74LS06 faulty twice in a row?

[mikro]

Guys, I'm getting desperate. A few days ago I have finally decided to give my Rigol scope a go because I got something simple to diagnose (at least so I thought...) - a Falcon where the MIDI test fails (M0 data not received).

After some initial struggle to understand how it all works I have got a pretty reliable setup - I could see that MIDI ACIA's TXD pin (6) does indeed send data over the serial line and that RXD pin (2) receives nothing. Step by step I managed to get it to this:

falcon.jpeg (85.76 KiB) Viewed 448 times

Based on:

inverter.jpeg (12.57 KiB) Viewed 448 times

So the trigger is happy on ACIA pin #6, U11#5, U11#6, U11#9 and it gives me nothing on U11#8. So clearly, U11 must be broken, right?

So I replaced it with a brand new 74LS06, Falcon boots, all good but ... the problem is still there, on the exact same pin. WTF?! My humble understanding of electronics is that even if that pin would be connected (as a short circuit) to ground or to some other pin, I should still see it triggering on that U11#8 or not?

Do you have any idea how something like this would be possible except betting on the fact that I purchased a faulty 74LS06 with the exact same fault?

EDIT: Just to clarify, I have verified the whole thing on a working Falcon, so cables, location of my scope etc is correct.

[PhilC]

Sounds to me like whatever is connected downstream from U11 pin 8 is pulling it low and this is causing you not to see a signal. Try lifting whatever chip pin that is and see if you then have a signal on U11 pin 8?

[exxos]

Open collector...

[mikro]

After finding out what an open collector means I can report success: the issue was indeed very simple, it was a broken coil (L55). Fortunately I had a few spares (purchased back in Australia, greetings @kodak80 ) so after replacing this one it all started working. So the Falcon is 100% bug free as far as the test fixture is concerned. Yay!