Pioneer SX-1980 von Receiver-Aus-Gangsleistung
Today were featuring a special monster receiver…the Pioneer SX-1980! The SX-1980 was introduced in the late 1978 to compete with the 2600/2500 Marantz, Sansui G22-33K and Kenwood KR9600 monster receivers with an original costs of $1250USD.
The SX-1980 featured some incredibly factory specs and was is a total behemoth weighing at 78lbs! The SX-1980 shares some very distinct features with its kin and like the SX-1250 touts a massive toroidal transformer and four 22,000/100V filter capacitors. Restored to this degree these receivers can still easily fetch 3500-5500$USD…
Power Supply und Protection AWR-154
The power supply on the Pioneer SX-1980 leaves a lot to be desired sadly. The original design suffers from inherent overheating issues and a failure prone JFET 2SK34 devices. A lot of the thermal issues can be addressed using better thermal transfer material like Sil pads with better thermal compound and more robust BJTs and 105C rated capacitors.
The large toroidal uses separated windings for each defendant channel! These are supplied to their own bridge rectifiers and a pair of 22,000MFD filter capacitors. The separate voltage driver stage, pre and RF sectional have a dedicated full-wave rectification network and regulating devices as well as a DC supplied lamp rails..convenient…
The protection consist on a auto inrush limiter during power phases and output /overload protection and significant DC deviation. A PA3004 IC triggers these responses on the power supply.
The first and foremost important item to tackle is the removal and upgrade of the original 2SK34 JFEt devices used to initiate the 80V supplies and provide a 2.2mA current source. I want to take a second and recognize the work of EchoWars & MarktheFixer’s work here in detailing a protocol and trial and error for this modification, only minor differences have been installed but the base is his design.
The modification is comprised of a pnp/npn diode drop network using an array of 1.2K emitter resistors a 1M (current feed BJT) using two sub mini 1.9V green LED’s such as; The 2SK34 zener protect network is then removed from this assembly as its unused.
((1.9Vfd-.6Vfd)/1200ohms X2 for a 2.166mA current source.
The electrolytic capacitors were replaced with high temp (105C) long life Nichicon PW capacitors with an increase in operating voltages. New coupling MKP polypropylene films were installed for the rectification stage. The three large axial capacitors were replaced with TVX Nichicons with an increase in operating voltages. Research will show that often the 35V zener fail or are damaged as well, these were replaced with two diode in series to a 500mW diode. Several resistors exhibited scorch marks or were entirely blown open, these were all replaced with 1/2W metal film KOA resistors. R102 was removed and a lead ran to the 330MFD axial capacitor to accommodate the new 24V relay without damaging it. Both relays were updated to LYS and MY2 24/48V low noise Omron relays.
The large regulator TO-220 devices were all replaced with On-Semi heavier duty TO-220 MJE devices with SIL TO-220 inserts to help dissipation including the both heatsink mounted 2SD325s to MJE devices. All other transistors were replaced as well, upgraded to TO-92L and TO-126 devices with modern low noise, more robust equivalents.
Power Amplifier Recht und Links GWH-109
The initial driver stage is comprised on a single package 2SA979 to a cascade array with a constant current controller (Miller Effect circuit,) The 2SA979 was replaced with two modern low noise Fairchild PNP devices with a 1% matched Beta range with a shared emitter, all associated above BJT’s were replaced with low noise modern /FairchildOn-Semi TO-92 and TO-126 devices. The pre-drivers and associated network were all upgraded to modern low noise Fairchild TO-126 (KSA1381/3503) devices. Both drivers were upgraded to heavy duty, robust On-Semi TO-220 MJE devices with new thermal compound. Both 150K/100 bias/offset pots were updated to precision Bourns trimmers and 1S2473 diodes to modern 4148 type.
The electrolytic were replaced with high temp (105C) low impedance Nichicon PW 100Volt capacitors. the .22MFD and all remaining films were replaced with audio grade high end WIMA and MKP polypropylene film capacitors. The original bat-winged outputs were removed and new thermal compound applied with MICA pads.
The output meters read a logarithmic .01-540W scale without the need for calibration range using a logarithmic Compression circuit for the peak indicator feature. The .47/220MFD were all updated to low impedance, high temp (105C) Nichicon PW capacitors and ECW PP films. I did note the initial meters were off calibration.
AF Circuits: Flat Amp AWG-058 und Tone Control AWG-059 W/ AWM-119
The AF circuit int he SX-1980 is very close to the SX-1250 when we compare the designs, where as the RIAA EQ stage greatly differs with a flat amp (22.5dB Gain stage)
The flat amp is built on an initial dual PNP device like the driver PCB using a 2SA979 device which was replaced with a 1% matched BETA low noise Fairchild TO-92’s with a shared emitter. The remaining gain stage comprises a low impedance SEPP output, all of the transistors were updated to modern low noise Fairchild TO-92 and TO-126 devices.
The electrolytic capacitors were replaced with a low impedance Nichicon PW and audio grades Nichicon KT audio grade electroltics with an increase in operating voltages. The coupling capacitors were replaced with high grade MKP polypropylene film capacitors.
The input is then fed to the AWG-059 tone control stage which does feature a bypass defeat option. The SX-1980 uses a main and sub-line attenuator circuits for the baseline twin controls and a sub-sonic hi pass control.
The compliments of 2SC1312 transistors were all replaced with a low noise Fairchild TO-92 devices. Electrolytic’s here were all replaced with low impedance Nichicon PW and audio grade KT capacitors and two 10MFD Fine Gold Nichicon’s with an increase in operating voltages.
The Filter amp is a-12dB octave NFB active filter circuit with RC passive filters. As above AF stage the electrolytics were upgraded to KT and Fine Gold audio grades and a low impedance filter PW capacitors. New low noise Fairchild TO-92 devices were installed.
EQ RIAA AWF-030
The SX-1980 houses one of the better RIAA gain stages I’ve seen yet with two handy adjustments of the input resistance (all tested at 47K(50K) standard) and input capacitance.
The initial gain stage of the AWF-030 is a 2Sk131 JFET which was a ultra-linear dual device and a NPN differential stage which was updated to two 1% matched Beta low noise Fairchild TO-92 devices. The constant current and voltage stage were all updated to new low noise Fairchild TO-126 and TO-92 devices with the driver SEPP stage to new heavy duty KSA/KSC 940/2073 TO-220 devices.
The electrolytic 2200MFD filters were updated to Nichicon Fine Gold audio capacitors with the remaining replaced with low impedance Nichicon PW and audio grade KTs capacitors.
RF Stage / Filters und MISC.
The 22Kmfd filter capacitors were all replaced with new Nippon Chem-Con 22,000MFD 100Volt capacitors with new capacitors mounts to adapt better to modern available dimensions and future placement.
The X4 bulbs were replaced with multi-facet SMD LED’s for long life use.
The front end /RF tuner is one of the best designs out there in these receivers, as Pioneer often has. With extremely low noise qualifications (M-50dB/St-30dB) and a unique automatic pilot canceling circuit to suppress multi-plex noise and maintaining separation and responses output.
The remaining RF stage capacitors were replaced to low noise, high temp (105C) Nichicon PWs and we tried new Vishay tantalum capacitors.