Pioneer CT-F750

A two-motor auto-reverse cassette deck whose logic-controlled transport and 16-pin PA4001 IC made it a workhorse of the late-'70s tape era.

Overview

The Pioneer CT-F750 is a stereo cassette deck with a high-torque DC servo motor and a separate motor for the reel drive. A logic-controlled transport handles auto-reverse, so the user never has to flip the tape. It records and plays back in both directions. Its four-channel head is a single recording/playback head plus two erasing heads, and it supports the full range of tape formulations available at the time: standard ferric, LH, ferric-chrome, chromium dioxide, and metal.

The key part is Pioneer's own PA4001, a 16-pin dual in-line IC that packs the head amplifier, line amplifier, recording amplifier, and headphone amplifier into one package. The recording and playback circuitry lives inside this tape deck system IC, which Pioneer developed in-house. The input stage of the recording amp is a differential amplifier, and the output stage is complementary. Inside the negative feedback loop sit a low-range compensation circuit and a bias trap, which keeps recording bias current out of the output circuitry. High-range peaking for STD, CrO₂, Fe-Cr, and Metal tape is set by four electronic switches using NPN transistors. The peaking frequencies are approximately 15 kHz for standard tape, 17 kHz for chromium dioxide, 17 kHz for ferric-chrome, and 18.5 kHz for metal. The equalizer switch S502 supplies +B to one of the transistors (Q106 through Q109) to engage the right peaking circuit.

The equalizer amp and microphone amp share the head amp circuitry of IC4001. During recording, negative feedback is switched in to get flat frequency response for microphone amp operation. During playback, the NFB circuit works as a time constant circuit for equalizer amp operation. Playback equalizer switching between STD (120 µs) and Fe-Cr/CrO₂/Metal (70 µs) is done by transistor switch Q101, which controls the CR series circuit (R112, C110) at the equalizer amp output. The deck uses a four-channel head, with playback amp gain of 40 dB and microphone amp gain of 32 dB.

The transport is built for convenience as much as fidelity. Fast winding time is approximately 90 seconds for a C-60 tape. Wow and flutter is rated at no more than 0.05% (WRMS). A high-torque DC servo motor drives the capstan, a separate motor drives the reels, and the logic-controlled mechanism manages auto-reverse. The motor switch is adjusted so that, with no tape inserted, it switches on when the play key is pressed and off when the play key is released. The muting switch is set so that in the stop position there is a gap of 1-3 mm in the connection, with a firm connection made when the unit goes to the play position. The play lever locks when the switch has moved 0.5-1.0 mm after making contact.

Frequency response is quoted at -20 dB recording level. For standard and LH tapes, response is 25 to 14,000 Hz ±3 dB. Ferrichrome tape extends that to 20 to 17,000 Hz ±3 dB (25 to 16,000 Hz ±3 dB). Chromium dioxide tape covers 20 to 17,000 Hz (25 to 16,000 Hz ±3 dB), and metal tape reaches 25 to 17,000 Hz ±3 dB. At 0 dB recording level, chromium dioxide tape manages 20 to 8,000 Hz and metal tape 20 to 11,000 Hz. Signal-to-noise ratio is more than 59 dB with Dolby NR off and more than 69 dB with Dolby NR on (measured over 5 kHz). Harmonic distortion is no more than 1.2% at 0 dB.

The deck has two line inputs on pin jacks, with 65 mV sensitivity, 25 V maximum allowable input, and 56 kilohms impedance. The microphone inputs are on 6 mm jacks, with 0.3 mV sensitivity, 100 mV maximum allowable input, and 10 kilohms impedance (reference microphone impedance 250 ohms to 10 kilohms). Outputs are two line outputs on pin jacks with 450 mV reference level, 640 mV maximum level, and 50 kilohms load impedance, plus a headphone output on a 6 mm jack with 60 mV reference level, 85 mV maximum level, and 8 ohms load impedance.

The calibration procedure shows how much precision the deck expects. For playback equalizer adjustment, set the control screws VR101 (forward left), VR201 (forward right), VR102 (reverse left), and VR202 (reverse right) to their mechanical centers. Then adjust so the 6.3 kHz reading is +0.5 dB compared with the 333 Hz reference. Playback Dolby level is set with VR103 and VR203 (forward) and VR104 and VR204 (reverse), so the meter reads 1 dBv (1.12 V) for playback of 333 Hz at 0 dB on the STD-341A test tape. Line output level is adjusted with the output level control at the click-stop position (No. 6) to read -3 dBv (710 mV). Level meter 0 dB adjustment uses a 333 Hz, -10 dBv (316 mV) signal, with VR701 and VR702 set so the 0 VU segment lights.

High Fidelity (1980, p.134) listed the CT-F750 in its equipment directory with a price of $400, three uni-crystal ferrite heads, flutter of 0.03% (WRMS), play response of 25 Hz to 16 kHz ±3 dB, rewind time of 85 seconds for a C-60, Dolby noise reduction, and fluorescent peak-reading meters calibrated from –20 dB to +8 dB, along with an opto peak level display. The same directory noted the presence of LSI tape transport mechanism and a nine-position APLD (Automatic Program Locate) function. High Fidelity (1981, p.48) again listed the deck with 45 dB separation at 1 kHz and 70 dB erasure at 1 kHz, confirming the fluorescent peak-reading meters and hold switch, the LSI transport, the opto peak-level display, and the nine-position APLD feature.

The APLD system, short for Automatic Program Locate, was one of the deck's more practical touches. It lets users skip between songs by detecting the blank gaps between tracks, which beat fast-forwarding and guessing at the counter. The logic-controlled transport and auto-reverse mechanism, combined with the LSI control system, put the CT-F750 near the top of Pioneer's cassette lineup for its time. It cost $400 in 1980, roughly the price of a decent turntable and cartridge. For that money it handled every tape type then available, in both directions, with no user intervention.

Collectors today say the CT-F750's complexity is both its appeal and its burden. The auto-reverse mechanism, the logic control, and the dual-motor transport all depend on belts, idlers, and switches that are now decades old. The motor switch and muting switch adjustments in the service manual are not just factory calibrations. Owners report having to revisit them. The PA4001 IC is a proprietary Pioneer part, and if it fails, replacement means scavenging from another unit. The four-channel head holds up well but is hard to source if worn. The fluorescent meters and opto peak-level display are generally reliable, but the hold switch and the various calibration potentiometers can get noisy with age. The play/record head is a single combined element in a four-channel configuration, so head wear affects both directions equally.

A properly serviced example will still make good recordings. That means belts replaced, idlers rebuilt, heads cleaned and demagnetized, and calibration checked. The 0.05% wow and flutter figure was respectable for a two-motor transport of its era. The Dolby NR system, properly calibrated with the STD-341A test tape, delivers the 69 dB signal-to-noise ratio Pioneer claimed. The auto-reverse function is genuinely useful when the mechanism is in good order, and the APLD system still works when the logic board is healthy. None of that is guaranteed in a forty-year-old deck, so a buyer should expect to pay for service.