Midnight Resistance Repair Logs

From Arcade Otaku Wiki
Jump to: navigation, search

Repairer: Womble

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!!

Repairer: Womble

Yup another one, I now have far too many Mid Res boards  ;)

I promise this is the last Midnight Resistance repair log for a while, I have no idea why so many faulty ones have popped up over the last few months, haven't seen any workers for sale anywhere.

Anyway - this board was mint, absobloodylutely mint, could have been made yesterday, not a scratch on it. The only thing I could see that worried me was 2 blacked pins on one of the custom LB0072 custom chips...


...this is a sure sign that something dropped on the board while it was running and shorted these pins out, OR more likely, someone trying to fix the problem jammed their probe between the two pins. When this happens its really a case of luck as to how much damage you can do. If someone is stumbling around probing stuff and they bridge two data pins then this wouldn't do much but if one of those pins was +5V and the bridge gave that a nice easy path to ground then the PSU will gladly step up to the challenge and supply all 15Amps of current at its disposal along that path, until something nominates itself as the fuse and burns out, usually instantly with a puff of smoke and a bad smell, incidentally if you do manage this (we have all done it once or twice) its always useful to postpone panicking or swearing and take notice of where the smoke came from, often a track on the board will have gone from stone cold to red hot and vaporised in a fraction of a second, the burnt out gap can be tiny and hard to spot, so follow the smoke!!! The burnt out section is not likely to be the only fault but at least if you know where you buggered it you can fix it and move on, if you have no idea where it was then its going to be a struggle.

Anyway I had a feeling that the custom was toast and was preparing to have a go at replacing a 160 pin surface mount chip as I have another Data East board as scrap with these ICs on it.

The game booted ok, but some of the gfx were not right.






The in game looks OK but the backgrounds are missing a lot of the colours other than green even though it actually looks pretty good! Not overly surprising seeing as these LB0072 customs control banks of graphics EPROMs (I worked out its pinout from a couple of schematics of other games - should document it for here).

First step when troubleshooting graphics issues is to check the ROMs, usually they are socketed and its quick and easy, even if it is a bit boring on boards with lots of ROMs. The gfx roms on this board were all mask ROMs, is ROMs that were made with the data already in them, etched in with a photo resist mask that defines the internal structure which defines the data. The were all TC541001P chips, which my reader didn't support, however they are pin for pin compatible with 27C100 EPROMs odd pinout really) so it kinda did support them. Anyway - they all dumped fine except for ROM 9 and ROM 10, both showed up as having blown address pins, #9 was missing 2 pins and #11 was missing 9. Both were ROMs associated with the suspect custom chip.

So I did a check on the board to see if there were any shorts to either ground or 5V on any of the ROM pins, (to make sure the board wasn't going to destroy any chips put into these sockets) burnt new copies of #9 and #11 into a couple of AM27C100 chips using the data from the MAME set, dropped the chips in


and fired her up....fixed!




Am lucky that custom wasn't bad considering the violence it was subject to, surface mount customs are generally very hardy beasts anyway thankfully. Plus it would have been a shame to have to scuff up such a mint board tho I admit I was up for a go.

Right that's my last MidRes board, seriously I don't have any more ;)