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 The protocol-specific capabilties are now outdated, so companies are dumping these units to the used-equipment market or donating them.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.

Partial Restoration of an FT-101E

I finished today a partial restoration of an FT-101E. Here it is, restored and in action, receiving on the 14 MHz band.

As you can see in the video, the unit is labeled Sommerkamp FT-277EX; This is a Yaesu FT-101EX, rebranded for sale in Europe. The transceiver was made in the mid 1970s. It is a hybrid transceiver, using mostly transistors, except for 3 vacuum tubes, the driver and RF power amplifier. The EX model suffix designates an extreme economy model (no DC power supply and speech processor). 

The unit belongs to a friend but had no power cord. Restoration therefore started with the power connector. I was fortunate to find components from which to make the connector in a store in Berkeley.

תמונה יכולה לכלול: ‏‏‏‏אדם אחד או יותר‏, ‏‏שמיים‏, ‏עץ‏‏‏ ו‏פעילויות בחוץ‏‏‏

The store did not have the exact connector I needed, but the sales person (probably the owner) found a cable-end male and a panel-mount female and suggested that I modify them to make a cable-end female connector, which is what the FT-101E requires. I did as he suggested. You can see the componets I bought in the picture below.

Here is the final connector powering the radio.

Once I had the connector, I started attending to the other problems the radio had. The most obvious one was a stuck tuning knob. I partially disassembled the tuning knob and discovered that the problem was a 6-to-1 reduction drive. Once it was removed, the rest of the mechanism rotated freely. Almost incredibly, I had in my junk box an identical British-made unit. Unfortunately, the spare unit did not turn smoothly either. I repeatedly cleaned both units with WD-40 and lubricated them. Eventually one started to turn smoothly (I think it was the spare one). I installed it in the radio, which now tunes beautifully.

The next step was to open the radio and to take out the plug-in printed-circuit boards. At this point, I also read other restoration reports and forum posts. As expected, I read that electrolytic capacitors often need replacing. I have already replaced electrolytic capacitors on other old radios, so this was not surprising. Some people wrote that faulty high-voltage capacitors can cause failure of the power-supply transformers, so operating the ratio with the old capacitors seemed outright dangerous. 

One difficulty with this radio is that it has A LOT of electrolytic capacitors. Still, I bought the capacitors and removed the plug-in circuit boards from the radio.

Inspection of the boards showed that the coating of some film capacitors has cracked and peeled. 

Possibly a lot of capacitors to replace. I proceeded to removed two electrolytic capacitors from one of the boards and characterized them in a component tester. I also characterized the brand-new replacements. Here is one pair; the original is grey and the new one red.

Interestingly, the old capacitors tested better than the new one; comparable capacitance and lower ESR.

I soldered the new replacement capacitors to the board but decided not to replace all the capacitors pro-actively.

Still, I did replace the two high-voltage capacitors that smooth the supply voltage of the tube RF power supply (~600V), in order to avoid transformer failure. The new capacitors have more capacitance and are much smaller than the old one.

The clamp on the new capacitor is from the old one; they have the same diameter. Replacing them was a bit trick as they are soldered and screwed to the chassis.

The picture above shows the chassis with the old capacitors; the one below shows the new ones installed.

I initially powered the radio with a 12V AC transformer, rather than with 220V, to check that there are no shorts or other catastrophic failures. I was able to measure voltage (about 46V DC) on the high-voltage caps, so I assumed the radio was not shorted.

I wired the connector to a three-prong AC cord and plugged the unit in. Initially the radio seemed dead, but after a short while I realized that the main power switch made intermittent connections. After flipping it a few times the radio woke up and the tuning-dial lamp lit. The AF and RF gain knobs make some scratching noises but there was really no audio. I connected a signal generator; the S-meter reacted but still no audio. Flipping the mode knob between different modes (USB, LSB, CW, etc) resulted in sound in one of them, sometimes. I realized that this switch is also not reliable, so I cleaned it with a contact cleaner, as well as the gain potentiometers.

At that point the radio seemed reasonably functional. I connected it to an antenna and got the results you saw in the video above. The VFO seems very reasonably calibrated; the calibrator works, and so does the preselector and clarifier. The noise blanker seems dead. Volume seems a bit weak, but the radio seems reasonably sensitive. Here are videos that show the preselector and calibrator in action.

I tried to go through the transmitter-tuning procedure. The meter did not react as it is supposed to, so I left this alone for now.

I do not think that the radio is fully function or up to spec, but it does receive, and this is satisfying (I assume it was not turned on for decades). It can probably be fixed further, except if the final tubes are gone or close; they were inexpensive 40 years ago, but replacements are expensive and hard to get today.

It’s been fun.

Toaster-Oven Reflow with Normal Solder (no Solder Paste)

I recently successfully soldered a 4mm-by-4mm QFN chip using my toaster oven but without solder paste. There were two reasons to avoid solder paste. The first is that somebody threw away the solder paste syringes that I stored in a fridge (the solder paste was also beyond the expiration date, but still worked okay). The other reason, which is more important, is that reflowing QFN chips without a stencil using solder paste proved difficult. The problem is to put an appropriate amount of solder paste on the pads (that is, to put little enough). When I put too much, which happens very easily, solder bridges form and are hard to remove.

I needed to solder a QFN chip to an evaluation board I designed and I decided to try to do that with normal solder. I tinned each pad with normal solder, including the center pad. I made sure that the amount of solder on the pads was uniform; every pad had a little hill of solver (probably 0.25mm or so high). I then added flux, placed the chip on the board, the reflowed it.

