. Perfect.

[@samuelemmettbray]

I watch these all the way through, every time.

[@yaghiyahbrenner8902]

20:06 when running parallel mosfets, the gate capacitance will increase, this will increase gate drive requirements. This was a great video. thank you.

[@manfredhund6681]

The same problem with my SFH6. Replacing the MOSFET with IRFTS8342TRPBF (30V, 8,2A, 2,0W), and cutting the short from pin 4 to pin 6 on the board and the machine works! Thanks for the video,

[@mark314158]

Very good as usual. Length is fine. With your high quality a video that goes on for hours is great. On the other hand for producers who's waffle-to-content ratio is far too high fifteen minutes seems like hours!

[@silentkiel]

Great video! After watching the video I put my old FSH3 under the microscope and found exactly the same defect. A few years ago when I tried to repair it I only noticed that the power supply circuit got hot when connected to the battery or wall adapter. The original MOSFET NDC651N is obsolete. But I found a much better transistor in the same SuperSOT case from ON Semiconductor (ex-Fairchild). The big difference of the NDC651N and all modern MOSFETs in this proprietary case is the usage of pin 6. But this can quite easily be changed on the PCB. The FDC8884 and FDC8884 have much better parameters, except from a slightly higher input capacitance.

[@deborahlacroix3869]

Exactly the same fault with my FSH3, I used the FDC855N (ON Semicond.) former Fairchild, works perfectly with pin6 bent up due to changed footprint but same package size. Thank xou so much for publishing this video, was really lucky when I observed the pulses after dead battery replacement. Changed just the NMOS after checking the resistors R31, R85 and R79 to be found ok. Great job, Shahriar !

[@karlfell3768]

Love the repair videos. You are the best on the net. Totally in a league of your own when it comes to diagnosis and descriptions. Keep up the good work. Karl

[@materialsguy2002]

Bravo!! This is the kind of in depth repair and analysis that one can learn from.

[@pezikon]

I was responsible for calibrating/repairing these units years ago. FSH6 to be precise. R&S fielded a power plug that didn't fit very well into the jack which left people shoving and jamming the plug into the jack. When i worked on them, I would remove the top and use mini-grabbers to feed power to the unit.

[@ebayscopeman]

Clark, I have one of these but I believe that the RF attenuator or input path is faulty as the RF attenuator does not track. Any suggestions on how to fix this? I have two units one with no options that probably has the FET problem and one that powers up and has the tracking generator and preamp. Can I transfer the front end components from the bad one to the good one? Any hints?

[@OnTheRocks71]

Great repair video as always. I love seeing these poor abandoned instruments spring back to life.

[@stevecanny1583]

Sweet! I love a story with a happy ending! :) The unit cleaned up very nicely. I love a dirty eBay find; I get to see it look so much better after some gentle scrubbing and I figure it brings the price down. Looking forward to your next one :)

[@noyoo153]

Thanks for sharing, love your nonchalant approach to RF voodoo..! Peter

[@jaa93997]

Yes we are still watching, but we need to see more of that cat-o-scope meter, a rather fascinating piece of equipment! Thank you for sharing this content.

[@Moonblade042194]

Great High-quality content as always!

[@BruceNitroxpro]

"You are a better man than I, Charlie Brown!" LOL Honestly... what a MESS! You figured it out, without even a schematic. OMG... I'm impressed! Loved this one the most of any you've done up to May of 2019.

[@techr9186]

This content never gets old

[@UA4HLQ]

Good business! Thank you very much for this film. Unfortunately, my English is very poor. You speak very quickly to my understanding. And I do not understand everything. But today I finished repairing my FSH3. NDC651N FET, was also faulty. I replaced it with STN4NF03L. Now, the device works. :)

[@7m4mon]

My FSH6 was broken with same issues. And I repaiered it with 2SK4033 instad of NDC651N. Thank you for video! You're AWESOME Engineer!!

[@AI7KTD]

I can't believe it worked perfectly fine after you took care of the power supply considering the initial state it was in!

