Galaksija typing random characters by itself after power-on

[Desperado]

Hello everyone,
I’ve just completed building my Galaksija replica (based on the original 1983 schematic) and I’m facing a strange issue I can’t figure out.


The symptom:
When I power it on, instead of the normal “GALAKSIJA BASIC” screen, the computer starts typing by itself — random characters keep appearing continuously on the screen as if a key were being pressed repeatedly.
The keyboard is not actually connected , so the input seems to come from somewhere else inside the circuit.

• If i lift pin 5 of the LS251 (the D0 output line toward the CPU), the random typing completely stops.
• When the pin is connected again, the ghost typing immediately returns.

• I’ve attached the original Galaksija manual and schematics I used for the build.
• Here’s also a video showing the issue in action.

Thanks so much!

 

[daver2]

Ah, a homebuilt machine this time...

First thing (with no keys pressed) is to check pins 1, 2, 3, 4, 12, 13, 14, and 15 of the SN74LS251 (U12 connected to the keyboard) for a permanent logic HIGH using your oscilloscope. Look for any signs of pulsing. There should be none.

With no keys pressed, each of these pins should be pulled up to +5V via the resistor pack (RP).

Dave

 

[Desperado]

Ah, a homebuilt machine this time...

First thing (with no keys pressed) is to check pins 1, 2, 3, 4, 12, 13, 14, and 15 of the SN74LS251 (U12 connected to the keyboard) for a permanent logic HIGH using your oscilloscope. Look for any signs of pulsing. There should be none.

With no keys pressed, each of these pins should be pulled up to +5V via the resistor pack (RP).

Dave

Hi Dave, how are you??
Im been In Brighton for 4 days last month! Nice city

 

[daver2]

>>> Hi Dave, how are you??

I am fine thank you. And you?

>>> I've been In Brighton for 4 days last month!

I have never actually been to Brighton myself. I have been close to it. My parents met in Brighton...

Dave

 

[Desperado]

Ah, a homebuilt machine this time...

First thing (with no keys pressed) is to check pins 1, 2, 3, 4, 12, 13, 14, and 15 of the SN74LS251 (U12 connected to the keyboard) for a permanent logic HIGH using your oscilloscope. Look for any signs of pulsing. There should be none.

With no keys pressed, each of these pins should be pulled up to +5V via the resistor pack (RP).

Dave

Yes these pins are all HIGH

 

[daver2]

OK, so what is pin 7 (of the SN74LS251 U12) doing?

This pin should be HGH to disable the U12 output on pin 5, and LOW to enable the U12 output on pin 5.

If this pin is permanently LOW - or going LOW at the incorrect time, the pin 5 output will corrupt data bus signal D0.

If you lift pin 5 of U12 and your problem stops, then it is likely to be either something driving pin 7 that is at fault or U12 itself (or some funny interaction due to a solder splash).

Dave

 

[Desperado]

This is U12 pin 7

 

[Desperado]

I replaced the U12 with other ics but i have always same problem unfortunately...

 

[daver2]

That looks OK, but is only one pulse.

Can you post a trace of U12 pin 5 on the second oscilloscope trace please. Use the existing trace as a trigger and get a couple of yellow pulses on the oscilloscope screen.

Can you also post a photograph of the front and back of your PCB please.

I notice that the ICs are not all installed the same way round. Are you sure you have installed all of the ICs the correct way round?

Is this a single sided PCB with wire links or a double sided PCB without the links?

Dave

 

[Desperado]

I notice that the ICs are not all installed the same way round. Are you sure you have installed all of the ICs the correct way round?

yes they are correct! is this a single side pcb.

 

[Desperado]

This is Ch1 on pin 7 and Ch2 in pin 5 of U12

 

[daver2]

You need to speed up the timebase to get multiple yellow pulses on the screen.

It is interesting that there is a glitch on the yellow trace there - but there could be two (2) consecutive accesses.

Dave

 

[Desperado]

Better now?

 

[daver2]

Not really...

We get two (2) yellow pulses now, but the signal definition of the green trace is awful...

Perhaps go back to one pulse on the yellow trace, and extend it to cover a large portion of the horizontal part of the oscilloscope screen (so we can look at what is going on in 'high definition').

Put the oscilloscope onto manual trigger and capture a few traces. Post the traces.

Dave

 

[Desperado]

Not really...

We get two (2) yellow pulses now, but the signal definition of the green trace is awful...

Perhaps go back to one pulse on the yellow trace, and extend it to cover a large portion of the horizontal part of the oscilloscope screen (so we can look at what is going on in 'high definition').

Put the oscilloscope onto manual trigger and capture a few traces. Post the traces.

Dave

Ok !

 

[Desperado]

Other some traces

 

[daver2]

That is excellent.

Note that the YELLOW trace extends for the full 5V range (inactive=HIGH, active=LOW).

Also note that the green trace is very low (in terms of voltage swing). This may be due to the particular signal that U12 is outputting, but (as I understand it) all of the inputs to U12 are HIGH, so the output should be HIGH when U12 is selected. That, therefore, doesn't look correct...

Can you take a few more traces, and see if the voltage swing on the green trace increases. If so, post that trace. If not, we need to ascertain what is going on...

Where did you get the 74LS251 devices from? Did they all come from the same supplier? If so, are they fake/faulty?

EDIT: All three traces you have posted seem to show the same thing...

Dave

 

[Desperado]

Where did you get the 74LS251 devices from? Did they all come from the same supplier? If so, are they fake/faulty?

I had bought a couple from an Italian eBay seller, and this morning I purchased three more from an electronics store. I tried them all, but nothing changes.

 

[daver2]

So, lets go back to basics...

Use a multimeter and measure the voltage between U12 (SN74LS215) pins 8 (black/negative) and 16 (red/positive). You should observe +5V. Do not use your oscilloscope for this measurement (we need to make sure that the 0V pin is connected to PSU 0V).

Dave

 

[Desperado]

So, lets go back to basics...

Use a multimeter and measure the voltage between U12 (SN74LS215) pins 8 (black/negative) and 16 (red/positive). You should observe +5V. Do not use your oscilloscope for this measurement (we need to make sure that the 0V pin is connected to PSU 0V).

Dave

i have 5,1 V