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, or from the W6PQL parts page in the form of an upgrade kit. In the case of the upgrade kit, the coax is already prepared for you. To prepare it yourself, here are the instructions:
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
- The reference designations for R11 and R13 have been changed to R16 and R17. These are shown installed in this photo, as are C5 and C6, but are not actually installed until the last step in the main assembly guide
- R13 is now the 100 ohm resistor installed next to the RF input trace
Bill of Materials (BOM)
RefDes Value Name BALUN 4T, 50z rg402 FEP made with a Laird 28B1020-100 core C1 .1u 100v capacitor_1206 C2 .01u 100v capacitor_1206 C3 .01u 100v capacitor_1206 C4 1u 100v capacitor_1206 C5 1000pf ceramic capacitor C6 1000pf ceramic capacitor C7 .01u 250v capacitor_1210 C8 .01u 250v capacitor_1210 C9 .01u 250v capacitor_1210 C10 .01u 250v capacitor_1210 C11 .01u 250v capacitor_1210 C12 .01u 250v capacitor_1210 C13 .01u 250v capacitor_1210 C14 .01u 250v capacitor_1210 C15 .01u 250v capacitor_1210 C16 .01u 250v capacitor_1210 C17 .01u 100v capacitor_1206 C18 1u 100v capacitor_1206 C19 100u 63v capacitor - electrolytic C20 .01u 100v capacitor_1206 C21 1u 100v capacitor_1206 C22 100u 63v capacitor - electrolytic C23 240p capacitor-MC18 mica C24 240p capacitor-MC18 mica C25 240p capacitor-MC18 mica C26 240p capacitor-MC18 mica C27 .047u 450v capacitor C28 .047u 450v capacitor DZ1 6.2v Zener Diode Q1 BLF188xru DUALLDMOS R1 50 resistor_1206 R2 50 resistor_1206 R3 50 resistor_1206 R4 50 resistor_1206 R5 300 resistor_1206 R6 100 resistor_1206 R7 5k thermistor resistor_1206 R8 510 resistor_1206 R9 50 resistor_1206 R10 10 resistor_1206 R12 10 - 2w resistor_2512 R13 100 - 2w resistor_2512 R14 10 - 1w resistor, metal film R15 10 - 1w resistor, metal film R16 430 2w resistor, metal film R17 430 2w resistor, metal film T1 4 to 1, 2T/1T RG316 input transformer, made with 1 Laird 28B0562-100 core T2 5T x2 #14 200C magnet wire BIFILAR drain choke, made with a Laird 28B1020-100 core T3 4T, TC-18 coax coax transformer, made with a Laird 28B1020-100 core T4 4T, TC-18 coax coax transformer, made with a Laird 28B1020-100 core VR1 200 IDQ ADJ trimmer PCB Circuit board, TC-350 material available through www.w6pql.com TC18 18 ohm Teflon coax 29 inches RG316 50 ohm Teflon coax 7.5 inches RG402FEP insulated RG402 conformable coax 18 inches magwire #14 magnet wire, 200C 36 inches