Yaesu FT-902DM / FT-902D Series
Failure Prevention Kit — Component & Modification Design

Section 1 — How the FT-902 Differs from the FT-901

FT-902 Variants

Community Resources

Section 2 — Root Cause Failure Analysis

The FT-902 shares the FT-901’s power supply failure mechanism as the single most dangerous risk, but introduces several failure modes specific to its design. Failures unique to or significantly different in the FT-902 compared to the FT-901 are noted.

• 1
  HV Power Supply Electrolytics — Explosive Short, Transformer Risk Identical in mechanism to the FT-901: the HV electrolytic capacitors in the three rectifier boards (A, B, C) age and can short-circuit, potentially destroying the rectifier board, transformer, and contaminating adjacent boards with corrosive electrolyte. M1ABK specifically documents finding a capacitor with a “very slight bulge on its base” on the FT-902DM rectifier board during careful pre-power inspection — a pre-failure indicator that was only discovered because the Variac protocol was followed and the covers were removed for inspection. The FT-902 service manual also documents a specific Rectifier C board modification (replacing five resistors and adding a diode in series) that provides additional protection against capacitor stress — verify whether this modification has been made. Replace all HV and bias electrolytics on the three rectifier boards as the mandatory first action before any power-up of an unknown FT-902.
• 2
  PA Ceramic Capacitor Power Fade — 1 kV 1000 pF Red Lozenge Type This is an FT-902-documented failure mode: a ceramic capacitor on the Final unit board (PA board PB-1715B) causes a characteristic “quite severe power fade under all modes.” The symptom is full power output for a few seconds followed by the power fading to approximately half output — consistent across all modes (SSB, AM, CW). MW0UZO’s FT-902DM repair identifies this as “a bad capacitor in the PA, on the final valve PCB. It’s a red lozenge type 1 kV 1000 pF, the ones that seem to like dying in these old Yaesus.” The red lozenge description identifies a specific type of ceramic disc capacitor used in 1970s Yaesu PA circuits, characterised by its shape and colour. Access requires removing the PA tubes, unscrewing and desoldering two wires. This is an HV area: full discharge procedure is mandatory before working on the Final board.
• 3
  Curtis 8044 Keyer IC Failure — Continuous CW Keydown, Unobtanium Chip The FT-902DM (and FT-902DE) has the Curtis 8044 electronic keyer IC built in as standard equipment. The FT-902D has it as an option. The Curtis 8044 and its predecessor the 8043 (used in the FT-901D and early FT-902D) have both been discontinued. MW0UZO confirms: “Bad keyer chip a Curtis 8044 IC. These are unobtanium.” The failure mode is decisive: the FT-902DM transmits continuously in CW keydown from power-on, which if unrecognised and allowed to continue, rapidly exhausts the 6146B PA tubes. If your FT-902DM goes into continuous transmit immediately on power-up with no key connected: pull the keyer board (PB-1728A) immediately. The radio will operate in all modes without the keyer board; the keyer function is simply absent. DL4YHF has designed a PIC-based replacement circuit (qsl.net/dl4yhf/key_circ.html) that provides full Curtis-equivalent keyer function for this board.
• 4
  MSM9520 Counter IC — Failure-Prone Display and Frequency Counter Later production FT-901 and all FT-902 units use the Oki Semiconductor MSM9520 counter IC on the Counter unit board (PB-2086A). This IC is documented in the Antique Radio Forums as “troublesome” and “failure prone and obsolete.” Typical failure modes: no frequency display (blank or dead LED display); random gibberish on the display; counter locks up at one frequency. The good news is that a Japanese ham created a PIC-based replacement for this IC, and units of this replacement board have been sold to restorers. Verify MSM9520 health at first power-up by checking for a coherent frequency display that changes correctly as the VFO is tuned. The MSM9520 failure does not affect receiver or transmitter performance — only the frequency readout.
• 5
  Yaesu Vibration Adhesive on Capacitor Bodies — Cap Replacement Trap Uniquely documented in the FT-902DM (not the FT-901): Yaesu used a small amount of adhesive glue around each electrolytic capacitor body on the circuit boards to prevent vibration-induced failure during mobile operation. M1ABK documents this explicitly: “Yaesu put these capacitors in with a small amount of ‘glue’ around each one as the radio could be used mobile and it was an attempt to stop vibration based failure.” When replacing capacitors in the FT-902, this glue must be carefully removed before desoldering. Aggressive removal using a screwdriver or other tool risks cracking PCB traces. Use a fine PCB pick tool and PCB cleaner to dissolve and remove the residue. After removal, re-clean the board thoroughly before soldering new capacitors.