[@dwDragon88]

Awesome result! Thanks for another great video. The length of the video is totally fine. In fact, I wouldn't have minded a longer video if it included analysis on why the transistor failed in the first place, and performance verification at the end. But maybe you were just ready to wrap it up after scrubbing dumpster grime off of that nasty keyboard... :)

[@AdisKurtalic]

Last month we did repair two of this unit, but with track gen. It was the same problem + the diode next to the transistor. The transistor we used: FDC855N and the diode: BYG22DHE3/TR.

[@TKomoski]

More in depth repair then the last one, good to keep you on your toes. As for the description of video you gave away the plot. Thank goodness I didn't read it before watching, it would've spoiled the video. Great video as always look forward to the next one. PS you might want to put a SPOILER ALERT in the description of the video.

[@uporabnik62]

Your presentation is great. I like that you don't give up even if the situation like not to be so good.

[@pa4tim]

I am working on a Fluke 192A. There is a mosfet like that with a 0.1 ohm resistor and it is burned too. It is also connected to the transformer. But in my case it is missing a gate signal from the power BGA and there is a hardware related firmware problem. It looks similar build to the Fluke indeed. Keep up the good work, nice to see and great explanations. I do this for a living but I always looking to learn more techniques and ways to find faults. Thank you

[@gilgameshismist]

These broken plastic parts can often easily be welded using MEK (Methyl ethyl ketone (butanone), add a drop or two with some tweezers, let it melt the top layer of the fracture (on both sides), push the parts together, wait 10-24 hours, and the part is bonded like new. I have used this technique to weld all sorts of plastics, eg. motorcycle and car parts, but also for housings of remotes and other gravity hungry stuff. The MEK can be bought at any shop supplying Lab equipment and chemicals, I bought mine for about 15 euro's for a liter at "labstuff".(a internet shop in my country) Thus far only used 200ml, and saved me a ton of money already.

[@thomasguilder9288]

I really appreciate the long videos! In my opinion this is by far the best youtube channel on repair/review of r&d gear! Go on like this! I'm really excited whether our FSH6 will exhibit the same fault one day :)

[@emcgon]

I absolutely love these repair videos - very exciting. I have to admit, I was holding my breath when you were shorting out that 5ohm resistor, half-expecting a bang and a puff of smoke !! The thrill when instead the thing actually powered on was great. When you are doing things like powering up the device after putting in the MOSFET, do you "cheat" and try it off-camera first or do you actually record the "real" test? It certainly seemed like you were genuinely surprised when it powered on with that second MOSFET. That sense that we are all along for the ride with you is definitely part of the fun. Thanks for making these videos - its always something to look forward to when the notification of a new Signal Path video comes though.

[@lechulsk4845]

Yes we are still watching. When you trying to fix something like this, interesting thing is if it works is hard to believe that it can break, but when it's broken it's hard to belive that it ever worked...

[@andymouse]

One of your best ! (I'm a DC man !) love your repairs...cheers.

[@LukaszTNT]

We use such a one at work. Nice to see it inside. Thank You :)

[@opelize]

Good score! Looks like it refreshes pretty fast on the full span

[@Shyam-ym3em]

Great work. Most would have given up at the point where the power supply connector was missing !!!

[@sandraledger2612]

Another in a great group of teardown and repair videos! Keep it up!

[@mariocordina5262]

Great to watch another repair video! Always very informative. May I suggest some tutorial video on the following topics: oscillator design, mixers and multiplier for vhf/uhf and microwave design starting from the theory and moving on to the practical implementation and characterization. Would make a great tutorial series for viewers starting in rf design.

[@IvanStepaniuk]

Awesome video as usual. Thanks for going over the RF voodoo, I miss a bit of depth in the overview of each RF component. The trouble-shooting process is cool but at the end it's just a simple SMPS. I wish you could go over the basics of how these microwave components work 8n the signal path, no pun intended (IF, couplers, different filter types, cavities, mixers, etc.) for us simple mortals they look like alien markings in the PCB ;-). Cheers!