An inspection of the joints under the microscope showed that there was too little solder on the pads; the solder was completely flat on the pads. I added more solder to the pads with a soldering iron with a needle-point tip (Hakko 888 with a T18-I tip). I also added solder to the center pad through a via to the back side of the board. Following this fix, the chip was soldered correctly (this was verified by correct operation of the board). The rest of the components were soldered using an iron, but perhaps I should have reflowed them too.

The is the picture of the board; it was designed to evaluate an AX5031 transmitter using a Texas Instruments Launchpad. I forgot to order a 16MHz crystal with a footprint that matches the boards, so I improvised with a larger crystal.


In a discussion in Facebook, Ohad Miller commented that he solders similar QFN chips using a soldering iron. He solders the pads pretty much like I added solder to the pads. He solders center pad using an large via (he makes these vias large specifically so that it is possible to solder through them). I didn’t try this but it seems like a good way to go.

Surface-Mount (SMD) Prototyping and Hacking

A huge range of electronic devices now come only in surface-mount (SMD) packages, so techniques for prototyping with these devices are often useful. Also, most ready-made boards use these devices, so if you need to modify such a board, similar techniques are helpful.

2017-06-01 17.41.39

The picture above shows such a prototype. I built the circuit on a piece of SMD prototyping board made by BusBoard. The squares are 50mil (0.05 inch) by 50mil. The small square device at the top is a 1.2mm by 1.2mm Hall-Effect sensor; its 4 contacts are spaced 0.65mm apart. The larger device next to it is a MOSFET in an SC59 package. The brown capacitor next to it and the two black resistors are in 0603 packages. It’s a simple circuit. It was not hard to prototype like this, and testing it gave me very useful insights and a lot of confidence before designing a PCB and ordering a batch of assembled units to test.

Obviously, some SMD packages are easier to prototype with than others. The same MOSFET (a DMN1019) also comes in a smaller 1.95-by-1.95mm package, so in the PCB design intended for automated assembly, I used the smaller package to save space. I bought the SC59 device simply to make prototyping easier.

The picture below shows another example of the same technique on the same type of prototyping board. This circuit is a 1.8V regulator, and here too the capacitors are 0603.

2017-06-28 09.42.43

The picture also shows how I hacked a ready-made board, a USB-to-UART bridge with an FTDI FT230X chip. The chip can support a variety of I/O voltages, but the board is wired for 3.3V and there is no easy way to change this. I needed the UART to work at 1.8V, so I added the regulator on the BusBoard. But I also needed to cut a trace between the 3.3V supply on the ready-made board and a pin of the FT230X, and to connect to that pin the 1.8V supply. The red wire carries this supply voltage to the pin.

The thin hookup wires I use are old wire-wrap wires. They are often convenient. To connect to the pin of the QFN chip the thin wire was certainly helpful.

These techniques are useful for simple circuits (or in these case, for small parts of circuits that are attached so a hacked board), but it is hard to scale this up to more complex circuits. For complex circuits, it is better to design a PCB and have it manufactured, even for a prototype. A PCB makes it possible to use a much wider variety of packages than is possible with the BusBoard boards, and the solder mask makes solder bridges much less likely; the lack of solder mask is one of the disadvantages of the BusBoard technique.

To end, here is another example of SMD prototyping, here an SPI NAND flash chip, placed upside down on a conventional 0.1″ prototyping board. It’s ready to be hooked up to a microcontroller for testing. It’s 8 pads are spaced about 1mm apart.

2017-06-29 18.27.04

J-Pole Antennas for GPS

I normally use commercial antennas for GPS (e.g., patch antennas with a magnetic mount), but I needed more antennas than I had so I built a couple of J-Pole antennas for 1575 MHz.

I started with dimensions from an on-line calculator and built an antenna using a 1mm enameled copper wire. The return loss (SWR), measured with a portable VNA, was not good. I started again with a slightly longer antenna, which resonated about 100 MHz too low, and started cutting it until it resonated on 1575. The return loss is about -15dB; not great, but good enough. Given that there are 3 different dimensions to worry about (the length of the radiating half-wave section, the length of the quarter-wave matching section, and the location of the tap on the matching section), I decided not to try to tweak it to perfection.

Here is the result:

2017-06-28 16.31.18

The antenna is soldered directly to two terminals of an SMA connector. This is a through-hole PCB connector, which has 5 terminals (5 for ground and 1 for the center conductor). I chopped 3 of the ground terminals so only two were left protruding, and I soldered them to the antenna.

I then made a second antenna with the same dimensions; it produced a -15dB return loss on the first try.

The length of the λ/4 stub in my antennas is 45mm, the lenght of the λ/2 radiator is 102.5mm, separation between the arms of the sub is about 3mm, and the feed point about 8mm from the shorted end of the stub.

In one of the antennas I inserted a 1000pF 0603 ceramic capacitor between the center conductor of the connector and the antenna, so that it does not short at DC. This allowed it to be used with GPS receivers that provide bias voltage to an LNA. Without the capacitor, the antenna shorts the GPS receiver, since the bias voltage is usually not current limited.

With this antenna connected to a NavSpark Mini GPS receiver outdoors, I was able to easily get a fix.

2017-06-28 14.21.35.jpg

Replacing Fans in USRP N200s

Two fairly old USRP N200s radios (about 4 years old) had issues with their fans. One was completely dead and the other was operating, but noisy. I was not able to get replacement fans from the manufacturer, but it turned out that form factor is completely standard, so I ordered fans with the same size and similar specs from Digikey. They installed just fine in the radios. Here is a picture of the old fan (top) and the replacement. The part numbers are easily readable.


A Second-Generation Front-End Unit


Itamar Melamed and I designed a second generation front-end unit. It contains a SAW filter, an RF limiter to protect the filter and the radio, and a bias tee with a DC limiter, allowing the unit to be used with both active antennas and antennas that present a DC short. It is small and fits within a USRP N200.

The full details are available at