8 bit computer, output module, annoying problem

Annoying ghost displays of ‘-‘ sign

As you can see in the picture on the right, the output displays two little ‘-‘ signs – and sometimes even more segments – in a dim, lightly flickering way, to the left of the actual output digits. Funny thing is this seems to happen only when bit 7 of the output register is on (values x’80’ and above).

The circuit

The circuit used to rotate power to each of the four 7-segment displays consists of:

  • a 555 timer chip that produces a 500 something Hz square wave
  • a 74LS76 dual JK Flip-flop to divide this pulse in two and then in two again, making it a binary 0-3 counter
  • a 74LS139 demultiplexer that decodes the binary 0-3 to four individual signals that drive the 7-segment displays

Measurements

“To measure is to know” is a famous Dutch saying. So, I started measuring things. See pictures below.

Solutions?

I have tried al kinds of things, amongst others adding a 100nF capacitor over all chip’s power leads, adding pull-down-resitors on the cathode side of the 7-segment diodes etc. The signals that are supposed to give ground to the 7-segment displays are particularly messy, and they seem not to reach 0 volts by a longshot, more like their minimum is around 1.3V.

Any suggestions would be greatly appreciated.

 

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8 bit computer, output module, annoying problem