[@LarsBerntzon]

Everybody loves movies with happy endings Thanks

[@hannonm]

Repair Video!!!! WOOT! Thank you!

[@hannonm]

Claps.....

[@hoodaly]

Great repair and interesting short walkthrough - by now it all starts to look familiar, thanks to your videos -, though I don't yet know why the original transistor could have failed.. maybe too high supply voltage when the previous owner tinkered with it, or just aging?

[@NoahFect]

hoodaly Job #1 on this sort of repair is to check every tantalum and aluminum electrolytic for both shorts and excessive ESR. The original part probably failed so spectacularly because of a bad cap. With luck the new part will be OK, but I wouldn't be surprised if he has to open it up again. A shorted diode could also have been at fault, but that would have kept it from working at all, of course. Ideally, it failed because whoever rigged up the kludged power connection hooked it up backwards or to the wrong supply altogether, and it will be fine now. Heck of a nice piece of hardware for $400!

[@ronniebrown8775]

all ways watching and learning . thanks for sharing.

[@pdmtrs396]

I;ve just seen this repair of your FSH. Although it's been more than 4 years, I have to say that replacing a N channel MOSFET with a P channel MOSFET (regardless that it had also lower Id maximum) and be surprised that it burned at once, is a bit silly at least. However good that after all you managed to repair it...

[@kmacriver]

With a switching transistor the biggest losses are usually generated during the switching transitions. Ron is usually very small and the losses negligible when on. To upgrade the transistor find one with the current handling and low gate capacitance for shorter switching time.

[@mattdudamusic]

My workplace has a couple of FSH3 spectrum analyzers and we figured out how to smoke them. The method to reliably blow one up is to run it with batteries that no longer take a charge, then pull the power while it is powered on. R&S charged over $2k each to fix a battery and that power IC.

[@deborahlacroix3869]

I had similar experience with R&S, usually the have really high performance equipment and good response to problems. For this part it seems different, high charges for repairs - obviously it took some time for them to realize they have a built-in bug here...

[@dmwtech4495]

Very Well Done. You have very good diagnostic skills,

[@alexreeve]

I love those exciting "ITS ALIVE" moments :D

[@sudiptaghose5247]

I have just got one of these. The outer cover is brittle may be because of tropical climate! Will use this video. Thanks de VU2UT

[@luisalbertolopezgarcia294]

I congratulate you friend, very good video and explanation. Well done!

[@ender1296]

Really enjoyed your fault finding

[@thierrydebakker]

Did you notice that the pinout for the MOSFET that was on the board is slightly different then the 1A MOSFET you put on. On the replacement the pins 1,2,5,6 are Drain, 3 is Gate and 4 is Source. While the old MOSFET only had 1,2,5 as Drain, 3 is Gate and 4 AND 6 are Source.

[@47smitten]

I love your videos and thank you for the blog very educational.

[@witeshade]

it's amazing how often it ends up being one dirt cheap component that kills the entire unit... makes you wonder though, what was the sequence of events that killed the component in the first place, and will it happen again

[@senohpi]

I have one bad FSH3 too, i repleced the FET with a SI3456DDV (Farnell 1840419). Now the instrument boots but unfortunately it has a lot of other issues to it, i guess the reason for this FET to die is when the battery goes very low the switch on circuitry starts to oscillate and then the poor FET dies.

[@baardegil]

Great video, interesting to see how you troubleshoot. I am holding a dead FSH3 in my hands, but I do not have the same knowledge and skills as you. Would you happen to have the schematics for the power section?

[@hdmalpas]

Excellent video.

[@maherawni]

great job, apparently very skilled & knowledgeable .

[@JustSomeGuy1967]

I agree with Komoski, spoiler alert if you allude to the issue because these videos are like a drama tv show..."who done it" what killed the circuit ! Great video !!

