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

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Greg's Sansui 9090 Receiver Driver/PS Focused Rebuild

Sansui 9090 Receiver

Today we’re showing off a sansui 9090 that can in for a power supply, driver rebuild and testing.

The 9090 is an extremely poplar Sansui we get in for restorations along with the DB version. The major differences are the integration of the Dolby network, driver design and form factor and the a complimentary output vs the quasi output of the 9090. 

F2546 Power Supply

The main PCB has the AF/Ax power supply as a main large PCB in the centre of the unit which also houses a PCB card slot for the driver to sit perpendicular to the F2546.

BEFORE

AFTER

All of the electrolytic were replaced with high temp (105C) low impedance PW Nichicons with an increase in operating voltages including the 1000MFD capacitor. The BJT were all replaced with NPN KSC TO-92L Fairchild devices along with the TO-220 voltage regulator transistor and two ultra-fast 4007 diodes for a common cathode replacement. 

MOLEX Modification

One of the common issues on the 9090 is the fact that the AC lamps is ran through the selector switch as well which causes arching to occurring within the phenolic switch destroying it! This simple mod changes the pinout location keep the dial illuminated while the units is always on just like the 9090DB version and saves your expensive hard to find selector switch! New custom SMD type LED's were installed as well.

F2463 DRIVER

The driver assembly on many of the original 9090 and 9090db we get in exhibit the sam exact issues. heat damage and cascade failures from the original fusible resistor design. Typically someones been in there trying to fix it and mucks it up worse.

BEFORE

AFTER

All of the fusible resistors as well as glue covered resistors were replaced with KOA carbon film type 1/2W as well as the epoxy diode VD1212 to modern 4148 type in series. All of the electrolytic were replaced with Nichicon high temp (105C) VP BiPolar/PW with an increase in operating voltages. The .47 input and .1 mylar were both replaced with high grade WIMA polypropylene film capacitors.

The TO-92 differential pair NPN were replaced with 1% matched beta low noise Fairchild TO-92 transistors and bias/ pre-drivers to modern low noise and robust TO-126 Fairchild transistors. Both sets of potentiometers were replaced with person Bourns devices.  The TO-3s were removed and new compound applied as well.  Often times Ill find fuses half assed soldered on this board which is cleaned up and presented better.

Cleaning Tone Assemblies

All of the original tone control boards were point inspect and cleaned for better operation and the PCB flushed with lab grade anhydrous alcohol 

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Fisher 800C Receiver Restoration

Fisher 800C Receiver 

Today were featuring a very special tube receiver..the Fisher 800C considered by many to be one of the best receivers ever made available. The Fisher 800C was released after the early 500/800B in 1962 as an upgrade to the Fisher 500C which did not include AM reception and MPX integration.  The Fisher 400/500/800C utilize a compliment of 7591 power tubes for a 30-35W .7% THD rated output factory output with a fixed bias.

As often with these old tube amplifiers, and receivers their are a few major contenders that need to be addressed such as the coupling network for the output stage, modify the 7591 fixed bias topologies amongst others outlined below.

7591 Output Stage

Several modification were integrated into the output stage which allows for bias matching between 7591, regulating the negative bias voltage and changing the coupling networks and grid resistors.

The original designed uses a .047/400V coupling capacitors with a 330K grid resistor. For more reliability and to handles todays tubes better we replaced the grid resistors with 200K ohm resistors to limit the value of reverse grid current  voltage under maximum rated dissipation. In connection we replaced the 7591 coupling capacitors with four .1/630V high grade MKP polypropylene film capacitors with a noted increase in operating voltages in order to help maintain time constance in the RC coupling circuit at the driver/output stage with the decrease in grid resistor resistance. 

After the coupling stage is complete we move onto the 7591 screen stability modifications which is straight forward; I isolated each 7591 from the original buss lead and daisy chained the Pin 1 input with a inline 100OHM resistors to pin 4 after relocation the Pin#8 feed, this effectively eliminates transient arching that can severely damage tubes.

Finally for the output stage we integrate what called a IBAM designed by Drew Bolce for early Fisher versions. The IBAM (Independent Bias Adjustment Modification) removes the fixed bias circuit and allows the individual bias adjustment of each 7591 output respectively to 32mA draw. The IBAM was built using low impedance high temp (105C) Nichicons after trying out FC Panasonics first and carbon film resistors then  installed on the chassis through a new mounting point near the output tubes…along with the Bias 10OHM cathode resistor modification allows for easier and accurate bias settings.  Last checked I was able to adjust to a perfect -22.05Vdc @ 30mA-30.5mA for all four 7591 :) ... nice

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PS Modifications

With the above IBAM modification we installed a way to attenuate the negative dc voltage rails for the bias circuit and pre-tubes using a 10K precision Bourns potentiometer along with replacing the original Siemens selenium bridge rectifier with a modern BR3 type 8A 400V+ bridge rectifier. As the negative voltage is increased this reduces our current draw allow to dial in the appropriate voltage for biasing modern tubes. Often times the required -22V is lacking in unrestored models causing bias issues. 

The original cardboard dual capacitors was replaced with 1000MFD low impedance high temp (105C) Nichicon PW capacitors as well as the associated .1MFD film to a high grade Mundorf Tube M Cap 630V. The B+ developing diodes CR6/7 were replaced with modern 3A Fairchild diodes. The .01 large axial capacitors were replaced again with high grade Mundorf EVO Oil type .01/630V film capacitors for better protection and filtering. 

The remaining capacitors for the 12AX7 drivers were replaced with high grade MKP polypropylene film capacitors with an increase to 630V operating voltages. The original axial electrolytic were replaced with low impedance high temp (105C) Nichicon PW capacitors with an increase in operative voltages as well as associated carbon film resistor changes. Also replaced with the C111 8MFD  to a high grade MKP polypropylene 630V film capacitor. 

The front end AM/FM capacitors were all replaced with high grade WIMA polypropylene 630V rated and MKP polypropylene 630V film capacitors including the MPX 1MFD and all associated .022MFD/.047 couplings. 

New LED’s installed in both front glass slides and Meter indicators.

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