Overview
When the X6100 transmits, it signals external equipment by pulling its ACC PTT line to ground (a low level of roughly 0.1 V or less). Many 100 W amplifiers want the opposite: they switch to transmit only when +12 V is applied to their PTT input. The two polarities do not match, so a small interface is needed.
Radio side
Ground = transmit. The line is idle in receive.
Amplifier side
+12 V = transmit, 0 V = receive.
The interface
A 2N2222A holds a 12 V relay on in receive. Ground on PTT releases it, and its normally-closed contact sends +12 V to the amp.
Where it lives
On a small perfboard mounted inside the amplifier, powered from the amp’s own 12 V.
Safety and ground rules
Disconnect the DC supply (and mains, if the amplifier has an internal supply) before removing the lid or soldering anything. A 13.8 V supply can deliver very high currents, so a dropped wire or screwdriver can cause burns or fire.
The X6100 produces up to 10 W from an external 13.8 V supply (5 W on its battery). For Xiegu’s own XPA125B amplifier the X6100 manual recommends setting the radio to 2.5 W or less. Check the maximum drive level in your amplifier’s manual and set TX POWER on the X6100 below it before the first transmission.
Do every test transmission into a 50 Ω dummy load at minimum power. Connect an antenna only after the PTT switching has been proven.
This is an amateur design, supplied as-is and not endorsed by Xiegu or any amplifier manufacturer. Opening equipment may void its warranty. Build and use it at your own risk, and verify each stage with a multimeter as described below.
How it works
- Receive. The X6100 leaves its PTT line idle. R2 feeds base current to Q1, so Q1 conducts and holds relay K1 energised. K1’s normally-closed (NC) contact is therefore open, and the amplifier’s PTT input sees 0 V.
- Transmit. The X6100 pulls the PTT line to ground. That removes Q1’s base drive (the base falls to about 0.2 V), so Q1 turns off and K1 releases. Its NC contact closes and passes +12 V from the amplifier’s own supply to the PTT input.
- Fail-safe. If the cable is unplugged or the radio is off, R2 still holds Q1 on, K1 stays energised and the amplifier stays in receive.
| Stage | Receive | Transmit |
|---|---|---|
| X6100 ACC PTT line | idle | grounded (≈ 0 V) |
| Q1 base voltage | ≈ 0.7 V | ≈ 0.2 V |
| Q1 (2N2222A) | ON | OFF |
| K1 relay coil | energised | released |
| K1 NC contact | open | closed |
| Amplifier PTT input | ≈ 0 V | +12 V |
A single NPN transistor with its collector pulled up to +12 V does the opposite of what you need: it gives +12 V in receive and 0 V in transmit. A small pull-up resistor also cannot supply the current some amplifiers draw on their PTT input. Using the relay’s normally-closed contact gives the extra inversion and a proper switch for the +12 V.
Parts and tools
| Ref | Part | Value / type | Qty | Notes |
|---|---|---|---|---|
| Q1 | NPN transistor | 2N2222A | 1 | Pin order differs between makers and packages (for example PN2222A and P2N2222A). Check your part’s datasheet before soldering. |
| K1 | Relay | 12 V coil, SPDT (changeover), coil ≥ 300 Ω, contacts ≥ 1 A | 1 | Any small PCB or panel relay. Pin order varies, so use the datasheet to find coil, COM, NC and NO. |
| D1 | Diode | 1N4148 or 1N4007 | 1 | Flyback protection across the coil. The banded (cathode) end goes to +12 V. |
| R1 | Resistor | 220 Ω, ¼ W | 1 | Series protection for the radio’s PTT line. |
| R2 | Resistor | 10 kΩ, ¼ W | 1 | Base pull-up. Gives about 1 mA of base current, enough to saturate Q1 for a coil drawing up to roughly 50 mA. |
| C1 | Capacitor (optional) | 10 nF ceramic | 1 | RF bypass, base to ground. Recommended inside an amplifier. |
| R3 | Resistor (optional) | 10 kΩ | 1 | Output pull-down, only if the amp’s PTT input has none of its own. |
| F1 | Fuse (optional) | 500 mA with holder | 1 | In the +12 V tap. Cheap insurance. |
| – | Perfboard | about 50 × 70 mm (16 × 11 holes or larger) | 1 | 2.54 mm pitch. |
| – | ACC plug and cable | 3.5 mm plug matching the X6100 socket, thin single-core screened cable | 1 | Core = PTT, screen = ground. Keep it short. |
| – | Hardware | hook-up wire, heat-shrink, stand-offs, cable clamp | – | For mounting and strain relief. |
Tools: soldering iron and solder, multimeter, side-cutters, wire strippers, a 50 Ω dummy load, and the manuals for the X6100 and your amplifier.
