exxos's Atari, Amiga & retro forum

Well, 2.2K SILs are in, but still the same problem, a completely black screen on power up. Since I now have a decent de-solder gun I decided to bite the bullet and remove the socket and re-solder in the CPU.

Trouble is, I did that and it's still dead.

Loolking at the board, it seems to have had a lot of re-work done over the years: The DMA and Shifter have been removed and refitted in sockets at some point, the YM was missing on arrival, and there's extensive signs of reflow work on the underside of the GLUE socket and at various other locations about the board.

I'm beginning to think that at some point it was 'Left for dead'. Therefore I guess I have a choice now of deciding wether to have it donate it's parts so that other STs may live, or go for it as a long term project to help me learn some stuff electronics wise, and to see if I can ressurect it at some point once I have amassed some more knowledge of the ancients!

I think since it's a Mega, It's worth keeping for now, and I can start to learn to use my scope and potentially get to fix it at some point in the future.

Bloody annoying though, when it had been running for a while. TBH I was a little concerned about stability, so it'd be no surprise at all if it has some other problem going on with it. :(

EDIT: @exxos I was reading your article about Mega ST issues and was wondering if it matters if you ise non Mega MMU and GLUE? It seems to hint that it does in your article.

I've swapped MMU and Glue from other machines into my Megas. I think as long as you use the same type you're ok, ie dont use IMP ones etc.

You can swap GLUE, MMU etc around from any ST machines. You may need my mega clock buffer board on that machine. Generally IMP chips aggravate issues.

I would also suggest you check the signals from the CPU to MMU. I had a machine where /AS was broken but remarkably machine generally ran fine but had some weird issues. This wasn't anything to do with the CPU, there was literally a bad via somewhere on the board.

I had a STFM a few weeks back which had never been touched, and yet I had to re-solder literally half the board because of bad solder joints.

@exxos Thanks, I'll check the signals, see what's what. Also going to swap out the reset switch, not convinced it's good, it's quite wobbly.

Will be resoldering the reworked areas to start with and then looking around for dry joints.

I suspect it's possible that all the manhandling of the board could have upset joints on the board, so out with the big magnifying glass.

I'll compare signals and voltages at the CPU with my working Mega one. It still seems to think its got no CPU, which is weird.

I soldered both sides when I put the CPU back in in case there was an issue with the vias too, as the board is pretty poor quality.

PhilC wrote: 24 Dec 2019 11:17 I've swapped MMU and Glue from other machines into my Megas. I think as long as you use the same type you're ok, ie dont use IMP ones etc.

I thought that was the case, I don't think I have any IMP chips at all.

Always useful to ask the collective brain 😁

Have you tried pressing reset and pressing down on various chips and PCB locations at the same time ?

exxos wrote: 24 Dec 2019 14:06 Have you tried pressing reset and pressing down on various chips and PCB locations at the same time ?

I have.

Sounds like its scope time to me.

just checking though, are you saying its a white border with black background or just all black screen?

@PhilC No, no border, just a black screen. I've currently got a hires cable plugged in, so I'm going to try one of my VGA switchers at lo res. I doubt it'll make a difference, but worth a look. Scope time indeed.

Ok, so its very unhappy if you're not getting at least a white border, as you should see that even if the cpu is halted.