[sdiy] 4017 sequencer

[Theo]

Hi Gavin,
Seems you mis interpreted some things on the schematic.
Please see inline.

From: TheMysticSatin <elmystico at earthlink.net>

<snip>
>  the levels
> at the pins of the tl072 socket without an op amp in it are
> 1. out 1  +5v
> 2. in -1 +3.5v
> 3. in +1 0v
> 4. Vcc- -15v
> 5. in +2 0v
> 6. in -2 0v - +1v depending on the pots
> 7. out 2 +5v
> 8. Vcc+ +15v.

Now this looks not good.
Both pin 3 and pin 5 ( the + inputs of the opamps) should be connected to
GND, also 0V.
Please check your connections!
But it gets worse ;)  , please read on.

> I can't figure out what about this would cause the chip to
> overheat... but with it in there is virtually no range in the CV outputs.
> with the pot at the lowest setting voltage measured at cv out is +5v, with
> the pot at its highest setting the voltage at cv out is barely + 6v.
these
> measurements are made with the range switch at +5v.  perhaps the problem
is
> related to that range switch?  I have actually dropped the switch and
> variable range option in favor of a constant +5v going to all the points
on
> the schematic that the range switch is drawn as going to...
>

This does not look good either.
>From what you say it looks like you feed 5V straight in the outputs of the
opamps.
This won't make the opamps very happy, rather fry them alive.
(Imagine yourself sitting in a electric chair with 30000V on your head,
won't make you happy would it?) ;^)

Leaving the range switch out is no problem, but you have to understand the
schematic.
The 10V, 5V and 2,5V marks relate to the maximum CV output level each
setting gives.
The range switch does NOT connect to any (supply) voltages!
A switch is simply a connection that can be open or closed.
The Range switch is a 3 position switch, ON-OFF-ON.

* In the 10V position the range switch connects the "vertical drawn" 47K
resistor in parallel with the 47K resistor to the - input of the opamp.

* When the range switch is in the 5V position it simply connects to NOTHING.

* In the 2,5V setting the switch connects the "vertical" resistor in
parallel with the feed back resistor of the opamp.


If you leave the switch out, you should also leave the "vertical" resistor
out, as it has no function.

<snip>
>
> also seems
> strange that the 100k resistors drop the voltage into the opamp so low,
any
> suggestions?
>
<snip>

In this case the minus input of the opamp works as a "virtual ground point".
That is, a opamp with a feedback loop always tries to make both inputs the
same voltage.
Because the + input is 0V (gnd) the opamp tries to make the - input also 0V.
The output of the opamp is connected with the - input (thru a resistor),
this is the feed back connection.
When a voltage is applied to the - input, the output gives a opposite
voltage,
so the sum of the voltages on the - input becomes 0V.


Hope this helps,
Theo