by 5B4AIY » 01 Jul 2013 07:17
OK, let's see if we can explain what's going on here. It will be essential to download the schematic for the MAIN BOARD, T/R, ATTENUATOR, CURRENT SENSE. The logic associated with the transmit/receive switching is on the lft-hand side, and is the T/R socket with !KEY-OUT and !KEY-IN (NOT-KEY-OUT, NOT-KEY-IN - negative logic, active low.) TR1, TR2, and the TRX socket.
Since you have not said with what transceiver you are using the amplifier, there are two possibilities, JUMA TRX-2 or TRX-1 or a non-Juma transceiver. In any case, do the following, connect the linear amplifier to a 50 ohm dummy load. Remove all cables including the RF input, except for the power cable. Switch the amplifier on. If you are using my firmware, then the amplifier will power up in the STDBY mode, if you are using the original firmware, it will power up in the last selected mode, which may be OPER.
Select the OPER mode. Does the amplifier still go into transmit? If so, then the fault is with the internal logic, which I will describe later. If not, then the fault is in the external connections.
Internal Logic
If the fault is internal to the amplifier, then this is how the internal logic works. There are two ways for the amplifier to be placed in the TX state.
1. Via a keying voltage on the RF input connector (This is the standard Juma TRX-1/TRX-2 transceiver method.)
2. Via a ground applied to the tip of a 3.5mm plug inserted in the T/R socket, the !KEY-IN line.
Method 1 - RF I/P Connector
The Juma TRX-1 and TRX-2 transceivers apply a +12V keying voltage to their RF O/P sockets when they are in the TX mode. This voltage is current-limited to about 1mA by a resistor in the transceiver so that no damage occurs if this voltage is shorted to ground. The voltage is sensed by R1 at the TRX RF I/P socket of the amplifier, and this resistor is bypassed to RF via C2. This voltage, identified as JUMA KEY IN on the schematic, forces D1 into conduction and is applied to the gate of TR2, forcing it into conduction, which repeates the inverted keying voltage on the !KEY-OUT line. In addition, this voltage is passed via pin 10 of J5 to the DDS/Display board where it is connected to pin 17, the KEY input sense line of the microprocessor. When the microprocessor senses a logic high on this line, it activates the transmit circuitry.
Method 2 - External Keying
In this mode the transmit/receive state is governed by the logic input to the !KEY-IN line of the T/R socket. In the receive state, this line is high or open, and as a result the gate of TR1 is held high by R3, forcing TR1 to the OFF state. With a logic low or a ground applied to the !KEY-IN line, TR1 is forced into conduction, and the voltage at R4 is high, placing a high on the KEY line, which is sensed by the microprocessor as before, asserting the transmit state.
Diode D1 in the JUMA KEY IN line is reverse biased in this state, thus ensuring that no DC key voltage is reverse fed to the connected transceiver.
Fault Finding.
It is necessary to first determine whether the fault is with the RF I/P connector line or the T/R line. First, short the RF input socket centre pin to ground. Did this clear the fault? In other words, if the RF input has a DC short on it, does the transmit/receiver logic now work correctly?
It is unlikely that there is a DC voltage present at the RF input socket, but it is necessary to make sure.
Now, plug a 3.5mm stereo patch lead into the T/R socket, and with a multimeter measure the DC voltage on the tip - it should show a logic high - if the multimeter has a 10M-ohm input impedance, you will see +12V on this pin.
Now measure the DC voltage on the ring terminal, it should show zero. To further check, set your multimeter to the low ohms range and with the negative lead connected to ground, and the positive lead connected to the ring terminal, you should measure a very high resistance, essentially an open-circuit.
To prove that TR1 is working, when you ground the tip of the patch lead the resistance measured between the ring and ground should drop to a very low value, typically between about 2 ohms and 5 ohms.
In all probability there is a fault associated with TR1 which is causing it to conduct and place a logic high on the KEY line. If this is not the case, then you may have an open circuit connection between pin 10 of J5 and the DDS/Display board. If this connection is open, then the microprocessor will sense a logic high on pin 17 and that will be interpreted as a transmit command. In this case, carefully check the ribbon cables, as well as the continuity of the connections on the DDS/Display board. It is not unknown for a poor soldered joint to initially work, but to go open after being moved or otherwise disturbed.
If you need any more assistance, let me know, and I'll try to help.
73, Adrian, 5B4AIY