Burndept BE515 Switching Scheme

It looks like the switching scheme used in the SARBE-6 is very similar to that used in the SARBE-5, for which I have the circuit diagrams. Switching is controlled by six glass reed switches.

The two SPST reeds for volume and power im not concerned with - their operation remains the same. It is the four SPCO reeds that im concerned with.

The plan is, if at all possible, to make the unit work almost exactly as a live unit, but with no ability to actually transmit. If I want it to be able to still pass all the self tests, then the Tx has to remain physically intact, and electrically operational, but with absolutely no way to access the antenna.

There is some concern over this proposal. As Ive already found, even in test mode, with the case fully closed and with NO antenna, the signal has enough leakage to be detectable on a close by receiver. I can pick it up with my discone antenna from downstairs in the sitting room, as distance of some 25ft! I will need to do some tests to ascertain just how much RF leakage there is in test mode!

Of the four SPCO reeds, two control switching for test mode. The first is an antenna changeover switch, which in test mode disconnects the antenna and connected the LPF (low pass filter) to the BITE circuits. The LPF is part of the antenna matching, and there is no further switching between the Tx and Rx circuits - just a shared common point. If I want test mode to remain workable, then I have to leave this as it is.

The second test mode reed controls power, and is essentially in parallel with the on/off switch reed. This again has to be left alone for test mode to work.

So that leaves the Tx and Rx switching reeds. These control the power supply to the receiver(s) and transmitter, and are wired such that the normally closed contacts are in series, with the Tx reed first in line. This means that, in the absence of a magnetic field from either the Transmit or Receive buttons being pressed, power is routed to the beacon circuitry. With the Receive button pressed, the beacon supply is cut and diverted to the receiver circuits. With the Transmit button pressed, power to the second reed is cut, so removing both the beacon and receiver power supply lines, and diverted to the transmit circuits and AM modulator.

So a simple, and I hope workable, modification scheme develops. The 1st Tx/Rx reeds NO (normally open) contact is disconnected from the rest of the circuitry, this preventing the transmitter being activated by the Transmit button. The 2nd reeds NC (normally closed) contact is disconnected, thus isolating the beacon circuitry.

In this state, test mode should still work as normal, but the beacon and the voice transmitter should never activate. The Receive button would still need to be pressed to listen though. However the radio could be set to permanent receive, without needing the button pressed, by adding a short wire to connect the 2nd reeds NO contact to its common contact, therefore permanently powering the receiver and audio amp when the unit is switched on.

I think though that a permanent receive modification would mean that in test mode the Transmit and Beacon functions would not be testable.

It is my intention to modify my unit to require the Receive button to be pressed, at least at first. When later, I work out how to retune the unit for a different frequency, at that point it will probably make good sense to go to a permanent receive function.