ROHDE & SCHWARZ SMS - Page 1
Forum section: electronics / repair

author: RichardM
author_rank: Frequent Contributor
timestamp: November 29, 2022, 09:33:10 am
content: Hi All,I have one of thes units and the power supply capacitors have leaked (oil?) everywhere. I have managed to seperate the rear of the unit that holds the power supply including transformer. I need to remove the power supply pcb to unsolder the bad capacitors. The pcb screws have been removed and all that seems to be holding the pcb in place now are the TO-3 cans on the heatsink. I am ussure whether they are soldered to the pcb or socketed. Does anyone have experience with these units. Any help appreciated before I try to desolder them.CheersRichard

author: R-1125F
author_rank: Regular Contributor
timestamp: November 29, 2022, 06:52:49 pm
content: All of the chips are socketed, so it is likely the TO3 transistors are too. If the schematic is correct, they are shown as socketed.

author: RichardM
author_rank: Frequent Contributor
timestamp: November 29, 2022, 08:11:50 pm
content: Ahh the schematic. Thanks very much.

author: tinkera123
author_rank: Newbie
timestamp: November 29, 2022, 11:16:21 pm
content: Mine is a 302.4012.25 ... transistors at rear are socketed.Filter caps were okay; many of the cylindrical brown caps (ROE's ??) were cracked and crusty/sweating; I replaced them ALL. My power transformer went up in smoke ... and I had to rewind it by hand. Other faults also. Took me awhile to fix, but is now a going strong.PM sent also.Cheers,Ian

author: RichardM
author_rank: Frequent Contributor
timestamp: December 01, 2022, 05:02:56 am
content: Thanks IanI have sent you an email

author: george79
author_rank: Contributor
timestamp: February 08, 2024, 02:09:47 pm
content: sorry for bringing back this from dead,Im trying to restore my SMS, the brown ROE capacitors, that they are everywhere, are electrolytic or tantalum ones? im thinking to replace them with 105c ones

author: RichardM
author_rank: Frequent Contributor
timestamp: February 09, 2024, 02:27:15 am
content: My understanding was these are aluminium electrolytic capacitors and I replaced them all as such. I found many of the smaller ones were totally leaky.Richard

author: cncjerry
author_rank: Unknown
timestamp: February 09, 2024, 12:48:14 pm
content: I have an R&S FMS spectrum analyzer that had all those brown caps that pissed everywhere. I caught it in time and replaced 9 in each supply (RF and display units).

author: ealex
author_rank: Frequent Contributor
timestamp: February 09, 2024, 04:03:01 pm
content: hellothey are electrolytic caps in some sort of plastic casehere is a list of replacement parts that I did when I worked on an unit I owned.https://docs.google.com/spreadsheets/d/11Odn-jWp-MgOMi0aTUD0uU4Mmuf1b6VcPVbLELHsMJg/edit?usp=drive_linkin my case all had their top crackedi don't know if the part numbers are still available, but you can use the values and dimensions.i have some high quality scans over here:https://drive.google.com/drive/folders/0B0CB5A9yU03XbnRSa0xVZ04yaXc?resourcekey=0-Wn5mhCsNZ2XOmfe3UF9wuQ&usp=drive_link

author: george79
author_rank: Contributor
timestamp: February 14, 2024, 11:19:11 pm
content: so after finished the powe supply, replaced the brown small electrolytics, the bigger aluminium ones and all the tantalum, im continue to replace all the tantalum and check the brown in all the modules. Thats a lot of capacitors!!!!! Im using new tantalum capacitors KEMET brand, will see the results on the endThank you for your help

author: george79
author_rank: Contributor
timestamp: February 15, 2024, 12:34:03 am
content: it was a manner of time to make my first fault and trying to fix it now.Please im asking for your help, because generators are little complicated for my knowledge. Where to focus to find50khz my fault, here are the results, i have recapped the 50khz oscillator, reference board, frequency converter and phase control, didnt touch the other modules. Have checked the tantalum capacitors for polarity, and the only fault known that i cant find, in the phase control board, there are four 1mf capacitors under the board, on the tracks side, one of them maybe cold solder, fall off and i cant find where it was was.Now the generator power on, everything on the front panel works, output and attenuator works, modulation works, i can change frequency from the buttons and see it on the screen BUT the frequency is unchanged on the output. There is a standar frequency around 66-66Mhz and is unchanged. it is locked there. I can add modulation to this frequency and adjust the output but not the frequency!!In the photo is the output on the oscilloscopePLEASE give me some help where to start and search, as i sayed the only fault that i can found now is a 1uf tantalum capacitor that is under the phase control, on the track side and i cant find its position, but can made all this mess??Thank youGeorge

author: george79
author_rank: Contributor
timestamp: February 15, 2024, 12:38:07 pm
content: any help with this problem please?Im desperate from where to start and what to look

author: ealex
author_rank: Frequent Contributor
timestamp: February 18, 2024, 03:02:43 pm
content: hellosorry for the delay, my job kept me away for a few days.i don't have the unit so I can't help you by comparing signals.the scope screen capture looks like an unlocked PLL - the fuzzy waveform around the trigger point.can you try with a longer time-base ? to see if the signal is even more fuzzy further from the trigger point ?did you wrote down what you replaced ? I don't remember seeing tantalum caps - there where some small value caps that where part of the PLL loops - I did not touch those (at leas one if explicitly marked in the spreadsheet shared above)start by downloading all PDF's.they have the schematics and board views - you can see where parts should be installeddo a visual inspection - maybe you bumped something, or ripped a trace - start from the last board you worked on.then start reading "SCAN A4_OCR.pdf" - that's the entire theory of operation - it will show you what gets enabled for each frequency range.with that you can quickly check that the digital section is selecting the proper oscillators / mixers / PLLs, etc.then look at "regulator.pdf" - there are several test points that allows you to check all power rails.the unit uses some old / odd ECL logic, and that needs the -15 rail.my unit had a "service adapter" - basically an extension card that would allow working on the boards in system - do you have it ?it looks like this:http://ealex.ro/wordPress/wp-content/gallery/rohde_schwarz_sms/top_view.jpgtake your time and work trough the theory of operations - the unit is quite simple and build with discrete blocks - start working trough them once you understand how and when they get enabled.