Pioneer TX-9100 (1973, $350, photo, schematic, alignment info, specs and graphs) search eBay
A very nice, solid, but not fancy tuner, the TX-9100 has 5 FM and 3 AM gangs, 4 ceramic filters and 3 IF gain stages, and is very sensitive and has decent selectivity in stock (unmodified) form. Unfortunately, it can't be modified for optimal selectivity without taking it apart because of the way the circuit board is constructed and wired. The TX-9100 uses discrete transistors in its output stage (no op-amps) and has a smooth, powerful sound that some prefer to that of later Pioneers. It has exactly the same RF front end as the TX-9500 and 9800, with differences only in the IF and audio circuit areas. Our contributor David Rich describes the TX-9100's front end as follows: "One tuned stage at the antenna followed by the first RF amp. Then comes a double-tuned stage followed by another RF stage (never a good sign - this means they were going for a good sensitivity number at the cost of IM [intermodulation - Editor] problems). The final RF stage is single-tuned, as is the oscillator. This is the same front end found on most Pioneer tuners and receivers including the SX-1010, SX-1250 and SX-1980 receivers. The 4 ceramic filters in the IF are followed by a ratio detector. The unit uses a PA1310P MPX chip with PLL. It has a strange noise pulse blanking circuit, said to cut pulse noise from automobile ignitions?!?" The TX-9100's 3 AM gangs and a tuned RF stage give it an excellent AM section as well, although it does not have wide and narrow IF bandwidth settings for AM as the TX-7800 and TX-9800 have. The TX-9100 is one of the few tuners with a front-panel headphone jack with volume control. Other controls include separate output level controls for AM and FM, an MPX filter, and a muting switch offering two levels of muting or off. Of course, there are center-tuning and signal-strength meters. On the rear are fixed and variable RCA outputs and multiplex outputs for an oscilloscope. Our panelist David "A," who believes that many Pioneer tuners are highly underrated, reports: "I bought one TX-9100 recently for $10 mint and it blew away every [Yamaha] CT-7000 I have tried! [Editor's Note: David acquired a couple of superior CT-7000s after writing this.] This TX-9100 was clearly better than the other three 9100s that I have owned." David adds that the TX-9100's quieting curve is steeper than that of the Yamaha CT-7000, and comparable to the Kenwood 600T's. Our contributor Tom B. says, "I have dusted off a TX-9100 that I had recapped a while ago and find that it sounds fantastic. When compared to a Sony ST-S730ES, the Pioneer has more detail and depth of soundstage. I also find the Pioneer to be in a league with my Sanyo Plus T55. The Sanyo is perhaps a little smoother in the midrange." Our contributor Robert agrees that his stock TX-9100 "sounds fantastic; I find it particularly rich in the midrange and powerful in the lower registers." Robert posted in our Yahoo group that he finds the TX-9100 "remarkably unelectronic. Harmonically rich, full and with quite good low end extension and spatial portrayal - polite and lifelike."

Our contributor Bill Ammons adds, "The TX-9100 is not a bad-sounding unit, but the measured numbers (separation, distortion) are nothing special. It wins the award for the hardest to service/modify. When serviced, it's an excellent RF performer; its weakness is the audio section. Bypassing most of it makes it a great-sounding tuner! But I would have to add two hours of bench time just to service this one. I could bypass about 75% of the audio path to make a much better-sounding unit. The TX-9100's biggest flaw is the noise reduction that it uses. A FET in each channel is used as a variable resistor with the deemphasis network that follows the FET. The normal value of deemphasis (assuming R ON of the FET = 0 ohms) is 72.5 microseconds. Turning off the FET slightly so that R ON is 10 K or so increases the deemphasis to a few hundred microseconds. Not a very good design based on a very nonlinear control signal being applied to the FETS. This is followed by an older style LPF filter block that is not overshoot compensated. And lastly, there is a 12 dB per high pass filter in each channel with a -3dB point of 23 Hz. Add in the unshielded wiring going to and fro in the unit and you get marginal performance. I increased some of the AF coupling caps, dropped the HPF to 5 Hz and used shielded wiring in a few places. I also upped the power supply and bypass caps. RF-wise, with a Filter Adder PCB in the first IF filter position and a narrower IF chain, this one will do as well as the TX-9500." Our contributor Eric B. responds to Bill's criticism of the stock TX-9100: "The control signal mentioned is nonlinear because the FETs are switches, not variable resistors. They are not used in a slightly turned off mode. They totally blank the output signal if pulse noise is detected and the pulse noise filter is set to the 'cut' position. Since there is a finite time delay needed to detect a substantial pulse and generate the blanking signal, the signal path is delayed by a delay line (filter). This circuit removes unwanted pulse noise such as that from a weed eater. I have recently aligned several TX-9100s and could not find ANY unshielded wiring in the signal path! Pioneer's fully shielded coax may be mistaken for plain wire because it is smaller than typical RF coax and there is a bulge along the side for a solid wire used for ground connections. The unit is hard to modify because it is so well shielded, top and bottom of all circuit boards. But it has no need for modification, since the IF strip has four filters and three limiters yielding 90 dB alternate channel selectivity. In stock form this model performs very nicely." Sale prices for the TX-9100 on eBay often defy logic: while the normal range is $100-130 for one in "average" condition and much less for one with minor cosmetic issues (we've seen as low as $42 in 5/04), the price can skyrocket for a nice one. Two nice TX-9100s sold for $190 and $271 in 5/04, but another one that seemed to be in good shape went for just $54 in 6/04. [DA][BF][EF]