User manual MCINTOSH MR80

[. . . ] THE MclNTOSH MR 80 DIGITAL FM TUNER Reading Time: 32 Minutes Price: $2. 00 VARIOUS REGULATORY AGENCIES REQUIRE THAT WE BRING THE FOLLOWING INFORMATION TO YOUR ATTENTION. PLEASE READ IT CAREFULLY. WARNING: TO PREVENT FIRE OR SHOCK HAZARD, DO NOT EXPOSE THIS UNIT TO RAIN OR MOISTURE. The Mclntosh you have purchased is a Model MR 80. Record that serial number here: Serial Number The model, serial number and purchase date are important to you for any future service. Record the purchase date here: Purchase date Upon application, Mclntosh Laboratory provides a Three-Year Service Contract. [. . . ] This meter can be set to give a full scale indication on a signal as low as 2mV. This allows setting the signal meter to indicate full scale on the strongest station in the users area. Then the signal strength meter indicates the RF level of all stations received relative to the strongest in the listener's area. The signal strength voltage is also used to control mono/stereo switching, stereo filter insertion, muting, and automatic scan stop. Because of it's wide range and high repeatability this control voltage is also used to adjust the stereo separation at low RF signal levels. This produces the maximum signal to noise ratio possible for all signal levels. The use of very high gain in the limiter circuit produces hard limiting with very good impulse noise rejection. Limiter bandwidth is greater than 50MHz, producing excellent detector capture characteristics. This detector coupled with the broadband limiter produces unmeasurable noise and distortion. A high speed buffer amplifier following the detector isolates the detector from all varia- tions in load impedance. This prevents changes in audio cable lengths outside the tuner from affecting the detector performance. FM STEREO MULTIPLEX The heart of the multiplex section is a new third generation phase locked loop (PLL) stereo decoder integrated circuit (IC). This PLL IC incorporates two special systems, an automatic variable separation control circuit to reduce background noise when receiving weak stereo stations, and tri-level digital waveform generation which eliminates interference from SCA signals and from the sidebands of adjacent channel FM signals. The variable separation control is operated from the IF amplifier's signal strength detector. A smooth transition is provided from mono to stereo or from stereo to mono at weak signal levels to provide the optimum signal to noise ratio and best stereo separation for the prevailing signal conditions. It switches automatically to monophonic if the 19kHz pilot tone is absent. In the PLL the internal oscillator operates at 228kHz, locked onto the 19kHz pilot tone. The 228kHz feeds a 3 stage Johnson countervia a binary divider to generate a series of square waves. Suitably connected NAND gates and exclusive OR gates produce the tri-level drive waveform for the various demodulators in the circuit. The usual square waveforms have been replaced in the PLL and decoder sections by tri-level waveforms. These tri-level forms contain no harmonics which are multiples of 2 or 3. This eliminates frequency translation and detection of interference from the side-bands of adjacent stations since the third harmonic of the sub-carrier (114kHz) is excluded. It also eliminates interference from SCA broadcasts since the third harmonic of the pilot tone (57kHz) is excluded. [. . . ] During the turn on delay the auto track ensures that the integrator's output voltage is within the band limits. This circuit allows the tuner to start searching for the next station from the current location. FREQUENCY COUNTER 5, 15 and 30v use integrated circuit 3 terminal regulators, while the 3ma current source is made with discrete transistors because of the high voltage on the input terminal. The remaining voltages are used for the headphone amplifier and the touch control reference signal driver. LAMP DRIVERS The counter and display form the digital equivalent to the slide rule dial and pointer, but with greater accuracy and readability. In the RF section a small portion of the oscillator signal is coupled into the buffer amplifier on the counter circuit board. [. . . ]