Marconi 2955B Radio: Transmitter test

There I had, with the same main components as the receiver used before, a wonderful MW-oscillator to which I added a buffer to drive the signal into 50 ohms. Alas, this was beyond the range for the TX-test of the instrument; I needed a higher frequency.

The device under test

ICOM IC-706

Luckily, I could borrow a transceiver that covered most amateur bands. As displayed above, for this demonstration I chose 3.6 MHz for a receiving test and performed as in the previous blog. The decoded signal can be taken from the phones output on the bottom left of the panel. The HF-antenna input was fed by a short cable with UHF-connectors (PL259) to the 2995B equipped with an adapter from UHF male to N female.

Interestingly, in the beginning I saw quite some distortion at the higher modulation levels. I put the output signal on the oscilloscope and there it turned out that the output amplifier of the ICOM was clipping. Reducing the output level amended this otherwise abnormal behavior.

Transmission test cable.

To prepare the instrument for the test, the standard microphone does not suffice. A cable was made to connect the af-signal from the 2995B to the transceiver. The PTT (transmission) switch was exported as well. The required connector is a standard RJ-45 found on UTP-cables used for internet connections. To make life simple, I just cut an old one in two and connected pins 6 (signal) and 5 (ground) to the AF-out connector of the 2995B. On my cable these were the green and white/green wires respectively. The PTT switch is between pins 4 (signal) and 7 (ground); on my cable blue and white/brown respectively.

The test itself

The test itself is described in the 2995B manual extensively. Starting in the middle of page 3-10 is the section AF generator operation which is actually the transmission test itself.

ICOM IC706 in transmission mode

Before the actual test, the IC-706 was set to low output power (L) and FM. The power meter was switched on in the screen. With the PTT switch on, the above picture was made. The transmission frequency turned out to be set 100 kHz lower than the receiving frequency.

Marconi 2995B in TX test

From the above figure it is seen, that the af-signal was set to 1 kHz at 1 mV and that the unit received 2.3 W at 3.5 Mhz.

Conclusion

With this, the basic demonstration of the instrument is completed. More options are available albeit that some are missing. SSB as well as digital modes are not present which makes the instrument less useful as it could be.