The circuit
| Part | What it does |
|---|---|
| R1 (220 Ω) | Limits current into the radio’s PTT line if anything ever drives it high, and isolates the radio from the board. |
| R2 (10 kΩ) | Pulls Q1’s base up to +12 V so Q1 is on whenever the radio is not grounding PTT. The radio only has to sink about 1 mA to switch. |
| Q1 (2N2222A) | Switches the relay coil. Fully on with the base pulled up, fully off when the base is pulled to ground. |
| K1 (relay) | Its NC contact connects +12 V to the amp’s PTT input when the coil is released. It can supply far more current than a transistor and a pull-up could. |
| D1 (1N4148) | Absorbs the voltage spike from the coil when Q1 turns off, protecting Q1. |
| C1 (10 nF) | Shunts stray RF on the PTT line to ground. Keep the lead lengths short. |
Build steps
Check your amplifier’s PTT input
- Read the amplifier manual and confirm the PTT input needs +12 V positive to transmit and 0 V (or open) to receive.
- Note how much current the input draws. K1’s contacts are rated for far more than a typical PTT line, but you should know the figure.
- Find the existing PTT connector or terminal inside the amplifier and decide where the interface output will connect.
- Find a +12 V point that is live whenever the amplifier is powered (an internal 12 V rail, or the DC input). If you use the raw 13.8 V supply, confirm the PTT input tolerates it.
Identify the PTT and ground contacts on the X6100 ACC socket
The X6100 manual shows the ACC pin-out only as a picture (page 8), and it describes the socket as a 3.5 mm “stereo” type, while Xiegu’s L4001 amplifier cable is listed with a 4-pole 3.5 mm plug. Check the diagram in your own manual, then confirm it with a meter so you are certain which contact is PTT on your radio.
- Connect a 50 Ω dummy load to the ANT socket, switch the radio on, set TX power to minimum (GEN → Radio Setting 1 → TX POWER), then switch the radio off again.
- Solder a short, labelled flying lead to each contact of a plug of the same type as the socket, keeping the leads well apart, and push it into the ACC socket.
- Find ground: with the radio still off, use your meter’s continuity range between each lead and the shell of the ANT BNC socket. The contact that beeps is ground.
- Switch the radio on. Set the meter to DC volts with the black probe on the ground lead, touch the red probe to each remaining lead in turn, and note the idle voltage.
- Press and hold the radio’s own PTT button (top panel) and measure again. The contact that falls to about 0 V while PTT is held, and returns when you release it, is the PTT line.
- Record your result below and mark the plug and cable so it cannot be mixed up later.
| Plug contact | Role | Idle (V) | PTT held (V) |
|---|---|---|---|
| Tip | |||
| Ring 1 | |||
| Ring 2 (4-pole only) | |||
| Sleeve |
The X6100 has a PTT MODE item in its General Setting 1 menu. Check the manual for what each option does, and make sure you are pressing the radio’s PTT, not a CW key.
Build the interface on perfboard
Figure 4 shows one compact layout. Components are seen from the top (component side); the coloured lines are wires on the underside of the board.
- Fit and solder the passive parts first: R1, R2, D1 (band to the +12 V side), then Q1 and K1.
- Lay the +12 V bus along one edge and the ground bus along the other with bare wire or hook-up wire.
- Make the connections in the table below. The net list is authoritative if you choose a different layout.
- Solder C1 (10 nF) directly between Q1’s base and emitter leads on the underside, as short as possible.
- Check the solder joints, trim the leads and make sure no joint bridges to its neighbour.
| Net | Connects |
|---|---|
| +12 V | R2 (top end), D1 cathode (banded end), K1 coil A, K1 COM, +12 V input |
| GND | Q1 emitter, C1 (if fitted), X6100 ground (cable screen), amplifier ground |
| BASE | Q1 base, R2 (lower end), R1 (right-hand end), C1 (if fitted) |
| PTT in | R1 (left-hand end) and the cable core from the X6100 PTT contact |
| COLLECTOR | Q1 collector, D1 anode, K1 coil B |
| OUT | K1 NC contact, amplifier PTT input (and R3 to ground, if fitted). K1 NO is left unconnected. |
Make the ACC cable
- Use thin single-core screened cable: core = PTT, screen = ground, using the contacts you identified in step 2.
- Fit the 3.5 mm plug that matches the X6100’s ACC socket and use a cable clamp for strain relief.
- Keep the cable as short as practical and add a small clip-on ferrite near the plug if you can.
- Label the plug “X6100 PTT” so it is never confused with other 3.5 mm jacks on the radio.
