Repairing an R&S CMU200

A few months ago I got a Rhode & Schwarz CMU200, a combined RF test instrument. It has both general-purpose capabilities, such as a signal generator, spectrum analyzer, and power meter, and capabilities that are specific to (now old) radio standards, including 2G and 3G cellular standards and to Bluetooth. The protocol-specific capabilties are now outdated, so companies are dumping these units to the used-equipment market or donating them. I got a donated unit, but these units appear to be widely available on ebay, and there are lively discussion of this instrument on blogs and discussion forums.

The CMU200 is not one specific instrument but a family of modular instruments that came in many different configurations; there are many options, some involving hardware modules, some software, and some both.

I selected my unit from several donated ones. Some of them appeared dead, some had very dim screens. This one seemed relatively healthy so I took it.

When I started testing it, I discovered that some of the keys malfunctioned and caused the unit to reset, rather than to do what they are supposed to. The service manual is available, so I consulted it, removed the front cover, the rubber keys, and cleaned both the rubber keys and the flexible PCB that they press on (the one you see below the unit in the picture above). This did not work and from discussions on the EEVblog forum it became apparent that there is no way to clean or fix it, but you can order a replacement both from R&S (original) or from a Chinese website (aftermarket replacement). I did not want to order from the Chinese replacement and did not want to pay for an original PCB, but after some searching I also found the after-market PCB on ebay and ordered one.

The CMU200 can also use an external keyboard. My unit has a PC/2 keyboard socket, but other units have USB sockets. I did try to use an external keyboard. It works, but it’s annoying to use, since you must remember not to press any of the built-in defective keys, to prevent unwanted resets. It’s not a good solution. (I think the external keyboard was meant mostly to ease interaction with the embedded PC inside the instrument).

The replacement PCB arrived a few days ago and today I repaired the unit. The service manual is not terribly easy to understand, and although there is a section on replacing this PCB, there are not diagrams to guide you in this particular repair (I think that if you read the entire manual you do get a visual understanding of how its done, but I did not read the entire manual). I was hoping that I would be able to do the replacement from the front, without removing the cover of the unit, but I did not succeed (it’s not possible). Removing the cover is easy if you have the appropriate Torx screw driver, but I did not. Eventually I bought Torx screwdrivers and was able to remove the back feet and then the cover. (A video on YouTube suggested a T15 screwdriver, but the right one is T20; I think it’s possible to open the screw with a T15, but it’s not the right size; fortunately, I decided to buy a set, not just a T15 screwdriver; I assume that the service manual specifies the size, but again I did not read it carefully enough).

The CMU200 with the cover off.

In the picture above you can see the unit with the cover off. You can also see the front panel and the rubber keys that I removed from the front side, although I think you are supposed to remove them after the cover.

Ribbon cables that connect the PCMCIA socket to the motherboard.

With the cover removed, you get access to a cage of modules that sit on a motherboard, and you can open a screw that allows the front unit, which is actually a PC computer, to slide forward. To release it you need to detach three ribbon cables that connect the PCMCIA socket.

I then proceeded to open the cover of the front unit, which reveals an embedded PC (the CMU200 runs DOS). Now I could replace the flexible keyboard PCB. I also replaced the CR2032 backup battery, which was dead.

The front unit (embedded PC) of the CMU200, open and ready for replacing the keyboard PCB (and the backup battery).

That’s it. I re-assembled the instrument, checked that it works, and that’s about it. I still intend to open it up again and to duplicate the hard drive (and to test that the duplicated drive works) because it is difficult to repair a unit with a broken disk drive.

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