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GFX Noise on an Osman [fixed]

ekorz

ekorz

Multi Boyz 4 Pi
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I'm poking around at a converted Osman board. I recapped it, but I don't think I goofed up anything there...

There's noise in the gfx, and I believe that the effected data comes from 9A (the background errors) and 13A (some miscolored sprites I didn't photograph). But from there, I'm pretty lost. Should I try to see where those chips' address lines go and then probe to see if those are stuck somewhere?

Interestingly the background noise gets heavier or lighter if I give the board more or less voltage. Not sure if that helps diagnose it. Anyone have a lead?

HCvFT9E.jpg
MWgKKU6.jpg
 
No answers to offer, but watching this thread since I want to do my own Osman conversion someday. The board worked with the original game without issues right?
 
yeah. probably next step is to put it all back to the original... but thought I'd see if anyone had any ideas first
 
I had similar on an Irem board. Not Osman I know, but hear me out.

First I pulled out one rom at at time to isolate where the gfx or palette issues were coming from (hopefully your roms are socketed).
I did manage to isolate it down to a single chip.

No rom burner / reader, so I had to just go on visual inspection. Critically if I flexed the board a little around that chip socket, I could reliably fix the palette and gfx issues.
This lead me to reflow the solder points on the chip socket in question - it was right on the edge of the board, so more suceptable to a few dings by an operator.
Lucky, but it worked - 100% fine board afterwards.
 
Cool. Yeah I don't think this is an osman specific problem at all. The dominant issue is on one chip, so one socket, and I'll give it a reflow tomorrow.
 
reflow sadly didn't seem to do anything. Looking at some photos online some small differences on my board. Mostly a few small capacitors in different spots, but they look like they're in different spots on different boards. I moved a few around but it didn't do anything, so I figure they're just helping clean the power in general.

I see one 74F157AN chip that has a ceramic capacitor joining two legs for some reason. I'm ordering that and some other nearby chips to piggyback, maybe i'll get lucky.
 
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Yeah the small caps are just tanking caps to regulate voltage to the VCC pins of the chips they are near.
I doubt moving them would help much.

Might be a bad video ram or a ram controller / timing chip, such as that 74F157AN multiplexer. No idea why you'd bridge the pins on it with a cap though.
Probably time for a logic probe and data sheets :(
 
You joke, but I have a magical drop just sitting here...
 
Hmmm... and Joe + Mac Returns is on the same HW too.. How hard is it to convert between games on this boardset 8o
 
i cant see you finding noise with a logic probe, you need a scope.
 
You can convert a Joe&Mac Returnns (or Magical Drop) to Osman/Cannon Dancer, I have done it but it's not a simple ROM swap, hardware needs some modifications.
 
start with the rgb & sync looking to see which lines are effected - then follow it back.
 
To me it seems not a problem of RGB or sync but a graphical issue related to data corruption.
 
cool, thanks for the tips all.

well I rebuilt the original, and it works as far as I can see. So I'll check all the roms (again) and maybe i'll get lucky. Also got a new programmer coming in so maybe that will help.
 
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