Paul's Pioneer SX-780 Receiver

Pioneer SX-780 Receiver

Today were showcasing a customer Pioneer SX-780 Receiver. The SX-780 is in the series of black dual black lit output meters, as well as tone defeat, integrated protection with a rating of 45W @ .05%

This unit came in with a faulty STK-0050 Power Pack transistor, which is fairly common on these as any speaker shorting and excessive current can easily blow these devices. As with most Pioneers, the power supply originally exhibits significant radiant heat, tied to the output devices and a single heatsink the ambient operating temperature of the SX-780 is rather hot!

Power Supply  (+Protection) & AC/DC Rectification Modifications

One of the biggest improvements Ive noted is upgrading the rectification stages in these Pioneers, the entire circuit relies on clean, constant power which starts at rectification.

AC/DC Rectification Modification

The SX-780 uses a full wave 1A bridge for the regulated power supply as well as a full wave 3A bridge for the driver rail supply.  The old axial type 10E & SR3 diodes were replaced with a custom PCB fitted  modern On-Semi TO-220 MUR type Ultra-Fast, Soft Recovery rectification diodes as well as new .047MFD high grade Panasonic ECW polypropylene film capacitors. The smoothing capacitors were also increased to 330 and 1000MFD low impedance, high temp (105C) Nichicon PW’s resulting in a cleaner, smoothing DC ripple voltage.  The 8000MFD -/+40V smoothing capacitors were replaced with modern Nichicon capacitors, which fix to a PCB mount.

BEFORE

 AFTER

Ripple Voltage

The regulated Power Supply is separated into several stages, with the front end delivering the initial -/+39.5Vdc supply, further down the PCB is the -/+23V supply for the AF/RIAa stages (highlighted below under RIAA stage) This area of the PCB also houses the new  MY4-48V type relay which was installed.  The SX-780 uses a unique monolithic IC PA3004 designed by Pioneer which includes; DC deviation, B-/+ detection, AC detect, discharge/switching and power muting. 2Sk34 JFETs also populate this circuit. 

All of the electrolytic capacitors were replaced with a high temp, low impedance Nichicon PW capacitors with an increase in operating voltages.  Small signal devices were all replaced with better dissipating TO-92L/TO-126 and TO-92 modern Fairchild devices as well as both both NPN and single PNP voltage regulators with new performance grade Wakefield thermal compound. 

BEFORE

AFTER

Meter Circuit
(Spec’d for 8OHM Impedance Loads) The electrolytic were replaced with a low impedance, high temp (105C) Nichicon PW, with both adjustment pots replaced with precision Bourns devices. 

Drivers Section

The PS/AF/RIAA/Driver/Protection are all populated on a single PCB with the RF stages in the rear, separate of the MAIN PCB.  The initial PNP dual devices makes up the initial differential with a dual non constant current stage, driven by the pre-drivers Q11-15. The output is composed of a hybrid monolithic IC STK-0050 power pack transistor comprised of a darlington complimentary design.

The dual PNP and dual NPN devices with a common base and common emitter pinout were replaced with low noise, modern TO-92 devices with a matched Beta (see HFE mapping below). 

Constant current drivers/pre drivers were all replaced with modern, low noise TO-126/TO-92L Fairchild devices. The STK packs were removed, with new performance Wakefield thermal compound applied and joints reflowed. New precision Bourns potentiometers replaced the offset adjustments. 

BEFORE

AFTER

New STK-0050 Hybrid monolithic IC' (45W @ .05%THD) installed due to defective STK-0050 found in circuit, copious amounts of performance grade Wakefield thermal compound applied across both devices.

AF Stage + Secondary PS Circuit

As mentioned above the regulated power supply develops an initial dual rail 40Vdc source, as well as a secondary 23.7/-23.1V supply for the front end/RIAA monolithic IC’s The DC coupling/filtering  capacitors here were all replaced and increased from 100MFD to 220MFD low impedance Nichicon capacitors with an increase in operating voltages for better filtering. 

The tone control, flat amplifier stage uses two input TA7136P monolithic Pre-Amplifier IC’s with a nominal voltage gain of about 31.4dB, also employing a CR-type tone network with a defeated switching option.

All of the electrolytic including the tone control (which also employs two 2SK34 JFET’s) were replaced with a audio grade KT capacitor with an increase in operating voltages.  All the 1MFD> electrolytic were replaced with a high grade WIMA polypropylene film capacitor. The 100MF DC filter capacitor was replaced and increased with a 220MFD low impedance Nichicon PW. New low noise TO-92 Fairchild devices were also installed replacing the old PNP transistors.

BEFORE

AFTER

RIAA EQ (PHONO)

The RIAA EQ stage utilizes two monolithic SE HA1457 IC’s with an open-loop gain of 82dB, featured around a NFB type EQ, providing a nominal gain of 35.5dB. 

All of the electrolytic capacitors were replaced with audio grade and low impedance type PW and KT Nichicon capacitors with an increase in operating voltages. The IC coupling ceramics were replaced and increased with a high grade WIMA polypropylene to a .01MFD value.

BEFORE

AFTER

RIAA Curve Response Tested

RF Stage

The RF stage is the only isolated PCB section due to potential instrinsic parasitic noise or RF inteference. The AM features a 2-gang tuning capacitor and HA1197 monolithic IC and single ceramic filter. The FM consists of a 3-gang tuning capacitors, dual gate MOS-FET, initial gain amp circuit, multi ceramic filters, HA1201/PA3001-A monolithic IC’s. 

All of the capacitors were replaced with a low impedance Nichicon PW with an increase in operating voltages.

New custom 8V SMD Wedge type warm white LED's installed for a nice even glow and all glass/acrylic cleaned

AUDIO NOTIZEN