In Step 2 and Step 3 you will be building the TR switch. There are two modifications made in this section.

First, the bottom lead of the .001 (C17) input capacitor for the RF switch is moved to the trace below the one shown in the standard layout from the ARRL. It is soldered to the trace that goes to the bottom terminal of the relay, as shown in the picture below.

Second, that trace is cut (about 1/16" wide) after the .001 capacitor. If the trace is not cut, the output of the 1 Watt MRF237 amplifier will be connected to the receiver input when in receive mode decreasing receiver sensitivity.

R7's value can be raised to increase sensitivity of the TR switch. This may be needed when driving a QRP transmitter when you have only the output of the VFO amplifiers to drive the TR switch. A 100K to 1 meg resistor can be used.

When driving QRP transmitters with the SLR, this circuit is built dead bug style as a standalone board for TR switching. This can also be done with the WA2EBY to improve input/output isolation and placed at the output of the transmit LP filters.

Be sure to cut the trace as shown above that goes to the bottom relay contact on the left side.

The Zener function is unque. The Zener connects to the 24 Volt line and decreases the voltge to the relay according to the value of the Zener. In this case, the relay has a 5V coil, so 24V minus 18V (Zener value) equals 6 Volts (measured 5.5V) for the relay coil.

Bend pins as shown below and solder to traces.
Notice trace for upper right coil connection is missing. Diode wire has to be bent down and soldered to the relay pin as shown in second picture.

The cores shown below were some surplus cores I had on hand during the build of this amp. Both are -43 material, though larger than the ones specified in the original article. They worked well even though oversized. The cores in the kit are the specified cores, FT50-43.

The yellow lines represent each wire winding and where they are soldered. Notice the T1 input transformer windings go straight across and T2 windings cross over.

NOTE! It is important to get the T2 connections correct as a mistake here will blow a MOSFET. You have to check if the ends of the wires are correct on T2 before you solder them to the board. After they are soldered, since one on each side goes to B+, you can't check them after they are soldered onto the PCB.

There are two coil winding forms you can use to make L1, L2, and L3. Dowel sticks from the hardware store or screwdrivers with shanks the correct size, one 1/4" and one 3/16".

The winding of L3 is shown below. I used electrical black tape as shown in the second picture and then placed a small drop of Super Glue on the windings to hold the coil together. Don't let it dry overnight or it will be stuck to the screwdriver. Remove after about an hour and it will slip right off.

Solder the coils L1, L2, L3 and transformer T3 to the PCB.

In the picture below, notice the solder blobs between the traces on the upper left hand side above the transformer connection. Look out for them and remove them as necessary.

Note! This would be a good time to double check your soldering and look for solder trash lingering around the board.

___2 - 10K Trim pots

In the picture above notice how the outside leads are spread apart slightly to fit the traces when they are soldered to the board.

Since the SLR does not do switching from receive to transmit at the Antenna terminal, the transmit signal is taken off the end of the VFO amplifier chain, the connections are different from the original design.

The normal transceiver connection goes to the Antenna input of the SLR. The VFO output goes to a MRF237 amplifier (2 Watt Output), a 3 dB Pad, then to the input of the WA2EBY amplifier. One modification is needed to the trace of the input of the TR relay. See the second picture below.

The output connections are as designed. The output of the WA2EBY amplifier goes to the input of the LowPass Filters, then the output of the LowPass Filters goes to the TR relay. The top connection of the relay contacts is the RF Output which goes to the outdoor antenna.

A trace is cut after the .001 input capacitor for the TR relay. This modification helps isolation between the input and output of the amplifier. Also, the output of the MRF237 amplifier would be sent to the receiver input during transmit - not a good thing - would upset the input impedance of the WA2EBY. See the picture below.

Shown below is what is needed to mount the MOSFETs. The MOSFET leads are cut short as shown below, Some Silicon Heat Treatment Compound is used, and the tab is isolated from ground (the tab is hot with 24 volts and the RF Output).

Shown are two different types of insulators. A TO-220 mounting kit can be purchase from DigiKey (and other companies) or the insulators can be robbed from discarded computer power supplies or audio amplifiers that use TO-220 output transistors. You will need the insulator and the insulating washer that will be on the mounting screw

The insulator shown on the left is made of mica and has excellent heat transfer characteristics. Almost as good is the grey colored one which is thicker and more durable. Both have worked well in this amplifier.

The screw used to mount the MOSFETs to the heat sink must have an insulating washer to keep the metal of the screw from touching the MOSFET tab. Note the arrow pointing to the washer. It has a smaller section that goes inside the hole of the MOSFET tab to prevent a connection.

A 1/2" long 4-40 screw is used to mount the MOSFETs. After they are soldered to the board, the board is laid down on the heat sink and a pointed punch is used to mark the center of the hole on the heat sink.

There are two ways to mount the MOSFETs to the heat sink.

A 1/8" hole is drilled through the heat sink and is secured with a nut underneath the heat sink. This works with heat sinks that have an area thin enough underneath the MOSFETs to drill and mount the screw/nut.

Heatsinks that have a very thick area underneath the MOSFETs, a drill and tap are used to mount the MOSFETs. Use a 3/32" drill to make the holes and make the threads with a 4-40 tap and tool. A 1/4" or 3/16" long 4-40 inch screw will probably work best in this case.

Picture shows completed dead bug build of the MRF237 2 Watt amplifier.

First step is to use a piece of fine emory cloth (or sandpaper) to roughen up the side and bottom edge of a small area on the MRF237 so it will take solder easily.

The whole bottom of the MRF237 does not need to be soldered to the board. Only a small, but good solder joint, will be needed to be effective in grounding the emitter (which is the case) and providing a good heat sink.

I cut this piece from the corner of the PCB that already had a hole that could be used for mounting the board.

Orient the MRF237 so the B connection is on the input side of the board and the C connection is on the output side of the board. First solder a blob on the PCB and then put the MRF237 against it and melt the solder together between the PCB and the MRF237.

The output of this amplifier is a little over 2 Watts. With the 3dB pad the output is 1 Watt. If you need 2 Watts for your transmitter, do not install the 3dB pad on the board.

Solder C on the MRF237 after the other parts are soldered. They are supported so that they can be carefully placed at the pin for soldering so that no shorts occur on the case of the transistor.

Step 1____ Determine if you want to add a connector to the input side of the amp. In the build below an RCA connector is used and supported by a triangular side piece of PCB.

Cut a small piece of PCB the width of the amp PCB and about 1" tall. Drill a 1/4" hole for the female connector. Then use at least one side support to hold the small piece of PCB steady. Without the side support it will bend easily and eventually break off.

Solder at the edges at enough places to hold the pieces securely.

Step 2____ Solder the bottom of the resistor on the Input assembly to ground near the MRF237 as shown below.

Step 3____ Solder the top of the resistor on the Input assembly to the B pin on the MRF237.

Step 4____ Solder one end of a standoff resistor to ground between the input connector and the coil assembly as shown below.

Step 5____ Solder a .01 capacitor between the input connector and the standoff resistor.

Step 6____ Solder the top of the 9 turn input coil to the standoff resistor and the .01 capacitor.

The input is connected to the transmitter output of the T/R switch at the end of the DDS VFO amplifiers on the BLT (Transmit Out).

The output side is connected directly to the input trace (where L3 is soldered) of the WA2EBY or to the input of any transmitter that you are using that needs this amplifier.