s a small number of favorite stations and selects definite programs. The scan feature on the other hand might appeal to the less critical listener who usually scans the band until he hears something he likes.
Other Features. One of the extremely useful by-products of this system is automatic interstation muting. The tuner just won't tune between channels. Noise muting is also provided however for silencing empty channels; this type of muting is defeated by a front panel switch. During the automatic "Stereo Station" scan mode, all mono stations are muted. When tuning from one station to another regardless of which tuning mode is used, the sound disappears without the usual transient swish or thump and reappears out of complete silence again with the absence of annoying noise bursts and distortion.
All muting is done after the multiplex decoder by two FET series-gate switches which reduce the signal by at least 60 dB in the muted condition without introducing a dc transient.
The r-f section in the tuner employs selected high-gain, low-noise FET's for both r-f gain and mixer functions. A FET is also used for impedance matching and low noise in the first stage of the i-f amplifier. Two 6-pole elliptical filters shape the passband of the i-f and achieve a selectivity in excess of 70 dB which allows this tuner to select any one station from a crowded area on the band.
The "Station" light on the front panel indicates the presence of a carrier and is actuated by a zero-crossing detector coupled to the output of the ratio detector; the station light is also a double check on the PLL and reference standard because it is actuated only if the station is tuned to exact center. On noise which is present on empty channels, the station light is extinguished automatically. The "Stereo" indicator will light up in the presence of a 19-kHz subcarrier when the signal level is sufficient to give an acceptable signal-to-noise ratio. A "Card Program" indicator shows at a glance whether or not card tuning is being used. Aside from providing instantaneous pre-selection of station frequencies, the card serves as a permanent memory since the code generator's volatile memory loses the station code when power is turned off.
Trend to Complexity. The trend in consumer electronics is toward greater circuit complexity made possible at low cost due to the use of integrated circuit technology. The circuit designer gains flexibility in achieving performance goals and operating convenience for the customer. Unless proper steps are taken, however, servicing of this type of equipment can become a problem as troubleshooting time and test equipment expenditures increase drastically.
The best approach seems to center around modular construction with each module representing a functional sub-assembly which can be replaced with no more effort than the vacuum tube in an old TV set. Fault location is greatly simplified by the fact that each module performs a definite function which can be monitored individually with a minimum of test instruments. Once the faulty module is replaced by the service shop, it is sent back to the factory where automated test facilities localize the fault to a component on the subassembly and it is either repaired or scrapped. This module exchange policy has been used for some time and is gaining in importance as equipment complexity increases. The customer also has the advantage of knowing exactly how much the repair will cost if it is done after the warranty period has expired because definite module exchange prices have been established. Even service shops with limited facilities can repair a unit as complex as the digital FM tuner if a set of PC modules or even another operating unit is available. Each module is simply interchanged with a new one until the fault disappears.
Although the complexity of circuitry has increased, reliability has increased also. Through the use of MSI (medium scale integration), the number of hard-wired interconnections has actually decreased thus avoiding a significant number of failures. By screening TC's i