Removing onboard RAM

[beachycove]

I have an LC520 logic board that gives the chimes of death no matter what is connected to it. I am not ultra-keen to throw it out, as it is probably worth real money in the open market as a Color Classic upgrade, assuming it could be made to work.

Now, of course other problems could be the explanation, but my suspicion is that the onboard RAM is bad. What would happen if I were to remove it? Could that potentially "fix" the board?

 

[Kai Robinson]

Easiest way is to examine the address and data bus with it powered on - if one line looks squiffy, you can usually pin it down to a single chip. These can be replaced easily enough by any 4Mb x 4 DRAM chips - i recommend the Toshiba TC514400's - as they're cheap and plentiful. You'll need a decent hot air station, flux, tinfoil, kapton tape and a bit of practice. I also recommend a SOJ-26 solder stencil, so you can use solder paste rather than drag soldering, although that's an option too, using a K-Tip on a decent iron. 

 

[maceffects]

I agree with @Kai Robinson's suggestion.  And the stencil should be cheap, it will save you from messing up the drag soldering and ruining pads, like I did...  I've messed up 3-4 boards with my attempt at SMD soldering, so I decided to stick with hot air rework or through hole soldering only. 

 

[beachycove]

Well I figure I can get the RAM off reasonably easily, as I do have a hot air station and am not a complete neophyte in soldering terms. I have zero conception how to examine the address and data bus, though.... There I be an absolute beginner.

My question was really just, would it boot if I removed all the onboard RAM? The idea was to remove an obvious potential problem in order to see if the board is otherwise viable. 4MB is not a big loss, and then if I wanted to, I could solder 4MB back on. Otherwise, I have some 32MB chips that would make that missing 4mb moot.

Alternatively, anyone want to buy it on spec? I reckon it would sell on eBay for $125+ if it could be made to function. I am moving and having a clearout.

 

[Kai Robinson]

In theory you could remove it all, then rely on the SIMM only...but i wouldnt confirm that theory. I'd still get an SOJ stencil and replace the damaged chip or chips. 

To diagnose, you'll need an oscilloscope and/or a logic probe. You want to check each pin of the address and data bus - see if the waveform looks solid, ie
 

___/‾‾‾\__/‾‾‾\___

Rather than 

___/WWNLS\__{Y}_V\




(it's hard to draw in ascii - but if you look at Adrian Black's TI/99A repair video's you should get an idea.