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Re: Mega ST4 - no video output at all
Posted: 18 Oct 2024 18:16
by stephen_usher
asapreta wrote: 18 Oct 2024 17:10
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.
Re: Mega ST4 - no video output at all
Posted: 19 Oct 2024 23:01
by asapreta
stephen_usher wrote: 18 Oct 2024 15:00
asapreta wrote: 18 Oct 2024 14:50
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?
Re: Mega ST4 - no video output at all
Posted: 19 Oct 2024 23:22
by 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
Re: Mega ST4 - no video output at all
Posted: 19 Oct 2024 23:37
by asapreta
stephen_usher wrote: 19 Oct 2024 23:22
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?
Re: Mega ST4 - no video output at all
Posted: 20 Oct 2024 00:36
by 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
Re: Mega ST4 - no video output at all
Posted: 20 Oct 2024 01:02
by ijor
asapreta wrote: 19 Oct 2024 23:37
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.
Re: Mega ST4 - no video output at all
Posted: 20 Oct 2024 01:22
by asapreta
ijor wrote: 20 Oct 2024 01:02
asapreta wrote: 19 Oct 2024 23:37
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
And to update the to-do, I have checked all RAM ICs and same readings at all address lines
Re: Mega ST4 - no video output at all
Posted: 20 Oct 2024 01:42
by ijor
asapreta wrote: 20 Oct 2024 01:22
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.
Re: Mega ST4 - no video output at all
Posted: 20 Oct 2024 01:43
by 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
Re: Mega ST4 - no video output at all
Posted: 20 Oct 2024 01:44
by asapreta
ijor wrote: 20 Oct 2024 01:42
asapreta wrote: 20 Oct 2024 01:22
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