Yaesu FL-101 Transmitter
Failure Prevention Kit — Component & Modification Design

Section 1 — Technical Background and Community Resources

Circuit Architecture

The FL-101 is a hybrid transmitter: solid-state driver chain feeding a two-tube PA. The driver section uses plug-in modular boards (shared with the FT-101E) for the balanced modulator, SSB filter, IF board, RF driver, and ALC/processor. The PA section uses a matched pair of 6JS6C sweep-tube pentodes driven by a 12BY7A pre-driver, outputting 130 W PEP SSB / 90 W CW / 30 W AM into a pi-network output circuit. The pi-network transforms the approximately 5500 Ω plate impedance of the 6JS6C pair to 50 Ω. High voltage is approximately 600 V DC at the PA plates (supplied via a voltage doubler from the power transformer secondary, filtered by C77 and C78).

The FL-101 has its own VFO (temperature-compensated LC, shared with an FR-101 in transceive mode via the FL-101/FR-101 cable), built-in RF changeover relay (RL-2), and a MUTE output for muting the companion FR-101 during transmit. In standalone operation (without an FR-101), the FL-101 uses its internal VFO. In transceive operation, the FR-101’s VFO is used and both radios track the same frequency.

Plug-in Board Complement

Community Resources

Section 2 — Root Cause Failure Analysis

The following ten failure modes account for the overwhelming majority of FL-101 restoration casualties. All involve the transmitter section. Read the HV safety warning at the top of this document before working on any failure mode that involves the PA compartment.

