A500: Green Screen/11 blinks, reseating chips didn't help...

[spirantho]

Piggy-backing chips often is worth a try - just stick the known good chip on top of the chip to be tested, such that all the pins are making contact correctly between the two. That way, if one chip has lost a connection internally, the good chip will set the output correctly as it'll be floating in the dead chip.

Testing for heat is not reliable. If it works, great, but the only time a dead chip will get hot is if there's a short circuit somewhere. If it's just a lost connection it'll be normal temperature.

The only surefire way to check for RAM being bad is to take the chip out and put it in a known good machine. One thing you can try though is to check the data line out of the RAM chip against ground - each of the chips should have roughly the same resistance between it and ground. If one is different, check that chip first.

[DanAdamKOF]

I had compared the PDF to my A500 earlier, it's for sure unmodified.

Piggy-backing chips often is worth a try

Thanks a lot for this!  At the least I'm going to check the chips with a multimeter.

So just to make sure about piggybacking, if I were to just reuse the RAM chips already in the A500, would it boot with only 2 chips in there normally + the dead chip with a good one piggybacked?  If I did this, would I need to do anything with those capacitor in the slot of the now-disconnected chip?


Again, thank you guys for your help

[DanAdamKOF]

An update, I checked each chip's pins' resistance against ground (and VCC), and found no difference in resistance from chip to chip.  They were left in the board for what it's worth.

[ikonsgr]

I had EXACTLY the same problem as you with an amiga 500 (rev 6A). I replaced all possbily bad ic's with good ones of another working amiga. I also checked the ohm resistance of all 4 ram chips like you,and found no difference from chip to chip. It seems like a "dead end"...
NOT QUITE!
I finally found what the problem was, by "thermal" testing the ram chips! I simply felt with my finger the temperature of each ic on motherboard. Normally after some time on power, there must be SOME heat on every ic .I take this as a practical way to see if an ic is dead or not. If it's too hot OR no hot at all this probably means there is a problem.
Anyway, what i noticed was that one of the 4 ram chips was slightly warmer than the other 3 (3 was ~30-35c and 1 about ~40-45). This temp difference put me in thoughts: although none of the ram chips was too hot or no hot at all (which normally indocates a problem), this difference seems very suspicious. So i took off a ram chip from a spare A502+ ram expansion i have, place it above the "suspicious" chip and then press it down untill it wedged and couldn't move.
Power up, and... NO MORE BLINKING POWER LED!
Amiga worked like new!
Funny thing is that the ram chip sitted so well and tight above the dead one (i also cut the ground pin of the bad chip, so that will not heat up the good one above it), that i left it as is! To unsolder the bad one and solder back the working chip requires a lot of trouble if you only have a soldering gun and a suction pump. First you must remove the aluminum back plate (which requires to unscrew all the connectors on the back side of amiga) in order to have access to the back side of motherboard, as the 20pins of each ram chip is soldered on both sides of mb. Then you must do 40 unsolderings and 40 solderings (20pins on both sides of mb=40)!

I hope this info will help you too. Power up your amiga, and after 3-4 minutes check how warm are the ram chips. If you notice any temp difference then you might have found the "culprit"!

[DanAdamKOF]

snip

Hey, that's promising.  As I recall, I did thermal test, but none varied greatly in temperature.  Maybe I didn't give it long enough, maybe I'm less sensitive in my fingertips than I assumed, in any case, it's something I should look into again.

An online friend has a half-working A500 he said I could have, but we haven't arranged shipping or anything like that.  Truthfully I'm not 100% sure I'd hold onto the Amiga stuff after I move soon (more of a c64 guy) but I do like repairing things regardless.  Hopefully we can arrange shipping and I can cobble together one fully-working A500 in the future.

[Zac67]

The temperature test is a good trick to start with but it doesn't work all the time. (It's surprisingly good though - I've repaired a couple of machines w/o any schematics that way!)

If everything else fails, there's no hint of bad contact on Agnus' socket, no bad solder points on the RAMs or the buffers (U34/35 - thermal?) you'll be left with desoldering each RAM chip and test it elsewhere...

Dunno if it's been covered before: doing the 1 MB chipram mod doesn't require you to have the RAM expansion in place all the time. If it's not present there's simply no difference to an unmodded board.

Then you must do 40 unsolderings and 40 solderings (20pins on both sides of mb=40)!

Don't really get that - you desolder und resolder from both sides?
Actually, 20 pins is pretty fast if the vias aren't too tight and you've got a bit of practice - once you know which one's wrong.

[ikonsgr]

Don't really get that - you desolder und resolder from both sides?Actually, 20 pins is pretty fast if the vias aren't too tight and you've got a bit of practice - once you know which one's wrong.

 Every pin is soldered on BOTH sides of motherboard,so you must unsolder/solder each pin twice. For unsoldering, maybe you can heat up with soldering gun many pins together (since they are very close to eachother) and then push the ram chip underneath with a small flat screwdriver to get it off. To tell you the truth, it wasn't the unsolder/solder procedure, but rather  the tedious task of unscrewing/rescrewing all the connectors on the back of the amiga, in order to remove the aluminum back plate! Amiga really has a "hell of an armor" around her!

[Zac67]

Every pin is soldered on BOTH sides of motherboard,so you must unsolder/solder each pin twice.

No. The pin's soldered into the via. The through hole has a metal rivet inside, connecting all layers (there may be more than two). For a reliable solder joint you need to solder a pin in one piece. Anything else may trap air in the via and/or cause a cold joint, leading to various trouble!

I can't even imagine desoldering in one step - how do you get a clean via without heating it thoroughly? And how do you desolder from the component side? :confused: No offense, but I've never ever seen that before.

For unsoldering, maybe you can heat up with soldering gun many pins together (since they are very close to eachother) and then push the ram chip underneath with a small flat screwdriver to get it off.

Wouldn't exactly try that either. It's a good way to tear off traces that haven't completely separated yet. If you're using a special desoldering tip or an air gun, that's something completely different of course.

To tell you the truth, it wasn't the unsolder/solder procedure, but rather  the tedious task of unscrewing/rescrewing all the connectors on the back of the amiga, in order to remove the aluminum back plate! Amiga really has a "hell of an armor" around her!

Yeah - and then disassemble an A3000 for a change - and you'll never complain about the 500 again...

[ikonsgr]

Yes i know all that my friend...
The thing is, that i DONT have any "special desoldering tip or an air gun" to do the job properly,and surely i'm not going to spent any money for any of these tools, just to unsolder a f@cking ram chip! :laugh1:  
So the "never heard before" procedures i was describing to you, was the "alternatives" I had, which of course are dangerous and crappy methods, an so  that's why i didnt even try to practice them on my precious amiga!

[Zac67]

Reread the post: I said unless using special tooling I wouldn't recommend the methods you described...

I've been working with a suction pump for most of my 'career' - a 20€ iron and a 5€ pump - and think I'm fairly acquainted with it. And I can't really make out what you're doing...

Doing it the right way, desoldering with a suction pump isn't overly dangerous or crappy. Provided the solder points in question are somewhat cooperative is doesn't take too long either, you won't need more than maybe 10 seconds per point.