Strange Test Results with the SARBE-5

I seem to have something a little odd going on with the SARBE-5!

Testing it today, to see if all is well before attempting to retune it, ive found a few curious anomalies. First, the AUX channel receiver gives audio output whether the receive button has been pressed or not! The EMG channel doesnt do this and works just as expected - needing the receive button to be pressed to listen.

But more of a worry is that as soon as I connect the unit up to the Marconi 2955 test-set, both receivers give a roughly 1kHz audio tone. Now, this would be fine, if that tone was the 1kHz 30% AM modulation of the test signal from the 2955, but it isnt! They give that tone regardless of what frequency the test set is set to, or what signal level, or even if the test set has its signal generator turned on! In fact, it only stops if I either disconnect the BNC connection to the test-set, needing only to do so far enough that the center pin un-mates, but the barrel can remain connected, or if I turn the test-sets power off!

Strange...!

I have a few theories. Im not too fussed over the AUX channel's indifference to whether ive switched to receive or not - thats a minor trouble. Its the inability to test with a direct signal from the test-set thats the big concern. Using a very limited test regime, due to the frequencies being used, I tried the receivers by close coupling between antennas - the SARBE-5's own rubber ducky antenna and a telescopic whip on the test-set. On both channels I was able to get normal receive performance, i.e. just background noise until the signal level was sufficiently increased (around -65dBm) and then the modulating tone was heard. So it seems whatever is causing the problem is a result of direct electrical contact between the SARBE-5's antenna connection and the circuits of the Marconi 2955. The ground connection from the coax patch-lead doesnt seem to be an issue. My thoughts are that, either there is a loop of some sort being formed, possibly including the bench PSU that is powering the SARBE-5, or there is some strange feedback occurring. Bear in mind that the receiver stages are self-quenching super-regeneratives, it could be that their own oscillations are somehow being fed back around!

So, apart from the AUX channels switching, it looks like the receivers work, but the test set-up is causing trouble! Probably the easiest next step will be to eliminate the PSU as a suspect, by rigging the unit to battery power. Proper SARBE-5 batteries are virtually unobtainable in a working condition, but luckily the radios work on a nominal 12v, so one of my spare 12v SLA batteries will do.

If need be, I will have to mock-up some sort of 1:1 RF transformer to isolate the radio from the test-set.