Yamah R-9 need to know bias current

[TA7205]

Hi;

Thanks to everyone who helped with suggestions
on this one. I think it may be fixed !

The protect relay would not kick on I thought due
to something I missed after replacing the outputs and drivers
and resistors, etc. I checked every damn component
on that board !

In desperation, I tried turning the bias pot down (a pinch)on the
offending side and ...what do ya know ?! it came on sounded
fine (even with a 100watt bulb for a fuse)and did not draw undue current
either.

I'm still not sure if VR501 is idle current or not. Maybe I still have
a problem ?! Can anyone tell me where the current should be set
at and where to sample this reading.

Thanks again guys !
Rick

 

[Tim Schwartz]

Hello,

According to the service manual, the left channel bias is done by
measuring across R553 and adjusting the pot for 10mV +/-1mV, after 2
minutes on, no signal, no load and with the "AUTO CLASS A" in the OFF
position. Then confirm the bias is between 30-60mV a minute after
switching to "AUTO CLASS A" operation.

The right channel is the same, except measure across R554 and adjust
VR502.

Lastly, did you make sure that the pot itself is OK? I've sometimes
seen bias pots damaged in a major amp failure, especially if the bias
transistor needed to be replaced.

Regards,
Tim Schwartz
Bristol Electronics


TA7205 wrote:

Hi;

Thanks to everyone who helped with suggestions
on this one. I think it may be fixed !

The protect relay would not kick on I thought due
to something I missed after replacing the outputs and drivers
and resistors, etc. I checked every damn component
on that board !

In desperation, I tried turning the bias pot down (a pinch)on the
offending side and ...what do ya know ?! it came on sounded
fine (even with a 100watt bulb for a fuse)and did not draw undue current
either.

I'm still not sure if VR501 is idle current or not. Maybe I still have
a problem ?! Can anyone tell me where the current should be set
at and where to sample this reading.

Thanks again guys !
Rick

 

[TA7205]

Tim;

No, I didn't think of checking the bias pots themselves. I know that the
fixed
portion of the pots measure about 260 ohms. Would I check the slider for a
linear
resistance ?

Thanks for the bias info. I'll set it up after I confirm the pots resistance.

Rick

 

[TA7205]

Just my luck !

The resistors don't have location numbers
on the pc board. Only transistors and diodes !

Do you by chance have the values of those resistors ?

Thanks again,
Rick