Atari 1040ST Green Screen..

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mikasonic1
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Atari 1040ST Green Screen..

Post by mikasonic1 »

Hi

I have an Atari 1040ST which i use for music production which i have had since the early 1990´s. It worked flawless up until around 2010 when it was put in storage until late last year when i took it out again to use on a project and it didn´t start up. Being an audio electronics tech i decided that before i would go any further, due to its age, i would change all of the Electrolytic caps in the PSU and on the Main PCB and check/resolder all of soldering on the back of the pcb.

After doing all of this i realised that the Floppy drive had seized up and my monochrome monitor was also dead, so i decided to go for a Gotek emulator and a VGA Monitor solution which i foolishly thought was just a VGA converter cable.. After going through all of that 15khz nonsense, i finally purchased a GBS 8220 VGA converter PCB and then found out i needed to build a V Ref + H ref Merge circuit which i did and finally got a 'picture' from the Atari on the VGA monitor which was just a white border and black box on boot up, no desk top icons etc..

So after reading internet forum posts which seemed to point to a DRAM issue, i decided to buy the EXXOS 4mb RAM upgrade which arrived last week and installed no problem. Now all i get when booting is either a completely grey or green screen (depending if i use Monochrome or RGB cable wiring). For a few seconds on power on, there is a vertical pattern that moves fairly fast upwards and is the same size square box as the black square box in the center before the RAM upgrade. Also on some power ups it has a light grey border with a darker grey box and shows that vertical pattern for a few seconds. Mostly it is just a totally grey or green screen. So something has changed since the RAM installation as there is no longer a black box.

Being a very experienced tech, naturally i have checked all of the simple and stupid stuff like voltages, broken traces, bad solders and cleaned/re-seated the chips that i can, unfortunately i have the version with the surface mounted MMU and GLUE. Most of the chips and processors warm up to normal temp after being left on for a while.

The next stage is to start probing but despite having a great knowledge of electronics i am not familiar with the Atari logics and troubleshooting it so does anyone have any experience with my issue? I know it could be caused by a million different things but i would say 99% of the silly stuff is already ruled out so this is likely something more serious.

Any help more than appreciated.

Thanks

Mike
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exxos
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Re: Atari 1040ST Green Screen..

Post by exxos »

https://www.exxosforum.co.uk/atari/ All my hardware guides - mods - games - STOS
https://www.exxosforum.co.uk/atari/store2/ - All my hardware mods for sale - Please help support by making a purchase.
viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
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rubber_jonnie
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Re: Atari 1040ST Green Screen..

Post by rubber_jonnie »

Are there any socketed ICs? If so you should check/clean/tweak the sockets, see here: https://www.exxosforum.co.uk/forum/viewt ... hilit=plcc

I would suggest that it would have been better to troubleshoot with the existing RAM installed before upgrading. I found myself in a similar situation with my Atari 600XL, and I ended up reverting to get a result.

If there is an option to set the RAM to 1MB it may help narrow things down.

A diag cart might be of help here, you can connect via serial to a terminal session on a PC. It sounds like a RAM issue based on what you've said, and this may well give you some output that will point at where the problem is.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
mikasonic1
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Re: Atari 1040ST Green Screen..

Post by mikasonic1 »

Thanks for your reply!

As i mentioned above, this is a C070859-001 rev C PCB which, apart from the 6 x TOS, it only has one other socketed IC and i have checked all of those.

The Expansion RAM has only been set to 1MB, i haven't even tried 4MB yet as i figured it was best to start with 1MB until i get it working.

All 32pc of the original DRAM are removed so there is no chance of putting those back. I have also changed and socketed the 4pc flip flop 74SNxxx chips when the old RAM was in d before i removed it and installed the upgraded RAM.

After everything i have done so far, if i had to take a calculated guess i am really suspecting the MMU is the problem and unfortunately it is the SMD type so no swapping it out to try or fixing the pins in the socket as there are none. I can honestly say that 99.9% of the silly stuff has been checked.

I don't have a Diag cart but i am certain it wouldn't boot with this fault.

I really appreciate your reply and links but it seems i am on my own with this and i will just have to figure it out.
Steve
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Re: Atari 1040ST Green Screen..

Post by Steve »

@mikasonic1 do you have a heatgun or a fine soldering tip? Perhaps the handling of the motherboard could have fractured a solder joint on the MMU. You could easily give it a blast of air, or put some flux around the chip and just run your iron over the pins/edge pads just to 'reunite' something that may have fractured.

Besides that suggestion - which might actually make matters worse so maybe don't do it straight away. With these things you gotta think basic, use Occam's razor etc. I think literally a few days ago there was another guy who was struggling after the same ram expansion installation. 99% of the time it's simply a broken trace on the drams (on the motherboard side)

I see you've done a few perhaps unnecessary things on top of changing the RAM. You've changed the 74SNxx chips before swapping the ram, and another thing or two earlier on in the post. You just need to retrace your steps, maybe put the old 74 chips back, etc etc.

Edit: just re-read your post. You were getting this issue before changing the ram. The things you did before changing the ram was just a re-cap right? It's too easy to just blame the ram here. You do need to get yourself a diagnostic cart and a null modem cable I think otherwise I doubt you'll move forward.
mikasonic1
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Re: Atari 1040ST Green Screen..

