UMK3 board playing blind 7407 inputs shorted to ground

pzlate

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UMK3 board playing blind 7407 inputs shorted to ground

Need a bit of guidance here. I have a board where everything works fine, but it's playing blind, it's definitely passing the hardware test and it has sound. It's outputting the correct resolution and refresh rate but just a blank image.

I've already swapped U45 and U47. I saw a post saying that replacing 7407 fixed a loss of color, but that didn't fix it. However I did notice that all the inputs on the 7407 at U91 right now are pegged low and shorted to ground. I traced the input back to the hex inverter on U75 and that input (pin 13) is also shorted to ground.

Replacing both did not change anything, and I can't seem to find the source of the short to ground. This board is 99% there, just can't figure this out and it's bugging me, it's very clean board otherwise.

e: A bit of extra info if I short the ALS541(U82 and U97) VCC and OE2 pin next it with my probe it will also have continuity with ground.
 
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Decided to work on this board again.. traced it back to U32, replaced U32 and I am now greeted with video, at bootup the screen stays solid purple for about 3 seconds, then the garbage comes up, then it blanks out before rom test..

Kind of stumped on this one
 
Fix it
Sell it <i'm interested
Or send to that new quy who fixes for cost of parts... But may have a queue already.

Sorry to hear its causing trouble.. But a working umk3 is not outrageous in price.
 
Decided to work on this board again.. traced it back to U32, replaced U32 and I am now greeted with video, at bootup the screen stays solid purple for about 3 seconds, then the garbage comes up, then it blanks out before rom test..

Kind of stumped on this one

is there enough of a visual where you can see it complete all self tests?
Thanks
Blake
 
Decided to work on this board again.. traced it back to U32, replaced U32 and I am now greeted with video, at bootup the screen stays solid purple for about 3 seconds, then the garbage comes up, then it blanks out before rom test..

Kind of stumped on this one

so you traced the short back to U32? and did that stop the other ic's from shorting to ground? Also I'm sure you already know this and have verified but the input 5vdc on all MK machines is very picky at the board. ive found both my mk3 ultimate and mk2 both have boot issues if not at 5.2vdc.
Thanks
Blake
 
so you traced the short back to U32? and did that stop the other ic's from shorting to ground? Also I'm sure you already know this and have verified but the input 5vdc on all MK machines is very picky at the board. ive found both my mk3 ultimate and mk2 both have boot issues if not at 5.2vdc.
Thanks
Blake

So U32 was shorting to ground, I removed the 0ohm resistor before it that goes down to pin 13 on the hex inverter at U75 , and that stopped the short to ground on those chips. So my U32 was faulty, I ended up removing U32, removing the short completely, put in a replacement U32 from a parts board, the game would boot up but cut out before rom test.

Today I just threw in new revision roms and tossed in a different U45 and U47 and it works, so I'm happy. I'll have to test the U45 and U47 I took out later, because it might have just needed to be resocketed.
 
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So U32 was shorting to ground, I removed the 0ohm resistor before it that goes down to pin 13 on the hex inverter at U75 , and that stopped the short to ground on those chips. So my U32 was faulty, I ended up removing U32, removing the short completely, put in a replacement U32 from a parts board, the game would boot up but cut out before rom test.

Today I just threw in new revision roms and tossed in a different U45 and U47 and it works, so I'm happy. I'll have to test the U45 and U47 I took out later, because it might have just needed to be resocketed.

Hey that's great!!! I have had problems with those sockets on an NBA jam board before so that could be the case.
Thanks
Blake
 
Noted :) I'm going to have three of these buggers to fix soon...

Good luck.. if you run across boards with no sound and manage to fix it (outside of it being a bad cap/dead GAL) please tell me how. I have another board in the pile of shame (that I stole my U32 from) that I could never fix the sound on.
 
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