• 1
  HV Filter Capacitors C77/C78 — Risk of Power Transformer Damage The high-voltage supply to the PA plate circuit uses a voltage-doubler arrangement, filtered by two large electrolytic capacitors C77 and C78 (typically 150 µF / 450 V each, in series for 600 V total). After 50 years, these capacitors have degraded. A failed HV filter capacitor allows excessive ripple on the 600 V supply, degrading transmitted audio quality with power-supply modulation. In the worst case — a catastrophic short-circuit failure — the rectifier diodes and the power transformer primary can be damaged or destroyed. Replacing C77 and C78 is a mandatory pre-operational task on any FL-101 of unknown service history. The replacement procedure requires removing the regulator and noise-blanker boards, inverting the chassis, and working with care near the HV wiring. The replacements must be correctly polarised; incorrect polarity will cause immediate explosive failure. Use modern 105°C high-ripple-rated electrolytics at values equal to or greater than the original specifications.
• 2
  6JS6C PA Tubes Exhausted or Mis-Matched — Low Power or No Transmit The FL-101 PA uses a matched pair of 6JS6C sweep-tube pentodes. These tubes have a finite service life and are consumed by the combination of high plate dissipation (30 W each, maximum) and the AC filament heating cycle. Exhausted 6JS6C tubes are the most common cause of low output power, inability to tune the PA to a proper dip, and complete transmit failure (no plate current reading). The tubes were originally manufactured by NEC or Toshiba (identified by a green stripe around the base). NEC/Toshiba 6JS6C tubes have slightly different characteristics from US-manufactured equivalent types (GE, RCA, Sylvania). When fitting non-Japanese replacement 6JS6C tubes, the neutralisation capacitor C125 must be changed from 100 pF to 10 pF. Failing to do this with non-Japanese tubes causes incorrect neutralisation, premature tube failure, and potential oscillation issues. Always use tubes in matched pairs; never mix types between sockets. Budget for neutralisation after any tube replacement.
• 3
  RF Changeover Relay RL-2 — Contaminated Contacts, Failed Transmit or Receive Path The FL-101 uses a sealed RF changeover relay (RL-2) to route the antenna between the transmit and receive paths. After 50 years of use and storage, the relay contacts develop an oxide film that increases contact resistance. A contaminated relay produces inconsistent or failed T/R switching: the transmit path may have high insertion loss (low output power), the receive path may show reduced sensitivity when the FL-101 is used in transceive mode with an FR-101, or the relay may not switch at all. The relay must be disassembled and the contact surfaces inspected and cleaned. The relay contacts are silver-plated and should be wiped — not abraded — with a clean contact-treatment tool. NW2M’s professional service specifically includes relay disassembly and contact inspection as a standard step. Original relays were available NOS from Yaesu but at high cost and limited supply.
• 4
  C131 and C13 Coupling Capacitors — Catastrophic Failure Risk C131 and C13 are coupling capacitors in the driver-to-PA grid circuit path. These capacitors are subjected to the combination of RF voltage swing and proximity to the PA tube heaters, which raises their operating temperature significantly above ambient. At elevated temperature over many years, the capacitor dielectric degrades and can suffer a sudden short-circuit failure. A short in C131 or C13 applies plate or grid supply voltage directly across the driver stage in a configuration not designed for it, causing immediate and usually catastrophic circuit damage. The K3JLS conversion guide specifically identifies these as “known to fail with sometimes catastrophic results” and recommends replacement with 1 kV silver mica capacitors at the correct values. Do not substitute ceramic disc or polyester film types in these positions — the heat environment in the PA compartment will degrade them and cause premature failure.
• 5
  R30 Screen Resistor Wrong Value — PA Tube Flashover Risk The screen grid supply resistor R30 (47 kΩ / 1 W in the original design) provides decoupling for the PA tube screen grids. This value is documented as a design weakness: when screen grid voltage is rapidly removed (at TX-to-RX switching), a transient overvoltage can appear across the screen grid, causing the tube to flash over internally. The correction is to replace R30 with a 470 kΩ / ½ W resistor. This higher value prevents the screen transient flashover phenomenon by limiting the discharge current path. The K3JLS conversion guide describes this as “VITAL.” This modification is applicable to any FL-101 that has not already had it performed. Check the value of R30 before any transmit operation: an original 47 kΩ unit is both a potential tube killer and evidence that other preventive work has not been done.
• 6
  Pi-Network Coil Burned Connections — Low Power on Specific Bands The pi-network tank coil connections underneath the coil assembly can develop burned or oxidised solder joints from heat stress over years of transmission. A bad solder joint in the pi-network produces reduced power on the band(s) using that coil section: often seen as a single band or two adjacent bands with substantially lower power output than other bands, while everything else appears normal. Inspect all pi-network coil connections underneath the coil assembly for discolouration, oxidation, or visible heat damage. Re-solder any suspect joints with the PA completely discharged and powered off. The connections operate at RF (up to 130 W) and must be low-resistance mechanical connections with adequate solder fill.
• 7
  Plug-in Board Edge Connector Corrosion — Intermittent Driver and ALC Faults All driver-chain, ALC, and audio boards connect to the FL-101’s chassis motherboard via gold-plated edge connectors — the same modular architecture as the FR-101. After 50 years, these connectors corrode and produce high-resistance or intermittent connections in the driver chain. The consequence for a transmitter (unlike a receiver) is more immediately obvious: intermittent audio, erratic ALC metering, or bands that transmit at very low power despite correct tuning all point to driver board edge connector problems. Remove all plug-in boards. Clean all edge connectors and motherboard socket contacts with DeoxIT D5. Re-seat each board with uniform firm pressure and verify all hold-down hardware is present.
• 8
  Electrolytic Capacitors — Driver Chain, Regulator Board, and Audio Sections Beyond the HV filter capacitors (Failure Mode 1), the FL-101 contains electrolytic capacitors throughout the driver chain, regulator board, audio amplifier, and bias supply. The regulator board electrolytics are the most critical: they filter the regulated rails that feed the driver stages, balanced modulator, and VFO. A noisy regulated supply on the driver chain affects SSB carrier balance and audio quality. The mylar (polyester film) capacitors used in the FL-101 are also known to split open and fail with age — documented in multiple FT-101E restorations. Inspect all visible capacitors for swelling, split cases, or electrolyte leakage. The regulator board electrolytics should be replaced as part of every complete restoration.
• 9
  Band Switch and Driver Trimmer Contamination — Weak Output by Band The FL-101’s band switch is a multi-wafer rotary switch with 13 sections (wafers A through M) routing DC voltages, crystals, and RF circuits for each band. The switch wafers are the same design as the FT-101’s band selector, which is described as the most complex mechanical component in the radio. After 50 years, the switch contacts develop an oxide film that increases contact resistance non-uniformly across the wafer positions. The result: reduced transmit power on specific bands while other bands operate normally, with the reduction varying as the switch is cycled. Clean all band switch wafer contacts with DeoxIT D5. After application, rotate the switch through all band positions a minimum of 20 times to work the treatment into all contact surfaces. Also clean the individual mixer and grid trimmer adjusters for each band — these are preset trimmers and should not be re-adjusted unless alignment is also performed.
• 10
  160 m Crystal Absent or Spurious Emissions — Band-Specific Transmit Issues The 160 m crystal (7.52 MHz) was an optional accessory at time of purchase and is absent in many FL-101 units. Without this crystal, the 160 m band cannot be used. If a crystal is present, Yaesu specifically documented a service note requiring the plate current to be reduced to 140 mA when operating 1820–1900 kHz to limit spurious radiation on 160 m. Additionally, the FT-101 / FL-101 series is known for spurious emission problems, particularly at 10 m where trap alignments in the main chassis must be correctly set. These traps eliminate harmonic and intermodulation products from appearing in the transmitted output. Any FL-101 that has not had a full alignment check with a spectrum analyser or a calibrated wattmeter plus low-pass filter test should be presumed to have unchecked spurious emissions.

