Trying to fix a Yamaha CS-15

[Marjan Urekar]

Gronda Andrea wrote:
 

>  Hello, I'm a beginner,but I would like to learn something. I'm
> courrently studying the schematics of the CPB board on the Yamaha CS-15
> trying to figure out how the EG1 works.... actually my target it's  to
> detect why it doesn't work.
> In the schematic of EG1 circuitry there are TP19,TP20,TP21,TP22; are
> these  test points to check with a multimeter?

or oscilloscope

> Are they to check in numerical order (es°: if every think it's ok at TP
> 21 the wrong component is between TP21eTP22)?

Usually not .You need correct voltages/waveforms for those points 
to compare them with your measurements.

> I was told that the Yamaha custom JNM4558DN is replaceble with a
> Fairchild 741C

Nope. 4558 is dual opamp (like two 741 in single case) but  741 is
single opamp 
Well that's if yamaha's 4558 IS normal 4558...
If so ,you can substitute it for any regular 4558.
You can also use TL 072/082 (almost always ,but someone
with schematics check this).
or maybe you can send schems to synth diy archive site  :)

 
> but in my ignorance I guess they have different pinouts.

yes

> Yamaha JNM4558DN:
>              ______
> OUT A -|1    8|- +Vs
>    -IN A -|2    7|- OUT B
>   +IN A -|3    6|- -IN B
>     -Vs   -|4    5|- +IN B
>                 ------
> 
> FAIRCHILD 741C:
> 
>                   ______
> -offset nul -|1    8|- NC
>          -IN A -|2    7|- +Vs
>         +IN A -|3    6|- OUT
>           -Vs   -|4    5|- + offset null*
>                       ------
> 
> Are these schemes right?

yes

> 
> *what does offset+/- null do?

Briefly ,opamp aplifies the voltage difference between 
+ and - input,but it's not ideal so when it's 
0 volts you have some voltage at the output
which you set to 0 with offset inputs.

hope this helps a bit...


marjan