Paul's Pioneer SX-850 Receiver

Pioneer SX-850 Receiver

Today were showcasing the restoration of a customer Pioneer SX-850 Receiver. The SX-850 is very closely akin the SX-950 short of the stepped up rails and driver compliments.   Rated at a nominal .1% continuous power the SX-850 series has a fantastic RIAA stage and robust Power supply.

Power Supply Board (AWR-101)

Power supplies are one of the most critical areas of a units operation, without a clean and stabile rail source the unit cannot perform optimally.  During this restoration particular attention was paid to significant upgrades done on the power supply.  Using todays modern components we can dramatically update the rectification stages for a MUCH more quite, cleaner and reliable AC/DC conversion. 

The SX-850 uses a 1A Full Wave Centre Tap, 1A bridge and 3A bridge rectification for the established  -/+ 60/63V and -/+45V rails.  The Full Wave Centre tap rectification was replaced with two robust, Ultra-Fast low switching noise On-Semi MUR TO-220 rectifiers and a WIMA polypropylene bypass. The 1A bridge rectification was replaced with a custom PCB populated with again Ultra-Fast, uber low noise axial Fairchild rectifiers and the 3A bridge rectifier was rebuild, again on a custom PCB populated with On-Semi TO-220 type Schottky rectifiers. The resulting product is an incredibly clean, fast and low transient AC/DC conversion which made DRAMATIC effects on the over presentation of this. This has been one of the most impactful upgrades I’ve notated to date for the record short of low noise, better 1st-4th order driver upgrades. 

Rail Confirmation

The remaining devices and voltage regulators were all replaced with modern, robust Fairchild TO-220 regulators, Fairchild TO-92L and TO-126 devices for better dissipation.  The original electrolytic capacitors were all replaced with a high temp (105C) Panasonic FC and Nichicon PW with an increase in operating voltages. Due to a failure noted, several 1/2W carbon film KOA resistors were installed. 

BEFORE

AFTER

Filter Capacitors

Both of the original 15,000MFD filter capacitors were replaced with a computer grade Nippon 18,000MFD capacitors for an increase in current handling and power reserve. 

Protection Board (AWM-062)

The Relay to the SX-850 is a chassis mounted 24V device, the original case is retain due to the odd mounting pattern and wired directly in circuit.  The electrolytic on this assembly are known to become leaky and fail in circuit causing a random dis-energize cycle from the timing capacitor.  The protection incorporates a overload and short detection. 

The electrolytic capacitors were replaced with a high temp (105C) Panasonic FC and Nichicon PW  capacitors with an increase in operating voltages. T]wo .22MFD WIMA  high grade polypropylene film capacitors were installed.  All of the transistors were replaced with better dissipating, robust TO-126 and TO-92L modern Fairchild devices.

BEFORE

AFTER

Power Amp Board (AWH-059)

The SX-850 and 950 share almost identical driver stages, short of the increase rail voltages and more robust output devices on the SX-950. With the below rebuild the same standards are achievable in both units.  This series uses a single symmetrical assembly with a full NPN/PNP output compliment stage. 

The initial differential pair transistors were matched within 1-2% as shown below in their respective curve mapping.  Current, Pre-driver and Driver devices were all updated with modern, low noise TO-126 and TO-92 Fairchild devices. 

New precision Bourns potentiometers were installed for adjustments and several Fairchild axial diode array.  Electrolytic capacitors were all replaced with a low impedance, high temp (105C) Nichicon PW and audio grade Nichicon KT capacitors with an increase in operating voltages. 

BEFORE

AFTER

Bias Confirmation

Equalizer Amp Board (AWF-011)

Two versions of the RIAA stage are known in the SX-850/950 series with only slight differences in the AF stage. The original PNP compliment populated are known to develop some noise associated issues.

All of the low signal devices were replaced with TO-92/TO-92L low noise, modern Fairchild devices for a significant reduction in hum noise. The electrolytic capacitors were all replaced with a audio grade Nichicon Fine-Gold and KT along with low impedance Nichicon PW capacitors with an increase in operating voltages. A slight modification to the circuit was integrated to achieve the noted below response, the end product is a very linear RIAA Curve with only slight deviation at 15Khz. 

BEFORE

AFTER

RIAA Curve Response

Flat Amp Board (AWG-038), Tone Amp Board (AWG-039) and Switch Assembly Board (AWS-094)

The SX-850 uses a NFB centered circuit with a 2stage direct coupled gain stage and a 2Sk30 JFET per channel, the resulting frequency response is very suitable with only a deviation of -0.5%dB. The Tone control assembly has a variable Bass and Treble control a tone defeat is employed as well.

All of the electrolytic capacitors on both assemblies were replaced with a Nichicon Fine-Gold and KT audio grade along with a low impedance Nichicon PW with an increase in operating voltages. High grade WIMA polypropylene film capacitors were installed as well.  

All small signal transistors were replaced with a low noise, modern TO-92/TO-92L type Fairchild devices and new Vishay general axial diodes. 

BEFORE AWG-038

AFTER

BEFORE AWG-039

AFTER

Hardware

New LED’s installed to give it a more cleaner, define glow across the dial using custom SMD warm white LED’s along with new LED indicator lamps. 

AUDIO NOTIZEN