The Marconi 2955B Radio Communications Test Set is a very useful instrument for testing radio receivers and transmitters and combinations thereof. It is a relatively dated instrument, the manual is from September 1991, but there are quite a few on offer second hand. The manual is available on line. In the coming few blogs I will discuss some – presumably simple – experiments I did with it.
The device under test
The first involved a simple crystal radio for the Middle Wave band (500 – 1500 kHz).
The larger part is the – at least in the Netherlands well-known – Amroh 402N coil set. Interestingly, the coil set is still being made and sold by Reinhöfer Electronic GmbH. In the Netherlands they are also sold by Cardan Educatie. The reason for taking this set-up is that as a 7-year-old youngster I tried – together with my father – to put it to work … but failed. In hindsight this was probably due to the low-impedance headset.
Anyway, I put the parts together – on purpose using old-fashioned ones: the extra 100 pF capacitor is not necessary, the diode – I took the germanium OA81 – is mounted inversely (do not see a good reason to do as in the schematic) and the headset is replaced by a 100 kΩ resistor. The HF-signal from the 2955B is fed to the antenna-coil of the 402N through a 68 pF capacitor.
The test itself
The manual is extremely comprehensive and in actual fact explains everything one would like to know about the instrument. Starting page 3-22 Receiver testing is described. The first part contains a description of all controls and connectors. Although useful, for a slightly experienced electronics engineer this is trivial. The real thing comes on page 3-24 where the section RF generator operation starts; admitted, the name is a bit confusing but here we start nevertheless.
To supply the signal from the 2955B to the antenna input, I used a coax cable with on one end a BNC connector and on the other hand two banana plugs, red for signal and black for ground. The BNC connector goes to the bottom left output on the instrument front panel. Toggle the SELECT key over it until the LED over the connector is lighted. The demodulated output is fed into the AF INPUT entry using an oscilloscope probe (1:1 setting).
The settings I used are displayed on the screen: GEN FREQ initially on 1.0 MHz with a step size of 2.5 kHz. With the up/down buttons left over the output connectors one can fine tune both generator frequency and amplitude until a reasonable output is achieved. The level I started with was1 mV with step size of 5 dB (mV’s are not possible here). The modulation frequency was set to 1 kHz with a 50% modulation depth.
There are two ways to view the results: in oscilloscope mode as in Figure 1 and in bars mode (with numbers) as in Figure 4. There is the option to measure noise and distortion (starting at manual page 3-31) but I did not bother for this simple radio.
Of course, I could not leave looking at the actual signals and so I connected a “normal” oscilloscope as well. The top trace gives the AM modulated HF signal and below the decoded signal.
In conclusion, this first test appeared to be quite easy to do and gave me confidence that proceeding would not be too prohibitive.