Repairing CMU200 - Page 1
Forum section: electronics / repair

author: YU7C
author_rank: Regular Contributor
timestamp: August 29, 2019, 03:48:52 pm
content: Hi,Recently I bought CMU200 for a $300 plus $300 shipping, and I knew it wasn't completely correct.It has next H/W options: B12, B17, B21v14, B41, B52v14, B54v14, B56v14, B66, B68, B96, PCMCIA.It has U65.Also noticed that RF1 has sticker 2W but it doesn't have U99 option?SW: V5.12 FMR6 256MBFirst, I noticed that some buttons don't work and thanks to forum members Bicurico and ps, connected external keyboard and mapped fault hotkeys.Then I performed RF test using instrument SG and SA:RF1 -> RF2-30dbm -53dBm TX-72dBm -72dBm AUXTXRF2 -> RF1-30dBm -53dBm TX-72dBm -72dBm AUXTXRF3OUT -> RF2-30dBm -53dBm TX-72dBm -72dBm AUXTXRF2 -> RF4IN-20dBm -92dBm TX-60dBm -113dBm AUXTXOn a first shoot, it looks that one amplifier in TX stage is damaged, and RF4IN is damaged.It seems that SA and AUXTX work. The question is where is damaged amplifier in TX stage: frontend or RXTX board1? ~20dB amplify could be one stage.Next task will be connect CMU200 on external SA and SG.Thanks,Vojislav

author: capt bullshot
author_rank: Super Contributor
timestamp: August 29, 2019, 04:17:39 pm
content: From my experience (CMU200 owner and repaired the frontend), there are two MMIC amplifiers within the RF frontend module that could be easily damaged by external mishaps, the input amp on RF4 and the output amp on RF3. I've replaced both in my unit (one was defective when I got the CMU, the other one way my fault).See this post:https://www.eevblog.com/forum/repair/rohde-schwarz-cmu200-repair-(analyzer-levels)/msg1375652/#msg1375652Edit: My unit doesn't have the AUX generator, so I can't tell if your readings are plausible with these amplifiers defective. As these are the first / last stage, beeing defective should affect anything regarding the RF4 input / RF3 output and should have no effect on RF1 / RF2.

author: YU7C
author_rank: Regular Contributor
timestamp: August 30, 2019, 07:28:31 am
content: Did you make photos of the bottom side of frontend pcb?I have checked all photoes but couldn't find them. Perhaps I didn't check carefully?Did you find which componet (amplifier) is with label S4 (black micro-x) housing?Thanks!Vojislav

author: capt bullshot
author_rank: Super Contributor
timestamp: August 30, 2019, 09:27:45 am
content: A few of the photos show the bottom side (P1130565 ... P1130569).I never investigated the S4 components, I'd guess it's a MMIC amplifier too. Somewhere one can find a block diagram (within the service manual) of the RF front end, this shows a few amplifier stages (but no component identification).One can test the Rx/Tx board quite easily by removing the hard-lines from that board to the RF frontend (accessible from the bottom side of the CMU), and e.g. apply a direct loopback from Tx to Rx of the RX/TX module.

author: YU7C
author_rank: Regular Contributor
timestamp: August 30, 2019, 10:05:20 am
content: Ok, I found it: S4 = Sirenza SNA-486 MMIC Amplifier.Thanks for photos!

author: YU7C
author_rank: Regular Contributor
timestamp: September 01, 2019, 01:33:01 pm
content: Thanks to photos of capt bullshot, here is block diagram of frontend with location on photos of rf switches and amplifiers.Less detailed diagram could be find in service manual.Vojislav

author: YU7C
author_rank: Regular Contributor
timestamp: September 04, 2019, 06:53:22 am
content: Hi,yesterday CMU200 was connected with external SA E4406A and SG 8656B.Its SA works fine, I have tested it with various levels and frequencies and it followed 8656B.Both RF1 and RF2 is OK.RF4IN doesn't work, 60 db lower. Input amplifier has to be replaced.Its SG was about 22dB lower in all level range from +13dBm to -110dBm. I was expected that in one moment the amplifier in TX stage will be bypassed, and level will be ok?But perhaps CMU200 doesn't bypass amplifier at all, during normal work?Next action is replacement of Amplifier in TX1 stage with the Amplifier from TX2 stage (which is not used). Both are from top of PCB.Does anyone have a picture of PCB of RXTX board?Thanks,Vojislav

author: YU7C
author_rank: Regular Contributor
timestamp: September 29, 2019, 04:05:03 pm
content: Last Friday I was playing with CMU200.First, was measured signal from Aux TX generator in front of FRONTEND module and that was a first surprise! Regardless of setting of levels, the signal had constant level of -55dBm. All adjusting of levels is done in FRONTEND module. On the other hand FE module doesn't have programmable atteunuator so it must be happened in first amplifier? Plese note that Aux TX works fine and generate right level on connectors.Second, was measured level from RXTX1 module in front of FE module and that signal was changing depending of set level.Then the FE was opened and that was second surprise from R&S. The PCB is a bit different then I saw on pictures from bullshot. It's revision with different amplifiers and some rf switches.Amplifiers are SGA-6289 and SBA-4089.Third, was traced signal from RXTX1 through FE module and easy noticeable when amplifier was on/off depending of required level.I am 95% sure that FE module is OK, except additional amplifier in RF4IN line, which was noticed at fist power up.For 100% I need additional measurement.Thanks,Vojislav

author: YU7C
author_rank: Regular Contributor
timestamp: September 29, 2019, 04:06:04 pm
content: Some pictures:

