You hit the mic, the radio is putting out power, but the amp just sits there — no relay click, no boost, dead. A CB amplifier not keying up is one of the most common complaints that lands on the bench, and the good news is that it almost always comes down to a short list of suspects. The trick is to work it like a decision tree instead of shotgunning parts. Grab a multimeter, a 50-ohm dummy load, and about twenty minutes, and you can usually isolate this to one cause.

Before you open anything up, understand that "keying up" is really two jobs happening at once: the amp has to sense that you are transmitting, and then it has to switch its transmit/receive (T/R) path over to the amplified side. If either half fails, you get the exact same dead symptom. So the first thing to figure out is which half is broken.

HOW T/R SWITCHING ACTUALLY WORKS

Most mobile CB and 10-meter linears key one of two ways, and knowing which one you own decides how you troubleshoot.

RF-sense (VOX) switching is the common one. A little of your transmit RF is tapped off, rectified into a small DC voltage, and that voltage trips a transistor or comparator that energizes the T/R relay. No control wire runs back to the radio — the amp simply "hears" RF on the input and flips over. The catch: it needs a minimum amount of drive to trip. Too little RF at the input and the sense circuit never wakes up.

Hardwired PTT switching uses a control line — a keying wire from the radio's PTT jack or an external foot switch — to ground the relay coil directly. It is more reliable because it does not depend on drive level, but now you have a wire and a connector that can fail on you instead.

Identify your box first. If there is a keying jack or a wire running from the amp back to the radio, you likely have PTT switching. If the only connections are coax in and coax out, it is almost certainly RF-sense. That single observation tells you which branch of the tree to walk.

FIRST, RULE OUT THE EASY STUFF

Do not chase a phantom relay fault when the real problem is a cold solder joint on a PL-259. Confirm these three things before you go any deeper:

  1. Power at the amp under load. Measure voltage right at the amp's power terminals during key-down, not at the battery. A solid-state linear can pull serious current, and a corroded ring terminal or undersized wire will sag the voltage the moment you transmit. Measure it live.
  2. The radio is actually transmitting. Put the radio into a dummy load on its own and confirm forward power on a wattmeter. If the radio is down, the amp was never going to key.
  3. Coax and connectors. A bad PL-259 center pin — not soldered, pushed back, or shorted to the shield — kills drive before it ever reaches the sense circuit. Wiggle-test both jumpers and inspect the solder cups.
Safety first, every time. Tube linears run hundreds to thousands of volts on the plate, and those filter capacitors stay charged after power-off. Solid-state amps are lower voltage but can dump huge current into a slipped probe. Power down, unplug, and bleed the caps before your hands go inside. RF burns from a live output terminal are no joke either — never key into an open antenna jack.

THE DECISION TREE: WHY YOUR CB AMPLIFIER IS NOT KEYING UP

Now work the suspects in order. Each step either clears a cause or points straight at it.

1. Not Enough Drive to Trip the Sense Circuit

This is the number-one reason an RF-sense amp will not key. The sense circuit needs a threshold of input power before it trips — commonly on the order of a watt or two as a design target, though the real figure depends entirely on the build, so use your build's spec. If the radio is barely making carrier, or you are running low power or a dead-key only, the amp may never see enough RF to flip over.

Test it: with the radio in a dummy load, key down and read forward power, then hold a steady tone and watch for the relay. If it keys on voice peaks but not on a bare carrier, your drive is sitting right at the threshold. Bring the radio's carrier up within its own spec, or hunt for loss between the radio and the amp input.

2. Dead or Sticking Relay

The T/R relay is a mechanical part and it wears out. Listen and feel for the click when you key. No click means either the coil is not being energized, or the relay itself is dead. With power off and caps bled, you can often check the relay coil for continuity with your meter — an open coil is a dead relay. If the coil reads fine but never energizes, the fault is upstream in the sense or keying circuit that drives it, not the relay. A relay that clicks but passes no RF (you hear it switch but get zero boost) has burned or pitted contacts.

