[sdiy] SH-101 help

[media at mail1.nai.net]

Does turning VR2 have any effect??  It should change the pitch.  Do the
power supply pins to the op-amp measure OK??  If not check the bypass cap
(C24).

I'd also check the 4052 mux and "range width" circuitry after it.  Does the
CV out work??

At 7:51 PM -0800 01/28/02, patchell wrote:
>
>    I am helping a friend fix his SH-101.
>
>    #1..Is there a copy of the schematic on line.  Every link I have
>followed for a schematic has ended in a dead end.. :-(
>
>    #2..Symptoms: The instrument seems to work except for when you press
>down a key, the pitch does not change.  But this action does generate a
>trigger.  When an external CV is pluged in, you do get a pitch change.
>The CV out, however, is stuck at the same voltage level no matter which
>key is presses.
>
>    #3...What I do know...I managed to trace the pitch CV a bit.  The
>SH-101 has a very crude DAC made out of resistors and a CD4050.  When I
>look at the output of the "DAC", I can observe everything working here.
>Tracing further down the line, I get to a Sample-Hold which does vary in
>voltage as you press the key.  The hold capacitor then goes to a TL062
>dual opamp.  However, this is where the voltage gets lost.  While I can
>see the keyboard voltage on the + input to the opamp, the output and the
>- input, which seem to be connected through a 1K resistor, are stuck at
>a negative voltage.  Off hand, I would say that this opamp is bad,
>however, something else could be dragging it down.
>
>    Does any of this sound familiar to SH-101 experts?  If nothing else,
>I will try changing the opamp out tomorrow, but, in my experience,
>opamps almost never fail...so I suspect a different problem.