Section 3 — Kit Component Reference

Section 4 — Pre-Operational Safety Protocol

Visual Inspection Checklist

• Check the AC voltage tap selection on the power transformer. The FL-101 supports 100/110/117/200/220/234 V AC. Setting this incorrectly will produce over-voltage or under-voltage on all supply rails including the 600 V HV.
• Inspect C77 and C78 physically (with power off and HV discharged) for swelling, electrolyte leakage, or any sign of previous thermal stress. Any sign of distress: replace before power-up.
• Inspect the 6JS6C tubes for physical damage (cracked envelopes, darkened areas around the plate structure). Verify heater pins are intact at the tube bases. Check that the top-cap plate leads are firmly connected.
• Verify R30 value with a DMM with power off. If it reads approximately 47 kΩ: the screen resistor value correction (K-003) has not been done and must be performed before transmit operation.
• Inspect C131 and C13 visually for burn marks, discolouration, or split bodies. Any visual damage: replace before transmit operation.
• Verify the pi-network coil connections are intact and undamaged.

Section 5 — Circuit Modifications

  PA TUBE NEUTRALISATION GUIDE — FL-101 / FT-101 SERIES

  CONTEXT: The 6JS6C grid-to-plate capacitance provides a feedback path
  that can cause oscillation, especially on the upper amateur bands.
  Neutralisation adds an equal and opposite path to cancel this feedback.

  C125 SELECTION:
    Original NEC/Toshiba 6JS6C tubes: C125 = 100 pF / 1kV silver mica
    Replacement non-Japanese 6JS6C:   C125 = 10 pF / 1kV silver mica
    (Non-Japanese tubes have different inter-electrode capacitance)

  R30 SCREEN RESISTOR (must be checked before any transmit):
    Original value: 47kΩ / 1W   ← REPLACE — causes tube flashover risk
    Correct value:  470kΩ / ½W  ← prevents screen transient overvoltage

  NEUTRALISATION CHECK (by band result — post-restoration target):
    160m:  plate current dip symmetrical, power 120–130W SSB ✓
    80m:   plate current dip symmetrical, power 130–150W SSB ✓
    40m:   plate current dip symmetrical, power 130–150W SSB ✓
    20m:   plate current dip symmetrical, power 130–160W SSB ✓
    15m:   plate current dip symmetrical, power 130–150W SSB ✓
    10m:   plate current dip symmetrical, power 100–120W SSB ✓
    (Power values from community-reported post-restoration results)

  ⚠ PA COMPARTMENT WARNING: 600V DC present. NON-METALLIC TOOLS ONLY.

Figure 1. PA neutralisation guide and R30/C125 specification reference.

Section 6 — Installation Sequence

• 1
  Documentation and community resources Download the FL-101 service manual from foxtango.org, ManualsLib, or lost-manuals.com. Bookmark the K3JLS 6146B conversion guide (k3jls.net) for its detailed component notes. Join or consult the Fox Tango Club archive at foxtango.org for historical modification and service notes.
• 2
  Visual inspection and AC tap verification Verify the AC voltage tap is set for your supply. Inspect C77/C78 and C131/C13 visually. Check R30 value with a DMM (power off). Inspect pi-network coil connections. Photograph the interior.
• 3
  R30 screen resistor replacement (K-003) and C131/C13 replacement (K-002) Replace R30 from 47 kΩ to 470 kΩ. Replace C131 and C13 with 1 kV silver mica types. These are safety-critical items; do them before any transmit testing.
• 4
  Replace HV filter capacitors C77/C78 (K-001, MOD-1) Follow the safe replacement procedure in Section 5. Verify HV reads zero before touching any PA compartment component. Double-check polarity before power-up. This is the highest-consequence replacement in the FL-101.
• 5
  Clean edge connectors and band switch (K-005, K-009) Remove all plug-in boards. Clean all edge connector contacts. Clean all band switch wafer contacts. Cycle the switch through all positions 20 times. Re-seat all boards.
• 6
  Clean RF changeover relay RL-2 (K-006) Disassemble and clean RL-2 contacts. Verify contact resistance below 100 mΩ. Reassemble and verify reliable switching.
• 7
  First Variac power-up and HV/rail verification Raise mains from 0 to full over 15–20 minutes. Verify HV at the rectifier board test point. Verify all regulated rails. No smoke or burning smell.
• 8
  Replace regulator board electrolytics and calibrate bias (K-007, MOD-3) Replace all regulator board electrolytics. After replacement: verify all regulated rails, then calibrate PA idle bias to 60 mA with no drive. Monitor for bias creep over 10 minutes.
• 9
  Replace or verify PA tubes (K-004) and C125 value (K-008) Verify 6JS6C tube emission with a tube tester. Determine tube origin (Japanese vs non-Japanese) and set C125 accordingly. Install tubes as a matched pair.
• 10
  Neutralise PA (MOD-2) and perform initial transmit alignment Perform cold-cathode neutralisation. Then perform transmit alignment: crystal trimmers for each band, preselector coil alignment, grid tuning and plate alignment. Record power output per band.
• 11
  Spurious emission check and trap alignment (MOD-4) Verify spectral purity on 10 m and other bands using a dummy load and spectrum analyser or calibrated harmonic measurement. Adjust traps as needed. Confirm 160 m crystal presence and observe plate current limit for 1820–1900 kHz operation.

Section 7 — Verification Tests

HV Supply and PA Idle Current

PA Neutralisation Verification

All-Band Power Output Verification

Relay and T/R Switching Verification