A500 rev 6a problems

[curtis]

Okay, I've had the motherboard for a while waiting on my workbench for a free moment to work on it.

It started out green screening on me.  Swapped out with another Agnus and got as far as a black/dark gray screen and nothing.

Checking the boot colors it would appear to be a bad cpu, right?  Nope.  Dropped in a known good and same thing.  Put the old cpu back in to keep things straight.

Then just for s&g's, I swapped out the ROM from a 1.3 to a 1.2.  Green screen back!

Put the 1.3 back in, black!  Tried another 1.3 known good ROM.  Black.

Also tried piggy-backing RAM chips to no avail.

I've swapped out every socketed chip on the board all with the same results, black.

Board looks absolutely virgin.  No cut traces or replaced RAM chips.  Nothing!

Keyboard LED's are solid power and nothing else.

Power supply is a modified ATX power supply and it checks out on another 500 I've got.

Frankly, I'm at a loss.

Any suggestions?

Curtis

[Tenacious]

Good chips in bad sockets still look like bad chips.  Pull the CPU and Rom chips and examine each pin well with a magnifying glass.  Look for wide, loose sockets and contamination and corrosion, even dust. Carefully look over the side expansion connector.  Check for solder bridges and ensure the bottom shielding is not shorting anything.  Look at ALL the sockets, even with the square chips installed, for irregular pins.

If you have a multimeter, check the power supply rails on the motherboard (not the power supply connector).

[curtis]

Didn't have a chance to work on it last night, but I did a scan of the sockets as I was checking chips and didn't notice anything amiss.

However, will get it under the magnified light probably tomorrow after work.

Hadn't thought about the power rails on the mobo.  Will check that also.

Curtis

[curtis]

Okay, the line starts here to slap the snot out of me!

Finally got a chance to work on this unit again and while I was examining ALL the socketed chips, I noticed I had put the ROM chip in wrong, ie there were 2 pins on the front end just kinda hanging out!

Resocketed the ROM and no change.  Tried a new ROM, making sure all the pins were in holes!), and suddenly I've got a very bright GREEN!

YAY a symptom change that I can work with.

Piggybacked the 4 RAM chips and no change.

Pulled the articulated, illuminated magnifier and started checking each RAM chip very carefully.  Lo and behold, one of the RAM chips is missing a leg!  REALLY!

Looking carefully at the bottom of the mobo, it is now very apparent someone along the way had replaced the RAM chips.

Guess my next step is to remove the gimpy RAM chip and replace it with a good one, check the soldering on the other 3 chips and see what happens next!

Curtis

[curtis]

Well, worked on it last night and the results are disappointing to say the least.

Using a brand new pair of nippy cutters(love those things!), I snipped out the suspected bad chip and replaced it with a known good chip.  No change.

Got out my lit magnifying glass and looked VERY CLOSELY at the traces and whoever replaced those RAM chips should be SHOT AT VERY CLOSE RANGE WITH A 50 cal!

From what I can tell, there are lifted traces, burnt out traces, and just a general hash made of the job.

To quote Leonard McCoy, "It's dead, Jim!"  

I'm pulling the (hopefully) good chips off it and will check them on a known good board to determine how many other chips took the trip down the river Styx.

Such is life.

Curtis