520ST piggyback RAM mod - "impossible to repair"?

[oracle_jedi]

So I have a 520ST with the infamous "piggy-back" RAM expansion,

 

And sure enough, I power up to a colorful screen of garbage.

 

Reading a post wood_jl made over at atari-forum, I noted he reported something Best Electronics apparently said:

 

"A. Our Atari 25+ year Super Tech does not recommend this 520ST Ram Upgrade. When they do go bad (and they do) they are about impossible to repair. Our Super tech will not even touch one that comes in for repair."

 

Impossible?

 

I can image heat management may have caused one or more of the RAM chips to go bad. There may be some bad soldering in there too.

 

But if I desolder all the RAM chips and install new ones shouldn't that correct the issue? Or does the piggy-back mod typically impact other components too?

 

To be clear, I am hoping to restore this machine to a stock 520ST with just 512K of RAM. I am happy to sacrifice the extra memory. I have other STs for software that needs extra RAM.

 

[ParanoidLittleMan]

Anyone listens to Best Electronics nonsense ? I read plenty of crap and false claims on their WEBsite.

Saying that 'will not touch' means that they don't like to work. They like easy money.

Piggy back is not most elegant way of RAM expansion, but it is OK and reliable - of course if it is soldered well - what stays of course for any expansion.

Desolder chips and test it .It's not sure at all that problem is in RAM, or if it is RAM it may be basic 512KB, and not expansion.

[rcgldr]

Attached is an image of the piggy back 1024 MB upgrade to a "developer" Atari 520 ST, where the upgrade was done by Atari itself. Ram chips in my 520ST are all NEC D41256C-15 .

 

 

[ijor]

Attached is an image of the piggy back 1024 MB upgrade to a "developer" Atari 520 ST

 

Would you mind posting a higher rez image of the motherboard so such that the chipset legend is readable. Seems like you might have the first generation chipset there, pretty rare.

[rcgldr]

 

Would you mind posting a higher rez image of the motherboard so such that the chipset legend is readable. Seems like you might have the first generation chipset there, pretty rare.

 

Link to higher rez image (1.6MB):

 

http://rcgldr.net/atarist/at520st1.jpg

 

It's a "developer" Atari 520 ST, it came with a few "developer" manuals, and developer took kit with Alcyon C compiler, some version of assembler, and I don't recall what else was included.

Edited July 22, 2018 by rcgldr

[oracle_jedi]

@rcgldr

 

Can you tell me to which pin of the MMU the Pin 4 of the extra memory is wired?

 

I have a cut trace on this motherboard that routes to pin 4 of U16. The line then disappears under the MMU and I can't see where it goes. I am not enthusiastic about desoldering the entire MMU to figure it out.

 

Having removed all the memory, installed sockets and new RAM, I am now powering up to a blank black screen. So that's progress...

Edited July 22, 2018 by oracle_jedi

[ijor]

 

Link to higher rez image (1.6MB):

 

http://rcgldr.net/atarist/at520st1.jpg

 

It's a "developer" Atari 520 ST, it came with a few "developer" manuals, and developer took kit with Alcyon C compiler, some version of assembler, and I don't recall what else was included.

 

Thanks. Yes, it has the first generation chipset. As I said they are pretty rare, probably used only for a few months before the second generation was available.

[rcgldr]

@rcgldr

 

Can you tell me to which pin of the MMU the Pin 4 of the extra memory is wired?

 

The way my board is connected to the case makes it risky to remove. Couldn't you use a needle as a probe and use a beep box or an ohm meter that uses a small voltage (<= 5 volts) to determine which pin is connected to the wire used for the piggy back ram chips?

[ParanoidLittleMan]

No need to remove anything. Look at ST schematic. Extra signal (not used with only 512KB RAM, so single bank) is RAS1, pin 18 . Accessible only from bottom side - so actually will need to remove something - board from case bottom

[oracle_jedi]

No need to remove anything. Look at ST schematic. Extra signal (not used with only 512KB RAM, so single bank) is RAS1, pin 18 . Accessible only from bottom side - so actually will need to remove something - board from case bottom

 

You sure about that?

 

I found this site:

 

http://www.gossuin.be/index.php/520-et-1040-stx

 

With schematics for an original 520ST, and it shows Pin 4 of the RAM (RAS) going to Pin 8 of the MMU.

 

It also has the original Atari technical bulletin that explains the piggyback RAM expansion, but it is in French.

[ParanoidLittleMan]

Pin 8 of MMU is RAS0 - that's for factory RAM bank. Pin 18 is for expansion - RAS1. Maybe the problem is that they use RAS1 and RAS2 instead RAS0 and RAS1 ?

Of course I'm sure - I expanded many Ataris back in time . I have here board with expansion, and that's pin 18 .

[rcgldr]

I see 3 wires coming out from under the board. One wire is common to pin 4 on all piggy backed ram chips. The other 2 go to pin 10's (?), one for pin 10 (?) of the left 8 piggy backed ram chips, one for pin 10 (?) for the right 8 piggy backed ram chips.

Edited July 23, 2018 by rcgldr

[ParanoidLittleMan]

Folks, look schematics, read some literature. There are total 6 lines from MMU to RAM. CAS0L. CAS0H, RAS0 - that's for first bank - what is there for sure by factory.

CAS1L, CAS1H & RAS1 - for second bank. Each bank can be 512KB or 2MB . Although, combination of 2MB + 512KB works not with IMP MMU.

In motherboards I upgraded CAS1L and H were available on top side of motherboard, only needed to access RAS1 at bottom side.

[oracle_jedi]

She lives!

 

After removing all of the RAM chips and replacing them with new socketed RAM, I was greeted with a black screen.

 

I then found a broken trace that leads to pin 4 of the first memory chip. You can see the trace in the picture below.

 

 

So I repaired that, and next I got a different screen of garbage to the one I got before removing the original RAM.

 

 

I tested the RAM chips in my 256K modded 1200XL and found it was all good, so it wasn't the memory chips. Then I watched the Youtube video by the 8bitguy where he shows using a logic probe to diagnose a faulty VIC20. I got a probe from Amazon, they are not expensive, and clipped it onto the cartridge port pins 1 and 39 for +5V and GND, and them probed each RAM chip.

 

Everything north of U38 was faulty on the A8 and A4 address lines.

 

So more careful examination and I found more broken traces. You can see the line to Pin 1 of U38 under the magnification is broken, the break occurring right next to the blue wire wrap that is another repair I had to do.

 

 

Here is a shot of all of the patches that were needed:

 

 

To be fair, some of the patches were necessitated by me. Pulling the original RAM chips out damaged some traces, as the RAM chips had melded themselves together and glued themselves to the PCB. I've learned that I need a hot air station before I try this again. A Hakko vacuum solder gun, flux and a roll of braid isn't enough with these fragile ST era boards, and applying heat to a pad for any length of time may cause it to lift and break.

 

But beyond the learning experience, I also now have a second working 520ST motherboard, which is cool.