exxos's Atari, Amiga & retro forum

To jog your memory. 1mb pads desoldered, but not shown in the photo are the 256kb solder bridges I had to bridge but I mark them on the diagram (I'm away from home right now and unable to take a newer photo).

retrotronic wrote: 19 Nov 2024 16:24 To jog your memory. 1mb pads desoldered, but not shown in the photo are the 256kb solder bridges I had to bridge but I mark them on the diagram (I'm away from home right now and unable to take a newer photo).


20241114_232800-picsay.jpg

I've placed your picture inline as per the forum rules.

Also, the 2nd solder bridge in from the left looks like it might still be bridged by a whisker.

rubber_jonnie wrote: 19 Nov 2024 16:29
I've placed your picture inline as per the forum rules.

Also, the 2nd solder bridge in from the left looks like it might still be bridged by a whisker.

Cheers Johnny. I did a continuity test and all of the desoldered pads registered as open circuit, I shall get the microscope on that pad later. To rule out bad eproms, I now am awaiting replacement Eproms. I guess if that not solve things then it time to purchase a diagnostic cart.

retrotronic wrote: 19 Nov 2024 16:41

rubber_jonnie wrote: 19 Nov 2024 16:29
I've placed your picture inline as per the forum rules.

Also, the 2nd solder bridge in from the left looks like it might still be bridged by a whisker.

Cheers Johnny. I did a continuity test and all of the desoldered pads registered as open circuit, I shall get the microscope on that pad later. To rule out bad eproms, I now am awaiting replacement Eproms. I guess if that not solve things then it time to purchase a diagnostic cart.

I could only zoom in so far, so a magnifier would be the best option :)

I always use 27C256 EPROMS or EEPROMS, what are you using?

I am using AM27C256 eproms (AMD) dated 1986.

retrotronic wrote: 19 Nov 2024 19:45 I am using AM27C256 eproms (AMD) dated 1986.

I use those (Amongst other things), should be fine.

With the 256K rom pads soldered, and if the 1MB pads have been desoldered and have no continuity between them, then check continuity from the 256k jumpers to further points along the tracks just to be sure they are properly connected down the line.

Also check your PLCC chips are properly seated and making good contact.

An update... after checking my soldering, and continuity testing the tracks to the ROMs, I found the problem; faulty EPROMs - got them replaced with new ones, and now we have TOS 1.04 running.

Next question; I now need to get off the metal retaining clip off he MMU to fit Exxos 4mb RAM upgrade - what's the easiest way to remove retaining clip on the PLCC socket on the motherboard?

Glad you got the machine up and running.

As for the spring clip... I've carefully used an electrical screwdriver to prise it up, but there's probably a better way.

Thank you to everyone guidance so far.

I have Exxos 4mb upgrade which is going to be fitted into my 520stfm. I note Exxos mentions about disabling the on board 512kb ram by either resistor pull up and connecting to +5v or by removing the on board RAM. I've chosen to remove the 512kb RAM using plenty of flux and a heatgun. To protect other components from heat whilst heating up the ram chips for removal, I'll probably use some capton tape and aluminum foil to protect surrounding components.

SMT desoldering is new to me, but have watched hours of YouTube videos to familiarise myself with the process

It going to be a while until I can afford a heatgun. I got my eyes on a Duratool D03167 for the job. I shall post back here again once I get the 512kb RAM removed from the board ready for Exxos' 4mb upgrade.

Is it through hole RAM or smt?

I would suggest caution with a heat gun. I did some tests with a dead ST board and they popcorn very easily.

By popcorn I mean the air trapped in the fibreglass heats up and expands which can cause internal damage in the PCB and can break connections especially if there are vias in the location.

Although the ST pcb is only double sided and as such less at risk of damage to internal traces, via damage aside you can end up with bulges that make the board unusable.

I certainly wouldn't use a heat gun on its own to remove through hole RAM.

Another option is to just cut the legs on the RAM then you can use a standard soldering iron to remove them one at a time.

If you're going to start using heat guns I'd suggest you watch a lot more videos on YouTube first and practice on something you don't care about before you start work on your ST.