Post by mikasonic1 »

Thanks for your input Steve!

As i mentioned in my first post, i repair electronics every day as my job and have done for the past 20 years so i have no doubt that i will get this sorted, my problem is i don't have the time to get up to speed on the logical workings of the Atari circuitry and was hoping there may have been a short cut via this forum. I will just have to get my head around the logics and start probing etc..

I will post the problem when i get it sorted with a very clear explanation for the benefit of others who may find themselves in my position.
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Re: Atari 1040ST Green Screen..

Post by rubber_jonnie »

mikasonic1 wrote: Mon Mar 21, 2022 6:59 pm Thanks for your input Steve!

As i mentioned in my first post, i repair electronics everyday as my job and have done for the past 20 years so i have no doubt that i will get this sorted, my problem is i don´t have the time to get up to speed on the logical workings of the Atari circuitry and was hoping there may have been a short cut via this forum. I will just have to get my head around the logics and start probing etc..

I will post the problem when i get it sorted with a very clear explaination for the benefit of others who may find themselves in my position.
Unfortunately there are no shortcuts, just good old fashioned troubleshooting. People are often looking for 'silver bullets' to fix things, but sadly it just doesn't work that way.

Yes, there are well known symptoms like the black screen with white border, and a completely white screen. These generally point at RAM and ROM, but as per my recent fix for Brett, there was also some luck involved when the machine booted whilst I was probing the CPU LDS line and I'd bridged the CPU to the socket with my probe, and that meant the socket had problems.

It's probably taken me a good 10 years to get my head into the ST space, and I still take a long time to figure stuff out, so whilst there are some pointers as to certain faults, you still need to do the necessary troubleshooting to narrow things down I'm afraid.

So logic probes/scopes/diag carts/schematics etc.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
mikasonic1
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Re: Atari 1040ST Green Screen..

Post by mikasonic1 »

It's very confusing as, due to my SM124 monitor packing up, i had to go through that pathetic VGA crap which i was told by a weekend warrior that was 'just a cable' which eventually after months of messing turned out to be GBS 8220 and a DIY V ref + H ref merge circuit to finally get a picture on the VGA monitor. It was only then that the black screen with white (or green) border problem became apparent and due to all of the messing with the GBS8220 i wasn't sure if it was that or the computer causing the issue.. I eventually come to conclusion that it could be RAM and instead of messing with the existing DRAM i decided to just put in the upgrade and hope it solved the problem. After the RAM installation the black screen white border has gone and now it is just a completely light grey (or green in RGB) screen, no longer a border on boot up.

You keep mentioning sockets but as i mentioned earlier, aside from the 6pc TOS and one other IC every other IC is soldered in, so the CPU/GLUE/MMU/SHIFTER socket issues are not a possibility in this case. I have checked all of the TOS sockets and resoldered most of the soldering on the PCB. Re-capped. I have tried to boot with all of the 6pc TOS removed and it had the same grey screen. I also have checked the connections and traces between the new RAM and the 74SNxxx IC's and CPU as well as the all of MMU to the vias.

As there are barely any socketed IC's, all of the voltages are correct, re soldered and re-capped, diodes, capacitors and resistors all good and the emitter follower transistors in the Shifter circuit all behaving correctly, it stinks of a failed IC IMHO.
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Re: Atari 1040ST Green Screen..

Post by stephen_usher »

It's probably time to get an oscilloscope out then.

Without being able to look at the signals it's at the point of reading tea leaves or animal entrails. You need proper evidence.

First check the clock on the CPU... and then the reset line to make sure that's working. I take it hitting the reset button does nothing?

Then I'd have a look at the data lines and address lines, both at the CPU end and at the ROM. See if the /CE and /OE lines on the ROMs are going low, as without those you're not going to get anywhere, and they're generated by the MMU.

The MMU is the key chip for starting up as it does all the address decoding and other glue, such as generating RAS/CAS for the RAM etc.

Is the RAM getting the RAS/CAS signals?

These are the basics. Once you can tell at least the autonomic nervous system is working we can build upon that.

P.S. Oh, and having a green screen suggest something iffy with the Shifter, unless your dodgy VGA adapter cable is broken.
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mikasonic1
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Re: Atari 1040ST Green Screen..

Post by mikasonic1 »

Hi Stephen

Thank you!! This is exactly the info i was hoping to get when initially posting on here.

I will start probing and put the scope on it and post my findings with reference to what you mentioned.

As it was such a run around hooking up the VGA via the GBS8220 and Href Vref Merge and i don´t have any other monitor i can test with, i have to assume everything is working. I have the VGA cable wired for RGB at this moment that is why it is green but the problem was exactly the same but light grey when wired as Monochrome.

Oh, i did forget to mention that the reset button has been working since the start of all of this. Randomly, when booting i do get a light green screen with a darker green (standard desk top green) border (which is lighter and darker grey in Monochrome) and when pressing reset button the screen goes all green and instead of the lighter green square a weird vertical pattern moves upwards for a few seconds in its place, sometimes when pressing reset the lighter green disappears and then reappears after reset so i assume the reset is working but i will probe it to be 100%.
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