Looks like I'll be getting a Mega ST4 soon

[rubber_jonnie]

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.

Indeed, and I swapped to a VGA switcher, and just get a plain white screen in lo res.

It's interesting, when I use the diag cart with a working machine, I see the red LED on the cart flash as the ROM loads, on this machine, nothing, which makes me wonder if the ROMs are visible to the machine. WIll have to do some scoping again, see what I can see.

I also swapped out the reset switch, which fell apart as I removed it, but no change even with a brand new one fitted.

[Tomswork]

Hi I'm working on a mega ste with the same status I did manage to get a red boarder up around a white box on the screen. I have straighted most of the pins in the sockets and now start the re wetting all the solder joints as many have corrosion on them. I had the reset switch disintegrate as well


Tom

[exxos]

@rubber_jonnie I don't remember if this has been mentioned before or not ? https://www.exxosforum.co.uk/forum/viewt ... =78&t=2362

[rubber_jonnie]

@rubber_jonnie I don't remember if this has been mentioned before or not ? https://www.exxosforum.co.uk/forum/viewt ... =78&t=2362

@exxos looks good, I will have a read through.

[rubber_jonnie]

Hi I'm working on a mega ste with the same status I did manage to get a red boarder up around a white box on the screen. I have straighted most of the pins in the sockets and now start the re wetting all the solder joints as many have corrosion on them. I had the reset switch disintegrate as well


Tom

Looking at some of the sockets they are in just plain awful condition, so I will probably replace now I have my de-solder gun since it will be much quicker and easier.

[rubber_jonnie]

Ok,so I have started working through here: https://www.exxosforum.co.uk/forum/viewt ... =78&t=2362

Since I replaced the pull ups with new 2.2k SILs, I did that first, and get either 1.8k-2k or 1k with the exception of the pins mentioned. I do get 4k on VMA though, which isn't mentioned. I will check a working machine to see what it reads there.

@exxos Should I be seeing 2.2k exactly here? the 1K vaues seem good, but the others reading below? Is the 4k value for VMA reasonable? Also, I buzzed out the power input sockets to see if there is a short there. If I buzz between 12v and GND nothing happens. If I do the same with 5v, I get a buzz which then stops and gives me a reading of something like 68R. It seems like a short, but could it simply be the 5v connected to GND via the capacitors? I will also check on my other Mega see what's what.

I'll work my way through the rest of the checks, see what's what.

I have also reflowed the GLUE socket, which was a bit nasty, but the dodgy parts did buzz out ok. The ROM sockets are really pretty awful TBH, they are heavily oxidised, so they will be getting replaced, so thank god for the new desolder gun, it really makes light work of this sort of thing.

Ho hum :sigh:

[PhilC]

Hi RJ,

Is it worth binning the rom chips in favour of an exxos rom adaptor?

I've probably said it before but I've found the rom address lines get easily corroded on the Megas I've worked on but still visibly look fine.

As for the 5v to gnd, that's normal that you'll get a reading when the meter is in beep mode but you'll get a different reading on ohms as you've seen.

If you are still just getting a black screen though, it really does sound like something is wrong with the shifter as you should hey a white 60hz screen before the glue asserts 50hz which as you know is the indication that the rom is being read.

[exxos]

@exxos Should I be seeing 2.2k exactly here? the 1K vaues seem good, but the others reading below? Is the 4k value for VMA reasonable? Also, I buzzed out the power input sockets to see if there is a short there. If I buzz between 12v and GND nothing happens. If I do the same with 5v, I get a buzz which then stops and gives me a reading of something like 68R. It seems like a short, but could it simply be the 5v connected to GND via the capacitors? I will also check on my other Mega see what's what.

Looks like they on their own anyway.

1.jpg

Capacitors on the 5V rail etc will measure short and resistance will go up as they charge.

Have you checked reset voltage is switching ? Machine sounds totally dead if your only getting a black screen. You probably going to have to hook up a LA to it. Do you get any pulses on the CE pins on the ROM sockets ? Or the ROM outputs form GLUE ?

Can you post a image of the shifter area on your board ?

[rubber_jonnie]

@PhilC @exxos

I have switched over to Lo-Res and I get a completely white screen, and flicking over to Hi-Res gives me a totally black screen, so staying on Lo-Res for now.

I get the same result with or without the ROMs fitted.

As for the 5v line, the cap charging after a short did seem logical to me, so thanks both, it's nice to have confirmation I'm thinking right. Also that VMA reading makes sense now looking at the schematic.

The Shifter is good , I've tested it in another machine, and also the DMA, MMU and GLUE, and all were fine. Shifter and DMA have been desoldered and sockets fitted, but prior to my ownership and also have bad socket contacts, though not as bad as the ROM sockets. I'm considering replacing them too, as quick and easy to do. It'll give me a chance to check under the sockets for damage. A lot of the vias seem to be in really poor order where the board has had rework done.

As for the ROMs, I may ditch them in the furture, but I am going to replace the sockets first, since I have the spares and and like the Shifter and DMA, I can check the tracks and vias for damage.

I think manhandling the board when I originally removed the CPU has done something. I just don't know what. I certainly had it working, albeit a bit unstable, before I pulled the CPU.

I may get an adaptor later from exxos, we'll see as have a Mega 1 with German TOS, though that is kind of working with the PAK and IDE board right now, so on English TOS 2.06 or 1.04.

I will check the reset voltage is switching, as just not got round to it yet. I also want ot buzz out all the data, addresses, RAS/CAS lines etc,see if any are broken.

I also need to scope out the CPU and compare to a working machine, and whilst I do have an LA, that's still a work in progress for me in terms of learning how to use it. I'll see what I can see on the CE lines too, as I got the ROM pinouts today too. Probably should start reading my Atari ST Internals book a bit deeper I reckon!

I'm no electronics expert, and this is the deepest electronic troubleshooting I've ever done, so your patience and assistance is most welcome, thanks.

I'll get some pics too.

[exxos]

The first thing the CPU will do after reset is put all address lines to 0's... At that point GLUE will decode that address and put CE low on the ROMs.

So regardless of anything else, if the CPU signals are all checked to GLUE, and ROM CE is checked to GLUE.. Then the machine should be at least trying to boot... It really doesn't matter what any other IC is doing at that point, not even RAM.

Really just go around the address pins on GLUE with the scope, keep pressing reset and make sure they all switching at least. /AS,/LDS,/UDS,/DTACK should all be going low at the same time after reset...

If you never see /AS go low after reset then the CPU isn't running, like its held in HALT or RESET mode. If /AS does go low, the CPU is running, so the CPU will put all 0's on the addressbus.. if it doesn't, then GLUE won't decode and GLUE wont set ROM CE low.

If you see /AS going low and no ROM CE going low, then GLUE isn't decoding the address, so maybe a broken address line to it, or it never sees, AS,LDS,UDS going low.

You need to check if ROM CE is triggering, then at least you know how far you are getting during stuff after reset.