[@jonka1]

@44:00 Please remind us why they 'up convert' the signal before down converting it. There must be a good reason for this. BTW this video is brilliant as always. Thank you.

[@keesvanaalst3063]

Actually this is a known problem and there is a factory modification kit for it with the change of several components. I can sent the data if you want.

[@TRANSMIFM]

Hi, could you release that info for us? Regards .

[@keesvanaalst3063]

Sorry, just saw this recently, here it is: www.signalhound.eu/download/Rohde_Schwarz_FSH_Mod_FET.pdf

[@silentkiel]

Thank you very much!

[@aci5818]

Thank you...great video

[@grantfullen9559]

I really like these repair vids. Great Job :)

[@naterobinson9506]

I love watching stuff like this....Don't understand it but it sounds cool xV

[@butlegeo]

Another great video! Thank you

[@andrewmcneil]

I was watching one of these last year, smashed screen and did not power on but 2 people started bidding early and it went for stupid money.

[@czorgormez]

i have the same unit and the exactly same mosfet was failed in my device too . but in my unit mosfet was short and battery was died. probably batteries was empty when mosfet failed and in your unit batteries were charged and mosfet blow up.

[@reallyniceaudio]

excellent video many thanks

[@marco56702]

really really interesting video! PS:that cat is absolutely wonderful

[@NicuIrimia]

That is some great info. My unit still works but it is good to know where to look for if it develops the same problem. Thank you. How much did you spent on it?

[@susantowaluyo5078]

thx for video, i have one fsh3, power good but keypad can't used,

[@keesvanaalst3063]

For those interested: hereby a copy of the official R&S modification to prevent the burned FET: www.signalhound.eu/download/Rohde_Schwarz_FSH_Mod_FET.pdf

[@tonydarby9096]

Very well done on the repair, most informative. Did you check the temperature of the replaced fet? I wonder if there is a design error that resulted in the original oveheating.

[@BarriosGroupie]

Could you also tell us the price you pay for these? It's interesting to see the economics behind this also where the seller is trying to get as high a price for something that doesn't work, and you're taking a risk buying it.

[@Thesignalpath]

Barrios Groupie I think this was about $400.

[@superdau]

The Signal Path Blog How much would the device now go for? (if you include the facts of it being repaired and having broken plastic inside)

[@NeuroMod]

www.ebay.com/itm/ROHDE-SCHWARZ-FSH3-FSH3-03-FSH-03-SPECTRUM-ANALYZER-100KHZ-3GHZ-FREQ-RANGE-/381330948021?pt=LH_DefaultDomain_0&hash=item58c9187fb5

[@BarriosGroupie]

ChantaFlaite Wow, $2,000 for a refurbished unit. No wonder Shariar was over-joyed at repairing the unit so easily.

[@JGunlimited]

Wouldn't mind a second 30min+ extra video on the RF section in lieu of the rushed description at the end.

[@nomeanlol]

RF is always interesting

[@AureliusR]

Sweet! A reason to stay up even later :)

[@babybearkill1]

It's quite strange that you need to parallel mosfets. Sniff around the inductor a bit, perhaps shorted turns

[@Richardincancale]

Great video as always! Question - on the schematic of the IF section, just before the ADC, there's a mixer fed from a block marked 'Noise' and a 100 kHz LPF - what's that for? Is it to do dithering? Can you explain how that works? Cheers Richard

[@sav56125]

28:09 Most important part during the repair

[@Qyonek]

You are definitely skilled. But you can not be so lucky! I bet you have room full of unrepairable ebay scores ;)

[@Thesignalpath]

Qyonek No, no full room. But I do have one semiconductor laser source which is driving me crazy. I am very close to giving up on it.

[@Qyonek]

The Signal Path Blog when you decide to give up I suggest some epic demolition video ;) Thanks for all your videos!

[@Qyonek]

DRC Paintball $450 is what? a 1/3? A quarter of working one?

