Pioneer SX-1250 Receiver
Today were showcasing a customers Pioneer SX-1250 Receiver restoration. The 1250 was one of the last few TOTL receivers made along with the SX-1980. Manufactured from 77-79“. The 1250 features some impressive points such as a total of 88,000M Microfarads of filtering capacity and a massive toroidal transformer. Each Driver is symmetrically isolated along with the power and protection assemblies. The front RF stages each have their own potted shield assembly as well. Similar to the other 1250's we've done we featured this unit with the multi-facet SMD warm white custom LED's.
Power Supply AWR-107 and AWR-106 Stabilizer PS
The large toroidal is preferred due to the winding characteristics which is touted to display almost negligible inductance slew. A large 20W 3.3OHM current limiting resistor is used, thermally coupled to a thermistor diode. Each 44,000MFD filter section is dedicated to the independent rectification.
The AWR-107 uses large 470MFD axial’s which were replaced with Nichicon TVX 470/100 axial capacitors, along with the smaller 35V 470MFD to Panasonic FC high temp (105C) capacitors.
The AWR-106 Stabilizer assembly controls the rail points and driver stability. As above the 1000MFD axial was replaced with a Nichicon TVX variant. The remaining electrolytic were replaced with a high temp (105C) Nichicon PW and Panasonic FC capacitors. All the regulated TO-220’s were updated with high quality robust MJE150/32/33 BJT’s and 869/945 type TO-92’s to 2383/2690 BJT devices. 1S2473/and s10B type didoes were all replaced with 4148 standard and ultra-fast 4007 type diodes.
The four original 22,000MFD 80V Nichicon capacitors were replaced with high grade Nippon Chem-Con 22K/100V filter capacitors and new .1MFD polypropylene rectification bypass films.
The protection circuit of the 1250 is comprised of a multi facet identifier; DC Deviations, DC Overload saturation and a Soft Start Relay. The overload consists of a resistor array and the relay driver circuit which shunts excessive current flow through Q1-D3. The Original was replaced with a 48V Omron. A new MY2-24V relay was also installed not he AWM-091 PCB. All of the electrolytics were replaced with the same above high temp (105C) PW and Panasonic FC type capacitors. Driver transistors Q5/ were updated to TO-220 2690 BJT“s and the the 945 to a 2383 type.
The AWF-021 circuit uses a tied emitter differential transistor group for 1st order gain while employing a SEPP )(single ended pp) complimentary circuit for voltage gain drive, high grade polystyrene and precision 1>% resistors results in incredible noise reduction and almost no discernible deviation (>.2dB) The larger filtering input capacitors were replaced with 470/220MFD Panasonic FC type capacitors and low impedance, audio grade PW/KA’s for the AF path. The original input films were upgraded to a polypropylene Solen (1MFD). Initial PNP Gain pairs were replaced with 1% matched 992’ TO-92 BJT’s and the original 2473 diodes to modern 4148 type.
AF PATH — FLAT AMP (AWG-042/AWS-094/ CONTROL PCB (AWG-041) FILTER (AWM-089)
The Flat amp assembly is comprised of a complimentary FET transistors in a direct coupled stage, the control assembly which features a stepped multi frequency attenuators in a two stage NFB circuit. Dont forget the 3 electrolytics on the switching assembly PCB's either, this is the time to clean each switch and contact using Caig Labs D4/F5.
The original tone and flat assemblies were comprised of a large assortment of tantalum type capacitors which were replaced with audio grade Nichicon KA capacitors, remaining electrolytics were replaced with Nichicon PW low impedance capacitors. Original AF path mylars and polypropylenes were updated to WIMA and Panasonic ECW polypropylene dielectric capacitors.
Designed around a classic a/b configuration the x1250 encompasses a 1st order differential pair gain stage tied to a constant current mirror design which mutually phases signal for a more stabile drive via Q3-4 und Q5/6 through a/b biasing resulting in a PP bias scheme. 3rd order drive is comprised of a tiered complimentary darlington arrangement via BJT’s through to outputs.
The differential pair transistors (TO-92) were repalced with 1% matched 992fbu transistors along with he Q11 to a 1845 device. Pre-drivers were updated to TO-220 2690/1220 BJT and the final driver to a high grade robust MJE15032/33 TO-220’s with new thermal compound and insulators. All of the 2473/and 1886 diodes were updated to 4148/ and ultra-fast 4007 type diodes. The original bias/offset potentiometers were upgraded to multi-precision Bourns trimmers. All of the electrlotics were upgrade to low impedance, high temp (105C) PW’s and remaining input film to a WIMA PP film capacitors. Remaining AF films were upgrade to high quality Panasonic ECW polypropylenes.
Tuner Assembly AWE-068
The SX-1250 has an incredible FM/AM sectional, with a highly sensitive 5-gang FM tuning capacitor and 3 gang AM, the fm is driven from a dual stage gain circuit while employing an array of MOS/FET and AM IC gain stages (HA1138). Both tuning capacitors were carefully cleaned and treated.