Michael S. Pioneer SX-828 Receiver Restoration

Pioneer SX-828 Receiver

Today were show casing a Pioneer SX-828 Receiver restoration. The SX-828 was released for a few years from 72-74, touting one of Pioneer’s best for the time, with impeccable FM/AM circuit design, robust build with plenty of AF stage shielding, complimentary output stage. MC/MM adaptations and a reserved power rating the SX-828 is a personal favorite from Pioneer along with the SX-850.

Power Supply & Filtering

The power supply in the SX-828 is sadly known for some faults…the large voltage regulator to-220 and resistors tend to run very hot burning up the 330 1/4W and damaging the PCB. As with this unit the 330 shunt had already been burned through, it was evident someone had attempted a “repair” on this unit as well.

The 3A diodes were all replaced with Fairchild 3A 1kV diodes along with the FR2-02 diode to a Ultra-Fast (UF) type diode. The common cathode diode was replaced with two UF type diodes.  The affected resistors replaced and increased to 1/2W carbon film type KOA. The electrolytic were all replaced with high temp (105) long life Panasonic FC and Nichicon PW with an increase in operating voltages. The original 2SC373 was replaced with a modern Fairchild TO-92L KSC type NPN transistor and both voltage regulating TO-220 replaced with modern OnSemi TO-220 devices of a more robust rating, along with the new thermal compound and redirected.

BEFORE

AFTER

The large filter capacitors were replaced with high temp (105) 10,000MFD increased from 8,000 and 80V capacitors. The chassis mounted 1000MFD capacitor was kept and leads were cut out of the circuit for cosmetics. Under the chassis a new phenolic strip was installed and a high temp (105C) Nichicon PW 1000MFD/63V capacitor installed, this includes the 220/470MFD capacitors to high temp (105) PW capacitors at the wire-wound bank under the chassis for the lamp + supply. 

Protection

The relay on the SX-828 is unique in that it uses a tab-mount chassis style relay cover vs a PCB fix device. To mount the new low noise Omron relay the original cap was carefully removed from the original relay socket base and exchanged with a new Omron MY 24V relay.

Their are two different design for this PCB seen like the PS. This PCB housed a population of what appear to be both designs. The first point was upgrading the .22MFD capacitors to polypropylene Vishay film capacitors 630V capacitors. The electrolytic capacitors were replaced with high temp (105) long life Panasonic FC and Nichicon PW with an increase in operating voltages. The original TO-92 transistors were a hodge lodge from a prior repair. These were replaced with a modern TO-92/TO-92L Fairchild transistors. A protection mod was installed by relaying a UF diode across the relay output pins for further protection (Flyback Diode.)

BEFORE

AFTER

Amplifier

The SX-828 uses a single symmetric PCB design to lay out the driver section with the compliantly PNP/NPN output bank of the heatsink mounted perpendicular to the assembly.  The electroytics on the driver stage were replaced with long life low impedance PW and KT audio grade Nichicon capacitors with an increase in operating voltages. The AF stage mylar films were all upgraded to high grade WIMA polypropylene film capacitors. The bias adjustment pots were replaced with precision Bourns potentiometers and dc fixed.  

  The drivers also have an epoxy diode that was replaced with two 4148 type general diodes. The initial differential pair were replaced with low noise Fairchild TO-92 transistors match to 1% with sourced NOS Sony driver devices installed from the UK as they do not have a good cross-reference I’ve determined yet. 

BEFORE

AFTER

Pre-Amp AF Stages 

The AF Pre-Amp stage is housed with the variable controls on a double solder PCB (which is a major PITA to work on… but very secure design…)

The differential stage is comprised of a FET array in the front end followed by a NPN gain stage. All of the electrolytic here were replaced with audio grade KT and low impedance Nichicons with an increase in operating voltages. The 1MFD and lower electrolytic and mylar were upgraded to high grade WIMA polypropylene film capacitors.   The NPN devices were all replaced with low noise TO-92 Fairchild devices. 

BEFORE

AFTER

RIAA

The phono stage is housed under the chassis with a shielded plate for added isolation for the sensitive RIAA nature. The SX-828 features a MM stage onboard and an option to add an inline step up transformer for MC carts (rarely found :( )   

The 220MFD capacitor was replaced with a low impedance Nichicon PW capacitors and remaining stages with a KT audio grade Nichicon with an increase in operating voltages. The NPN/PNP devices were replaced with low noise TO-92 Fairchild transistors as well.

BEFORE

AFTER

MISC:

The old wiring had been severely disorganized and burnt in places, I found it best just to re-route the harness are replaced all the PS stage wiring to the drivers etc.

BEFORE

AFTER

Removed Components

Audio Notizen