Atari 520ST White Screen Dead CPU ?

[axeman]

Hi,

 

I'm troubleshooting a white screen non booting 520ST. The CPU gets hot quite quickly and after testing glue mmu and shifter in another working machine these work fine. I've pulled out the 6 rom chips and tested the address and data lines which are all high is this normal ?

 

I've socketed the first ram chip which I thought may be faulty it was ok.

 

All the clock signals seem ok and the BERR line is low when all chips are inserted. 

 

The CPU is the only chip getting hot. Any other tests I can do to verify CPU is bad before embarking on desoldering it ?

 

 

[snarkdluG]

Yes, address- and data bus is all pulled high without activity. You need the ROM in the board. Get yourself a test cartridge. 68K proc is normally warm/hot but not burning.

 

Normally ram is a very common problem. Could also be rom. Check PSU? 5V ok? Is reset working or stuck?

[axeman]

Thanks for the reply

 

I've recapped the PSU and checked the voltages on a scope for noise and they appear ok.

 

Reset toggles the reset line on cpu.

 

Just tried swapping ROMS and they work ok. Its MT ram so a very likely suspect.

 

   

[Chri O.]

Link: Wizztronics  stock all custom IC's for the Atari computers some available in bulk.

[axeman]

I think I'm going to try just desoldering the ram data out pins initially as desoldering that first ram chip was a nightmare and I now realise why. There is solder on the top of the ram pins so there was probably a piggy back ram expansion installed at some point. 

 

Hopefully removing Data Out signals will identify bad ram chip(s).

[axeman]

I discovered data pins d0-d3(Pin 11,12,13) on rom sockets  U2, U3, U4 don't connect to the same pins on U5,U6,U7. Is this normal ? 

 

Difficult to tell from the schematic but I thought all the data pins would be connected together on all the roms 

[ParanoidLittleMan]

ROMs are 8 bit, data bus is 16 bit. So, 3 ROMs go on lower half of data bus (d0-d7), other 3 on higher half (d8-d15) .

 

[axeman]

Thanks ParanoidLittleMan.

 

Had some damaged tracks. Repaired them but still getting white screen with data lines high on the CPU and Roms but not on the RAM.

 

 

[ParanoidLittleMan]

If you don't have schematic, get it here:  https://docs.dev-docs.org/

 

[EzdineG]

Check the GLUE socket. Remove the IC, spray contact cleaner, have a good look at the pins.

I have had two STMs that white screened from gaps in the pins between the socket and IC here. Swapping GLUE chips make them boot, but bending pins ultimately fixed them for me.

[axeman]

Swapped a couple of ram chips that had slightly fuzzy data out signal and now getting white border with random colour pattern.

 

Could also be dodgy glue as there was a ram expansion connector stuck on top of it which I removed. Cleaned the pins but will try bending them back a little.

[axeman]

Apologies the RAM expansion connected into the top of the MMU not the Glue

[axeman]

Yahoo it's back from the dead. I very slightly bent out some of the more bent in pins in the mmu socket and voila!

 

Thanks for all your help and inspiration guys. Another old ST lives.

[TGB1718]

Nice one, glad you got it going ?

[calimero]

On 7/30/2021 at 8:16 PM, axeman said:

Yahoo it's back from the dead. I very slightly bent out some of the more bent in pins in the mmu socket and voila!

 

Thanks for all your help and inspiration guys. Another old ST lives.

Does MC68000 still get hot (did temperature now go down)?

[axeman]

No the temp was the same but my other st is the same. The cpu gets hot but you can still touch it with your finger.

 

I decided to put some heat sinks on.

Edited August 1, 2021 by axeman