Mega st 2 boot problem

[Gotang]

Hi guys

Like to say thanks for the welcome to the forum thank god i found you lot was starting to pull my hair out trying to find info on the megas

ok the story so far
have a mega 1 that is working and am using this as a base for things to try
picked up a mega 2 that's been upgraded to 4meg
it originally had a Viking card in and on power up white screen (sometimes pink ) corrupt graphics won't go any further
i also have a grabbed a exxos diag cartridge to help (thanks to Jonathan been a massive help)

so what i have done so far first took the Viking card out pushed it to one side for now no change
took out all the socketed chips and tried them in the mega 1 all work find and cleaned the contacts no change
have checked the whole board for cracked or broken solder joints no change
de soldered the roms and fitted sockets tried roms in mega 1 all good but no change
de soldered the shifter chip also fitted a socked tested in mega 1 all good still no change

my skills set
im good and soldering and de soldering lol
ive just picked up a logic prob it came with no instructions
but have got it going and am at moment testing the data lines but to be fair its over my head the readings
im just looking for the same readings at each end of the data line (think thats what you do )

ordered a cheep little oscilloscope never used one but for 15 quid its got to help lol

ive also ordered a few spare good working memory chips to try the gadget tech of placing them on top of current chips to see if i can work out if its memory ( hopeing this works )
im also going to order a new 68000 just in case after memory tests it turns out to be the cpu
reading are totally different fromt he mega 1 cpu

i have a de soldering station
hot airgun and iron
my knowledge of electronics is low but im keen to learn have had a lot of success following videos from gadget etc on you tube learned a lot
from watching them a lot does go over my head but managed to fix a few amigas, bbc, speccys and cpcs the basic stuff this has got me stumped
the more i work on the Atari the more I'm falling in love with it

all help and things to try would be greatly appreciated keep it dumbed down for now lol
thick Geordie hear
again thanks

Gotang (dave)

[Gotang]

Little update

Did a load of data line checking not found a problem
so thought i would change cpu and fit a socket while doing it did that nothing
also managed to get 2 memory chips so tried piggy backing to test memory nothing all same probs
just before testing memory turned on and smoke came from l50 no idea what this is resister ?

atari1.jpg (389.53 KiB) Viewed 3462 times

have a old stfm board that's snapped that i used to harvest parts from and on l50 on my mega one was this chips had one on the harvesting board so took it off and fitted

atari2.jpg (328.12 KiB) Viewed 3462 times

also can someone shine a light on this i think diode on my mega one this is not on the shifter looks like a trace has been cut and this fitted
I've been told by two people different things one its factory done and should be like that
and someone else told me to take it off and fix the data line what's correct

atari3.jpg (423.23 KiB) Viewed 3462 times

im a bit stuck on what to try now waiting on some memory for a old pc to fix up with xp so i can null modem it to the st to try and see what the diag card is saying but got a feeling this will be a battle in its own to get going
again many thanks to anyone who can help out
also like to put out a massive thanks to exxo this webpage shop is awesome prices are very fair and shipping service is awesome very impressed

cheers
dave

[DoG]

The diod bodge job is factory. I have that fix in one of my Mega as well.

Is there a 33ohm resistor on R71, R72 and R74? Why does it look like there is not enough solder on the new memory chips? Must be a bad connection somewhere on those. White screen, black border is often memory fault. Or ROM fault.

Is L50 soldered in in the second photo or it is just a dry fit? Either way, it's an inductor, shoke.

[Tomswork]

It does look like ic u66 has very little solder on some of the pins

Tom

[rubber_jonnie]

The diod bodge job is factory. I have that fix in one of my Mega as well.

Is there a 33ohm resistor on R71, R72 and R74? Why does it look like there is not enough solder on the new memory chips? Must be a bad connection somewhere on those. White screen, black border is often memory fault. Or ROM fault.

Is L50 soldered in in the second photo or it is just a dry fit? Either way, it's an inductor, shoke.

If that diode is a factory job, be nice to know what the heck it is for, neither of mine have that. Good point about the resistors, very much needed. I have mentioned the solder on the RAM chips as I've spoken with Gotang on FB Messenger prior to this thread. In fact several of the RAM chips look like they're not soldered in properly
.

[czietz]

If that diode is a factory job, be nice to know what the heck it is for, neither of mine have that.

Reducing the supply voltage of the IMP Shifter.

From an Atari technical document:

modification_imp.PNG (236.9 KiB) Viewed 3433 times

[DoG]

Yes, got a IMP on both DMA and Shifter and both got diod fix. But mine is a Mega 1.

[rubber_jonnie]

@czietz Cool, that's interesting and very good to know.

@Gotang I'd suggest you leave the diode in place then if it's necessary for the IMP Shifter.

[Gotang]

all great feed back guys
the extra memory was fitted when i got it
this board is a bit weird if you solder anything to it it just does not seem to flow through the pcb very well
who ever fitted the memory used a load of flux that's really impeded into the board got about 80% of it off going to get rest off today
from the back all looks great but the front looks like there is hardly any solder on
i was going to remove the memory and fit sockets and re sit memory from there but i think i will just resolder the chips for now and flow them better
until i can get a pc to read the diag card and give me a better idea of what's going on as rubberjonnie has suggested
must say the mega 1 version 2.1 board is a much better quality board than this one
cheers again for all the info good to know that was a factory fix guys

gotang

[Tomswork]

Just make sure you have flux and solder that are leaded
Old solder can be hard to reflow it will have oxidized. Good cleaning

Tom