Pioneer SX-424 Receiver
Today were highlighting a small Pioneer SX-424 Receiver that came in for a quick repair. The SX-424 was produced during 1972-74 as the predessor to the SX-434 which came out after the SX-525. The SX-424 is a great solid unit, stabile and modest output with a good distortion factor.
The issue with this SX-424 was severe oscillation occurring in the left channel only noted after about 6dBW output. This was due to several poor electrolytic’s that some half-twit tech had incorrectly installed which stress the driver stage devices and caused some reverse voltage issues or premature breakdown.
Due to the single PCB topology the front end AF stage/Driver was rebuilt as high heat from the TO-220 output heatsinks position. All of the electrolytic capacitors were replaced with low impedance and audio grade PW/KT Nichicon capacitors with an increase in operating voltages. The input and coupling values were upgraded to a high grade WIMA polypropylene film capacitors as well.
The AF state front end BJT’s were replaced with low noise TO-92 Fairchild devices. The differential stage NPN, driver and pre driver BJT’s were all updated to TO-92/ TO-92L Fairchild low noise devices. This circuit utilizes a fixed bias idle.
All of the controls, contact were cleaned as well as new SMD Custom LED’s installed.