How to diagnose dead Videl?

[mikro]

Checking net alternately against VCC/GND helps build a clearer picture of whats going on. Check the net against GND and if you get a brief beep, check it against VCC and expect a long beep.

You literally means the multimeter's beeper here, correct? I.e. I'm checking pins on the EXP connector (say, one probe on a VCC pin and other on the rest) and due to capacitors in the way I can hear a beep which gets muted after the capacitor(s) get charged from the multimeter?

[mikro]

Also, while am I at asking dumb questions:

Since you have a stack of Falcons to compare with, you could check the resistance of these nets to GND/VCC (keeping in mind presence of capacitors!) and at least rule out any shorts but maybe could find differences that could point to some damage. If these nets don't show any differences, start looking at other pins on the expansion to compare this way.

When you say "nets", what exactly you meant there? I thought it's just a different term for EXP pins but then you mentioned EXP pins as the second thing to check, which totally confuses me.

[dml]

Updated a bit the nets of interest (for case of a mis-aligned booster accident :p)

EINT3 is tied to the COMBEL #175
XIPL2 tied to the COMBEL #166 and CPU #68
XMFPINT goes to the DSP GAL U44 #19 and the DSP #65 and to the MFP #36 itself

[dml]

When you say "nets", what exactly you meant there?

Sorry by 'net' I mean all pins/component connected together in the schematic. A net represents one signal, one set of connections - but possibly several ICs (and also expansion pins!). Checking one pin on one IC is effectively checking all/any pins on that net.

However - a net can be broken (bad trace, solder problem) so checking a net is a guide only. You can also check all pins in the net agree.

A net can involve pullup/pulldown resistors which won't affect the beep tests but something to keep in mind for resistance comparisons.

[mikro]

Many thanks, I'm glad I asked. :-P

[Badwolf]

When / if I find some energy tomorrow, I'll do more measurements. Would be great to know how far can actually system go without working Videl. RAM is of course out of question but maybe it could at least read a long of two from ROM? I guess it depends on relationship between Combel and Videl.

The jump to the diag cart is very early in the boot process. It's actually in that little ROM dump you posted a few messages ago.

I think you ought to give that a crack next. You may even get something on serial.

BW

[mikro]

It was a nice hope while it lasted. Both /ROM3 and /ROM4 (just to be sure I also measured them on Combel) are steadily high. So back to the awful measuring (tomorrow).

[Badwolf]

It was a nice hope while it lasted. Both /ROM3 and /ROM4 (just to be sure I also measured them on Combel) are steadily high. So back to the awful measuring (tomorrow).

Oh well. I don't think it can be too far away.

I'd suggest constructing the address and data buses for those first four AS assertions as in the other thread next, in that case. It'll cover Exxos' address bus test suggestion and also let you see if the data coming out of ROM is what you'd expect.

It's not perfect but it'll normally let you pick up a stuck data or address bit.

BW

PS. I've just started working on a Falcon motherboard restoration job too.You have a head start so: come on! Beat me to it. :)

vlcsnap-2025-07-21-22h29m34s288.png

[mikro]

Just a quick update: today I measured all address lines, got the satisfying $000000, $000002, $000004, $000006. Data lines tomorrow.

@Badwolf to show you how crazy is the idea of me beating you to it: in the morning I touched one of the GALs only to see a nice static discharge (idiot...), so the first thing I did was to verify that the Falcon works at least on the level as yesterday. Connected to scope to /AS and observed a nice, flat, 0V. Nothing helped. There even wasn't the transition from HIGH to LOW.

... it took me 10 minutes to realise that I'm measuring the wrong channel.

Your victory is safe, mate. ;-)

[Badwolf]

... it took me 10 minutes to realise that I'm measuring the wrong channel.

:shock:

Your victory is safe, mate. ;-)

Haha. Come on! I don't even have a CPU on mine! :lol:

BW