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 29, 2019, 05:37:50 pm
content: Quote from: YU7C on September 29, 2019, 04:06:04 pmWhat is going on in the first picture with the blue coax?

author: YU7C
author_rank: Regular Contributor
timestamp: September 29, 2019, 06:53:23 pm
content: Quote from: PA0PBZ on September 29, 2019, 05:37:50 pmSignal from Aux Tx (yellow) is feed to FE (blue) and Spectrum analyzer (N connector). It was not perfect matching but works.

author: Ct1jrz
author_rank: Contributor
timestamp: October 02, 2019, 12:59:51 pm
content: HelloDo you have fotos from the other side of the board?Mine has some burned resistors. I can´t see the values!Thanks

author: YU7C
author_rank: Regular Contributor
timestamp: October 02, 2019, 01:15:35 pm
content: Quote from: Ct1jrz on October 02, 2019, 12:59:51 pmStill no, but thanks to Martins you could find photos on the next link:http://wunderkis.de/cmu200/rawpic/A board has different revision than mine, but in basic it's a same board.Thanks!

author: Ct1jrz
author_rank: Contributor
timestamp: October 04, 2019, 01:23:31 pm
content: Quote from: YU7C on October 02, 2019, 01:15:35 pmYes, the board it´s different from mine also.The photos have good quality, but I still can´t see the value from some smaller resistors!Comparing the boards, I see different values.

author: elettronica52
author_rank: Newbie
timestamp: October 25, 2019, 04:30:58 pm
content: Hi HELLOI am an Italian amateur radio operator and I have a CMU200 R&S.it has no RF generator output while the spectrum analyzer works. Perhaps it is the faulty frontend in output. But is it possible to open it due to the fact of resistance with berilium oxide? It's dangerous?thanks a lotIZ2GGS

author: capt bullshot
author_rank: Super Contributor
timestamp: October 27, 2019, 09:42:29 am
content: Quote from: elettronica52 on October 25, 2019, 04:30:58 pmPeople opened and repaired their frontend modules, and still are posting here ...I don't remember if there was a beryllium warning on my RF frontend, anyway, as long as you don't break or grind the component containing beryllium, it isn't too dangerous.RF 3 output has a MMIC amplifier that could be defective, but RF 2 still would have its normal level then.

author: YU7C
author_rank: Regular Contributor
timestamp: October 27, 2019, 10:14:37 am
content: My FE module also doesn't have warning label.Also datasheet of MMIC amplifiers inside of FE doesn't have any remarks about handling of dangerous components. In fact one MMIC is RoHS.In TX path FE has one amplifier next to SMA connector (from RX/TX1) with an additional amplifier only at RF3 connector.

author: richnormand
author_rank: Unknown
timestamp: October 29, 2019, 01:40:06 pm
content: Quote from: elettronica52 on October 25, 2019, 04:30:58 pmObvious suspects were the RF amplifiers (with digital control attenuators) but it turned out to be a blown voltage regulator in the module (initially Rx/Tx module dead) and a cracked rf capacitor blocking the rf path (very low RF output).I remembered using a much higher frequency than 10MHz and needed my HP frequency analyzer to make sense of it while following the signal with a homemade RF loop connected to my HP SA.I assume you read most of the repair thread I posted then.Have a look at:https://www.eevblog.com/forum/repair/rohde-and-schwartz-cmu200-rxtx-module-issues/The module was dead initially and I found a voltage regulator open. Make sure all the voltages are OK before digging in too deep.After that I injected a 100MHz signal and followed it through all the various IF conversions through the board using my HP SA. Then I was able the generate a 100MHz via the CMU200 and do the same.PS1: BeO is an issue only if you file it, powder it or machine it. Otherwise it is a very stable ceramic with good dielectric and thermal property. Just leave it alone.PS2: your mailbox is full according to the reply I got.

author: Ct1jrz
author_rank: Contributor
timestamp: October 29, 2019, 01:51:33 pm
content: HelloI´m still trying to find some good resolution photos from the two sides of the front end unit!To see the borned resistors values.If anyone can help I appreciate.

author: YU7C
author_rank: Regular Contributor
timestamp: August 12, 2020, 02:05:37 pm
content: Hi,after almost a year I returned to repairing my CMU200.Now I noticed that PI attenuator is damaged in front of P4 Input.Does anyone know values of resistors?Thanks,Vojislav