That is the polarization voltage of the output coupling capacitor of the buffer. The voltage at the output of the buffer is about -1.5V referred to the ground. The typical signal amplitudes on the base are about ~35mV at 0dB, so the polarization voltage is enough to drive to reduce the distortions of the C161 and C163. It is also well self polarized by the input bias of the Q121 (about -0.7V on voltage drop on the R181). It was bad idea at all but it was required by the bipolar input design of the buffer. Starting with the line input buffer we can notice one electrolytic capacitor in the front of the buffer. Now lets comment the modifications by chains and the explanation why they are required. Some photos can be seen at the official Facebook page of the lab. rebuild of the mechanism – replacement of the idler and belts and new lubrication of the parts.replacement of all the electrolytic caps with new ones based on Elna Silmic II, Panasonic FC and Nichicon Muse (bipolar).replacement of all the orange caps with new polypropylene ones.The rebuilds made in the lab are based on some primary major steps: It is not bad at all but for now there a lot of better replacements. The used op-amps are made by JRC and the precise low noise are J-FET based on NJM-072D. Nakamichi used these polar capacitors in opposite connections to simulate bipolar capacitors and that approach significantly reduce the distortions but the signal path is full of coupling electrolytic capacitors. For me it is bad idea and increase the distortions. Never mind on the serial number the machines are made using polar Nichicon capacitors. The coil replacement in the Sony chip is made by electrically simulation of inductance and is more precise because the coils high tolerance. The traditional Dolby module use four 36mH coils, which are also high cost components. Also the Sony chip has build in spectral skewing circuit without coils. The external Dolby module is 2 channel and use only two Sony chips instead of the four TEA0652 chips. Maybe the cost of the early Dolby chips was the reason for this decision. The Dolby chains on the mainboard were removed and it was a strange decision. The late serial numbers use external Dolby module outside the mainboard. The early serial numbers up to 1’000 were full of issues and they were fixed on the fly by fabric patch on the mainboard: The serial numbers of 30’100 were manufactured about 1993 and that fact can be seen on the mark on the flywheels. The machine was manufactured in the early 1982 but it was manufactured up to the 1995. In the past several years we serviced around 10 machines and made full rebuild of 4 of them and that is why we decide to describe the construction of the machine at the same time we will describe the required modifications for improvements of the sound quality. The Dragon model made by Nakamichi is one of the top cassette decks and it is unique with its playback auto azimuth correction system which improves the high frequency loss due to the azimuth errors in the records.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |