Vintage Hi-Fi Audio Restorations: April 2018

Two Pioneer SX-1250 Receiver's

Today were showcasing two different customers Pioneer SX-1250’s receivers. The SX-1250 produced from 1976 to 1978 and was the largest Pioneer receiver to that date featuring robust built qualities like a total of 88,000MFD filtering capacity, independent rectifications and robust amplifier designs, plentiful shielding and a clean aesthetic.

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Power Supply AWR-107

The 1st tier power supply for the SX-1250 contains one bridge and one full wave rectifier 85/23V supplies. The On-Off inrush limiter is relayed via a Omron device and a 3.3OHM 20Watt limiter tied to a micro temp controller.

In one of the units the 3.3OHM had suffered massive current draw and dissipating significant heat which in turn destroyed the microtemp controller. Both were replaced in conjunction with a blow output bank.

The axial capacitors were replaced with Vishay computer grade high temp (105C) electrolytic with an increase in operating voltages along with remaining capacitors to a high temp, low impedance PW type. The 1A full wave was upgraded to a MUR type TO-220 Ultra-Fast soft recovery array with high grade WIMA polypropylene bypass films, the bridge was replaced with Vishay Ultra-Fast, soft recovery axial array with new Panasonic polypropylene films bypassed. As highlighted above a new wire-wound 3.3 ohm 25Watt resistor and NOS microtemp installed.

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Each channel operates off a separate full wave bridge rectifier tied to a pair of 22,000MFD filter capacitors. These were both replaced with new Nichicon computer grade 22,000MFD 100V type.

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Stabilizer AWR-106

The stabilizer circuit established the regulated -/+ 25 and -/+ 65Vdc and 13.5V(5.4V) supplies. This PCB often suffers from heat associated issues due to large amounts of dissipation from the large surface area TO-220 heatsink.

The axial capacitor was replaced with TVX 1000MFD Nichicon as well as all electrolytic with new high temp (105C) low impedance Nichicon PW’s with an increase in operating voltages. New precision Bourns voltage adjustments were installed as well as 24V Zeners. All small signal devices were replaced with TO-92/TO-92L Fairchild devices for better dissipation and all new TIP series TO-220 regulators with performance full synthetic Wakefield thermal compound applied.

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Protection AWM-091

The SX-1250 features several protection shunts such as the Muting operating, DC deviation, Overload protection and thermal cutoff.

All of the electrolytic here were replaced with a low impedance, high temp (105C) Nichicon PW with an increase in operating voltages. Due to a large current limiting resistor several nearby resistors had suffered significant heat radiance, these were replaced with 1/2W. carbon film type. All small signal devices were replaced with TO-92/92L and TO-126 Fairchild devices for better dissipation. A new MY4 Omron relay was installed.

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Drivers AWH-048

The core concept of the SX-1250 amplifiers are comprised of a differential pair drivers and a constant current mirror array. Voltage amp and 3-stage Darlington drivers and symmetric complimentary NPN/PNP outputs.

In both units, all four amplifiers had 1% matched differential pair transistors installed, current mirror, pre-drivers and drivers all replaced with a low noise modern TO-92 and TO-126 Fairchild devices, with robust OnSemi MJ series TO-220 driver devices performance with full synthetic Wakefield thermal compound applied. New precision Bourns bias/offset adjustments installed. One unit had suffered a catastrophic failure and required new OnSemi TO-3 output devices with performance full synthetic Wakefield thermal compound applied.

New low impedance, high temp (105C) Nichicon PW capacitors were installed with an increase in operating voltages, as well as films replaced with high grade WIMA polypropylene film capacitors and a 1MFD high grade MKP polypropylene film capacitors.

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Differential Matching Drivers

Bias Confirmations

Flat Amp AWG-042

The AWG-042 assembly comprises a standard NPN/PNP gain stage with a JFET coupled gain stage and NFB circuit.

Short of the Jfet’s all of the small signal devices here were replaced with a low noise, modern TO-92 Fairchild devices. All electrolytic were replaced with low impedance and audio grade Nichicon KA/KT and PW type with an increase in operating voltages. Films were all upgraded to a high grade MKP 1MFD polypropylene and all high grade WIMA polypropylene films.

The 22MFD electrolytic here are all tied in a “anti-phase” fashion, these were replaced with single 10MFD audio grade MUSE Nichicon bi-polar electrolytic capacitors.

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Control Amp AWG-041 & Filter AWM-089

The SX1250 employs Pioneer’s “Twin Tone” controls which sub-control a sub-sonic and high frequency range and NFB circuit design. The Filter control acts as a relay lo/hi pass network.

Like the flat amp, in both the control and Filter PCB’s the small signal devices were all replaced with low noise, modern TO-92 Fairchild devices. All electrolytics were replaced with a low impedance PW and audio grade KA/KT Nichicon with an increase in operating voltages. All new high grade WIMA polypropylene films were also installed.

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AWM-089

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FUNCTION SWITCH AWS-094

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RIAA EQ AWF-021

The phono stage is built around a PNP differential gain driver and “bootstrapped” circuit R/C with a SEPP complimentary output stage.

The differentials in both units were matched to 1% and all remaining small signal devices replaced with a low noise, modern TO92 Fairchild type.

Differential Matching RIAA EQ

Like the above Flat Amp stage, the 22MFD electrolytic here are all tied in a “anti-phase” fashion, these were replaced with single 10MFD audio grade MUSE Nichicon bi-polar electrolytic capacitors.

The 49V filtering capacitors were increased to 330MFD for better filtering and all electrolytics replaced with a low impedance or audio grade KA/KT and PW Nichicons with an increase in operating voltages. New high grade MKP polypropylene input films installed.