My Odyssey with the Mega ST & the dreaded bombs

[olfad]

HI there,

I'm 28 & from Germany and have had a huge interest in old hardware for quite some time. Currently I have a collection of around 30-35 Commodores, Ataris, Sharps, Apples, Amstrads etc. and I just have a blast working on them

I've been reading on this board for some time now. I'm glad there are still active and passionate communities for this old hardware The past few weeks I have been busy with restoring and repairing a few dozen (!) STs, STEs, Mega STs, Mega STEs and TTs. I'm getting ahead quite well and managed to get everything operational again (even the floppies), except this one case, which is slowly driving me insane.... A Mega ST 1.

This specific machine is.. weird. I don't want to bore you with a wall of text, so here is a rougly chronological order of what I did. This happened over the course of many days now where I have been busy with searching through schematics, service manuals, the internet and then some.

 

1. Power on the Mega ST: 33 Bombs

2. 33 Bombs apparently mean "Trap instruction vector #1", though none of the service manuals, 68k manuals and internet sources mention any further info as to why and what this error occurs.

3. Removed the Rom (6 Chip), put it into an 520ST+ - Boots fine with 1.04, ROMs seem fine!

4. Resocketed all ICs, same 33 bombs

5. Checked whether Jumpers W2,3,4 were set correctly for the 6 Chip upgrade - they were. U12 is disabled as well. Pins 12 and 14 disconnected!

6. 33 Bombs.

7. Checked all RAM, CPU, ROM solder joints

8. 33 Bombs

9. Tried booting in all combinations with or without FDD, Keyboard, Mouse

10. 33 Bombs

11. Checked booting with RTC battery mounted, checked PSU voltages (5.07 & 12.65)

12. 33 Bombs

13. Now, I tried booting while holding down different keys on the keyboard: Shift (4 Bombs), Control (fast resetting a few times, then 33 bombs) and ESC (11 Bombs) ????x

 

One thing I noticed: As the Mega ST 1 was originally delivered with only two ROM sockets, the other 4 were soldered in by a former owner. BUT: The small ceramic Caps are missing, C9-C12 if I remember correctly. Could that be the issue? Not enough juice to boot from the roms?

I probably missed a few steps I have undertaken, sorry about that. This thing is giving me a headache for sure

I would be happy about any suggestions you might have. Anything, really.

 

Another thing: I have two more Mega STs 1 & 2. I have refrained from exchanging parts to find the culprit, because: They are both originally sealed by atari, have never been opened since leaving the factory and still run great, even the floppy. Do you think I should open them to check on everything (caps, leakeages etc)? Or do you consider sealed Megas to have a higher worth?

Thank you

[snarkdluG]

The decoupling capacitors would provide a more stable feed and eliminate spikes. But it isn't necessary. 

 

Tried changing the 74LS244 and 74LS373? Otherwise try put your finger and feel if any chip is hotter then the others.

[olfad]

Hi Snark,

thanks for your reply :) Are there specific 74s I should check? A brief inspection revealed 4 in total:
74LS244: U32, U35

74LS373: U33, U36

 

Also, I just turned it on again out of frustration.. Still 33 Bombs, but the Colors are inverted. White Bombs, Black background...

 

Greetings

[olfad]

Me again,

I attached a photo of the 74s and the RAM area. There are clear solder residues around some of the RAMs and they seem to be 256k types.. which would make 2MB of RAM? Did a former owner possibly upgrade the RAM and not finish some other setup to get the ST to work?

[olfad]

Just managed to get the occasional 4 Bombs (Resocketing the roms once again) and a new record of 38 Bombs (no idea) ? Already ordered a Diagnostic Cartridge from Exxos, hope it arrives soon..

[Umberto]

Can you swap out the keyboard also can you scope the 5v and 12v to see if it is a clean source

 

Edited May 14, 2020 by Umberto

[olfad]

Hi Umberto, unfortunately I only have one Keyboard with the Mega-Connector right now - The others are still in storage. But the 33 Bombs stay the same with or without keyboard, do you think it matters anyways? I know the Ataris had "smart" keyboards which already processed some of the inputs.

I don't have access to a scope right now - I'm part of an IT-Club of sorts, but we're closed because of the pandemic. But what I do have are around 20 other Atari ST PSUs, I can't believe I haven't thought of trying one ? I noticed the Large resistors on the Mega PSU becoming really hot, starting to smell and darkening the PCB a little. Not sure if that has been there from the beginning.

I will try a different PSU and keep you updated, thank you for the tip! :)

 

[olfad]

Ok, tried a different PSU. Lingers at 2 Bombs for about 2 seconds, then the usual 30+ I just can't wrap my head around the problem.

[tjlazer]

See about getting a hold of the Atari diagnostic cartridge for the mega ST.

[olfad]

Hi tjlazer,

Exxos's Version of the Diagnostic Cartridge is already on the way I guess there's no use in fiddling around with the machine anymore until it arrives. Would you mind sharing your opinion on whether I should open up the two other factory sealed Megas?

Greetings

[olfad]

Hey,

The Cartridge arrived and I ran some tests on other devices. Unfortunately, the Mega ST won't output any video signal at all anymore - With or without cartridge Keyboard LED is on, voltages across the board seem fine. I'm at a loss here.

[snarkdluG]

Have you hold your finger on all the chips? Any of them getting extremely hot? Warm is okey, you should be able to hold your finger on them. If it is burning it is a sign that is is shorting. MMU or perhaps memory chips hot?

 

Otherwise you need to start checking if you got clock signal on all chips and if you got signal on address lines and data lines. If anything seems strange or a missing signal. Or checking cold solder joint or cracks. Anything suspicious really.

 

When does it start bombing by the way? At boot screen? Directly after power on? Does it have time to start accessing the floppy drive?

 

When booting with diag cart in you could connect to another computer via a null modem cable and see if anything appears on the other computer with a communication program on it. 9600 baud 8N1 (8 bit char, No parity, 1 stop bit). 

 

RAM seems to look as they do on factory Mega 1. I got 256 in my Mega 1 as well.

[olfad]

Hi again!

Thank you for the info Star Raider, I ran some further tests.

The PSU itself seems to be damaged, but still gives the correct voltages. It has heavy darkening & burning smell from the area around the central voltage regulator. The model is the Mitsumi SR151, as shown on exxoshost's PSU repair site: Mitsumi SR151

Regulator seems to be the 2SC3459, I will get one and change it.

In the meantime I ran your recommended temperature-checking with a PSU borrowed from a (also dysfunctional) 1040STF.  The only really warm chip was U37, a PAL Logic array.

This time I got a picture with the inserted Diagnostic Cartridge:

 

In case you can't read it, it says:

Testing MFP, "Glue timing, Video

E9 Ba? instruction fetch

 

At least something! Although the only info I get is that something is broken ? Does anyone know what "E9 Bad instruction fetch" is referring to? Google brought me some promising links to the atari-forum, but it has been down for weeks...

Thank you for your help :)

 

 

[olfad]

Seems like quite the rare problem ?

Anyone got any advice on where to go from here? I don't know what to make of "E9" and the glitched bits of text (?) on the right side of the screen.