Yamaha A-960II - No power

Jarod

New Member
Hello

Few weeks ago I've bought this amplifier in working condition. Anything was fine until I got this brilliant idea to look inside.
To be honest, I was little surprised by placement of some capacitors near "hi watt" resistors. Few of them looked really bad so
I decided to replace them to avoid future trouble. After giving it a second fought, it was clear that replacing most of the caps on
main PCB was no bigger trouble. After a week or so new caps have arrived by mail and begun work to replace old ones with new ones.
Basically all work was going smoothly, few hours past and it was done. I was time to face the "moment of truth"... the power button.
After deep breath, my shaky finger approach the power switch... click... and nothing happed (!@#!@#$ @ <- curs words ;). Now the amplifier
is completely dead. So far I've checked following things.

1) Power bulb is OK
2) The are no gaps in "electric path"
3) All caps, resistors and diodes on power supply board are OK

Now I'm and stuck and to many things became unclear, so I have some questions. First one is about "voltage reference line" and voltage
figures on the schematic. All listed voltage values are only true if i measure them in respect to the "voltage reference line"? I suspect
that there is a problem with SCR 501, SCR 502 and SCR 503 triacs. I've measured voltage values on them and it follows (regular multimeter,
no True RMS): SCR 501 (G/A/K) 0.93V 120V 0.84V / SCR 502 0.93V 132V 0.84V. I don't like the 120V and 132V values, should be 49.1 and 50V
(I don't know how they look, no oscilloscope at home). If I try to get voltage value from gate of SCR 503 something odd happens. F 501
sometime blows for no apparent reason (no short circuit to chassis etc.). This also happened (but not always) when I was trying to get
voltage values from SCR 501 and 502. I'dont want to kill the SCR 503 (maybe is already too late) because when that happens, the game is
over. All measurements where taken "in the air". The PCB has a screw that connect it to the chassis is that in any significance? I don't
understand in enough detail how this power supply works. Can someone explain it to me please! Any suggestion what to do next?
 
All measurements where taken "in the air". The PCB has a screw that connect it to the chassis is that in any significance? I don't
understand in enough detail how this power supply works. Can someone explain it to me please! Any suggestion what to do next?

I would start by connecting the PCB to the chassis.

This power supply is complicated. The controler IC501 in the power supply is opto-coupled to the audio signal and the primary of the power transformer is switched ON and OFF according to the amplifier's output voltage, more exactly to the rate of change of the output voltage, thereby anticipating the demand from the amplifier.
 
Double and triple check to make sure you installed al the caps in the correct polar position.Also look for possible broken wires-solder bridges-pinched wires.That screw you reference may or may not ground the PCboard to "chassis ground".I'm not sure , you'd have to look at the foil side of the PCB where the screw hole is and see if there is bare foil around the screw hole or not.
Dave
 
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