PPG 313 Sequential Switch

PPG 313

Introduction The PPG 313 analog switch dates back to 1976 and the beginnings of PPG when they were a modular synthesizer company. The first PPG sequencer in the 100 series was a straight copy of the Moog 960, but the 300 series sequencer was a more advanced design. The 313 is an improved version of the Moog 962 with 4 channels/steps rather than 3 and the ability to skip and repeat the step.

Brochure Description The PPG313 Sequential Switch is an electronic switch that connects one of four input signals to a common output. These signals can be audio signals, control voltages and trigger impulses. The main application is in conjunction with the 314 Sequencer, which can provide up to 32 pre-selected voltages, one after the other. The PPG313 has four stages which are sequenced in series.

The signal inputs are the lower 4 jack sockets labeled SW1, SW2, SW3, and SW4. One of these four inputs can be switched to the Switch Output either manually using the buttons or by trigger signals via the jack sockets SET 1, SET2, SET3 and SET4. The four positions of the electronic switch can be used one after the other, using the front panel button or via the SHIFT jack socket.

The four rotary switches called COUNTER PROGRAMMING are used to preprogram repeating patterns for each of the four stages. For example; with all 4 switches in position 1, the 4 positions are switched through evenly one after the other, according to the impulse that is given to the SHIFT input.

PPG 313 Buttons

How Does it Work The arrival of CMOS logic chips in the 1970’s really opened up sequencer circuit design, thanks to its low power consumption compared with TTL. The PPG 313 uses two decade counters (4017) to sequence the 4 channels and to provide the step repeats (1, 2, 3, 4, 8). The CLOCK and INHIBIT pins are used in reverse, with the clock signal being externally provided or created via the SHIFT button.

A 4011 quad NAND chip provides the debounce of the SHIFT button as well as a monostable circuit to stop the clock on the 4017 used to sequence the channels, when a SET input is triggered or a channel button is pressed. The channels drive transistors to provide the trigger signals and JFETs are used to switch the analog signals on/off into a merged output.

The PPG design has a large main PCB which contains most of the circuitry. A smaller PCB contains the 5x buttons and a 4017 and 4011. They are connected via soldered wires and therefore difficult to take apart. The PCB’s are single sided with no solder mask and with a lot of hand wiring to the jack sockets.

The PPG design is very elegant and clever, with a small number of CMOS chips and a 741 as a comparator. Some WIMA capacitor values have been assumed, whilst others like the 1000pF polystyrene caps used in the debounce and set circuits are clearly marked. Resistor values were read from the colour accurate photos.

AM313 Mockup

AMSynths Design The AM313 is an exact replica with the circuit traced from an original 313 using photos and tracing paper! There are two high quality PCB’s; the PANEL PCB holds the rotary switches, buttons and jack sockets. The MAIN PCB holds all the CMOS logic and transistors. They are connected using SIL headers and easy to take apart.

The tricky parts of the re-engineering job were the SKIP feature and getting the channels to SET with the buttons and external inputs. The SET feature was easily corrected  from my schematic error but the SKIP feature refuses to work, the pulse does not get to the 4017 to move it to the next step.  I am reworking this part of the circuit and the SHIFT button debounce which is intermittent.

The analog switching is done using P-channel JFET’s rather than a CMOS switch. The JFET’s work very well but don’t handle negative voltage very well, as there is a 10% bleed through when switched off. Given the 313 will be usually used with the 314 0 to +10V outputs this is not an issue.

Outcomes & Availability An initial prototype AM313 was tested in late July and corrected over the next few weeks. Careful testing was conducted on the STEP repeats to ensure they were correct and stable across all channels. Rework was needed on the SKIP and STEP features, and production PCB’s ordered in August.

Copyright AMSynths 2019