Wednesday, September 27, 2017

2-M EME Sequencer Modifications Complete

Most ham stations have no need for an external "Sequencer" as the functions of a sequencer are built into the rig itself.  However, it can be an essential piece of equipment for a VHF/UHF station or one used for EME.  What it does is to turn various pieces of equipment ON or OFF in a specified sequence when moving between Transmit and Receive.  This is done to protect some equipment from receiving RF when it is still in receive mode or to protect the transmitter from transmitting before the antenna change-over relay has completed switching the antenna to the transmit position.  Especially when you are dealing with rather high power in VHF/UHF frequencies, NOT using a sequencer can result in serious damage to your equipment.

Take a look at the station equipment diagram above and think about what needs to be turned on (or off) first, second, third and last when moving from Receive to Transmit and back.  If you are in RECEIVE, your LNA is being supplied with 13.8 VDC and the T/R Relay is in RECEIVE.  The Amp is OFF and the Rig is in Receive.  When you press the PTT, footswitch, or the computer tells the system to begin transmitting, FIRST you remove the 13.8 VDC from the LNA to turn it off, then you tell the T/R relay to move to the TRANSMIT position, then you tell the Amp to turn ON, and lastly, you tell the Rig to begin transmitting.  When returning to RECEIVE, all those items need to be performed in the reverse order.  By using a sequencer you can control the order in which all these devices are triggered and you can be sure that the LNA is OFF, the T/R relay is firmly in the transmit position and the amp is turned on BEFORE the Rig begins transmitting.

Many years ago (when planning an earlier EME station) I acquired a Model TRS, TR Sequencer, from Down East Microwave (DEM.)  It was still in the original box in my garage so I had a "NOS" (New, Old Stock) sequencer ready for my current 2-M EME station.  However, I needed to figure out the logic of which piece of equipment to turn ON (or OFF) in what order and how to control that switching.  DEM gives a "matrix" to help you layout the configuration.  There are four sequenced stages each with two separate outputs.  This means you can control up to 8 different pieces of equipment during four different time periods.  The switching time between stages is controlled by an R/C circuit with a user adjustable time constant of about 125 milliseconds.  That means you will have about 1/2 second between transmit and receive (and the reverse) for the full sequence process.

Sequencer Matrix Configuration
I copied the matrix so that I could work on it again and again by scanning it into the computer.  I quickly determined that I needed to control the pieces of equipment shown in the diagram at the top of this Post as well as one other T/R Relay on the Receive line.  That gave me the four sequenced stages and the order of the equipment.  Once that was done, I needed to determine how to control each piece.  For instance, when I wanted the station to be in receive mode, I needed to supply 13.8 VDC to the LNA.  Each stage of the sequencer can control two separate pieces of equipment and can deliver either a High Voltage, a Low Voltage (Ground), or an Open (no contact at all.)  Then, in Transmit mode it can again supply a High, Low or Open to the same piece of equipment.  In this example, the LNA needs a High when in Receive and an Open (no voltage) when in Transmit.  You can see on the Matrix at the right that I have circled the H in Receive and the O in Transmit for the LNA.  (Click on any image for a larger view.)

Inside Sequencer Showing Modification Board on Right
Once the sequence configuration was determined, I needed to modify the stock DEM TRS Sequencer to meet those conditions.  Some of the stock configurations were just what I needed but I had to make four changes to the wiring of the board.  This meant I had to remove the circuit board which was mounted with three nuts to the bottom of the case, remove the power switch plus the ON LED and XMIT LED from the front panel, and twist the board up to where I could un-solder (and re-solder) the connections I needed to change.  Since there were some ELEVEN wires coming from the board to the RCA connectors on the rear panel, this was not very easy.  And, it would have been really helpful to have someone to hold the board in place while I held the solder and soldering iron - Hi!  Still, I got 'er dun!

One change I had to make was to supply a different  High voltage for the T/R Relay than the 13.8 VDC available in the sequencer.  So, I added another RCA jack in an open hole on the back of the sequencer where I can supply the 20 to 28 VDC needed by the T/R relay.  That voltage was internally connected to one side of the second timing stage relay so when activated it would either supply 20-28 VDC to the T/R Relay or remove that voltage.

Sequencer Showing ON Above and XMIT Below
Once the above changes had been made and checked out, I needed to add some circuitry to the sequencer.  My rig (Elecraft K3S) has a provision called TX Inhibit where you can keep the radio from transmitting until the TX Inhibit line is released.  Turning on the menu function of TX INH (Transmit Inhibit Signal) allows you to prevent transmit by holding Pin 7 of the ACC connector Low (or High) through a 2.2-K to 10-K resistor to a 5 VDC supply.  I found a suitable circuit online from KL7UW where I just needed to build a 5 VDC power supply, add a relay, make a few connections and the result would be that the radio would not transmit until the sequencer gave it the proper signal.

I built the 3-terminal regulator power supply and other parts on a small circuit board and installed it inside the sequencer as you can see above.  I also decided it would be helpful to see that this circuit was indeed producing the 5 VDC and that the 20-28 VDC power supply was providing its voltage to the sequencer.  So I added two LEDs to the front panel along with a decal describing what they were indicating.  You can see those in the photos of the front of the sequencer on the right.  The upper photo shows the RECEIVE mode where the sequencer is turned on, 5 VDC is being made by my little power supply (as shown by the Blue LED) and 24 VDC is being sent to the T/R Relay up at the antenna to switch it into Receive mode (as shown by the Green LED.)  The bottom photo shows how the front panel looks when it switches to TRANSMIT.  The 24 VDC is removed so the T/R Relay switches to Transmit, the 5 VDC is still being provided, and the XMIT LED is illuminated to indicate that the sequencer has switched to TRANSMIT Mode.

NOTE: the T/R relay is POWERED ON for Receive.  This is done so that if you should lose power to the relay or the wires get cut or suffer an intermittent connection, etc., the relay will go into the TRANSMIT position and the expensive preamp and the receiver input will be protected.

All that is left now is to run a couple of wires to the K3S ACC Connector and connect them to the appropriate pins on my modification circuit.  Then the LNA, the CX-600NL relay, the T/R Relay, and the amplifier need to be connected to the RCA connectors at the rear of the sequencer.  At that point, the station is ready to turn each piece of equipment ON or OFF in the proper sequence when changing from Transmit to Receive.

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