Rohde & Schwarz CMU200: another PSU problem - Page 1
Forum section: electronics / repair

author: EB5AGV
author_rank: Contributor
timestamp: October 12, 2016, 10:58:56 pm
content: Hi all,I have just received a R&SCMU200which has a not uncommon PSU problem. I have read other threads with similar problems and this is not exactly the same: what happens is that when you press the POWER ON button, the PS reacts and starts, but both 5.2V and 3.3V rails LEDs do not light and, after just few seconds, the PS shuts down.I have removed it from the unit and have tried to power it up, but it does the same. I have measured voltage on the 5.2V and 3.3V lines and during the short time the PS is running, there is no voltage on them.I wonder if someone has found a similar problem. I have the PS dismantled and have started to look for problems but, without schematics, it is always harder to work, so any clue would helpThanks!Jose

author: EB5AGV
author_rank: Contributor
timestamp: October 12, 2016, 11:59:56 pm
content: Looking into detail, I have found a very good clue on Reply #74 of this topic:https://www.eevblog.com/forum/repair/rohde-schwartz-universal-radio-communication-tester-cmu200-psu-issues/50/On my unit (pictures will follow on a later post), there was some oxidization around the same transistor which had failed on that case. So, yes, I have checked it and it was open!. Luckily, on my parts stock I had the exact transistor type (BC846B) so I have cleaned the PCB, replaced it and... the 5.2V and 3.3V rails are backNow it is too late for safe working (close to 2:00AM) but I am confident the PSU problem is fixed. Tomorrow I will get the PSU carefully cleaned, back to its metal enclosure and then will power up the CMU200 for the first time... I will let you know what happens!BTW, my unit came with a broken display glass. Luckily, it has not damaged the LCD panel but I need a replacement for the glass. Any hint on where to locate one would be welcomed!Regards from a happy camperJose

author: richnormand
author_rank: Unknown
timestamp: January 09, 2017, 02:14:21 am
content: Great.Certainly spent a lot of intimate time with my CMU200 PSU!For the display glass, it might have a shielding coating but you might try getting a custom cut sheet from your local source to try and fit it? The EM protection might not matter much in a small lab?

author: CJay
author_rank: Super Contributor
timestamp: January 09, 2017, 11:42:14 am
content: Quote from: richnormand on January 09, 2017, 02:14:21 amI bid on a few CMU200s with smashed glass, wondering if some of the thermal coated glass you can buy these days would be effective for screening as it certainly seems to stop or impair the function of mobile phones.

author: RF_fanatic
author_rank: Regular Contributor
timestamp: January 09, 2017, 09:34:52 pm
content: The shielded glass problem of the CMU has been a known problem most of the time seen with many of the signal generators from the SMx serie see for instance the youtube movie of a SME03https://www.youtube.com/watch?v=cfh9bJBU_E8.Please find here the interesting article about the RF shielded glass and replacement and the effect of it.http://www.bymm.de/documents/30/Displayglas_V1_4.pdfI have some spare CMU's for spareparts, I might also have some good glasses as spare

author: EB5AGV
author_rank: Contributor
timestamp: January 09, 2017, 10:52:24 pm
content: Hello!Quote from: RF_fanatic on January 09, 2017, 09:34:52 pmAs the original poster of this thread, I am also looking for an spare glass so, please, let me know if you can supply one to me.Thanks a lot!Jose

author: CJay
author_rank: Super Contributor
timestamp: January 10, 2017, 08:26:27 am
content: Quote from: EB5AGV on January 09, 2017, 10:52:24 pmMyapologies, I wasn't tryijng to hijack your thread, I was musing about the possibility of replacing your smashed glass with thermally coated window glass, I didn't win the bid for any of the smashed ones but the glass on my CMU is slightly discoloured.If there's a cheap source for replacement glass then I'd be interested but I can live with the glass as is.

author: EB5AGV
author_rank: Contributor
timestamp: January 10, 2017, 08:44:21 am
content: Quote from: CJay on January 10, 2017, 08:26:27 amDon't worry, I was just updating my interest for a glass replacement. It is normal to add comment to threadsRegards,Jose

author: richnormand
author_rank: Unknown
timestamp: January 10, 2017, 05:35:54 pm
content: Has anyone tried glass pane with a TiO2 transparent tin oxide conductive coating as is commonly used in LCDs?It is readily available in fairly large sheets, cuts just like regular glass, likely much cheaper than the exact replacement from R&S and might still provide acceptable shielding.