NMK Thunder Dragon Repair Log

Some repairs can be very tedious and take you months (even years) of fighting with the PCB to complete. Others on the other hand can be both easy to diagnose and to repair. This is one of the latter. And I think it is important to tell about the full spectrum of difficultness. So here we go...

This little baby by NMK has actually already been on the table once before when I bought it as defective. But a couple of months ago, I decided to bring it to a cozy get-to-together with my dear friends at MadGearArcade (a private arcade set up with some candy cabs and SuperGuns in my friends living room };-P), as they all 3 love good shmups. It was just bobble wrapped and stuffed tightly into a bag-pack together 3-4 other PCBs. I know this is of cause not the optimal way of transporting arcade PCBs, but when you don't own a car and have to bike to get to your destination, and you also know how to fix brokes PCBs, this is actually how I usually transport them to different gatherings.

When I connected it in the a wonderful Sega Aero City and flipped the on-switch, I quickly discovered that something was wrong with the music and some of the sounds. Most noticeable was, that the intro music was played in level 1 instead of the normal music for that level (this video is not from that night, but shot with my tv and SuperGun).

After a quick visual inspection, I found this

The smoothening cap right next to the sound chip had been crushed during transport. It would seem very likely, that this had something to do with the problem I was experiencing. All the other disc-caps for smoothening spread around the PCB is 0.1uF (code 104), so I removed the 2 broken-off pins, cleaned the holes, and installed a fresh one.

And now the music was in the right place again.

After playing a few games, I wasn't actually completely sure that music was completely right, but after inspecting an OST I found on youtube, I realised that I just have a bad memory. The music is ABSOLUTELY right now };-P

NMK Thunder Dragon Repair Log

I have to be honest...ever since I learned, that one of my favourite games Psychic5 was made by NMK for Jaleco, I have completely fallen in love with both companies. So I try to snap up as many games from both (when I find them at a fair price) as I can. Here I'd found an original NMK Thunder Dragon with "graphical errors" on evilBay with a starting price of 2EURO + 28EURO P&P. As I was the only bidder, I got it for the 30EURO all-in-all };-P.

When first booted up, the main intro logo, text layer and sprites looked fine, but some of the background layer was pretty messed up.

So dumped all socketed ROMs (they were fine) and did visual inspection. The board was in very fine physical condition, and the only suspective thing I found, was this

But they all checked out just fine using the continuity tester.
After having eliminated ROMs and physical damage, I had a hunch, that this error was caused by a RAM issue. So I started shortening adjacent data pins on the different RAMs on the board, to see if I could somehow stir up the graphics on screen. I found that the 4 MB84256 at positions IC1-IC4 is involved in generating the different layers on the screen. I googled the data sheet for MB84256-12 (that btw is produced by The Evil F aka Fujitsu) and found that they are just the Fujitsu equivalent of the standard 62256 with 120ns access time. These are not very common (at least not at the boards in my scrap pile), but at last I managed to find 2 pieces at 70ns and 85ns respectively and they both tested good in the Top2005+ };-P. So I tried piggy backing the 4 RAMs one at the time.

I saw a change when piggy backing the one at IC4.

So I desoldered it

and tested it in the Top.

So fitted a socket and installed one of the scrap-RAMs. Now the background layer was perfect again };-P

However it was now evident, that the game also had a sprite problem. I just didn't notice that before because of the issue with the backgrounds.

They all had stripes through them! So I tried piggy backing the 3 other RAMs, and on IC3 I saw a change in the stripe-pattern. So desoldered that IC as well

and tested it in the Top.

So fitted a socket and the 2nd scrap-RAM IC

and presto! All the sprites were back to normal };-P

So the two culprits are ready for the bin

and the case is closed.
The game is very addicting, fast paced, and has a very cool soundtrack. I'm really pleased to have it in my collection };-P