R&S CMU200 - Attempt repair or sell - Page 1
Forum section: electronics / repair

author: turbo2ltr
author_rank: Newbie
timestamp: April 11, 2020, 12:07:43 am
content: I have one of these units. I swear it worked last time I used it. I ran a deviation test on a ham repeater. It's been sitting (plugged in) for a year or more.It boots fine, Fails all loopback tests. No TX on either output, no RX on either input. The voltages are all bad.I've been reading up on all the "journeys" people have been going through to repair these but I haven't seen a whole lot of success. I usually have no issues diving into something for a repair, but I have no other RF gear other than a 200mhz scope and my skills troubleshooting to the discrete level are not great.I did not test other frequencies, which now that I think about it, I should have. I was operating in the 440mhz range. I remember seeing a curious spectrum sweep that looked more like a square wave, which I think will mean something to those smarter than me. The Power meter did work.So1: is this something I should even attempt?2: how much is this thing worth with a bad TXRX if I were just to sell it? (if anything..)3: Is there anyone that has a working one, interested in trying to repair my TXRX?Thanks!

author: richnormand
author_rank: Unknown
timestamp: April 11, 2020, 08:20:57 pm
content: " The voltages are all bad."Where are the voltage bad? Are the motherboard LEDs OK?I assume if the CMU200 boots OK that is the case and the PSU is fine.Or are the voltages bad in the selfcheck reporting?If the TXRX module is found by the software that is useful but when you test it does the appropriate yellow LEDs on the front panel light up for each connector and function?If the voltages on the TXRX are bad there is two voltage regulators on board of the module. Odds are that it would affect both the RX and TX functions.In case you did not see this old post:https://www.eevblog.com/forum/repair/rohde-and-schwartz-cmu200-rxtx-module-issues/msg1020780/#msg1020780It has lots of info on the RXTX module.Have you tried (on boot screen before initialisation of modules) the option of resetting the whole instrument (when installing a new module)Warning: these are my opinions about your questions. So a grain of salt is expected....1."something to attempt"depends on how deep you want to go.First try to reset/remove and reinsert the module. There is lots of cabling connectors and contacts . You might get lucky.Try to remove unused modules etc...Get the R&S CMU200/CMU300 "service manual". it is not that good but will provide you with a block diagram, basic troubleshooting info and such. Simple visual check and DC readings are possible on the module once you remove the covers and solder some test wires to it. There is limited access when plugged back on the motherboard.If the cause is not a simple issue common to both the RX and TX channels ( DC supply or firmware) you will have to inject a signal in the RX and follow it from the plug via the several switching and IF stages. Similarly for the Tx you will have to trace it from the reference board through the digital board to the RXTX unit. You will need a second spectrum analyser and RF loop for that.2."worth with bad RXTX" This is a basic board for the unit. Perhaps eBay might have a RXTX module from someone parting a unit. I'd love a spare CMU200 for parts but just the cost of shipping to my location would be equivalent to the cost of the CMU200 I am afraid.3.I am happy to help. In the repair link posted above you will notice several members of this forum that do know a lot more about the CMU200 than me. They helped me a lot.

author: turbo2ltr
author_rank: Newbie
timestamp: April 11, 2020, 10:41:20 pm
content: By voltages I mean:This is the only failure on the high level self test.I have not reset anything. I'm scared of clearing out calibrations or something like that.When I do the loop tests, the yellow LEDs on the connectors do light up as they should.I guess, I'll start with basics and see if I can't make this an expensive paperweight. lol. I'll have to take a look at that thread again now that I've opened it up.Thank you!

author: turbo2ltr
author_rank: Newbie
timestamp: April 12, 2020, 12:14:15 am
content: So I took it apart. Took the covers off. Looked the board over under the microscope. Reseated everything. Still failed.Took it all apart again, located the transistor you had fail. I wanted to probe them with the scope, but I was afraid I would short the power and ground that are facing each other between the two tantalums with the scope probe, so I placed a small piece of tape over the ground side. I put it all back together, booted it and stated measuring. Got 8V at the collector. That's when I noticed it had booted right to the self test ...and it had passed!I think I'm going to pull the board again and hit that area with a soldering iron.So running through the RF tests. The internal loop passes.The 1-4,2-3 test partially fails but not terribly. The values are a few DB out of spec. Everything below 1200MHz passes. I'm using this for ham stuff so I don't really care about above 1.2GHz. Most I'll do is 900MHz. And if you saw the contraption I had to make to get the two looped with a short good quality SMA lines, I'm not surprised it failed at the higher frequencies.Going to touch up the solder in that area after a through inspection under the microscope and cross my fingers.

author: Eyecue
author_rank: Contributor
timestamp: November 21, 2020, 11:20:58 pm
content: What was the result and what transistor was giving you grief? I have the same unit doing the same thing on the self tests. Also you might want to use this forum for advice:https://groups.io/