How to diagnose dead Videl?

[mikro]

In other words - the 555 OUT pin should switch one time only

Unfortunately, it doesn't. It behaves exactly as Videl's XRESET (just inverted) ...

[dml]

Unfortunately, it doesn't. It behaves exactly as Videl's XRESET (just inverted) ...

I'll need to test my own machine now to see what it does. Will report when I have tried...

[mikro]

Just to be clear, I meant on my non-working machine. Of course on the working machine it mimics the *correct* behaviour, i.e. the reset pulse is as expected.

EDIT: Ah, wait. Perhaps you meant that the reset pulse on the 555 should be visible just once while on Videl's XRESET it should be twice (because of the RESET instruction) ?

Anyway, it is still true for the non-working machine as there is no RESET instruction reached, just the 2-3 peaks, same on the 555 and on XRESET.

[dml]

On pin 3 (OUT) of the 555 timer I'm getting +5V when I push the reset button in and 0v a short delay after I release it. I don't see any other oscillations.

I'm using ROLL mode on the scope to record the history over 200ms or so.

I'll check now what XRESET is doing on output from U11 - that could be different as its shared with the other ICS which can pull it low.

[dml]

EDIT: Ah, wait. Perhaps you meant that the reset pulse on the 555 should be visible just once while on Videl's XRESET it should be twice (because of the RESET instruction) ?

Yes - it actually seems to be less of a pulse and more of an on/off period with the pushing of the reset button.

Pushing the button turns pin 3 high, releasing it holds pin 3 high for a slight delay set by C7, then off. Just one lengthy on-off switch, for that signal.

[dml]

Now checking pin 2 of U11 (XRESET), I see exactly the same in reverse. Pushing reset pulls it low, releasing it sends it high - once - after a brief delay of a few 100 ms.

I'm a bit lazy to set up extra probes just now to measure the delta but it's a human-scale noticeable delay :)

I'll try to capture the low-high switch at a higher speed and see if I'm missing anything but on ROLL mode it just looks like a tidy up-down-up as i press and release the button.

[mikro]

Frankly, I'm not that bothered with that random (and very short) +5V pulse on the working machine (maybe I will once it stops working :D). If I ignore it, everything behaves exactly as you describe on your machine.

Clearly, the non-working machine never gets into that delayed state.

[dml]

Have confirmed that XRESET pulses twice, capturing at high speed.

A spike high of 10uS, low for maybe 80uS then it goes high.

[dml]

Yeah I'm seeing a tidy double-pulse with 10s'-of-uS-scale period on XRESET, at the end of the reset delay - but not on the 555 timer pin 3. Which makes sense to me.

So something funny going on in your reset circuit?

[mikro]

Have confirmed that XRESET pulses twice, capturing at high speed.

A spike high of 10uS, low for maybe 80uS then it goes high.

Actually, thanks to your measurement I understand better what's happening in mine:

On the working Falcon:
- 0V until the treshold
- 5V for a very short time
- 0V for about 1100us
- 5V for a very short time
- 0V for about 60us
- 5V for the rest

There is no "delayed state" as I wrote here:

Frankly, I'm not that bothered with that random (and very short) +5V pulse on the working machine (maybe I will once it stops working :D). If I ignore it, everything behaves exactly as you describe on your machine.

Clearly, the non-working machine never gets into that delayed state.

Basically what seems to be happening (unless I'm measuring it wrong, wouldn't rule this out :D) is that during the button release there's a short pulse (reason unknown and I don't like at all but -- machine works) which triggers the scope but it's not the final state of the button -- that state comes only when the "5V for a very short time" [the second one] (mine) / "spike high of 10uS" (yours) happens. After that our observations match.