R9 Negative Slew

From a 1970s catalog: The DUAL NEGATIVE SLEW (NEG) is one of the unique multi-functional patch-programmable modules in the Serge system. The module features two independent sections with wide range voltage controllable slew rates. The slew is active in the negative direction only and can be patched to perform a number of synthesizer functions. With the Pulse output applied to the input, the module will regenerate for use as a voltage controlled sawtooth oscillator or pulse generator. An audio signal applied to the input will be envelope detected, and the complex envelope will be available at the output. If a pulse is applied to the input, the unit will function as an envelope generator with a fast rise time and a voltage controlled fall time.


Left to right: 1973 panel with some rub-on transfer customization, 1973 PCB

 


Left to right: 1975 panel, 1975 PCB

 

The schematic is for the 1973 version with minor changes such as the addition of an LED driver.

The schematic is for the 1975 version. There were some minor changes made by 1975, such as deleting the +6V power rail and changing the values of some resistors, particularly the values of the output resistors, and the addition of a 4k7, which if omitted, caused some very strange behavior if hooking the two sections to each other. The 1973 version did not suffer from this problem. Of note is that this 4k7 is wired incorrectly on some boards, causing the above problem. Instead of acting as a voltage divider in conjunction with the 2k2 pulse output resistor, it is instead wired from 0V to 0V. See the photograph below.

Mods: Again, an LED has been added to the circuit. In 1975, LEDs were simply wired directly to the output sockets, and, in the case of the sequencer, shared a common current limiting resistor. If a high brightness LED is used in this way, the current limiting resistor can be in the 4k7 to 10k range depending on the LED used. The load is negligible.

When repairing this module, if possible, lower the large timing capacitors so that they touch the PCB. Maybe a touch of glue would also help. As originally assembled, they have a tendency to form dry joints.

Left: Incorrect 4k7 resistor position. Right: Correct 4k7 resistor position.

On the 1975 board, there is also a short trace missing between the 6k8 input resistor and the 100k pull down on the lower input. This may extend to boards from other eras as well (as is suggested by some healthy globs of solder). I expect the small piece of art tape fell from the artwork and was never noticed as missing.

 

PCB connections:
A = input (1)
B = output (1)
C = VC (1)
D = pulse out (1)
E = input (2)
F = output (2)
G = VC (2)
H = pulse out (2)
U = not used (0v)
W = 0v
X = +12v
Y = +6v
Z = -12v

 

 

Board with pots removed, courtesy of Dave Brown.

 


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