Bench-test the board (no radio, no amplifier)
Power the board from a bench supply or the amplifier’s 12 V with the current limit set low, and use the multimeter on DC volts.
| Test | Action | Expected result |
|---|---|---|
| A | Apply +12 V with the PTT input left unconnected | K1 clicks and stays energised. Q1 collector ≈ 0.1–0.3 V. OUT ≈ 0 V (or floating). |
| B | Connect the PTT input to GND with a wire | K1 releases with a click. OUT ≈ +12 V (the supply voltage). Q1 collector ≈ +12 V. |
| C | Remove the wire | K1 clicks on again and OUT returns to ≈ 0 V. |
If any result differs, stop and use the troubleshooting table before going further.
Test with the X6100 (dummy load, minimum power)
- Leave the amplifier disconnected and power the board from the bench supply at +12 V. Plug the ACC cable into the radio with the radio off, and note OUT stays ≈ 0 V.
- Connect the dummy load to the X6100 ANT socket, switch the radio on and set TX power to minimum.
- OUT should still read ≈ 0 V in receive.
- Press and hold the radio’s PTT: K1 releases and OUT rises to ≈ +12 V. Release: OUT returns to ≈ 0 V.
Install it in the amplifier
- With all power disconnected, mount the board on stand-offs, away from the RF stages and any heat sinks. An insulating plate or heat-shrink underneath prevents shorts to the chassis.
- Take +12 V from the point you chose in step 1, through the 500 mA fuse (F1) if fitted, and connect the board ground to the amplifier’s ground.
- Connect OUT to the amplifier’s PTT input in place of its existing PTT connector or wiring, or alongside it if you want both inputs.
- Route the ACC cable away from RF wiring and through a grommet or connector, with strain relief inside the case.
- Double-check polarity and the net table before reconnecting power.
Final checks into the dummy load
- Connect the amplifier’s RF input to the X6100, and the dummy load to the amplifier’s output. Set the X6100 power within the amplifier’s drive rating.
- Power up. With the radio off, the amplifier should stay in receive.
- Press PTT: the amplifier should switch to transmit (listen for its relays and check any TX indicator), and return to receive when you release it.
- Make a short low-power transmission and confirm output power and SWR look sensible.
- Only then connect your antenna and operate normally.
Start with a brief CW or SSB transmission at low power. Listen for any relay chatter, hum or instability in the amplifier, and confirm there is no RF-induced keying (the amplifier staying in transmit after you release PTT).
Troubleshooting
K1 never clicks when +12 V is applied
- Check Q1’s pin order against its datasheet. A mis-wired transistor is the most common cause.
- Confirm R2 is present and connected to both +12 V and the base.
- Check D1’s polarity. If it is reversed it conducts when power is applied, the relay will not operate and parts may get warm.
K1 stays energised when PTT is pressed
- Re-measure the X6100 contact in step 2. You may have picked the wrong contact, or the ground is not common with the board.
- Check that R1 is soldered between the PTT contact and the base.
- Measure the base voltage while holding PTT. It should fall below about 0.4 V.
OUT is +12 V all the time
- K1’s NC and NO connections may be swapped. Check the relay datasheet.
- The coil may not be energising. Repeat test A and check Q1 and the coil connections.
The amplifier keys when the radio is switched off
- Measure the PTT line with the radio off. If it sits low, the contact assignment is probably wrong (for example a ground or band line was chosen instead of PTT).
The amplifier doesn’t switch to transmit
- Measure OUT while PTT is held. If it reaches +12 V, the fault is in the amplifier-side wiring or its PTT requirements.
- If OUT stays low, check that K1’s COM is at +12 V and that the fuse (if fitted) is intact.
Relay chatters, buzzes or drops out when transmitting
- The 12 V supply may be sagging during transmit. Add a 100 µF electrolytic across +12 V and ground close to the board and check the supply wiring.
- RF may be getting onto the PTT line. Fit C1 if you have not, shorten the leads, use screened cable, add a ferrite and route wiring away from RF circuits.
Notes and limitations
- Switching speed. A relay takes a few milliseconds to operate and release. Most amplifiers have their own relays that are no faster, but if you use fast QSK CW or digital modes, check the timing and start with low drive.
- Standby current. In receive the relay coil is energised, so the interface draws roughly 30–40 mA from the amplifier’s 12 V supply.
- Solid-state alternative. A PNP high-side switch (such as a 2N2907A) is faster and silent, but it depends on how the radio’s PTT line behaves when idle. This relay design works whichever way the line idles.
- Ready-made alternative. For Xiegu’s own XPA125B amplifier, Xiegu sells the L4001 control cable, which carries PTT, ALC and band data between the X6100 and that amplifier.
References
- Xiegu X6100 User Manual (ACC port, TX power, Appendix 1: XPA125B connection)
- WiMo – Xiegu control cable L4001 (XPA125B ↔ X6100 & X6200)