Vintage Hi-Fi Audio Restorations: S. Crist Pioneer SX-626 Receiver Restoration

Pioneer SX-626 Stereo Receiver

The Pioneer SX-626 was released in the early 70's from 71-73. It featured the ability to maintain multiple sources including phono and a stabile and common quasi-complimentary amplifier output design. A classic Pioneer sound, accurate and detailed, while still in the blue face era of Pioneer aesthetics.

Power Supply AWR-029

The SX-626 has a very stabile and simplified single ended design. Composed of a 3A full wave bridge and a 1A full wave rectification stage. The 30V and 57V SE rails are established via the 3A regulator and the filter stage for the unregulated 57V supply. The AF and RF front ends are derived from the full wave for a single ended 13.5 and 25V supply.

The 3A and 1A rectifiers were all replaced with Ultra-Fast, Soft Recovery axial diodes and bypassed with high grade WIMA film capacitors, including a new 14V zener. The TO-220 regulators were replaced with modern, robust MJE OnSemi devices and a new TO92L low noise Fairchild device.

The capacitors were all replaced with a high temp (105C) low impedance Nichicon PW with an increase in operating voltage. The initial full wave filter capacitors were increased to 1000MFD for better filtering.

BEFORE

AFTER

SE Regulated Supply Confirmation

Filter Capacitor

The single 3300MFD 57V filter capacitor was replaced and increased with a computer grade 6800MFD filter capacitor. Also replaced with the axial 200MFD capacitor off the selector with a radial 220MFD PW Nichicon capacitor.

57V SE Confirmation

RIAA EQ W21-004

The RIAA stage in the SX626 is fairly accurate as shown in the RIAA Curve performed below after the rebuild of the stage.

Electrolytic capacitors here were replaced with a low impedance PW and a audio grade KT/KA Nichicons with an increase in operating voltage. The 30V dc coupling capacitors were increased to 330MFD for better filtering. The .22MFD and two ceramics were replaced with a high grade WIMA polypropylene film capacitors. New low noise, modern TO92 Fairchild devices installed.

BEFORE

AFTER

RIAA Curve Response

Pre-Amplifier AWG-008

The audio front of the SX626 employs a JFET input, high impedance output stage as well as attenuated tone controls and HI/low pass filters.

The small signal devices were replaced modern, low noise TO92 Fairchild devices. Electrolytic capacitors were replaced with low impedance PW and audio grade KT/KA capacitors with an increase in operating voltage. The 30V dc coupling capacitors were increased to 2200MFD for better filtering. High grade WIMA polypropylene films were also installed including the input bypass. JFET’s were removed and plotted to confirm solid, low noise operation.

BEFORE

AFTER

JFET Mapping

Driver Stage AWH-002

The driver stage is typical of a solid economical approach of the time due to the higher costs of PNP silicon transistors. Employing NPN voltage gain input, quasi complimentary design which is developed via a complimentary darlington design (Sziklai Pair) and 2200MFD cap coupled output.

Input devices and remaining TO-72 style devices were all replaced with modern, TO92 and TO126 low noise Fairchild devices. Bias and offset adjustments were replaced with precision Bourns potentiometers.

The large 2200MFD devices and remaining electrolytic were replaced with a audio grade KT/KA and low impedance, high temp (105C) Nichicon PW type with an increase in operating voltage. Bypass mylar films were replaced with a high grade WIMA polypropylene films.

BEFORE

AFTER

Bias Confirmation (17(-/+mV))

The SX626 RF section employs a FET front end and 4-gang variable capacitor, where as the Am front end uses the AGC “(automatic gain controller)”. All of the electrolytic capacitors were replaced with a low impedance Nichicon PW capacitor with an increase in operating voltage.

BEFORE