The Pioneer F-90 is a high-performance AM/FM tuner that features a balanced mixer, three narrow-IF filters, a pulse-count detector, and a novel switched-sinewave stereo decoder. It has good sensitivity, excellent overload resistance, excellent immunity to spurious composite signals, very high AM suppression, very high ultimate S/N, extremely high modulation acceptance, and extremely low distortion.
The negatives: With its brushed aluminum front panel, the F-90 does not blend well with black audio components. It has no signal-strength indicator. The IF bandwidth and mono/stereo settings are not saved along with frequency in the station memories. Two power transistors on the PCB generate enough heat to upset a careful alignment. The board must be unbolted and awkwardly tilted for modification because there is no bottom access. Finally, the memory pushbuttons often don't work due to decomposed foam between the front panel and switches.
The front-end uses a tuned antenna transformer with center-tapped primary. This provides both 75Ω and 300Ω input impedances without a lossy balun. The transformer feeds a dual-gate MOSFET RF amplifier followed by two tuned circuits. Forgoing AGC keeps an untuned signal from reducing sensitivity. The balanced mixer uses a pair of 2SK125 JFETs.
The mixer output is diode-switched to two IF paths. The wide path uses two ceramic filters and a tunable transformer. The narrow path uses three ceramic filters. Amplifiers that isolate all filters in both paths prevent undesirable interaction.
To greatly increase the modulation percentage, the IF signal is doubled to 21.4 MHz and then heterodyned to 1.26 MHz. This enhances S/N.
The patented Digital Direct Decoder uses two proprietary ICs. One includes a pulse-count detector. The decoder phase-locks a 38 kHz sine wave to the 19 kHz pilot. The pulses gate the sine wave to the left and right channels (the multiplication circuits are analog switches). Because the pulse rate is well beyond the spectrum of the composite stereo signal, no spurious mixing products result. In particular, the decoder is immune to HD Radio self-noise. The circuit includes pilot cancellation.
The FM IF amplifier uses a proprietary IC. AM uses a Sanyo LA1247, which has somewhat better performance than the usual LA1245. The tuning range is 520 to 1610 kHz. AM and FM share the 75 µs deemphasis network. It provides a reasonable approximation to NRSC AM deemphasis, at least until the IF filter takes over. A 10 kHz notch filter suppresses adjacent-channel heterodynes. An RCA jack provides 450 kHz IF output for an external AM stereo decoder.
The memory pushbuttons worked in none of the three F-90s I've restored. It's easy to replace the decomposed foam between the front panel and switches with something elastic, such as thin pieces of weatherstripping.
The power supply pass transistors mount on two PCB heatsinks. They generate enough heat to significantly raise the internal temperature with the unvented enclosure in place. This can degrade a meticulous alignment done with the enclosure off. Experiments with a heat gun suggest that changes in ceramic filter response are the main cause.
In one F-90 I remoted the pass transistors to the rear panel. This dissipates much of the heat to the outside. You must electrically insulate the transistor tabs from the chassis since they connect to the collectors. With this arrangement the alignment still changed somewhat when the enclosure was replaced, but much less than before.
To achieve a precise alignment without relocating the transistors, replace the enclosure, do something else while things warm up, remove the enclosure and quickly make an adjustment, and repeat. This procedure is tedious, but I found it necessary only for the wide distortion and narrow separation adjustments.
I've found ceramic filters with a dot color other than red at F105, the second wide-IF filter. This indicates that the filter center frequency is not exactly 10.7 MHz. R116, which determines the filter source resistance, is lower than the 330Ω Murata recommends. So is the load resistance, the parallel combination of R137, R117, and R118. L102 and C141 resonate at 10.7 MHz, presumably to tame spurious filter responses, but L108 will alter this resonance and seems out of place. I don't know whether any of these circuit oddities influenced the center-frequency choice for F105, but replacing it with a red-dot filter improves the overall IF passband shape, weak-signal performance, and distortion. I've always found a red-dot, low-group-delay, 250 kHz MX filter in the first wide slot.
I usually replace the narrow filters with two 110s and a 150. Three 110s provided terrific selectivity, but the audio occasionally clicked. I've also tried three 150s, which yields lower harmonic distortion and 36 dB of adjacent-channel selectivity.
To improve stereo separation in narrow IF mode, the F-90 compensates the stereo decoding matrix by switching in R306, 68kΩ, with a JFET. If you replace the narrow filters, the compensation is no longer correct. You can increase narrow separation to beyond 60 dB by adding a 100kΩ trimpot across R306, outlined in red below. Adjust it for maximum separation in narrow after making the wide separation adjustments. Make sure the signal generator is exactly on frequency.
The stock circuit rolls off the AM audio response at high and low frequencies. Removing C217, 0.047 µF, outlined in yellow above, extends the high-end response. I clip one lead and tilt the capacitor back as shown. Adding 0.47 µF across C356, 0.047 µF, outlined in green above, improves the bass response. Note the polarity. These changes noticeably improve AM sound quality.
Instead of steps 3–5 of the FM adjustments, I aligned the front-end by sweeping it. The result at 90 MHz is shown above. The horizontal span is 88–108 MHz and the vertical scale is 10 dB/div. Although the passband exhibited a narrow peak, tracking was excellent across the band.
I used L instead of L−R for the stereo test signal. I tuned T101 as well as T2 for minimum distortion in wide-IF mode. I turned VR101 all the way up to maximize weak-signal limiting in wide-IF mode. I turned VR501 all the way down. The two unmarked transformers to the left of TP17 are in the IF doubler circuit. A couple inches to their left is a 1kΩ resistor. Adjust the transformers to peak the 1.26 MHz signal at either end of the resistor. Just to the right of TP14 is an unmarked coil. Adjust it to maximize the 38 kHz sine wave at TP14. The AM alignment instructions neglect IF filter F201. Adjust it for minimum audio output for a low-deviation FM test signal exactly on frequency. This will minimize residual digital noise when receiving an AM HD Radio signal.
For the following FM measurements I used IEEE 185-1975, updated as described here. I used the test equipment listed here. The tuner had 110-110-150 filters and a separation trimpot for the narrow IF.
50 dB quieting sensitivity, mono W 18.8 dBf, N 18.8 dBf 50 dB quieting sensitivity, stereo W 39.4 dBf, N 37.8 dBf Adjacent-channel selectivity N 46 dB THD, 1 kHz, stereo W 0.02%, N 0.9% Stereo separation, 1 kHz W 56 dB, N 50 dB S/N, 65 dBf, mono W 84 dB S/N, 65 dBf, stereo W 74 dB S/N, 85 dBf, stereo W 76.5 dB AM suppression ratio W 82 dB, N 78 dB RF intermod W 88 dBf (97.7 + 98.5 ⇒ 96.9) RF spur W 111 dBf (95.77 ⇒ 96.9) RF image W 102.8 dBf (118.3 ⇒ 96.9) RF mismatch loss < 0.1 dB Noise figure, 96.9 MHz 7.1 dB Modulation acceptance W 270%, N 160% Minimum stereo pilot injection W 3.6%, N 4.8% Treble response W +0.2/-0.3 dB Bass response, -1 dB 22 Hz Output level W 0.6 V, N 0.5 V Output impedance 900Ω