NASA Active Dual Channel NAVTEX Antenna Repair

Despite what the title says, its unlikely this will ever be a dual channel antenna again!

For a start, the PIC 12C508 IC, which provides the switching control and the local oscillator for the channel conversion, is not working. Whether it is just physically damaged but the code can be read, will have to be discovered. Even if the code will read, chances are that its been protected and cannot be used.

The mixer IC MC1496 has also been removed from the board. I have no easy way of testing this, so have to assume for now that it is faulty. Ive removed it to prevent any damage from causing problems with the rest of the circuit.

And so we come to L4 (as I have designated it). This is the output tuned transformer. And its knackered! I had noticed some corrosion under it, as there was with the PIC and the previously rotted off crystal. On removing it from the board I found that the fine wires of the primary had completely corroded away.

This seems at first a major problem, but in reality probably not. The frequency its tuned to is known, 518kHz, and its series capacitance also known, 150pF, so its a simple matter to calculate the secondary inductance needed, which is 630uH. Replacing it is just a matter of finding a suitable 10mm can transformer with that impedance secondary and the same winding configuration.

But for now, I have the trouble of bypassing all this to try and get the direct 'pass-through' 518kHz channel working. All the transistors tested out OK off board on their DC parameters, so hopefully its just a case of getting the supply to them and the signal from them!

The secondary of L3 is the problem. This would normally feed the mixer inputs, but also has a DC level on it for one of those input pins. So, I need to isolate that DC level from the transformer, provide a DC block from the transformer to the output/supply wire to allow the signal out, and temporarily ground the other side of the winding. I 'think' that I can isolate the DC by lifting one leg of one of the 680 ohm resistors, and a 100nF cap should work for coupling the signal.