Vintage Hi-Fi Audio Restorations: J.Weeda's Pioneer SX-1980 Monster Receiver Restoration

Pioneer SX-1980 Monster Stereo Receiver

The Pioneer SX-1980…a major contender in the Receiver Power Grand Prix of the era and Pioneer’s top entry. The 1980 touted an impressive 270W output into 8 ohms at 0.03% THD! The 9180 had an advanced and accurate RF front end, consistent and clean pre-amplifier stage with a well shielded and robust RIAA EQ amplifier. The Drivers used unique outputs and 3-stage Darlington triple SEPP output all powered by a massive power plant with a combine reserve of 44,000MFD. The 1980 has the aesthetic and performance to continue to swoon Pioneer enthusiasts around the world, for generations…

AWR-154 POWER SUPPLY W/ JFET MODIFICATION

The SX-1980 has a very very robust power plant designed around a massive toroidal transformer. The 1980 employs a dual secondaries for the -/+ driver rails, each with its own independent full wave bridge rectifier and 44,000 microfarad total filter capacitors per bank! The RF and front end audio features and the 80V -/+ rail sources all utilize a full wave rectifier with a series of TO-220 regulators.

The AWR-154 employs the protection features such as 48V soft start relay engaging the on/off muting shunting the PA3004 IC used for the protection in the DC voltage detection circuit as outputs, a overload/current detection and finally a surge current suppressor due to the high inrush current of the toroidal at startup.

A common failure mode of the AWR-154 involves the 2Sk34 Jfet which acts as a 2.2mA trigger source for the -/+80V rail regulators. A excellent solution to this high voltage device is a custom made current source. Use a phenolic PCB that was cut and hand sanded to dimensions, we populate the device with an array of NPN/PNP devices and diode dropping 2.5Vf Leds’s a reliable 2.2mA design can be installed as we show below in action. (design courtesy:GlennMcDonald 07’)

Custom made 2SK34 Jfet Modified Devices

The massive 20W surge suppressor resistor over the temperature fuse was replaced with a more robust Wire-Wound 25Watt resistor. Both relays were replaced with a robust modern 48V Omron relays. All of the small signal devices were replaced with better dissipating, low noise TO92L type Fairchild devices. All the TO-220 regulators were replaced with a robust, heavy duty OnSemi type device with all new synthetic thermal compound applied to substrates.

On the AWR-154 the large value and high voltage 160V filter capacitors were all replaced with a high quality Nichicon TVX axial type with the proceeding 1A diodes to a Ultra-Fast Soft Recovery Vishay axial diode, bypassed with high grade WIMA polypropylene film capacitors. The remaining electrolytic capacitors here were replaced with a low impedance, high temp (105C) Nichicon PW type with an increase in operating voltages. Remaining 35V, 24V, 13V and 6.1V zener diodes were all replaced with a modern axial type diode as well as Vishay 4148 general diodes.

BEFORE

AFTER

Secondary Voltage Confirmation (34/80V -/+)

Filter Capacitors

The four large filter capacitors, used in pairs for the independent bridge rectifiers were replaced with computer grade 22,000MFD, 100V Nippon Chem-Con capacitors with new high grade MKP polypropylene film bypass capacitors.

BEFORE

AFTER

Filter Capacitor Rail Voltage Confirmation

AWF-030 RIAA EQ

The SX-1980 phono stage has some great features such as a variable load setting for the phono 1 input by compensating for the different input resistances and input capacitance of various cartridges. The circuit also piggybacks the MIC input via the eq amplifier, using an inverse input for a very flat response with micro levels of noise. The EQ stage uses a dual, “ultra-low” noise 2SK131 type dual FET for a differential stage and a cascade connected array proceeded with a current mirror fed PP and symmetrical SEPP final drive stages.

The dual 34V rail DC filter caps were increased for better filtering with low impedance Nichicon PW capacitors, including the 2200MFD capacitors. Remaining capacitors were replaced with a audio grade MUSE Bipolar in the tied 10MFD capacitors and audio grade KT/KA Nichicon capacitors all with an increase in operating voltage.

All remaining diodes were replaced with a modern axial Vishay 4148 general type. Small signal devices were replaced with low noise, modern TO92 Fairchild devices as well as new robust, heavy duty OnSemi TO-220 type drivers thermally coupled with synthetic Wakefield compound.

BEFORE

AFTER

RIAA Curve Response

AWG-058 FLAT AMPLIFIER

The initial input tone control circuit achieves the nominal output of 22.5dB for the proceeding control circuit at a low impedance. The Flat amp is comprised of a tiered direct-coupled driver, using a dual differential 2SA798 device and a complimentary symmetrical SEPP output stage. The quality stepped attenuator and other controls were removed during this stage and properly cleaned and lubricated with Caig Labs products.

