Jan 052018
 

Received today from Germany some faulty PCBs for repair, there was this Dragon Breed boardset (on Irem M72 hardware)

Sprites were glitched with jailbars thru them:

Sprites data are stored in four 28 pin 1Mbit MASK ROMs (Toshiba TC531000) located on top board:

Due the fragile nature of these devices I was pretty sure that there was a faulty one but I was wrong since dumps turned out to be good.Analysing them with a logic probe I noticed that data line D0 (pin 11) of the MASK ROM @IC49 was stuck low, measuring its resistance to GND gave me only few Ohms:

Since device was good there was clearly something forcing the pin low, most likely a short (not ‘dead’ though).I traced the involved pin back until I came across to this scenario:

Here’s a close-up under a microscope:

The capacitor @C62 was bended and its terminal connected to GROUND  was accidentally lented on the pad of the trace tied to the data line.I straightened the capacitor, this cleared the short and hence sprites issue, simple but effective!Job done.

 

 Posted by at 7:29 pm
Jan 012018
 

Over the years there have been people asking if they can submit posts of their own.
Aside from friends of mine I have never really wanted to open up registrations for everyone.
I have now finally got around to enabling a frontend to allow people to submit their own posts.
It can be found under the “Contact/Links” menu and is just a trial right now.
If it doesn’t take off or gets abused in any way then it will be removed and we will go back to business as usual.
Hopefully we will get some more diverse content going on as a result of this

 Posted by at 2:49 pm
Jan 012018
 

Happy new year everyone to everyone that visits this place and a big thanks to all that have either contributed or made a donation.
Hope you all have a great 2018

 Posted by at 12:48 pm
Dec 252017
 

In the past, every time I’ve undertaken a ROM hacking project i have ended up with a cluster of binary files all at varying stages of usefulness. This simple program is how I will now be working with binary file hacking.

The format of the changes is really simple and can all be put together in a text file or in the program itself.

As you can see, comments are supported.
Using this I can comment the changes I make and just ‘Autopatch’ a binary file after I’m finished and if I make a mistake then I can modify the text file and reapply to the original binary.

I understand that the way I approach things is not necessarily the way most other people approach things but it might be of some use to someone else out there.

The software has a simple HEX viewer in it, can byteswap the loaded binary and also supply an offset.
The offset is relatively untested and is intended to be used if you are loading a ROM file that normally starts at, for example, address $1000 in memory space. You can apply the offset and write values to the correct addresses. If in doubt then just avoid this altogether as it will cause problems and cant really do any error checking.

Merry Chistmas

 Posted by at 3:57 pm
Dec 242017
 

Received from Spain this faulty Rolling Thunder PCB for repair:

Board was not booting stuck on a static garbabe screen with some background sound noise (which seems a common issue on this PCB)

After ruled out all the custom ICs, few was left to test.The background sound noise very likely  suggested a problem in the digital sound circuit that on this hardware must be working for proper operation of the whole system.Probing this circuit with a logic comparator revealed nothing abnormal until I tested a 74LS257 @7C, I was warned about a problem on its output pin 12:

 

Also comparing with a scope this ouput with the one of the reference IC gave discrepancies, signal was not properly toggling (good on left of the below picture)

Looking at schematics pin 12 of this 74LS257 outputs the address line ‘A0′ to a 6264 static RAM @3F tied to the custom ’30’ which controls the sound system:

Sure enough I removed the TTL which failed the out-of-circuit exactly in pin 12:

After replaced the TTL the board successfully booted, no further issue were found so 100% fixed.

 

 Posted by at 4:23 pm