• 6
  Cracked Nylon PA Loading Capacitor Gear — Inoperable PA LOAD Control The FT-902 (and by extension some FT-901 units) uses a nylon gear in the PA Loading variable capacitor drive train. The LOAD control shaft connects to the loading capacitor through a chain and gear mechanism. N8YX’s FT-902DM restoration: “the PA Loading control wouldn’t turn its associated capacitor. Close examination of the drive train revealed a cracked nylon gear on the loading capacitor shaft. Many hybrids of the area used these; the all-nylon variety end up cracking due to age and stress.” A cracked loading gear makes it impossible to tune the PA LOAD control to a dip, preventing normal transmit operation. Reproduction gears in brass are available from NR6C (nr6c.com) who manufactured a replacement set based on the original geometry. If the LOAD control feels loose or ineffective: inspect the gear before suspecting any other cause.
• 7
  PCB Edge Connector Oxidation — 31-Board Intermittency Identical in mechanism to the FT-901: 31 plug-in boards with oxidising edge connectors. An FT-902DM owner in the Hamisland forum confirms: “The biggest annoyance was a need to occasionally reseat all of the circuit boards after a thorough cleaning of the contacts.” M1ABK notes: “I also cleaned and re-flowed the edge connectors — if you do this make sure that the re-flow is as thin as possible or you might open up the edge connector in the chassis and cause intermittent problems.” Use DeoxIT D5 for cleaning; protect with dielectric grease or a contact preservative after cleaning to slow re-oxidation.
• 8
  Double Balanced Diode Ring Mixer (PB-2154A) Schottky Diode Failure The FT-902’s RF board uses a Schottky diode double balanced ring mixer (PB-2154A) rather than the FET mixer of the early FT-901 (PB-1702). This improves IMD performance but changes the failure mechanism. Where the FT-901’s FET mixer fails with complete loss of sensitivity from static or overload damage, the FT-902’s ring mixer diodes can develop asymmetric degradation that appears as reduced sensitivity combined with elevated noise floor, or — in severe cases — complete loss of mixer conversion gain. The service manual calibrator test (S9+10 dB on 14.000 MHz with preselector peaked) applies equally to the FT-902. If S-meter reads low after this test and the RF amplifier FET is healthy: suspect the ring mixer diodes. Also check J02 and J03 connector orientation if any board has been removed and reinstalled — the positions are reversed between PB-1702 and PB-2154A.
• 9
  Mixed-Orientation Capacitor Trap on Rectifier B Board — Restoration Risk M1ABK’s detailed FT-902DM restoration photographs document a specific hazard unique to the cap replacement work: on the Rectifier B board, the electrolytic capacitors are not all in the same orientation. M1ABK warns explicitly: “TAKE CARE with the insertion of the vertical capacitors! This board is a little different — the capacitors are not all in the same orientation — check and re-check — as if you get this wrong it will not be pleasant.” Additionally: “The board has the polarity of the capacitors marked with + and − but you can’t see them when you put the capacitors in.” The consequence of incorrect polarity on an electrolytic in a HV supply is explosive failure. Photograph every capacitor orientation on the Rectifier B board before removal. Verify each replacement position against the photographs and the board markings before soldering.
• 10
  6146B PA Tube Failure and AUX Connector Link — No Transmit Like the FT-901, the 6146B PA tubes are a consumable that must be periodically evaluated. However the FT-902 has an additional no-transmit cause specific to the design: the rear AUX connector must have pins 1 and 2 linked together for the PA heater supply to function. M1ABK documents this: “the rear AUX plug — remember with this or the right pins (1 and 2) being linked together the heaters won’t work on the PA so no output.” An FT-902DM that arrives without its original cable assembly may not have this link in place. If the FT-902 receives normally but has no transmit output at all, and the 6146B filaments are not glowing: check the AUX connector link before any other diagnosis. After confirming the AUX link: check 6146B tube emission, PA bias voltage, and PA coupling capacitor as the remaining no-transmit causes.

