Looks like I'll be getting a Mega ST4 soon

[PhilC]

You've noticed as well then @rubber_jonnie that when it comes to ram diagnosis that the service manual isn't very useful but you are on the right track.

I've had the most ram problems of any ST on the four Mega's that I'd worked on (sold two). And if I'm honest, I'd probably dump the whole of the ram and use something like @exxos 4mb mmu ram adaptor instead as the type of ram used in may of the megas is very hard and expensive to find and not very reliable any more because of its age.

If it helps I have a couple of Marpet adaptors that I could send you. You'd need to source your own simms but at least this might help you. But in all honesty, best advice is dump all of the ram and replace with a modern solution.

[rubber_jonnie]

You've noticed as well then @rubber_jonnie that when it comes to ram diagnosis that the service manual isn't very useful but you are on the right track.

I've had the most ram problems of any ST on the four Mega's that I'd worked on (sold two). And if I'm honest, I'd probably dump the whole of the ram and use something like @exxos 4mb mmu ram adaptor instead as the type of ram used in may of the megas is very hard and expensive to find and not very reliable any more because of its age.

If it helps I have a couple of Marpet adaptors that I could send you. You'd need to source your own simms but at least this might help you. But in all honesty, best advice is dump all of the ram and replace with a modern solution.

As it goes @PhilC The thought of one of exxos adapters had crossed my mind. Whilst I have some spare chips I'll try and fix it, after all, at the moment I'm looking for things to do, and I've also managed to identify some alternative NEC chips that might fit the bill..

Your offer is very much appreciated and I may well take you up on it (Unless I feed harry first!) as I have a ton of SIMMS, but the RAM reliability on this Mega 4 is just terrible. I've had the odd problem on STF/Ms and the Mega1 I have is super reliable, but the 4 is such a lemon, and I think it's probably half the reason it was in quite a state of disrepair when I originally got it.

I had to dump the RAM on my 520ST, but fitted a Marpet in that (It's a snug fit!) but it's definitely the way to go when onboard RAM starts to go south. Didn't help the 520ST had a bad GLUE too, but that is all sorted and runs lovely now with TOS 1.04 and 1MB.

[rubber_jonnie]

Well, my order of sockets arrived, and I swapped out the bit 8 & 10 chips in bank 0, but I still have the same diag cart result. I can't help but think that there is something more fundamentally wrong here, and it reminds me very much of the 520ST that I got from @exxos last year.

That gave a similar on screen result when booting to the Mega 4, and it had a nicely done, but faulty piggyback ram expansion. I removed that, and it worked, for a while, and then died again. This really reflects the behaviour I'm seeing in the Mega4, and what fixed the 520ST was removing all the RAM and fitting a Marpet SIMM adapter I had spare.

I think as soon as the store is open again, I'm ordering the MMU RAM expansion, which will effectively sidestep the on board RAM circuitry. In the meantime I will probably change the bank 1 bit 8 & 10 chips because I may as well whilst I wait for the store to open.

[Icky]

These motherboards aren't getting any younger @rubber_jonnie it won't be long before a lot more of these ST's are suffering from issues like the ones you are seeing.

[exxos]

These motherboards aren't getting any younger @rubber_jonnie it won't be long before a lot more of these ST's are suffering from issues like the ones you are seeing.

I'm fighting a losing battle with my boards. Once they die, I just scrap them for spare now.

[rubber_jonnie]

These motherboards aren't getting any younger @rubber_jonnie it won't be long before a lot more of these ST's are suffering from issues like the ones you are seeing.

Absolutely. I want to get this going again as it showed promise and it was nice to have a 4MB Mega, but it has absolutely been the pits, dying almost as soon as I fix anything.

We'll see what happens, but if the worse case appears and it still doesn't work after fitting one of the MMU upgrades from exxos, then at least I'll know. I just need an H4 Mega variant now, since I already have a potential donor

[Icky]

These motherboards aren't getting any younger @rubber_jonnie it won't be long before a lot more of these ST's are suffering from issues like the ones you are seeing.

I'm fighting a losing battle with my boards. Once they die, I just scrap them for spare now.

I agree. I am following the same view as you @exxos. Once a board dies it becomes parts for the H4.

[rubber_jonnie]

These motherboards aren't getting any younger @rubber_jonnie it won't be long before a lot more of these ST's are suffering from issues like the ones you are seeing.

I'm fighting a losing battle with my boards. Once they die, I just scrap them for spare now.

Yeah, I had a CO70789 just stop working with any floppy drives, and having tested all possible options, it still just won't work, though the rest is fine, so it's my next donor board for the H4 I just received.

Hopefully I can do a mich nicer job of this H4 using the lessons from no1, and it can go into the spare case, with H4 no1 becoming my test board. Trouble is, I don't want to give up this Mega as a donor.

[PhilC]

Its knowing when to give in. Of the 4 megas I've had, only one is still using it's original ram. The other one I have is a mega 1 with soj22 to dip22 adaptors and 4 mg wired in that way. And that machine wasted far too much of my time but now it's running nicely with my Pak68/3, so was worth it in the end.

[PaulJ]

Don't know @rubber_jonnie , I'd be hard pressed to use a mega 4 as a donor. I would keep fixing even if it wanted to die.