C. Jones <Jones.125@osu.edu > wrote :

> Greetings.

> I have a general question about sample and hold circuits related to
>a  specific question regarding the Moog S/H controller.

> Firstly, what is the nature of electronics in the circuit that holds
> the sampled value (i.e. a capacitor coupled with a transistor, etc.)?

> Second, is the nature of these circuits fairly standard from synth to
> synth or can the design vary widely?

> These questions are prompted by the sometimes erratic nature of my Moog
> S/H controller. The ability of the unit to hold the sampled voltage is highly
> erratic, sometimes resulting in no hold at all... the sample source
> sometimes bleeds through continuously (i.e. continous saw scale instead
> of  trilling from a saw source and straight noise instead of random voltages
> from the noise source). Is it likely to be a faulty capacitor or something
> else....

> > Any advice or information would be immensely helpful.

Thanks.

> -Chris

Hi Chris,

A sample/Hold circuit consist of an input buffer, an electronic switch, a
storage capacitor and an output buffer.

In the SAMPLE mode, the switch is closed, allowing the input buffer to make
the storage capacitor voltage equal to the analog input voltage. In the HOLD
mode, the switch is opened, isolating the storage capacitor from the input
and leaving it charged to a voltage equal to the last analog input voltage
present at the input before entering the hold mode. It should be noted that
the output buffer amplifier isolates the charge on the capacitor from the
load.

This design is fairly standard and used in most analog synthesizer. However,
some brands like Buchla and Serge have modified this circuit in a form or
another.

First, test with a Multimeter if the connections between the jacks on the
patchboard and the circuit board are O.K (check for bad connections on two
important points : the analog and logic inputs on the board). Then, test
if the capacitor holds the voltage: if not, replace the capacitor. What kind
of chip is used in the Moog S/H circuit?: is it a LH0023C?

Hope this will help,

Andre'