Radiometer CMB12 1 MHz Capacitance Comparator / Kapazitätsmessbrücke

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OSNelektro
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Registriert: 16 Aug 2020, 16:10

Radiometer CMB12 1 MHz Capacitance Comparator / Kapazitätsmessbrücke

Beitrag von OSNelektro »

I have a Radiometer CMB12 Capacitance Comparator / Kapazitätsmessbrücke, which originates from the Danish manufacturer. I carried out an overhaul of it which I would like to share with you. If you have comments / questions, feel free to write in German :-)

The CMB12 can be used to match sets of capacitors, or to adjust trimmers. In the 1 pF range one can read differences with 20 fF resolution. My CMB12 has never been calibrated or adjusted while it has been in my possession, and on top of this, the calibration botton of the CMB12 started to behave weirdly, with the clear signs of bad contacts. In other words, the CMB12 needed some care.

However, I did not have a manual, and I have been looking in vain for a manual for years. To my huge surprise, I found a copy at Wellenkino, and by an extremely generous gesture by Martin, I now have the manual here! On top of that, I discovered that an LMS1 Limit Sensor, which I also once salvaged from being thrown out, was actually meant to be used with the CMB12. As a result Martin sent me the manual for the LMS1, too!!

Encouraged by this, the time had come to take a look at the CMB12, and to make it work with the LMS1...!

Initial test: A simple measurement of the test signal level revealed some unstability. The oscillator in the CMB12 is a common base LC oscillator, and the working point of the oscillator is defined by the oscillator itself. There's no leveling loop or crystal, and any component change will result in a level and/or frequency shift.
Niveau CMB12 efter koldstart målt med R&S URE.png
Niveau CMB12 efter koldstart målt med R&S URE.png (13.16 KiB) 2467 mal betrachtet

The graph above shows an example of how the level changes over time after power on. The measurement was made by a Rohde & Schwarz URE RMS voltmeter with the measurement data acquired by an application in LabVIEW. The ugly jumps were caused by an unstable 200 Ohm carbon trimmer potentiometer R128, used for adjusting the level setting, which I replaced with a fixed 110 Ohm metal film resistor. The level change was then reduced from about 2.1 % to about 0.12 %, and the jumps were gone.

From power on and over a 4 hour period the frequency changed from 1.000430 MHz to 1.000612 MHz, a shift of less than 200 ppm, and quite well for a free-running LC oscillator.

The unstable calibration button was actually an unstable relay being activated by the calibration button. In the manual the relay is merely listed as "24 V 1700 Ohm", but it's actually a Siemens V23154-C0722-B104. The '722' identifies the coil, but that's not one of the listed options in my Siemens Relay Data Book from 1988.

A measurement with a HP 4328A milliohmmeter showed that the contact pairs had a resistance between 3 and 10 Ohms, if any contact was made at all. A good rinse with Chemtronics "Pow-R-WasH" did the job, and now the contact resistance is between 20 and 30 mOhm.

I then made a test fixture to allow measurement of connectorized reference capacitors. This also allows me to carry out a calibration of the CMB12 and of its internal calibration capacitor.
Adaptor 20200807_191238.jpg
Adaptor 20200807_191238.jpg (206.74 KiB) 2467 mal betrachtet

Thanks to the schematics, I found this to be the way of calibrating the CMB12:
1) Mechanically adjust the needle to zero (center position)
2) Remove the lid, power the CMB12 on, wait for about 1 hour
3) Set to 1 nF range, adjust R107 for zero
4) Adjust R104 for zero on pin 3 of connector J1 on the back
5) Set to 1 pF range, adjust balance for zero
6) Set to 1 nF range, connect a 1 nF reference capacitor, adjust the calibrate trimmer on the front for 1 nF
7) Press 'Calibrate', and adjust the internal calibration capacitor C119 for full scale deflection

The graph below shows the measurement of a General Radio type 1403-K 1 pF air capacitor, when the temperature has settled. The DC output of the CMB12 is not calibrated for any nicely looking figure, but is close to 145 mV at full scale.
Udgang CMB12 1pF opvarmet.png
Udgang CMB12 1pF opvarmet.png (18.79 KiB) 2467 mal betrachtet

The next step was to make an interconnection cable between the CMB12 and the LMS1.
For the cable I used a piece of Kromberg & Schubert (Kroschu) Schaltflex CY 5-wire plus screen (only 3 wires are needed, but a 5-wire cable was ready at hand).
Male connector for CMB12: Amphenol-Tuchel Katalognummer T 3360 001
Female connector for LMS1: Amphenol-Tuchel Katalognummer T 3361 001 (though I had to use T 3361 002 as the T 3361 001 was sold out)

To my pleasant surprise the LMS1 worked with no issues, despite the fact that it had not been used for at least 25 years. The picture shows the CMB12 comparing the 1 pF General Radio air capacitor with a 1.8 pF ceramic capacitor.
CMB og LMS1 med GR 1403-K og 1p8 20200816_160027.jpg
CMB og LMS1 med GR 1403-K og 1p8 20200816_160027.jpg (579.88 KiB) 2467 mal betrachtet

In all, the CMB12 and LMS1 are now ready to be used again for matching capacitors, thanks to the manuals from Martin :-)

MfG Ole
Martin
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Re: Radiometer CMB12 1 MHz Capacitance Comparator / Kapazitätsmessbrücke

Beitrag von Martin »

mint condition :-)
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