exxos's Atari, Amiga & retro forum

I put a 1K across R5 and now the reset button is working..


:WTF: is up with these machines ?! :pullhair: :headbang: :pullhair:

I should have took it as an omen when I cut my finger on the metalwork and it drew blood. :hide:

exxos wrote: 14 Oct 2025 19:01 No problems with 4.50v.

Now upped to 5.80v and sdma clock is at 4.40v... Now another hour wait...

Given the low voltage of the clock signal and the potential issue with the rise time when you add the 47pF capacitor, would a pull-up on that clock line at the SMDA end, in addition to the capacitor to counter ringing, help with this maybe? It seems as if it may need the clock slightly ahead of the other chips, as in potentially pico Seconds earlier.

stephen_usher wrote: 14 Oct 2025 23:08 Given the low voltage of the clock signal and the potential issue with the rise time when you add the 47pF capacitor, would a pull-up on that clock line at the SMDA end, in addition to the capacitor to counter ringing, help with this maybe? It seems as if it may need the clock slightly ahead of the other chips, as in potentially pico Seconds earlier.

I tried earlier speeding up the SDMA clock by bypassing the clock patch and it made no odds :(

It seems to have some thermal fault now. It started resetting like mad.. I went around all the main chips with freezer spray.. Made no odds..

I left it all turned off for 5 mins.. Now it running fine again.. I don't get it :(

I'm using the diagnostic cart currently.

EDIT
Its died again. Reset is stuck low again even after powering up it doesn't shift. Thats the same problem I had when I added that 1k resistor. There's nothing there other than U2.. Maybe it's faulty :roll:

I'll try cutting the track to pin2 U2 tomorrow to isolate U1 output..

A quick throwaway video (won't have time to work on this today) that 0-1V only seems to appear after the Falcon has been turned on for several hours..

Note - This is not a fault with the DFB1X, I am just using it to show the reset signal on the white LED.





I did try freezer spray on all the main chips earlier (before I found it it was the same reset problem I had earlier again).

So basically I thought the 1K solved the problem, but actually it was because the Falcon was turned off for a few mins while I did the soldering.. So after the Falcon had been on for a couple hours again, it started resetting again!

So if I get chance tonight, I will cut the track between U2 (inverter) and U1 pin2 (555) to see what happens then. There is nothing which can pull that signal down as C7 is removed. So I assume U2 output goes faulty after several hours run time...

I'm defeated :cry: I did a video on this because it seems you can change the law of physics captain :stars:





Have I missed something obvious here and I'm being dumb or what :shrug:

.. and why is it always me who ends up with this weird issues :cry:


EDIT:

This is how the circuit is currently..

BLUE lines indicate isolation.
Plus I drew in (badly) the 1K also .

EDIT2
Only thing I can do is lift pin 3 also..
I measured across the 1k.. I get 150R one way and 160R the other. So it hints at some silcon connected somewhere.

Lifted pin 3 of the inverter, same problem :roll:

So basically I have a track on the motherboard which has a 10k and a 1k pull-up on it in parallel to 5 volts and it is still fluctuating all over the place.

:dizzy:

My new digital microscope came.. so hooked that up so I can actually see the track in its entirety. It is the track where U1 text is, and you can see everywhere it goes, and yet, I get that screwy signal on it even though nothing is connected to it.


@JezC unfortunately, I am probably going to have to start cutting some tracks on this in order to try and isolate the problem at this point. I am not sure really what else to try...

Is it worth taking the 555 off before that? I think you said you'd lifted the pin away from the pad...but maybe there's something lurking underneath?

I'm very conscious of how much of your time this is taking (which was never the intention) so do consider at what point you need to get some progress on the other projects instead!

JezC wrote: 16 Oct 2025 13:36 Is it worth taking the 555 off before that? I think you said you'd lifted the pin away from the pad...but maybe there's something lurking underneath?

I have wondered about taking both the chips off but that may be difficult because I think are glued down.. I cannot simply take the chips off. I would have to cut the pins off and try and get the chip off. But I am a bit reluctant to do that at the moment in case the glue is really strong and ends up ripping up some vias or something under the chip.

The 555 circuit seems to work as touching the trigger pin with my finger causes a reset pulse. The inverters are isolated..

I should pretty much only have a copper trace with a 1k pull upon it to 5 volts and the scope and yet it is pulsing like crazy.

There's also another possibility that maybe there was some manufacturing defect. I don't know, but I replaced the reset switch because the reset switch wasn't working. Maybe now it has a new switch that it has somehow contributed to whatever this problem is somehow.. But I don't really see what because the reset switch seems to work fine on the input of the inverter.

I'm very conscious of how much of your time this is taking (which was never the intention) so do consider at what point you need to get some progress on the other projects instead!

Yeah, I don't know what happened because it was basically running perfectly fine for several hours and it started resetting with this weird fault.

The problem is with the reset circuit malfunctioning, it might have invalidated the clock patch tests I was doing originally. When I was getting unstable results it could have been the malfunctioning reset circuit starting to rear its ugly head rather than what I was doing with the clock patches.

So I really need to figure out how to fix the reset circuit before we're doing anything else. Worst case is I will have to rebuild the circuit and probably just use a more modern reset chip like we use on the H5 motherboard.. annoying as it won't be a original but I am running out of options at this point.

That's why I mentioned that with your permission, I will just have to start cutting the tracks on the reset circuit in order to try and isolate at least where abouts the problem is.

It's rather frustrating because you can see the entire track on the monitor screen and there is nothing connected to it anywhere.. And yet it is randomly pulsing like crazy. There isn't exactly anything really near that track for it to even short out against.

I could only really come up with crazy theories at this point. Like maybe one of the vias that the reset switch is soldered into just happened to get aggravated by me changing the switch, and there's a internal short to the gnd plane on a inner layer of the PCB.. But I'm not really convinced about that either because surely I wouldn't get a consistent 150R to gnd.

I mean if falcons wasn't so valuable, I'd generally I would just connect the trace to my power supply and slowly increase the current and see what starts getting hot, but I'm not going to risk that and such an expensive machine.

I may get my low resistance meter out which is very sensitive, to see if I can see any slight resistance differences between each end of the track which may give some clues.

Well, although keeping it as near original as we can is desirable, I'm OK with you making some changes as needed to get it working & help to bottom out the clock patch situations.

So if a quick way to progress is to have a custom reset circuit (at least for now) then why not give it a go?

It dertainly does sound weird...and doubly so since it was apparentyl working and ersetting OK during CL 4 (and on testing before heading down to the venue).