May 282016
 

One of the most rare PPG units found its way to my workshop recently: the 390 Drum Unit.
PPG 390 Drum Unit

Basically, this is a very rudimentary 8 bit sample player. It features 8 drum sounds in EPROMs on 4 voice cards. Two clock sources drive the address counters on the boards, which means that Sample Freq 1 defines the sampling rate of the odd channels, while Sample Freq 2 is in duty for the even numbered instruments.
[Block diagram to follow…]

Upon arrival one card was missing, instead I found a non working wire-wrapped vero board with a prototype for a single 4kByte instrument in two 2716s, and two 2732 EPROMs with instruments samples that won’t fit anywhere. I designed a PCB to replace the missing voice card, providing sockets for 2764 EPROMs so experimenting with other samples woule be easier as they can be programmes with cheap USB programmers.

PPG 390 Voice Card

Once completed the design I realized that I cloned the original cards so perfectly that I still don’t have a use for the 2732s – but my layout provides an easy solution, with two piggy-packed 4040 counters and some wires I modified my new voice card to work with the Noise and Hi-Hat EPROMs. Some more wires would allow to experiment with 2764 or even bigger EPROMs.

The clock frequencies are in the range of 13 to 30kHz, so the sound duration is between 70 and 160ms for the originally used 2716 EPROMs and 140 to 320ms for the 2732 sounds and would scale accordingly when using a larger EPROM.

PPG 390 Overview

Apart from this addition, the cards got guides (the blue things) to protect the connectors, new capacitors, additional capacitors in the output lines (originally it had a 3V DC offset), a new transformer, IEC mains jack, a mains fuse and a mains wiring according to current standards.

It is probably not an impressive drum unit sound-wise, but an interesting collectors item and I somewhat like the shaker … 😉

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