[@Thesignalpath]

DRC Paintball Well, relative to a working one. :)

[@javierparra3482]

This unit was bougth to R&S Directly. I'm only buyed one item (sig. gen.Agilent 8648A )from ebay but it's working perfectly. One can buy but with strong guarranties.

[@k1mgy]

Any thoughts as to why the fet failed? What did you use for cleaning?

[@makutingting]

Hi Shahriar...as i understand it, you replaced the original mosfet with a higher rated one. so what makes the original part burn in the first place? I assume it worked for sometime with the original part, so what did happen internally for the original part to burn? is this a design flaw? great video as usual, thanks

[@cthree87]

Finally something to fix!

[@MegaQwerty2000]

Your main board appears to have the mod that is suppose to protect the V13 FET. Is the power supply for this unit a negative tip positive sleeve connector?

[@erikbertram6019]

You mentioned the final filter is very critical because it is only 2MHz, but on the other hand it is using a S/D ADC, so I would imagine that the requirements on the filter are actually quite relaxed no? I guess you won't be reading this on a 5 year old video, but maybe some other scavenger of old TSP content.

[@javierparra3482]

Hi, It's very usefull. Thanks. I have one FSH3-TV that, basically is one FSH3 width preamp, tracking generator and the capability of TV analog/digital measurements, but it can work as one FSH3 normal. I have a problem with it. It don't detect RF input. Usually, when the FSH3 is started, you can ear one relay click. This one don't make this. Because the R&S service it's very expensive, how I could repair it or where?

[@JustSomeGuy1967]

I understand that chip carries a significant amount of current but what do you suspect caused it to fail and nothing else fail with it?

[@teerosheyal]

Can someone elaborate on the noise source at the end of the RF chain shown in the schematic? Why is it required?

[@guyvanburen]

Most Impressive

[@sivabalanm9717]

love you bro...

[@PlasmaHH]

Hm, I think I remember this not being the first time of lost footage, what about an eyefi sdcard that automatically backups as you record? Anyways, you seem to never say what you pay for this stuff, when I go on my local ebay damaged things like those sell for prices comparable to usualy used stuff :/ No bargain to make for hobbyists ... Oh btw. on your scope you can increase the font size of the Vpp/Vavg etc. measurements so they are better visible for us.

[@gustavoassing4098]

Hi, can You tell me wich transistor was used instead ndc651 thanks

[@peterderidder6549]

Love the FSH3. I have two. One is cannabalised to repair the other. It has the TG and the pre-amp. Anybody knows how to open up the TDR option?

[@Nascarbrett]

I’m a freelance rf coordinator and audio engineer. Someone is selling one local for 950 US I am thinking about it just wondering what your thoughts are and recommendations. I need it to monitor 100mghz 1200 max and how is the pc interface in 2023? Software still work etc?

[@embeddedusystems]

great video....+1 for the cat!

[@davecc0000]

How much did you pay for the R&S unit? Great vid. Love the troubleshooting & repair vids. Keep up the great work. Thumbs up!

[@TKomoski]

davecc0000 He paid $410 USD

[@gustavoassing4098]

Hi, can You tell me the transistor MOSFET that You has used thanks

[@hanssee3677]

Thanks

[@lions8141]

Thanks for sharing, my FSH3 only starts-up on a power-supply, and there is no start-up on battery. Do you have a suggestion of the fault?

[@gustavoassing4098]

And a diagram of modification thanks

[@fleuroman]

you will probably find, like the old Bosch hand held scope-meter were identical to the Fluke, its just a badge engineered product, produced in bulk for R & S, however R&S wouldn't have been able to sell it at or below Flukes retail price.

[@fleuroman]

I purchased a Fluke scope-meter not working for $200au, 1/4 of the price of a working unit, I've chased the problem down to no -30v supply for the display output, it powers on, the screen lights and I can control the back light to off, it gives 1 beep which means the processor is alive, it just displays nothing.

[@RupertHandford]

Great video. Would love that in our charity workshop if you want a good home for it :-)