I have already started a topic under restoration section, but my RC760 has got technical problems, so here we are. I am not counting bad belts and dirty switches/pots under this. There are some pictures on that thread. Let me start: There was a certain amount of glue holding C760 and surrounding elements together, probably to increase stability and prevent tearing of the capacitor off the board during transit and use. Long story short - the glue solidified in dark brown brittle mass. After that, a lot of corrosion was under it, and R1000 got very hot
due to poor ventilation(see below).
This was the situation after I have removed the "glue" using small pliers.
TFK-RC760_corosion_andR1000.jpg 112.25K 16 downloads
There was corrosion above and under board. Clear sign that something went wrong - capacitors have leaked but there is still work to be done.
TFK-RC760_PCBcorosion_andR1000.jpg 120.67K 15 downloads
When turned on, the box produced distinct humming sound (50Hz in EU/Russia/Japan, 60Hz in USA/Japan). The humming has gone away after I have replaced 220uF and 2200uF filter capacitors. Also, after several minutes, R1002(R1000?) overheated and unit went silent until it was left to cool off. Unfortunately, I have a service manual that does not list this 270 ohm 1/4W resistor... as well as T904, T905 and other elements around it. Go figure.
TFK-RC760_C760R1000.jpg 30.7K 15 downloads
I have replaced both the capacitors and resistor. There is the board pictured from above, after the electrolytic caps are replaced, and 270 ohm is waiting underside to be cleansed.
TFK-RC760_R1000.jpg 119.08K 17 downloads
Now I have to find another schematic diagram or draw this part myself. Most of them on the 'net are exactly this one. It seems to be a copy from Portugal.
TFK-RC760_schematicdiagram.JPG 126.25K 21 downloads
There are two transistors, T904 (2SA966) and T905 (2SC1815), coupled together via this (270ohm, R1000) resistor. Base of T905 is connected to ground via 5k6 resistor, and connected to S004 via 10k ohm resistor. Therefore the T905 is driving T904 and it is basically turning power to IC701 (power amp) when the S004 is in appropriate position. To put it in perspective, the 270 ohm resistor gets rather hot in a mater of minute or so, and that heat is probably the reason why the glue got so brown and brittle, why the capacitors leaked, and why the box will go silent in minutes if I do not find what is wrong (the resistor will burn through, and T904 will loose drive and turn off the power to IC701).