Help with 4mb ram upgrade on 520ST C070243 REV. B

[stephen_usher]

Instead of removing the RAM you can disable it by snipping the the power pin on each of the chips with a fine side-cutters.

The power pin (VCC) is the bottom left one:

41256.png

Just cutting one pin on each of the RAM chips is probably the safest way to do this given your level of expertise.

[llanquin]

this is the one i got:

https://exxosforum.co.uk/atari/last/4MBMMU/index.htm

[Badwolf]

Instead of removing the RAM you can disable it by snipping the the power pin on each of the chips with a fine side-cutters.

Yikes, does that really work? I could imagine that might be safe enough for old TTL chips, but with voltage on the other pins CMOS chips will likley still have some level of (undefined) functionality in them.

I dunno enough about these particular DRAM chips to know where they fall in that regard, but I'd be concerned with that approach at a more general level.

In this case, I wonder if cutting the 'Q' pin would be the most expediant solution.

BW

[exxos]

Its been talked about before about cutting power pins. I've powerd up chips and had them malfunction with missing vcc power in the past as well as other people. As I said months ago, in some cases it might be safe, but in others not. Some people say its safe, but is it ? "It works" is not a valid argument. Unless someone extensitve tests every make brand/type of chip the fact is , we simply don't know.

This is why I opt to state cutting power is simply not safe to do. Some people disagree, but again "new comers" searching forums will latch onto the idea cutting power is generally safe and will incorrecly apply that tactic to every chip ever made without fully understanding the implications.

So this is why I always state its not safe and prefer people not to confuse matters by saying otherwise. It may or may not be technically correct, but we have to look at the bigger picture in all this.

What @Badwolf sounds like a better solution. I had not thought about doing that before.

But as said many time already. Just cut the old chips out. It would take like 5mins tops. I've got videos on doing that on my YouTube channel.

[stephen_usher]

The only alternative would be to cut them all out or target the CAS/RAS pins, which are harder to target. DRAM uses the same underlying silicon technology as TTL, i.e. MOS.

[Steve]

You mention that you're trying to install a 4mb kit from Exxos, but you realise he sells two different types? One is the MMU piggy back and the other is the direct soldered type where you remove the RAM and solder the PCB in. I assume you're doing the MMU kit since you mention not wanting to remove the RAM? Sorry for the basic question but I think it's always good to start off knowing the full situation for certain.

The solder-in board doesn't fit this motherboard, so it will be the plug-in.

Are you sure? I'm not talking about the STfm upgrade, if that's what you're thinking.

Soldered ram upgrade for STm

[rubber_jonnie]

The solder-in board doesn't fit this motherboard, so it will be the plug-in.

Are you sure? I'm not talking about the STfm upgrade, if that's what you're thinking.

Soldered ram upgrade for STm

The OP links to this: 4MB upgrade so it is the MMU type.

[exxos]

I asked my old boss about the cutting vcc as I know I was told this when I first started my electronics job. But I stopped doing that and don't remember why. It was almost 30 years ago.

HOWEVER.

He did mention it was VCC AND GND which were to be cut. I think this is where a lot of the confusion comes from about "cutting power".

Dave Jones did a video about chips still running without vcc . They can be powered via io pins, protection diodes etc. But that was documenting CMOS devices. If TTL have input clamp or ESD protection diodes ( I don't remember ) then you can end up in a similar situation. Maybe still possible to get some odd power up by high and lows on io pins. No idea.

My point remains that to newcomers who won't likely understand the differences in chip technology could end up in this trap especially as the "argument" for cutting power only seems to relate to removing 5v from the chips only.

Also as dram can fail, which is well known. We don't know the failure mode which could be anything. Maybe 2 inputs or outputs got shorted due to abuse or internal failure. Cutting power will not solve that problem. Then go shove working DRAM into the circuit and your in the area of killing that as well. I really can't be bothered to look into failure modes and chances of killing stuff.

This is again why I think its best to simply avoid this method . To many unknowns and implications in my book.

Now I think about it disabling ras/cas for a test run isn't 100% safe either. If a dram has failed and shorting something out, it could give the impression the new ram is faulty. The method likely is only safe if there is no existing ram fault. This is not always the case. So I may remove that information from my site or at least mention the machine needs to have no existing ram faults.

[stephen_usher]

@exxos given that the original posting didn't mention a fault I would assume that the current memory is operational, hence safer to leave in place than have someone with almost no experience attack the board and possibly cause terminal damage. I was looking for the simplest and most foolproof manner to help.

41256 chips would have to use Metal On Silicon technology in that era just to get the speed of operation. CMOS is/was just too slow.

With respect to failure modes, yes, if a chip fails it can pull high or low the line in a manner which could over time cause damage to other chips on that line by draining too much current and damaging the line drivers. If this is then main worry then cut the Dout pin (Pin 2) as this is the only pin with a line driver on it, all the others are inputs, and the DRAM effectively becomes a passive device on the bus.

[exxos]

With respect to failure modes, yes, if a chip fails it can pull high or low the line in a manner which could over time cause damage to other chips on that line by draining too much current and damaging the line drivers.

Yes indeed.

If this is then main worry then cut the Dout pin (Pin 2) as this is the only pin with a line driver on it, all the others are inputs, and the DRAM effectively becomes a passive device on the bus.

Yes.. But..Let's just clarify for future readers that is only true if the dram is working to start with. This isn't true of course for failure modes in the previous sentence above.

As for faliure modes, it cant be predicted and cutting Dout,VCC,GND might not be enough. You have to assume a chip fail has gone from being totally open circuit or fallen totally short. Like the IC is filled with solder. Then everything in between. Generally i personally assume the later.

Again for future readers :

If something has caused the output to fail you can't assume it hasn't blown the silicon and shorted other pins, ie inputs. The ic could have blown and have and even have a crack in it. Or simply failed totally as spikes on its inputs over the years have cause it to short out internally. Inputs to chips don't mean they cannot short out what's driving them.

I've worked on z80 systems where a blown PIO pin will either cause the output to remain high or stop the board from starting up at all. That's what I couldn't think of earlier. Cutting power to the chip never worked which is why I stopped trying it. It was a common fault and I've changed thousands of the chips which were killed in one respect or another. I learned the failure modes across the years. I even talked to maxim once about failure modes and they basically said you can't predict it.

The OP could easily cut every IC out if he's not happy about desoldering. If hes not good at soldering then he's probably not good at tracing the MMU pins to cut ras/cas either. A slip of the knife or wrong tracks cuts and hes going to end in a bigger mess. By the time spent doing that he could cut the chips out anyway.

In the case the RAM is proven to work currently then as suggested cutting Dout on every dram would be the quickest way. But those ram chips should be taken out anyway. Even a novice should be able to do that. Then heat the pins up one at a time and gently pull them out one by one. I demonstrated this in a video. He could cut the pins closer to the motherboard if he can't desolder them. But never try cutting close to the motherboard without first cutting out the ic first.

If the OP simply can't do the job then he should likely find someone local to do it for him.