Ok, so I'm not sure what you were told re VGA displays, and I'm sure you know that a 15Khz display is required for low and medium res on a VGA display, but the ST will drive a VGA monitor at high res with a simple cable and no special adaptors. I know it's a bit late now, but all you need for high res is one of these: https://www.exxosforum.co.uk/atari/store2/#0170 and any VGA monitor. What I would say though is that high res is not the most helpful for troubleshooting as you don't get plain white screens or black screens with a white border, so if you can use low res it is more helpful. Also, it is pretty simple to wire up a composite out connector. Assuming you have a machine with a modulator simply connect pin 2 of the 13 pin video connector to the signal wire of a composite cable, and the ground to pin 13. This is just stuff for reference in case you need it again.mikasonic1 wrote: ↑Mon Mar 21, 2022 8:01 pm 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.
I'm only mentioning sockets as a general troubleshooting thing, and I understand you have nothing in a PLCC socket now, but any IC's in sockets need to be pulled, have the legs cleaned and if possible give the sockets a squirt with DeOxit, Electrolube or something similar. If you've done that, fine.mikasonic1 wrote: ↑Mon Mar 21, 2022 8:01 pm
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.
What we need now are photos of the board so we can look see if there is anything we see wrong as folks that are used to fixing STs. There may be something obvious to us that's not to you.
Next you need to check the activity on the following pins of the 68000:
CLK - Are you seeing an 8Mhz clock signal?
VCC - are both VCC pins getting 5v?
All the address and data lines - do the signals look right during boot and when powered up? Are any stuck high or low? I'm assuming here that you understand 5v as high and 0v as low from a logic standpoint.
/HALT - is this low when you've booted? it should be high, so 5v.
/RESET - monitor this with power on and when pressing/releasing the reset button. It should go low then high, if not you may need to fix the reset circuit as caps can die and stop it working as expected
/UDS & /LDS - You should see activity similar to the Address lines here.
You should also check the continuity of the data and address lines between the ROMS/CPU/MMU/GLUE and RAM. It is possible for a trace or via to develop a hairline fracture due to manual handling since these boards are now pretty fragile. I had this on one of my Megas, I plugged something into the Megabus socket and it died because D2 & D4 fractured "somehwere". Thankfully an easy fix, but not so easy to find.
For reference:
If you have a scope (I'm guessing you do given your background) then get images of the signals and post them up here.
I'd also say to go through what @stephen_usher said as below:
And I agree that a diag cartridge is likely to be a big help. Even if you get nothing out of the serial port, given you haven't replaced the ROMs and the diag cart starts very quickly from the TOS ROMs it may point towards the CPU, which is something that happened on the last large repair I did.Steve wrote: ↑Mon Mar 21, 2022 6:35 pm @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.
Out of curiosity, do you have a bench PSU? If so then just set it up for 5v and current limited to 1.5A (You can tweak that up to 3A in stages if it doesn't play nice) and see if the machine boots. You only need 5v to boot, not 12v, and a bench PSU will at least rule out your own PSU, which might be supplying the correct voltages, but may not be delivering the correct amperage under load. It may be that the PSU has some sort of failure having been in storage for 12 years.
As has been said, there are no 'do this and it will fix your problem' answers sadly.