Pioneer SX-828 Receivers Part II
Here were showcasing three Pioneer SX-828 restorations broken down by customer, the first two will be showcased on this article, the remaining SX-828 will be showed on the proceeding article. Both feature identical build qualities/restoration approaches verbatim articles with unit specific photos and highlights.
You can read the first two restorations here; http://marantzhallo-fi.blogspot.com/2018/05/rtyner-two-pioneer-sx-828-receiver.html
The SX-828 is a personal favorite Pioneer, right next to the SX-850. Both have a very warm, classic 70’s tone and aesthetic, the SX-828/850 are probably the most referred Pioneer’s I recommend. Released from 1972 to 1974, rated at a nominal 55Watts @ .5%THD factory and very versatile with multiple phono inputs, dual dubbing and three outputs.
Power Supply AWR-010
The SX-828 Power Supply features both regulated supplies for the RF/AF stages as well as piggyback circuits for the protection circuit. The AWR-010 is designed around a primary -/+44V 3A bridge rectifier for the drivers supply. The regulated 31-35Vdc supply for the RF/AF stages are comprised of a simple full wave rectifier.
It should be noted almost every SX-828 I’ve restored I have seen significant heat stress on the PCB from R12 resistor which is current limiting supply for the 12V SE. R12 develops approximately 0.0606mA with a 1.188Watt dissipation considering a 20Vf. I recommend increasing the dissipation rating to 3-5Watts MOX or equivalent as we did in these units.
The TO-220 regulators were replaced with modern robust TO-220, TIP Series devices. The 1A diode was replaced with a Ultra-Fast, Soft Recovery type with its associated dc filtering capacitor increase to 1000MFD. The remaining TO-92 devices were replaced with modern Fairchild TO-92L devices for better dissipation.
The remaining electrolytic were replaced with a high temp (105C) low impedance Nichicon PW with an increase in operating voltage. Also replaced was the 13V axial Zener for the RF/AF stage supplies.
BEFORE
AFTER
Filter Capacitors
The -/+ 44Vdc filter capacitors were replaced and increased to 10,000MFD Computer Grade capacitors for better DC filtering and subsonic responses. The associated 1000MFD capacitor was cut and retained for aesthetics, a new 1000MFD Nichicon PW was relocated to a existing terminal strip under the chassis, along with the 470MFD indicator DC filtering capacitors.
-/+ 45V CONFIRMATION
Protection AWM-025
The SX-828 features, more often than not, two variants of protection; the AWM-025 (A/B/C) used in the 838 or 7XX series and the earlier designed W28-008 assemblies. Both monitor significant DC deviation at the output/input development. One PCB variant differs in compliment design and the use of a trimmer, in that assembly we replaced the adjust with a precision Bourns potentiometer.
The electrolytics were replaced with a high temp (105C) low Impedance Nichicon PW with an increase in operating voltages. The 0.22MFD films were replaced with a high grade WIMA polypropylene type. Small signal devices were all replaced with modern, low noise Fairchild TO-126 and TO92(L) devices with better dissipation.
BEFORE
AFTER
RIAA W21-002
The phono stage W21-002 uses a SE supply and a NPN gain stage input/output PP design for both MM and MC cart’s.
The DC filtering capacitor was replaced with a 220MFD low impedance PW Nichicon, remaining electrolytic capacitors were replaced with PW’s and audio grade KT/KA Nichicon’s with an increase in operating voltage. High grade WIMA polypropylene films were also installed. All small signal devices were replaced with modern, low noise TO-92 Fairchild devices.
BEFORE
AFTER
Tone AWK-012
The pre-amplifier stage (Tone/AF) in the SX-828 is a rather unique construction, using a double-sided solder technique (making a real PITA) and heavier layer construction than other PCB’s onboard. The AWK-012 utilizes a Jfet 2SK30 front end versus a NPN/PNP differential.
The NPN/PNP devices were all replaced with modern, low noise TO-92 Fairchild devices. 1MFD< mylar were replaced with a high grade WIMA MKP/PPE polypropylene film capacitors. Remaining capacitors were replaced with a audio grade KT/KA and low impedance PW Nichicon with an increase in operating voltage. Hardware such as potentiometers were removed/stripped and Caig Labs products applied.
BEFORE
AFTER
Driver AWH-010
The single symmetric driver format shares a large perpendicularly to the PCB. The AWH-010 featured a PNP differential gain stage thru a complimentary pp output.
The differential pairs were matched within 5% and pre-driver/drivers all replaced with modern, robust and low nose TO-92, TO-126 and TO-220 Fairchild devices.The bias and offset adjustments were replaced with precision Bourns potentiometers on the under of the chassis for easier adjustment near tests points. The VD diodes and Zener (13V) were replaced with a series and axial type Vishay general 4148 diodes. The electrolytic capacitors were replaced with low impedance, high temp (105C) Nichicon PW’s and high-grade WIMA polypropylene film capacitors.
BEFORE
AFTER
Differential Pair Matching
RF AWE-016
The AM/FM front end is shielded by a robust amount of metal covering unlike some other SX series. All of the electrolytic capacitors populating the assembly were replaced by low impedance, high temp (105C) Nichicon PW capacitors.
Display/Meters
Both Display and Meters had the original fuse lamps replaced with custom SMD/LED 8V type.
Sadly one of these units is tinted beyond cleaning, this is also a excellent example of why its smart to convert to LED as early as possible. Original incandescent lamps radiant a high degree of heat which damages the plastic housings and meter patina. In this case one of the meters suffered tinting from the heat permanently changing the hue.
AUDIO NOTIZEN
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