Trying to fix Mikro's wrecked TT board

[Steve]

If you get a bit stuck it might be worth talking to Matthias from the Thunder Storm team since they created the SpeedyTT booster (which entailed recreating the entire 'state machine') so he probably has a lot of knowledge.

[dml]

Confirmed just now from the back of the CPU board socket that RESET starts low on boot and goes high after a short pause - which is normal. BERR+HALT both remain high...

The gold top on the CPU actually looks somehow corroded - small bubbles under the gold plate and brown spots. Not seen that before.

[dml]

Thanks Steve, will keep it in mind.

[dml]

The CPU in question has strange rust-bubbles forming under the gold plating and brown marks around them. There is green copper corrosion bubbles around the edges. Not too clear in the photo with the sketchy lighting etc. but it's visible to the eye.

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[dml]

There is definitely corrosion from something affecting the CPU board - 3 rows of pins on the topside of the PGA header, near the front of board, are dull and crusty, won't melt and won't take new solder. Having to use a lot of flux and brushing to get through it. Two of the 'tinned' holes are rejecting solder completely.

[dml]

Super interesting.

After having a go at reflowing the crusty PGA pins, the machine is no longer presenting a black screen - now I get a white screen and corrupt graphics - but with a very TOS-like palette.

If I swap back to the 2nd CPU directly on the mainboard, it still boots to the desktop. So I didn't damage anything - the corrupt display looks like some kind of progress :p

Can only conclude there is/was something definitely wrong with connectivity to the board - corroded pins, tracks or other connections.

[dml]

.....and now the CPU board is working ...... o_0

[dml]

Some random things I fixed along the way, although not clear these prevented it from working.

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[dml]

Result....

Those 2-3 rows visible there at the front of the PGA grid and the 3 caps around them - all crusty and bad. It's fixed for now but will probably redo that whole area before moving on.

[mikro]

Excellent progress, Doug! Glad to see my poor PCB alive and kicking!