Tempo One / Yaesu FT‑200: Chasing Down “No RF Output”

This started as a bench question from Howard, working on a Tempo One that powers up, receives, and passes all voltage checks — but makes no measurable RF. His symptoms are textbook for one small, notorious part, so it’s worth writing up the full method. If you own any of these rigs, bookmark it: the fault mode below is common enough that you will meet it eventually.

⚠ High-Voltage Warning — Read Before You Reach Inside

The FT-200 family runs a hybrid chassis fed from an external power supply that develops lethal voltages. These are not “transistor-radio” potentials. Treat every session as if the set is live.

• The PA plate line is nominally ~600 V and can read 800–900 V unloaded. This is instantly lethal and will arc across a screwdriver. The 6JS6 plate top-caps sit at full plate voltage — do not brush them.
• There is also a ~150 V HT rail, plus screen-grid and bias voltages, and raw AC line voltage at the mains entry and power connector.
• The filter capacitors store a lethal charge after power-off. A supply switched off ten minutes ago can still bite. Always discharge the HV filter caps with an insulated bleeder (a ~10–100 kΩ / few-watt resistor on an insulated stick, then confirm with a meter) before you touch anything. Never assume the bleeder resistor did its job — verify 0 V with a meter.
• The HV lives in the separate power supply and travels down the interconnect cable. Never probe a live power plug or a disconnected supply-side connector — its pins can be hot. Power down and discharge before mating or unmating the cable.
• Work one hand behind your back where practical, wear insulated footwear, keep the other hand out of the chassis, and never work alone on a live HV set if you can help it.
• Remove rings and watch bands. Clip your meter leads with the supply dead, then power up to read — don’t hand-hold probes into a 900 V node.

A plate/cathode current that refuses to dip means no RF is being developed in the plate tank. In a pi-network final, the current only dips at resonance because RF is being generated and coupled into the tank. A flat, unresponsive IC is DC bias current with nothing riding on it.

So the real question is never “why won’t it tune?” It is “why is there no RF between the driver and the antenna?” — and that is the same question as “why is there no output?” They are one fault, not two.

1. C55 — the driver-to-final grid coupling capacitor (the notorious one)

C55 is the 100 pF / 1 kV mica that couples the 12BY7 (V5) driver plate to the 6JS6 (V6/V7) final grids. It is a well-documented frequent failure on this family: when it opens, drive stops dead at the finals — flat IC, no output — and when it shorts it can take out the finals, the power transformer, and the chokes. Worse, some production runs fitted a C55 rated at only 500 V, sitting in a circuit that swings far higher. If you don’t know the history of the set, treat C55 as a prime suspect and a proactive replacement: fit a 1 kV (or better) mica / silver-mica.

2. Plate tank path

If drive is present at the final grids but IC still won’t dip, the RF has nowhere to go. Check the plate RF choke (a classic open-circuit failure), the band-switch wafers (dirty or burned contacts — very common), the plate coil, and the plate blocking / output coupling capacitors.

3. Final bias & screen

A 250 mA idle is high — suspect insufficient negative grid bias letting the tubes run hot. Conversely, a missing screen voltage would give low, sluggish plate current. Verify both are present and sane before condemning anything downstream.

4. Tubes & neutralization (secondary)

A 12BY7 at ~5000–5200 µmho against a “good” card figure of ~5700 is roughly 90% — soft, not dead. It makes less drive, never zero, so it is not the no-output cause; leave it for now. Same logic for the finals: worth a match-check, but a weak tube reduces output, it doesn’t abolish it. Note that modern (non-NEC/Toshiba) 6JS6C tubes can require a neutralization tweak — but that shows up as instability on the higher bands, not a dead carrier on every band.

Use an RF probe or a scope with a suitable HV-rated probe, keying briefly into the dummy load. This is the step that pins the fault to one side of C55.

Key insight: with no output, the finals aren’t drawing transmit current, so a 900 V reading is essentially a light-load reading. Fix the no-output fault first — the HV may well settle toward 600 V on its own once the finals load the supply properly. Don’t chase the tap while the real fault is starving the plate circuit.

Order of work. Make it safe → confirm bias & screen → follow the RF to find which side of C55 it dies on → repair that stage → then re-address HV under load → finally re-tune and, if needed, re-neutralize. Resist the urge to start by changing the power-supply tap; that’s Stage E, not Stage A.

Disclaimer — Please Read

This article is provided for general educational and informational purposes only, as an account of one hobbyist’s troubleshooting approach. It is offered “as is,” without warranty of any kind, express or implied, including but not limited to accuracy, completeness, fitness for a particular purpose, or suitability for any specific radio, component, or situation. Vintage equipment varies between examples, production runs, and prior modifications; procedures that apply to one set may not apply to yours.

Servicing this equipment is dangerous. These transceivers and their power supplies contain lethal high voltages that can cause serious injury or death, and stored charge remains present after power-off. Working on high-voltage electronics, soldering, and using test equipment carry inherent risks of electric shock, fire, burns, and property damage. You proceed entirely at your own risk.

Nothing here creates a professional, advisory, or service relationship, and no engineering, safety, or repair services are being offered or rendered. This is not a substitute for the manufacturer’s service documentation or the judgment of a qualified technician. If you are not fully competent and equipped to work safely around lethal voltages, do not attempt this work — engage a suitably qualified professional.

To the fullest extent permitted by law, the author disclaims all liability and shall not be responsible for any injury, death, loss, or damage — direct, indirect, incidental, or consequential — arising from the use of, reliance on, or inability to use any information in this article. By continuing, you accept these terms and agree that you alone are responsible for your own safety and for any outcome of the work you undertake.