Thursday, 6 July 2017

Serge Positive Slew - Paperface - restoration

Some pics of the Positive Slew.
This is from panel 2.1 (Warren Burt paperface).

 The red IC socket holds a LM3900 opamp which I swapped.
No improvement though. :-(
The module is powered by -12V (bottom blue), +6V (Green), +12V (Red), 0V (Blue top)

Connecting the lower pulse output to START or SUSTAIN should cause the Positive Slew to operate as a VCO or LFO..

The module uses two OpAmp ICs:
a  LM3900 & a MC 3401

The above pin out is for the MC 3401 operational amplifier.
The Motorolar MC3401, is out of production.
It seems to be responsible for providing both pulse outputs .
The lower pulse output exits pin 9.
The upper exits via pin 10
It seems that the Opamp is being used as a Schmitt trigger circuit with positive feedback.
The Schmitt trigger will convert any regular or irregular waveform into a pulse or square wave.
The LM 3900 is used extensively throughout the serge modules... in this circuit its used in a negative feedback config. 
It's interesting that both Quad op amps have identical pinouts.
I wonder if we can swap the MC3401 for LM3900s . My next question is why Serge chose
to use the MC3401s in the first place.
Another possible substitute for the MC3401 could be the CA3401 ???

This style of banana socket are I think impossible to remove without breaking them.
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To be continued.

1. Ken Stones CGS - Positive Slew
2. DUSG - CGS 
4. Tim Stinchcombe

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