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Midnight Resistance Repair Log #3

Womble
(@womble)
Noble Member

Got some more faulty Midnight Resistance boards recently, this one did this on power up, totally frozen screen of rubbish and a gentle two tone warbling noise from the loudspeaker.

The board itself was in pretty good nick, one wrenched cap..

...and the carcass of a previous inhabitant in a rather webbed up corner.

First off I fixed the cap, sometimes these are involved in the reset system on the board but it was a fair way from the 68000 so not that unlikely, plus the reset pin on this board's CPU was stable in "not reset".

Having fixed a few of these now I knew that the screen of crap is what the board does when it is missing all or half of a bank of RAM comprising two TMM2063 chips at 11K and 12K ..

so I pointed the scope at the output of the two RAM chips and it was crap on some pins and awful on others.

So I desoldered both chips and put them through the SRAM test for TMM2063s on my EPROM reader. One could not be written to and the other gave an error saying the D6 and D7 pins were missing, so no longer connected internally it seems. What was odd was that one pin on the SRAM chip had been cut and soldered back in place, this is a common test trick to isolate a pin without removing the chip. You cut the leg off leaving enough of a stump on the chip to solder up the cut later.

So I fitted some machined pin sockets and a couple of Sony CXK5864 chips from a scrap board. Fired it up again and got the same fault.

So I pointed the scope at the address bus for that RAM pair, which it shares with 2 EPROMs and 2 mask ROMS, a few of the lines looked pretty knackered too..

Have seen this pattern a few times before where the bus transceiver chips either loose the oomph to drive the downstream chips from logic 1 to 0 or the downstream chips are faulty to the extent that they take more grunt to drive them. Often chips will actually pass tests out of the board but the original system on the board is too weak to drive them. I pulled all 4 ROMs, read them and they checked out against the MAME set, proved they held the write data but not that the chips were really in spec. Also pulled the new SRAM chips out again and pointed the scope at the address lines on the now totally naked bus, the same wrecked signal was present. Traced it back to a 74LS245 at K21, its inputs were clean, but some of the outputs were in the state in the above photo. Replaced that chip, and the address bus lines all looked healthy, so I put the ROMs and the RAM back in and powered it up.

The game booted, but there was a problem, when MidRes boots it should show the following screens and then go into the title screen for 15 seconds or so, and then into the game demo.

However this board showed a black screen for 10 seconds, then showed the split screen with the two fighters on the red background, then black again for 5 seconds, then the two fighters on the green back ground for 5 seconds, then black again etc. Every screen with text, plus the Data East logo was missing, I was only getting screen 4, 6 and 8, plus the delays while the others should be on-screen.

Once it finally got to the parallax scrolling splash screen all the text was missing, the "Data East Corporation 1989", and the flashing "insert coin".

In the game all the ammo, keys collected and lives left logos were also missing.

I knew from previous boards that all this is stored in 2 ROMs 4 and 5 at the rear of the board. On some MidRes boards most of the gfx data ROMs are soldered in, but ROMs 4 and 5 always seem to be socketed, sometimes they are actually EPROMs, its this pair that differ from the US, WORLD and JAPAN versions of the game, so socketted to let Data East change the versions on their stock as demand required. Anything text related is stored in these eproms and processed via another pair of TMM2063 chips at B8 and B9. Poking a scope at the EPROMs showed some decent signals and the RAM outputs looked clean. The only way to be totally sure was to de solder them and test them off the board, they both were working fine. Back to scoping around the ROMs address lines showed something that looked odd, a few of the address lines were tied low. These traced back to a pair of 74ls273s at B11 and B12, the outputs looked fine but the inputs were pretty messy. To cut a long story short the 273 inputs were outputs from the 160 pin surface mount custom at B3 and for a while it looked like the board was going to be scrap. I dug out a working MidRes and compared the signals coming from its custom and they looked identically messy to this board. I decided to pull ROM 4 and 5 out of that board to see what it did with the ROMs missing. On my board the lack of these ROMs made no difference, but on the working board all I got was a white screen with the game playing without a display. Clearly on a working board the overlay data really is overlayed, as all the data pins on the ROMs were tied high via resistor networks when the actual ROMs were missing it would appear to the game as if the ROMs contained all 1's, which would be full RGB at all points of the screen - whiteout!!!

So I went straight for the PROM at F22 knowing when that is missing you get a blank screen. I had already tested that PROM in another board by the way. I found that pin 13 (an output) was doing nothing, yet if I shorted pin 12 and 13 together I got this...

...the overlay was there faintly and somewhat corrupted. Tying pin 12 low briefly got me this...

.. the overlay in its entirety, but nothing else.

By comparing the working board to the faulty one I could see that one of the address lines was doing nothing on the faulty boards prom, but not as low as all the other logic level zero signals were, this one was about a 1.5Volts higher and not very stable. Traced this back to a 7425 at E22. All of this chips inputs were healthy but its output was stuck, piggybacking a 7425 from a scrap board...

.. fixed the issue!

Ironic that the chip causing this troublesome fault was located right next to the bashed cap I replaced at the outset.

So I desoldered the old 7425 and soldered in the piggybacked on and the fault stayed gone.

Took the board to my cab for a test blast and found fault number 3!

The rotary control from the twisty joystick (you turn the joystick shaft to determine which way the character points his gun) wouldn't always work. Sometimes you could get half way through level 1 before it would just stop working, and other times it wouldn't work at all. The game was still running and you could do everything except aim. Thankfully this was a simple fix, the pins poking through from the ferrite beads in the path of the rotary control inputs were rather long and some had been bent together and were slightly touching. Straightening these out fixed the last problem, can play the game all the way through without a single problem now!!

Sic Transit Gloria Atari

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Topic starter Posted : 09/02/2010 10:33 am
AndyRCM
(@andyrcm)
Member Admin

Mmmmm don't you just love midnight resistance - superb work again matey! 🙂

One machine we will HAVE to get when we have our museum! 🙂

"I could see the faces of those who led pissing themselves laughing" - Funeral Pyre by The Jam

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Posted : 09/02/2010 10:57 am
Womble
(@womble)
Noble Member

Got another MidRes board awaiting some attention, that will either be an easy fix or a real pain, scorch marks on a 160 pin custom surface mount chip, will need replacing if its shot - am up for a soldering challenge tho 🙂

If you can find a cab shell and the rotary sticks I will donate a PCB to the RCM cause 🙂

The sticks are often seen sold on ebay, they were bright yellow hexagon shaped joysticks, very iconic but people tend to want something more generic if its not a specific MidRes cab so they flog them off.

Sic Transit Gloria Atari

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Topic starter Posted : 09/02/2010 12:14 pm
AndyRCM
(@andyrcm)
Member Admin

We will find a shell! In fact we have two cab shells in my garage - not midres cabs but . . . In good condition! I will look for some of the joysticks! 🙂

Andy

"I could see the faces of those who led pissing themselves laughing" - Funeral Pyre by The Jam

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Posted : 09/02/2010 12:20 pm
Phu
 Phu
(@phu)
Member Moderator

*drafts an invoice to RCM for arcade board assembly work* 😛

-- Richard

8 End of File, RCM:1

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Posted : 09/02/2010 1:04 pm
AndyRCM
(@andyrcm)
Member Admin

PML! - I can probably manage most of that Richard! 😉
Although . . .

"I could see the faces of those who led pissing themselves laughing" - Funeral Pyre by The Jam

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Posted : 09/02/2010 1:05 pm
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