Section 3 — Kit Component Reference

Section 4 — Pre-Operational Safety Protocol

FT-902-Specific Pre-Power Checklist

• AC voltage tap: for the FT-902DM, voltage selection is inside the case connected to the power transformer primary. Verify the setting matches local supply before connecting mains.
• AUX connector pins 1 and 2: check the rear AUX connector for the heater link before first power-up.
• Keyer board: note whether a keyer board (PB-1728A) is fitted. On first power-up, be ready to power down immediately if the transceiver goes into continuous CW keydown.
• HV capacitors: remove all boards and inspect Rectifier boards A, B, and C for capacitors with bulged bases, discolouration, or any sign of electrolyte weeping. M1ABK found a bulged capacitor in a radio that passed Variac power-up — visual inspection with boards removed is the only way to catch pre-failure indicators.
• PA Loading control: test the PA LOAD control before applying transmit power. It should rotate smoothly and the loading capacitor should visibly move. A loose or non-functional LOAD control with no resistance to the capacitor: inspect the nylon gear before transmitting.
• Rectifier C board modification: inspect the resistors on the Rectifier C board against service manual values to determine if the documented modification has been implemented.

Section 5 — Circuit Modifications

  FT-902 RECTIFIER B BOARD — MIXED CAPACITOR ORIENTATION WARNING
  (M1ABK FT-902DM Restoration, photographically documented)

  ⚠ NOT ALL CAPACITORS FACE THE SAME DIRECTION ON THIS BOARD ⚠

  Standard practice: all caps oriented with – stripe facing same direction.
  Rectifier B: orientations VARY from position to position.
  Board polarity markings: present BUT HIDDEN under installed capacitors.

  PROTOCOL before cap removal:
  1. Remove Rectifier B board from chassis
  2. Photograph EVERY capacitor from BOTH SIDES before touching anything
  3. Mark a template showing + / - orientation for each position
  4. Remove caps one at a time; verify each replacement position
     against your template AND against the board marking underneath
  5. Re-verify before soldering — an incorrectly-installed HV cap
     will fail immediately on power-up

  Also: Remove Yaesu vibration glue before attempting to desolder.
  DO NOT lever caps out before glue removal — risk of PCB pad damage.

Figure 1. Rectifier B mixed-orientation capacitor protocol — FT-902DM specific.

Section 6 — Installation Sequence

• 1
  Documentation and FT-902-specific pre-checks Download FT-902DM service manual (foxtango.org library) and Survival Guide. Identify exact variant. Verify AC voltage tap inside case. Verify AUX connector heater link (pins 1 and 2). Note keyer board presence.
• 2
  Power transformer test and all-board visual inspection Remove all boards. Ohm all transformer windings to chassis ground (any short = transformer failure). Visually inspect every board for bulged capacitors, discoloured components, or cracked PCB traces.
• 3
  Rectifier board A, B, C cap replacement and Rectifier C modification (K-001, K-002, MOD-1) Photograph Rectifier B capacitor orientations before removal. Remove Yaesu glue carefully. Replace all HV and bias caps. Implement or verify Rectifier C board modification.
• 4
  PA power-fade capacitor replacement (K-006) Replace the 1 kV 1000 pF red lozenge ceramic on the Final board. Requires PA tube removal and wire desoldering. HV discharge mandatory. This prevents the power-fade failure that is common in aged FT-902DM units.
• 5
  Edge connector cleaning, Yaesu glue removal on all boards (K-003, K-007) Remove all boards; clean edge connectors with DeoxIT D5. Remove Yaesu vibration glue before desoldering any capacitors. Complete all electrolytic and tantalum cap replacement. Test each board before reinstalling.
• 6
  PA loading gear inspection and PA screen grid protection (K-009, MOD-3) Inspect the nylon PA loading gear for cracks. If cracked: order reproduction from NR6C before proceeding with transmit testing. Implement screen grid varistor protection modification on Final board.
• 7
  First dim-bulb + Variac power-up and HV/supply verification Use dim-bulb tester in series with Variac. Raise mains from 0 to full over 20–30 minutes. Verify HV reaches specification. Verify all LV rails. No fuse blow; no sustained bright-bulb condition.
• 8
  Keyer board test and MSM9520 counter check (K-005, K-008) Power up and immediately monitor for CW keydown. If present: pull keyer board. Verify MSM9520 frequency display is coherent on all bands including WARC bands.
• 9
  Calibrator test, 6146B check, pots/switches cleaning (K-010) Run S-meter calibrator test at 14.000 MHz (target S9+10 dB). Inspect 6146B PA tubes. Clean all pots, switches, and controls. Verify PLL lock on all bands.
• 10
  PA neutralisation and DL4YHF keyer replacement if needed (MOD-2, MOD-3) Neutralise PA with non-metallic tools. Set bias to specification. If keyer replacement desired: build and install DL4YHF PIC circuit.
• 11
  Full alignment including WARC bands (MOD-4) Complete alignment per service manual. Verify WARC band crystal trimmer alignment on 30, 17, and 12 m. Verify MSM9520 display accuracy. Record performance baseline.

Section 7 — Verification Tests

AUX Heater Link and PA Heater

Keyer Board CW Keydown Check

Calibrator Receive Sensitivity

PA Power Output Stability