Mega ST4 working but only showing 2mb

Problems with your machine in general.
Forgottenmyname
Posts: 282
Joined: Fri Mar 23, 2018 8:22 pm

Mega ST4 working but only showing 2mb

Post by Forgottenmyname » Sat Mar 31, 2018 6:08 pm

Hi All,

So I have a V4.0 Mega ST4, which I have saved from a fate worse that death. It's the 32 ram chip model.

When I first got the Mega it had lots of signs of corrosion, some of it breaking tracks, which is probably whats wrong with it now I suppose. I've fixed the many problems I could see and the Mega lives once again and works fine but only as a 2mb machine.

I'll be getting the scope out over the next couple of weeks whilst I build my Pak board and would appreciate any hints on the most likely places to look.

I've got a couple of schematics but the ones I have do not list which chips are in which banks for the ram. I'm presuming that the Mega is only detecting the first 2 banks, thus the diagnostic cart showing 2mb only (which tests fine.)

Any thoughts guys?

Phil C.

User avatar
exxos
Site Admin
Posts: 3541
Joined: Wed Aug 16, 2017 11:19 pm
Location: UK
Contact:

Re: Mega ST4 working but only showing 2mb

Post by exxos » Sat Mar 31, 2018 7:52 pm

You just need to follow the address and datalines to the DRAMs, you can measure connections between RAM banks as well. Starts with the MAD lines from the MMU, find one row of DRAM which connects to the MAD line, and check rest of chips in the row.. pinout for the DRAM will help as well.
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

https://www.exxoshost.co.uk/atari/ All my hardware guides - mods - games - STOS
https://www.exxoshost.co.uk/atari/last/storenew/ - All my hardware mods for sale - Please help support by making a purchase.

Forgottenmyname
Posts: 282
Joined: Fri Mar 23, 2018 8:22 pm

Re: Mega ST4 working but only showing 2mb

Post by Forgottenmyname » Sat Mar 31, 2018 8:22 pm

Ok thanks, makes sense to me.

Will have a look and see where the break is.

Forgottenmyname
Posts: 282
Joined: Fri Mar 23, 2018 8:22 pm

Re: Mega ST4 working but only showing 2mb

Post by Forgottenmyname » Mon Apr 02, 2018 9:08 pm

So I've had the scope and the multimeter out.

Two banks of 8 chips I believe are working correctly but 2 are not. The only difference I can find is that CAS1H and CAS1L have a roughly square wave trace at 2Mhz, whereas CAS2H and CAS2L are both logic high at the MMU but are high with a very slight square wave ripple around 3V at each chip of the two banks that don't work.

I'm guessing that CAS1H is correct with the square wave and CAS2H is incorrect with the logic high.

Whats made matters worse is I mixed some of the ram up and now the Mega doesn't work at all (oops) and no new ram to replace it with.

Just wondering whether to cut my loses and dump the 32 drams and fit something a little more modern.

Any suggestions guys?

User avatar
exxos
Site Admin
Posts: 3541
Joined: Wed Aug 16, 2017 11:19 pm
Location: UK
Contact:

Re: Mega ST4 working but only showing 2mb

Post by exxos » Mon Apr 02, 2018 9:48 pm

Forgottenmyname wrote:
Mon Apr 02, 2018 9:08 pm
Whats made matters worse is I mixed some of the ram up and now the Mega doesn't work at all (oops) and no new ram to replace it with.
If they are in sockets then you can test banks of chips even if its 512K bank. Then you can work out which chips are faulty if any. If all the chips work then you know you have PCB damage problems (or socket problems).
Forgottenmyname wrote:
Mon Apr 02, 2018 9:08 pm
Just wondering whether to cut my loses and dump the 32 drams and fit something a little more modern.
As the PCB is damaged then it might be easier just to go with one of my kits if you want something quick and simple.. viewtopic.php?f=15&t=267
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

https://www.exxoshost.co.uk/atari/ All my hardware guides - mods - games - STOS
https://www.exxoshost.co.uk/atari/last/storenew/ - All my hardware mods for sale - Please help support by making a purchase.

Forgottenmyname
Posts: 282
Joined: Fri Mar 23, 2018 8:22 pm

Re: Mega ST4 working but only showing 2mb

Post by Forgottenmyname » Mon Apr 02, 2018 10:00 pm

Yes I think your kit might be the easiest way to go, considering the replacement ram is hard to find anyway.

I'll get all of the old ram removed and wait for pay day to order the adaptor. At least I have the Pak assembly to keep me busy whilst I wait for pay day.

If only they were as easy to fix as an 800XL.

Forgottenmyname
Posts: 282
Joined: Fri Mar 23, 2018 8:22 pm

Re: Mega ST4 working but only showing 2mb

Post by Forgottenmyname » Sun Apr 08, 2018 9:11 pm

Hi Exxos,

Just removed all the ram from the motherboard of the Mega, Will I still need to put +5v to some resistors if I've removed the ram, or is that only if you have ram still in the sockets?

Will be ordering one of your 4mb upgrades in a mo and it looks fairy plug and play.

bye for now

Phil C.

User avatar
exxos
Site Admin
Posts: 3541
Joined: Wed Aug 16, 2017 11:19 pm
Location: UK
Contact:

Re: Mega ST4 working but only showing 2mb

Post by exxos » Sun Apr 08, 2018 9:33 pm

Forgottenmyname wrote:
Sun Apr 08, 2018 9:11 pm
Just removed all the ram from the motherboard of the Mega, Will I still need to put +5v to some resistors if I've removed the ram, or is that only if you have ram still in the sockets?
If old RAM is removed, then no need to do anything with the resistors :)
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

https://www.exxoshost.co.uk/atari/ All my hardware guides - mods - games - STOS
https://www.exxoshost.co.uk/atari/last/storenew/ - All my hardware mods for sale - Please help support by making a purchase.

Forgottenmyname
Posts: 282
Joined: Fri Mar 23, 2018 8:22 pm

Re: Mega ST4 working but only showing 2mb

Post by Forgottenmyname » Sun Apr 29, 2018 7:54 pm

So finally had a chance to do some diagnosis on the Mega.

With Exxos's 4mb upgrade and a Marpet 4mb fitted I get a black screen with a white background.

So I've tried a few new chips (the removable ones on my board) and all of the old Ram is removed.
The PSU was recapped.

The CPU seems to be working, Halt is high. The reset circuit seems to work ok.
I've tried a different set of Roms. I've tested the MAD lines and these look ok.
32mhz, 16mhz and 8mhz clocks seem ok on the scope on the MMU, Shifter and CPU.
I've put a diagnostic cart in the Mega and I get a red screen with a lot of vertical white bars.
Connecting my laptop to the serial port gives me a whole load of repeating codes. I'm not sure of the exact terminal emulation but most of the stuff I tried does not produce anything meaningful, v100 etc.

Any ideas?

User avatar
frank.lukas
Posts: 73
Joined: Fri Jan 19, 2018 11:52 am

Re: Mega ST4 working but only showing 2mb

Post by frank.lukas » Sun Apr 29, 2018 8:28 pm

Set your Laptop to 9600 Baud ...

Post Reply