Pioneer SX-1250 Another Restoration!

Pioneer SX-1250 Receiver. 

We’ve recently done a slew of these popular Pioneer SX-1250’s so this write up will be a little more brief.  To recap the SX-1250  was one of the foremost and sought after SX-series designs fetching still a more than attractive price for collectors. Built from 1977-1979 the 1250 features a massive toroidal transformer and a total of more than 88K microfarads filtering capacity.

New Custom built LED's

Power Supply AWR-107/AWR-106 PS and Stabilizer & AWM-091 Protection Assemblies

The large axial 470MFD capacitors were updated to a Nichicon TVX along with associated 470/35V to high temp (105C) low impedance/long life PW/FR capacitors. On this model the flyback diode was also updated to a ultra-fast #400 type diode. 

The AWR106 stabilizer  had the large axial electrolytic 1000MFD again updated to a Nichicon TVX and associated electolytics to a Panasonic FR/Nichicon PW type electrolytic. This particular stabilizer board had issues regulating the 65V B+/B- rails due to a leaking 2SB507 TO-220 driver device.  All of the small signal TO-92 and TO-220 devices were upgraded to 992/1220/1845/MJE on Semi devices and all WZ-140 with new new diodes and the 2473 to 4148 diodes. 

The AWM-091 assembly consists of the dc protection circuit.  The protection network detects dc deviations/overload parameters as well as a 48V Soft Start Relay.  The original MY4 relay was updated to a new low noise Omron MY-4 relay and associated electrolytic capacitors to low impedance, high temp (105C) Nichicon PW/FR Panansonics.  The drive transistors were updated to TO-220 2690 BJT’s and the 945 to 2383 BJT’s, remaning 2473 diodes replaced to 4148 type. 

Filtering

The X4 22,000 microfarad filtering capacitors were replaced with high grade Nippon Chem-Con 22,000/100V filter capacitors and the associated .01 film upgraded to a Panasonic ECW type.

AF Stages RIAA AWF-021 Flat Amp/Control AWG-041/AWG-042/094

The SX1250 RIAA design was well done and extremely quite. using a 1st order tied emitter/SEPP gain topology and 1% resistors. The initial PNP pairs were updated to matched ksa992 groups. The large 470/220MFD) input filtering capacitors were updated with FR Panasonics and remaining AF stage with low impedance PW Nichicons. Input capacitors were replaced with high grade Panasonic ECW polypropylene film capacitors all remaining 2473 diodes replaced to 4148 type.

Remaining tantalum and electrolytic on the AM AWG-041/042/94 PCB’s were updated to low impedance Nichicon PW type and Nichicon KA audio grade electrolytic. Mylar input/out films were updated to high grade WIMA and Panasonic ECW polypropylene film capacitors. Small signal transistors within the AF front ends were replaced with matched 992/1845 type TO-92 devices.  All the functions and front end switching were removed as a pcbs and cleaned and lubricated using Caig Labs products. 

Drivers AWH-048

In earlier post I break down the drive topology as;

“ 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 pairs BJT’s were replaced with 1% matched ksa992 devices and Q11 to a 1845.  Pre drivers were updated to 2690/1220AYS groups. All 2473 diodes to 4148 type and ultra-fast 400#. Bias potentiometer was replaced with a precisions Bourns trimmer, all electrolytic and large 470/100V were replaced with low impedance PW Nichicons and Panasonic ECW polypropylenes replaced the original and tantalums.

Audio Notizen

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