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Kenwood KT 917 - $550

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ToKenwood KT-917 (1979, $1,000, photo, schematic, detector/MPX scheme: pulse count detector, discrete CMOS switch driven with slimed pulses (no charge injection cancellation) generated by analog means, MPX PLL generated 38k with HA11223 chip) search eBay
The FM-only KT-917 was the successor to the 600T as Kenwood's flagship tuner. The KT-917's front-panel controls are identical to those of the 600T, which led some bygone internet commentators to assume erroneously that the differences between the tuners are merely cosmetic, but the KT-917 is somewhat larger and has very different circuitry inside. The KT-917 has a huge 9-gang tuning capacitor, similar to the 600T's 8-gangs-and-a-jumper capacitor, and the 917 also uses Kenwood's pulse count detector circuitry, but the similarity ends there. Unlike the 600T which uses two parallel filter paths (Wide/Normal and Narrow), the KT-917 uses a single serial IF filter circuit with taps for the three filter bandwidths. It starts with a single ultra wide ceramic filter (there are two other ceramic filters but they're used only for meters and are not in the IF path), followed by a single tuned LC filter in Wide mode, followed in Narrow mode by four Murata "Surface Acoustic Filters" (sometimes called SAW filters, for Surface Acoustic Wave) which were specially designed by Kenwood and are also used in the L-07TII. As each filter is normally two elements, or stages, this would give the KT-917 a 12-element narrow mode, similar to the 600T. (There are two SAW filters in the wide IF bandwidth mode.)

Our contributor David Rich observes that like the McIntosh MR 78, which is "double-tuned at the input, then has a cascoded (better linearity) RF stage followed by another double-tuned filter, the KT-917 is the same except the drain - source connection of the cascode is double-tuned and the output of the cascode amp is triple-tuned. The oscillator gets an extra tuned stage to reduce phase noise and improve matching. The mixer is passive to keep the good IP3 [third-order intermodulation - Editor] rejection. The KT-917's Stereo MPX circuit is a zero-order sample-and-hold. A small pulse generated by an analog circuit turns on the MOSFET switch for a very short period of time. At that time the output follows the composite. For the rest of the time the output stays stable. This is not a switching system where the polarity of the composite is switched on 50% of the time (the older Kenwoods do that). The zero-order sample-and-hold can be done with diode networks alone and it shows up in even early tuners such as the Marantz 10B I think. It is all explained in the KT-917's manual. IC 9 is the switch (TC4066), C35 and C38 are the hold caps, and the op-amp after that is a TL 072 (IC 10). It has to be a FET op-amp to hold the charge on the cap. IC1, IC5 and IC 12 are also upgrade candidates as are all the passive in the signal path except C35 and C38 which I would not touch. IC 10 needs high slew rate and fast settling time like all op-amps in a sample-and-hold application. See Linear Tech for an upgrade of the TC 4066 (this is a high-risk move but may offer lower charge injection. It should only be done if you really understand the circuit and how a part change in the switch could really kill its operation. Remember that Kenwood dumped the timing circuit to the switch in the next-generation tuner. The last Kenwoods used analog multipliers."

Our contributor Georges tells us that the KT-917 has "two RF amplifier transistors Q1 and Q2 (Q2 being a common gate amplifier) before the diode doubly balanced mixer." And the KT-917's service manual says, "The RF amplifier section has a wide-gap, 9-gang variable capacitor for the double-double-triple tuning system (one tuning stage for ANT, and two tuning stages for RF). The CC3588DE used as Q1 is a DD-MOS FET (selected SD-306) which features low noise and superior square response over a broad input level/frequency range. It also features a high power gain. VR1 adjusts Q1's input response to its maximum linearity. For servicing adjust VR1 so that the maximum deflection of the S-meter can be obtained. In the second stage, another double-tuning circuit is coupled to a common-gate amplifier, which features a lower input impedance and stable amplification with no influence from feedback admittance."p of the line kenwood tuner in excellent shape. No trades email phone number
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