beginner's question

[Martin Czech]

> Hello, I've to check this thing (4558) by a low budget DMM:
>             ______
> OUT A -|1    8|- +Vs
>    -IN A -|2    7|- OUT B
>   +IN A -|3    6|- -IN B
>     -Vs   -|4    5|- +IN B
>                ------
> Where have I to put the red and the black probes to check if it's alive
> or dead?
> What should  I read on DMM display?
> Similary is it possible to do the same operation to check diodes and
> transistors, soldered on the PCB?
> I'm going  to fry my synth?
> 
Unfortunately things are not that simple. Just measuring the ohms between
pins won't get you any further ;-> First you have to understand, what your
opamp is doing in the circuit. Then you can check if input and output
signals make sense.  If it's ac, well you might need an oscilloscope.
Same for diodes and transistors. If you know, what they are doing in
the circuit, you can measure the pin voltages, e.g. to check for the
0.7V drop at the diode, etc.

Schematics of the synth are a neccesary help, without you're in trouble.

Of course, sometimes faulty elements get very hot, so you may detect
them this way, and if they are in sockets, you could try to swap them.
As a rule of thumb: In linear applications + and - input have the same
potential, the voltage between them is 0. The output voltage is somewhere
between +-12V, if the supply is +-15V.

Voltage measurements with the standard impedance of 10 megohm (even cheap
DVM) is no danger for your circuit, as long as you don't provoke a short
with the probes. But you may be a danger, it is wise to use an anti-ESD
set to protect your synth against electrostatic discharge, either from
your body or the instruments you use. A human body can charge up to
3000V just by getting up from a chair! I use a anti static table math
(sp?)  and a wrist strap and a connection yo the soldering iron and all
is grounded with a special safety plug to mains earth. There are several
megohms between me and earth, this is safe and good enough to discharge
in a second or so.

If you don't understand at all what I'm talking here about, it is likely
that you won't succeed and maybe do something harmfull to your synth.
Depending on the value of the machine I'd get proffesional help.

m.c.