The initial dual package device was replaced with beta matched PNP devices, remaining electrolytic were replaced with TO-92/L low noise, modern Fairchild devices.

Hfe Matching 2SA798 Replacement

Capacitors here were replaced with a audio grade KT/KT and low impedance type PW Nichicon capacitors with an increase in operating voltages, Dc filtering for the dual 34V supply was increase to 470MFD for better filtering and bypass with high grade WIMA polypropylene film capacitors.

BEFORE

AFTER

AWG-059 TONE CONTROL

Following the Flat Amplifier, the ’Twin Tone” controls offer standard and parabolic crossover points. Designed around a simple C-B feedback, transistor circuit.

All of the electrolytic capacitors in this stage were replaced with a audio grade KT/KA and low impedance type PW Nichicon capacitors with an increase in operating voltages including replacing all tantalums. New low noise, modern TO92 Fairchild devices were installed. Open-Frame switches and controls were cleaned and lubricated with Caig Labs products.

BEFORE

AFTER

AWM-119 FILTER AMP & GWS-129 Function Selector.

The 1980 Filter amp are a -12dB octave NFB type filter with a simplified emitter/RC array with tailor cutoff points of the passive filters.

This stage uses tantalums, which are all replaced with audio grade KT/KA Nichicon capacitors with an increase in operating voltages as well as high grade WIMA polypropylene film capacitors. New axial general 4148 diodes installed, including low noise, modern TO92 type Fairchild devices. The function assembly had the 220MFD capacitor replaced and all push switch controls were cleaned and lubricated with Caig Labs products.

BEFORE

AFTER

BEFORE (GWS-129)

AFTER (GWS-129)

GWH-109 & GWH-110 Driver Assemblies

The SX-1980 features a very robust dual, symmetric driver stage design, designed around a unique output device stage and massive wrapped/finned heatsink. The concept of the 1980 operates off an initial dual different and cascade connection to a current mirror. Following to a push-pull pre-driver gain stage to a 3 tiered darlington array triple-SEPP output block.

The initial dual differential package was replaced by a beta matched PNP pair plotted below. All small signal devices were replaced with modern, low noise Fairchild TO92/L devices for better dissipation. New TO-126 and TO-220 drivers were also installed with ew synthetic thermal Wakefield compound. New Ultra-Fast, Soft Recovery axial and general axial 4148 type diodes. Idle and offset adstjuments were replaced with precision Bourns sealed potentiometers.

He Matching Driver Stage 2SA979 Replacements

Films were heavily replaced with high grade MKP and WIMA polypropylene film capacitors, including the 1MFD input coupling. The 80V DC filtering capacitors were increased to 220MFD low impedance Nichicon PWs for bettering filtering.

BEFORE

AFTER

Bias Confirmation @ 30mV -/+

AWM-122 Meter Amplifier

The two front output meters on the SX-1980 are calibrated for a 8 ohm load only rating from .01-540Watts RMS output. The AWM-122 uses a logarithmic compression circuit of emulation.

All of the axial diodes here were replaced with a general 4148 axial Vishay type, also replaced were remaining small signal devices with modern, low noise TO92 Fairchild devices. New precision, sealed Bourns potentiometers installed for calibration. Electrolytic were replaced with a low impedance Nichicon PW with an increase in operating voltages plus high grade WIMA polypropylene film capacitors.

BEFORE

AFTER

FM/RF & APC Control (AWE-093 & AWX-124)

The 1980 has a top notch tuner front end, with impressive S/n, THD/IMD separation specs far exceeding many counter components of the receiver grand prix. Featured with Pioneers proprietary Automatic Phase Control (APC) circuit which detects variances in phase thru the crystal oscillator via voltage signal input and touch sensor tuning lock working off the bodies own noise voltage and relayed. The FM front end uses a 5-gang tuning capacitor and dual-gate MOS FET- tiered RF amplifier (modified Clapp design) along with a HA1137 and PA1002-A AF amplifier/muting IC. The Am front end is composed of a 3-gang tuning capacitor and HA1197 IC current/rf amplifier chipset.

AWE-093

In this stage extensive films were used thru out for small value tantalums and ceramic replacements in IC coupling points, high grade WIMA polypropylene films were used. Low impedance Nichicon PW’s with an increase in operating voltage replaced remaining electrolytic.

BEFORE

AFTER

AWX-124

For the APC assembly all of the small signal devices were replaced with a low noise, modern TO92 Fairchild devices as well as a general axial Vishay 4148 diode. Films and ceramics were replaced with high grade WIMA polypropylene film capacitors including a single audio grade KT/KA Nichicon capacitor with an increase in operating voltage.