SARBE-5 (BE375) Very Odd Test Tesults!

[Martin G7MRV]

Im working on a BE375 SARBE-5 unit, with the intention of retuning it to monitor useful local UHF airband channels. Ive disabled Tx by disconnecting the associated reed switch connections for power to the Tx side, removing the crystals, and removing the auto-beacon control board.

So now im checking it over prior to attempting retuning, and im finding im getting some very odd test results!

To give a brief overview - these contain two UHF super-regenerative receiver stages (one for each channel), and also a pair of UHF oscillators (essentially the same as the receiver sections) that act as test sources. These test sources are selected by lifting a lever, which operates a pair of reed switches, one supplying power to the test oscillators and the other switching the antenna socket out and connecting the receivers to the test oscillators.

So in theory, either of the two receivers should only produce an audio output if it is the one selected, the receive button is pressed, and either there is a signal to receive, or the test tab is lifted, in which case the internal test should be heard.

Mines being a bit odd. The EMG channel, if selected, seems to work as expected - no audio unless receive button is pressed, but the AUX channel is giving audio whether it should be or not! Both give a tone if the test lever is lifted, but just noise otherwise...

...that is, until I connect my Marconi 2955 test set! Then, both receiver channels will give a tone output - as if they are receiving a signal. This happens regardless of whether in operate or test mode, and regardless of whether the 2955 is actually on frequency, or even providing an output! Turn the signal generator OFF and theres still a tone! But, turn the test set off, or disconnect the BNC connector enough to unmate the center pin - and the tone stops!!!

Im at a bit of a loss. I can only think that somehow the test oscillators are still activated and they are feeding back through the test-set somehow?

The only other thing I can think might have a bearing is that im running it from the bench PSU, as I dont yet have a suitable battery.

Any ideas? Im wondering if one of the two test mode reed switches is stuck!

Cheers
Martin

to add - just tried another test with the test-set using paralleled antennas for coupling, so the radio is isolated electrically from the test set - Both receivers work as expected in that configuration (other than the AUX channel not needing the receive button to be pressed first!). So it seems its a problem only when directly connecting? This is not an ideal way to test a receiver tuned to a distress frequency for obvious reasons!

[Martin G7MRV]

Well, ive solved my problem, though im still not sure exactly what was going on!

Where I had tack-soldered my coax to the test-set to the unit, was at the point the antenna connects to the reed switch, via a 10pF capacitor. I was connected at the reed switch side - bypassing that cap. Putting a 10pF cap in series with my coax connection resolved the issue, and has allowed me to test and partially retune the radio. Theres no DC at that point, so I can only assume that the direct connection was influencing the receiver circuit somehow.

So now, I have a SARBE-5, with no Tx capability at all, and its AUX channel retuned to a more useful frequency for me to monitor, working to the spec of 20uV for a near fully quieting signal.

Sadly the same cant be said for the EMG channel, currently of course on 243MHz, which is somewhat deaf, needing nearly 200uV input for the same result. Im not very familiar with self-quenching super-regeneratives, so im not sure yet where to start on fixing this! I will probably move it a channel or two away from 243MHz though while I work it out!

Martin