Yet another dead Mv4ft board

[raulos]

I managed to get another mv4ft board. The idea was that i could obtain some parts from it for the board I tried to repair a few months ago. Screenshot after startup (diagnostic bios). Lot of corrosion marks - fortunately only around battery.
How do you think it can be repaired or as planned in parts?

 

[ack]

The traces outside/around the battery area look ok to me in the picture. You will need to do a bit of cleanup of the parts in the battery area and that ULN2803 above. Pretty sure none of the effect parts should be preventing the board from booting.

Are you actually getting the click of dead or is that screen static?

 

[raulos]

Just static screen.

 

[ack]

A static screen like that can be an indication the board isnt getting enough power. When the board is powered on, use a multi-meter to check the 5V rail on jamma connector. If its below about 4.90V its likely the problem.

 

[raulos]

Yeah that was my first thought. I adjusted voltage to 5v (voltage drop is like ,4v on jamma - is it normal?) then i checked halt and reset pins on 68k. first one pulse like hell and second one is low. Anything before 68k can cause that or i should replace 68k in the first place?

 

[ack]

I've never seen a 68k die on a neo geo board so I probably wouldn't start by trying to swap that. You might check the clock signals on the NEO-D0, the 68K and other chips.

By chance is J2 jumper above NEO-B1 jumped?

 

[hatmoose]

I was super unlucky - My MV4-FT2 did in fact have a dead 68k, on the plus side I learned how to used a logic probe to diagnose/fix it.

If you dont have a logic probe you can get a crappy-but-functional one for under twenty bucks now.

More details here
https://www.neo-geo.com/forums/inde...hdog-even-with-neogeo-diagnostic-bios.267802/

 

[raulos]

I've never seen a 68k die on a neo geo board so I probably wouldn't start by trying to swap that. You might check the clock signals on the NEO-D0, the 68K and other chips.

By chance is J2 jumper above NEO-B1 jumped?

J2 is open.
Pin 21, 22, 23 on NEO-D0 pulse.

I checked AS and A1÷A4 pins on 68k and there is nothing at all.

 

[raulos]

Solved!
Faulty 74as245n located on reset line between 68k and Neo-B1. That's why reset was on low all the time. Replaced with 74act245pc. Now it boots ok and pass all tests on diagnostic bios. Time for upper board inspection.

 

[hatmoose]

Oooh that was a gnarly one, I would have suspected a bad 68k and replaced that - nice work finding the AS245 fault and working that first :-) Keep up the good work!