3. Bias Turned Off or Set Wrong

Some linears gate the bias through the T/R circuit — the finals only get their idle bias once the amp keys. If the bias supply is dead or the pot is backed all the way off, the relay can switch while the transistors never conduct, so you get switching with no output, which reads as "not working." Confirm the bias voltage appears at the finals on key-down, measured against your build's documented value. Never guess a bias figure — set it to the number your build sheet calls for and nothing else.

4. Blown Final Transistor

A shorted or open final kills output and can also load down the switching supply enough to prevent clean keying. With the amp unpowered, a diode-check across the final's junctions — compared against a known-good reference or the other device in a push-pull pair — will flag a dead part. A common CB/ham RF power transistor like the Toshiba 2SC2879 is rated around 100W per device; treat that as a datasheet ceiling, not a target, and always confirm the actual part number in your build against its own datasheet.

5. Bad Coax or PL-259 (Again)

Yes, again — because this one hides. Intermittent center-pin contact can pass enough RF to sometimes key and sometimes not, which sends people chasing the relay for hours. If the symptom is flaky rather than dead-solid, re-terminate the connectors before you condemn anything expensive on the board.

Use a dummy load, not the antenna. Every one of these tests should be done into a 50-ohm dummy load. It protects the finals, keeps you off the air and legal while you troubleshoot, and gives you a stable load so your readings actually mean something. Our free calculators can help you sanity-check power and impedance figures while you work.

ISOLATING IT WITH A METER AND A DUMMY LOAD

The whole method comes down to splitting the signal chain in half and asking "does the fault live upstream or downstream of here?" Order of attack:

  1. Radio into the dummy load alone — confirm it makes rated power. If not, the amp is innocent and the radio is your problem.
  2. Radio into the amp input, amp output into the dummy load. Key down and listen for the relay.
  3. Relay clicks but no boost? Suspect bias, a blown final, or burned contacts. That is a downstream problem.
  4. No relay click at all? Suspect drive level on an RF-sense amp, or the keying wire and jack on a PTT amp, then the sense circuit itself. That is an upstream problem.
  5. Meter the power terminals live, meter the relay coil for continuity cold, and diode-check the finals cold.

Nine times out of ten that sequence lands you on the cause without ever pulling the board.

Quick Sanity Checklist

  • ✓ Voltage measured live at the amp terminals under key-down, not at rest
  • ✓ Radio confirmed making rated power into a dummy load on its own
  • ✓ Both PL-259s inspected — center pin soldered, no shield short
  • ✓ Drive level above the amp's RF-sense threshold (RF-sense amps)
  • ✓ Keying wire and jack continuity confirmed (PTT amps)
  • ✓ Relay coil continuity good and the relay audibly clicking on key
  • ✓ Bias present at the finals per your build's documented value
  • ✓ Finals diode-checked against a known-good reference
  • ✓ All testing done into a 50-ohm dummy load, caps bled first

A CB amplifier not keying up feels like a big problem, but it is really a five-suspect line-up you can clear in an afternoon. Sense circuit, relay, bias, finals, and the humble coax connector — check them in order, into a dummy load, with a meter in hand, and you will find it. If you want the full schematics with step-by-step bias and T/R procedures for real builds, that is exactly what our Linear Amp Build volumes lay out.

Build it right — with the full documentation

When bench guessing runs out, real documentation ends the hunt. Our Linear Amp Build volumes — full troubleshooting trees hand you the schematics, T/R switching diagrams, bias procedures, and step-by-step fault-finding for real builds, so the next dead key gets solved in minutes instead of an afternoon.

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Bias cheatsheet, transistor substitution tables, and the 10 mistakes that kill amplifiers. Free — no spam.

Educational reference only. Always follow your specific device datasheet and build documentation, and verify all values independently. Working on high-voltage and RF equipment carries real risk — if you're unsure, get help from an experienced builder.

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