⚙️ FOSTEX G24S
24‑track · 1/2" tape · analog multitrack · Dolby S
Field‑collected data from technician forums and service manual excerpts. The G24S is mechanically complex; many issues stem from aging electrolytics and delicate transport setup [citation:2][citation:3][citation:4].
Common complaint & field‑issue analysis
🔹 Intermittent / no sound on channels
Sound fades in with crackle after warm‑up, meters stay dark. Fault migrates with channel card. Caused by leaky SMD electrolytics, cracked solder joints at edge connectors, or failing Dolby S chips [citation:3][citation:4][citation:6].
🔸 Tape transport / lifter problems
Tape lifter (lift arm) does not stay out, settles back — may be related to tension out of spec. Pinch roller pressure critical: 2.5–3.0 kg (5.5–6.6 lbs) measured with spring scale [citation:1][citation:2].
🧪 Dolby S chip failure (unobtanium)
Dolby S versions (cards PN 8251897) use Sony CXA14150/14160 chips that are prone to failure. No replacement source; cards can be used with Dolby off, but S/N degrades. Earlier Dolby C cards are more reliable [citation:3][citation:8].
🧪 864 capacitors ‼️
A full recap of a G24S means 864 electrolytics (24 cards × 36 caps). Different card revisions exist; boards PN 8251897 use 36 caps per card including many 10µF/16V, 1µF/50V, and sub‑µF values marked with R (e.g. R47 = 0.47µF) [citation:4].
Mechanical & transport system
PINCH ROLLER PRESSURE
service manual adjustment
Spec: 2.5 – 3.0 kg (5.5–6.6 lbs) using spring scale hooked to pinch roller shaft in Play mode, pulling opposite capstan until roller stops [citation:1]. Adjust by moving bracket, then set solenoid gap with no gap X between rubber and solenoid core [citation:1].
WOW & FLUTTER / FG ADJUST If wow/flutter exceeds ±0.05% peak WTD, the FG magnet/print coil may be off‑center. Loosen PCB screws, shift while monitoring wow/flutter meter for minimum [citation:10].
Calibration & alignment data
🎚️ Reference level:+4 dBu @ 0 VU (typical, align per MRL tape)
📼 Calibration tape:Fostex or MRL equivalent (1/2\", 15 ips, appropriate eq). Use for repro level and azimuth.
🎛️ Record calibration:Test tones via RCA, daisy‑chain first 8 channels to record all tracks simultaneously; adjust REC CAL, METER CAL [citation:3].
🧲 Head maintenance:Cleaning (isopropyl), degaussing. Dolby S cards need careful handling.
⚙️ Ground reference:On R/P cards use TP6 or chassis; never TP5 (erase current test point) [citation:3].
Tape selection & compatibility
Original formulation: 1/2\" tape, 15 ips, suitable for +6 or +3 level depending on card revision. Many users report success with RTM LPR 35 or ATR Magnetics [citation:2][citation:3].
Bias adjust: Cards have bias trimmers; alignment after recap is essential. Old tapes with sticky‑shed (white powder) must be avoided—they clog lifters and path [citation:2].
Parts sourcing & service notes
🔧 Capacitor plague: G24S channel cards use surface‑mount electrolytics that leak/drift. Replacement with radial electrolytics or film for sub‑µF values is standard [citation:4].
📘 Service manual: Fostex G16/G24S service manual (covering transport & electronics) is essential. Two main R/P card part numbers: 8251878 (earlier, Dolby C/daughterboard) and 8251897 (later, Dolby S single chip) [citation:4][citation:8].
⚡ Capacitor types: Use low‑leakage, audio‑grade, bipolar in audio path (Panasonic ECA or similar). Sub‑µF (0.1, 0.15, 0.22, 0.47) best as WIMA film (MKP2, FKP) [citation:4]. 10µF 16V: ~288 pieces for full deck. Voltage ratings higher than original OK, but watch physical size.
🎛️ R/P card capacitor guide (PN 8251897)
Per‑card electrolytic count: 36 · G24S total: 24 cards = 864 capacitors. Below are typical values for later‑style Dolby S card [citation:4]. Use 50V or 25V as marked, bipolar recommended for audio path.
| Designator | Value | Designator | Value | Designator | Value |
|---|
| C12 | 10µF 16V | C39 | 1µF 50V | C74 | 10µF 16V |
| C13 | 10µF 16V | C43 | 0.1µF 50V | C75 | 10µF 16V |
| C16 | 10µF 16V | C46 | 0.1µF 50V | C77 | 10µF 16V |
| C17 | 10µF 16V | C48 | 10µF 16V | C80 | 0.15µF 50V |
| C18 | 10µF 16V | C52 | 0.1µF 50V | C81 | 10µF 16V (yellow) |
| C21 | 10µF 16V | C56 | 1µF 50V | C83 | 10µF 16V |
| C24 | 4.7µF 25V | C68 | 0.47µF 50V | C84 | 10µF 16V |
| C29 | 0.22µF 50V | C72 | 10µF 16V | C86 | 10µF 16V |
| C33 | 0.22µF 50V | C73 | 10µF 16V | C87 | 0.47µF 50V |
List abridged; full per‑card includes 36 positions. C81 is a pre‑installed radial; often replaced [citation:4]. Always verify against your own card.
Punch in/out & cueing (from manual)
🎛️ Method 1 (buttons):Hold PLAY + press RECORD for punch‑in; PLAY + STOP for punch‑out [citation:5].
🎛️ Method 2 (SAFE/RDY):Set all SAFE, press PLAY+REC (READY LED). At punch point toggle channel SAFE/RDY [citation:5].
🎛️ Method 3 (footswitch):Fostex 8051 into rear jack; step toggles punch [citation:5].
🔍 Cueing:Press EDIT, rotate CUEING dial CW to move tape toward take‑up reel [citation:5].
Safety & service notes
⚠️ high voltages, delicate head wiring
Power supply rails must be checked after recap. Do not operate with cards removed — cooling and ground paths affected. Dolby S chips are proprietary and unavailable; if dead, card becomes non‑Dolby [citation:8].
🔒 Post‑service: check pinch roller pressure, back tension, and all tape guides. Calibrate using MRL tape for reliable response.
Key specifications
📻 Tracks:24 tracks on 1/2\" tape
🎛️ Speed:15 ips (38 cm/sec)
📏 Reel size:10.5\" or 12\" (NAB) [citation:9 context]
🔌 I/O:RCA line in/out, punch jack, rear controller switch [citation:5]
🎤 Noise reduction:Dolby S (later cards) or Dolby C (early)
📐 Dimensions:approx 19\" rack mount, deep chassis
* Capacitor quantities approximate; always verify card revision. Donor machines are best source for mechanics and Dolby S spares.
