Recently a Pioneer QX-646 was dropped off for diagnosing distortion when playing LPs through the phono preamp. The noise was described accurately as a static, crackling noise that was only present in the phono mode and occasionally in aux mode. The receiver was tested for output and then signal path was traced to determine the origin of the noise and repair plan. The power amp section of the receiver is still strong in 2 channel mode through the aux input the receiver still can produce 13wpc which is the factory spec. When signal is pushed into the phono preamp there is awful distortion causing the waveforms to bounces around sporadically.
Once narrowed down to the phono amp the signal was traced through the EQ board and the first section which is the phono preamp showed the distortion was present. A plan was established to replace all electrolytic capacitors on the EQ board and transistors which are vintage 2SC1312 and 2SA725. Modern Fairchild KSC1845 and KSA992 will be used for the transistors. For the capacitors Nichicon PW series electrolytics will be used for the large caps and the 8 1uf/50v low noise electrolytics will be replaced with Panasonic ECQ film caps. Parts were ordered and work began.
All capacitors, and transistors were replaced. There was a discrepancy in the pin outs of the original transistors between the schematic and what was actually on the board. The service manual lists the transistors as ECB but they are actually BCE. The modern substitutions are ECB. The screen print on the board was accurate. Here is the before and after of the board. Now on to testing.
