A modification to the input circuit was added to eliminate an instability discovered when operating above 20 MHz into certain narrow-band antennas. This instability became evident at power levels above 500w when driving these antennas, and were not present when operating into a dummy load or other broadband loads. The typical symptom was erratic reverse power readings, increasing at a non-linear rate as drive was increased. On a spectrum analyzer, these signals were as strong as -30 dbc, and numerous.
The problem was fixed by replacing the input transformer with a different type, and adding additional degenerative feedback components to the gate circuits. 4 components were removed, and different components were substituted for them. This modification takes about an hour to perform. It also reduces the overall gain of the amplifier a little; after the update, drive requirements will be approximately 2.5w (160m) to 5w (6m) for 1kw out.
These were the changes made:
Here are the parts you will need to get to make the change (kits and RF decks shipped after July 2016 include these parts):
All of the above parts are available from www.mouser.com.
To perform the modification to a previously assembled input board:
- Remove the old input transformer and the 4 resistors mentioned above
- Install the100 ohm resistor at R13 (not shown in this photo, it's shown in the first and last photo)
- solder the center of the coax to the board. Make certain to orient the shield stubs forward as shown here
- pass one of the coax ends through the ferrite core, then the other end through the other side (criss-cross)
- Solder the two shield stubs to the traces on the board where R14,15 and 16 used to be located
- Pass the wire on the right across the front of the core to the left, through the core from the left, and solder it to the ground pad on the right.
- Pass the wire on the left across the front of the core to the right, through the core from the right, and solder it to the RF input pad on the left above the core
- Bend the two solder lugs upward a bit and solder one end of a 1000pf capacitor to each lug.
- join the other end of the capacitors to their respective 10 ohm resistors using a hook in each component lead and apply solder to ensure a good connection. The shrink tubing (mine is red) I used to cover the resistor/capacitor connection is optional, but recommended
- solder the other end of the 10 ohm resistors to each gate pad
- install the lugs under the screw heads (these screws will not be installed yet if you are building this board from a new kit; in this case, just position the lug over their holes for now).
The modification is now complete. There is no need to re-adjust bias (IDQ) levels, as nothing in the D.C. bias path was changed. Future revisions of the PC board will allow the use of surface-mount parts to make assembly a bit easier.
A revised schematic diagram and bill of materials follow for the kits and RF decks as they are currently shipping. Some of the bias components have changed; these are minor adjustments to make IDQ settings a bit easier to do, and do not need to be changed if you are only performing the 08-2016 modification to an existing build.
If you are building a new input board, note these changes and use the photo here as a placement guide:
- R1, R2, R3 and R4 are now 50 ohm resistors
- R5 is changed to 300 ohms
- R6 is changed to 100 ohms
- R7 is changed to a 5k thermistor
- R8 is changed to 510 ohms
- R13 is now the 100 ohm resistor installed next to the RF input trace
- The reference designations for R11 and R13 have been changed to R16 and R17. These are shown installed in this photo, and were listed on the schematic, as were C5 and C6; these components are no longer used after this upgrade is made, and should be removed
As of May 2017, the bifilar drain choke was changed to a pair of separate drain chokes. First, remove the original bifilar choke.
Now make the new drain chokes; locate the two #14 tinned magnet wires, straighten them out, and pass each through a ferrite core (Laird 28b1000-000) as shown here. One end should extend out the end of a core by about an inch, the other by almost 2 inches. The wire should pass through the center of the core 9 times.
This photo shows how and where to mount these chokes. Leave as much space as possible between them, about 1/8 inch is enough.