[Video] Broken Falcon030 Motherboard

[mikro]

And of course, if you don't see anything on Videl's DE pin with ROM and RAM removed, you know what to add to your list of items to purchase. ;-)

[Badwolf]

So I started out with the best of intentions this evening.

Quick tests of everything that had been suggested here and on Youtube.

...but first I'll just probe RESET to make sure the software-driven reset is issued after the first ten or so bus accesses...

New problem found!

Not always, but often, RESET will reassert a few (as little as two) bus cycles after it's just come out of reset. Sometimes bouncing around a bit.

Probing around U11 it appears to be coming from the 555 side of things. Not really understanding 555s, I'm now staring at capacitors and resistors and wondering what to measure.

Any tips, hive mind?

And of course, if you don't see anything on Videl's DE pin with ROM and RAM removed, you know what to add to your list of items to purchase. ;-)

Good idea! I have nothing in the board that can be removed right now and DE is pulsing away at about 16kHz. :)

BW

[dml]

Probing around U11 it appears to be coming from the 555 side of things. Not really understanding 555s, I'm now staring at capacitors and resistors and wondering what to measure.
Any tips, hive mind?

555s are thankfully quite simple and more difficult to explode than most - so you probably just need to check the usual stuff and make sure the caps are not dried out as the RC setup decides the timing. And check for bouncy 5V rail near the chip in case that's causing it.

One of the axials there - C7 - definitely is a problem on most machines, it times the reset delay. You can just scope one end of it to see what its doing. It should charge in a normal curve during the reset delay.

There's another one pretty close by which is usually installed backwards with 12V across it, for biasing something. I forget what - but that also should probably get a look.

It could be something else of course but better check that whole area first.

[mikro]

Not always, but often, RESET will reassert a few (as little as two) bus cycles after it's just come out of reset. Sometimes bouncing around a bit.

Probing around U11 it appears to be coming from the 555 side of things. Not really understanding 555s,

That sounds awfully similar to what I have observed on my Falcons (two pulses: viewtopic.php?p=132261#p132261, many pulses: viewtopic.php?p=132262#p132262). For me it matters whether it is a long or short push of the button. In the end, I decided to ignore it before solving the bigger issues as the machines appear to work without further fixing.

[Badwolf]

That sounds awfully similar to what I have observed on my Falcons (two pulses: viewtopic.php?p=132261#p132261, many pulses: viewtopic.php?p=132262#p132262). For me it matters whether it is a long or short push of the button. In the end, I decided to ignore it before solving the bigger issues as the machines appear to work without further fixing.

Hmm, you're right. It appears suspiciously similar, doesn't it?

Perhaps this is merely a symptom and not the smoking gun I was hoping for, then.

I think I'll try to model the reset circuit a little and see if I can understand it better.

BW

[Badwolf]

Reset: only seems to affect a long press. Caps look fine. Without much evidence I'm eyeing the 555, but I think it's a red herring for the main issues.

So, forgetting reset:

► Show Spoiler

Screenshot 2025-08-17 at 22.20.09.png

:hide1:

BW

[exxos]

:lol: ras/cas issues?

[dml]

BW: Is that a binary representation of a moomin playing a keyboard?

[SteveBagley]

According to Acsi, this used to have a HS-32 (bus accelerator?) inside it, so I'll check the ACIA clocks, but would absent ACIAs exhibit this behaviour?

Not having the 500kHz clock to the keyboard ACIA definitely caused my Falcon to fail to boot when I tried to fit a Nemesis upgrade last year (I still need to go back and try and to get that working). IIRC, I looked through the TOS startup and it attempts to do something with the keyboard ACIA before it initialises the display which meant if the fifo never has any data (which if won’t if there’s no clock) you don’t even get to see a display on a VGA screen.

If the diag cart doesn’t something similar, that might be an issue…

Steve

[mikro]

So, forgetting reset:

It is so easy to move on once one checks the basics, isn't it. :D

If anyone can solve this mystery, it's you.

Spoiler alert: I had changed the 555 on the (formerly) bad Falcon but even that + recapping didn't resolve the issue 100% (although it's definitely the best behaving Falcon now).