Mega ST4 - no video output at all

[stephen_usher]

I never have piggybacked logic ICs before, wouldn't the outcome be the same as the badchip ?
DRAMs don't do logic, so piggyback is fine with them.

It will work if the bad chip isn't dragging the line low as TTL chips only very weakly pull up the line and the piggy backed chip will easily overcome that.

As with RAM chips, it only works for certain failure types but it's non-destructive, quick and easy and harms nothing.

[asapreta]

Hi stephen_usher, I was to ask about this, are the 41256 compatible with the memory on the Mega ST4?
Mine has the TC511000AJ10, which is 1Mbit x 1, right?
I do have some 41256 here, and will buy more, but as you said, piggybacking can help. I saved my A500 doing this.

Oh, of course, they're 1Mbit on the MegaST. oops!

About the CAS lines, pin16 I checked them yesterday and got nothing.
The RAS lines, pin 3, has activity.

With no activity in CAS I saw it was connected to the 74LS244 (U59).
Probably this IC is faling as the traces to CAS lines from it to MMU are ok and MMU IC is working, tested on my 520 ST.

Does this sound right?

Well, no CAS no memory. If you've got the CAS from the MMU entering that chip and nothing coming out then that would point to that chip being the problem.

I find this location has decent schematics https://docs.dev-docs.org/htm/search.php?find=_s. Specifically the "Atari Mega ST4 schematic (C100166, Rev A) [1987]" one is clear.

Hello!

Busy day today.
I have checked and there is no /CAS signal on MMU pins.
MMU IC os good, works in my 520ST.
I would say U59 was bad but the RAS signal is ok.

/CAS is only generated in MMU or at other IC?

[stephen_usher]

Only the MMU. Get some good contact cleaner such as Deoxit or Electrolube EML and clean the MMU socket pins. You MAY be able to use a fibre pen if you’re very careful and only stroke upwards away from the PCB

[asapreta]

Only the MMU. Get some good contact cleaner such as Deoxit or Electrolube EML and clean the MMU socket pins. You MAY be able to use a fibre pen if you’re very careful and only stroke upwards away from the PCB

Will do this cleaning again.
Which input signals in MMU should I check also to see to have CAS output?

[asapreta]

Checking other signals in MMU all address lines are stuck high.
But I think this já due HALT is stuck low.

I really must decide to look for a RAM problem (because of the white border with black Square screen or the HALT signal

[ijor]

Which input signals in MMU should I check also to see to have CAS output?

Checking other signals in MMU all address lines are stuck high.
But I think this já due HALT is stuck low.

Press Reset to take the CPU out of the halt condition and probe the CAS signals. The exact behavior depends on the MMU version you have, but at the very least you should see some CAS activity after Reset and until the CPU is halted. Check CAS0L and CAS0H, both should pulse after reset. In some MMU versions they might work even during reset.

[asapreta]

Which input signals in MMU should I check also to see to have CAS output?

Checking other signals in MMU all address lines are stuck high.
But I think this já due HALT is stuck low.

Press Reset to take the CPU out of the halt condition and probe the CAS signals. The exact behavior depends on the MMU version you have, but at the very least you should see some CAS activity after Reset and until the CPU is halted. Check CAS0L and CAS0H, both should pulse after reset. In some MMU versions they might work even during reset.

Hi!
Pressing Reset and checking CAS0L or H I get this pulse below:

VID_20241019_211524479_exported_1665.jpg (161.71 KiB) Viewed 311 times

And to update the to-do, I have checked all RAM ICs and same readings at all address lines

[ijor]

Pressing Reset and checking CAS0L or H I get this pulse below:

That means that MMU is working, at least it is not completely dead.

[asapreta]

O did another small test, lifted CPU pin 17, HALT and tied It to RESET.

In this test the screen was Full white and I started to have readings in CPU's address lines.
But still no CAS

[asapreta]

Pressing Reset and checking CAS0L or H I get this pulse below:

That means that MMU is working, at least it is not completely dead.

Yes, I tested it in my 520ST