Sansui 771: incorrect polarities for C17 / C18 on F-1504 preamp board and schematic ?

[david winter]

Hi,

I have been wondering what caused some low background noise on a 771 which I almost completely restored.
Two electrolytics on the F-1504 preamp board (C17 and C18) apparently have incorrect polarities both on the schematic and the pcb silkscreen. They had -5V accross them. The 2SC1313 were replaced by KSC1845 and the 2SK34 FETs are ok.

Anyone can confirm this ? Or did I miss something ? Thanks!

 

[Hyperion]

apparently have incorrect polarities both on the schematic and the pcb silkscreen.

Looking at the schematic for DC paths, and the polarity shown for C17 & C18 I would say that the schematic is correct. The resistor values and the positions of other capacitors (blocking DC) suggests to me that correctly the '+' of those capacitors (as marked on the schematic), will be more positive than the '-'

Additionally there is no split supply (i.e. +ve & -ve) to that board, only +ve referenced to chassis ground, so you cannot be seeing a -ve voltage, perhaps you had your multimeter probes reversed?

You know that the schematic designation for those (originally orange) capacitors is 'BRN' which means that they are 'low leakage' type, and need to be replaced with same low leakage type capacitors i.e. specifically 'low leakage' electrolytic, (or film capacitors) for best performance.

 

[EastPoint]

Put some Wima film caps in there and you'll be fine.