May 102015
 

I had this PCB since many years, honestly I couldn’t remember when I got it or where.The only sure thing is that it was faulty.Condition was pretty good for a such ‘aged’ board but it was missing a BPROM @IC101 as you can see:

Double_Dragon_PCB

Original part is a Fujitsu MB7114, pretty impossible to find nowadays so I opted for a 82S129 which an equivalent one.After burned the device with the correct file supplied by MAME, I powered on the board and some times it got stuck on RAM/ROM TEST showing error on WORK RAM:

WORK_RAM_ERROR

Other times it booted into game but sprites were missing, could not coin up, screen was unstable and speed was kinda accelerated.When I got this strange behaviours, the fist thing I check is the main CPU, in this case an Hitachi HD63C09.Probing it I found that PIN2 and PIN4 were stuck LOW, they are respectively the NMI and FIRQ interrupt lines:

HD63C09_IRQ

So this meant that CPU was undergoing a request of interrupt from external devices (an interrupt is a signal to the processor emitted by hardware or software indicating an event that needs immediate attention).As you can see from the above schematics these two signals are generated by a 74LS74 @IC73 so for first I went to check it and I found missing CLOCK and CLEAR signals input on PIN3 and PIN13 respectively.In particular CLOCK on PIN3 is the VBLANK signal and this is generated by a NAND gate of a 74LS00@IC70 on VIDEO board :

VBLANK_generation

Comparing it in-circuit revealed it was faulty:

74LS00@IC70_comparing

Once replaced it I got no more WORK RAM error and strange behaviours, game was playable but all sprites were missing:

no_sprites

This a common fault on all Double Dragon PCBs since object generation circuit is wide and made of many components (it occupies five sheets of the .PDF schematics).

So, following the schematics I checked this part of circuit and all was fine until I came across to this section:

OBJECT_generation

The 74LS86 @IC72 had stuck outputs and these are tied to some ADDRESS lines of a TMM2018 SRAM which had some silent DATA lines, too.So I prontly desoldered them and they failed once tested out-of-circuit:

first_fix

I thought my job was done but I was wrong since this improved the sprites but not fully restored as they still missed some lines symptom of missing DATA:

improved_sprites

So I kept to check the OBJECT circuitry until I come across the TMM2018 SRAM @IC20 which had four silent DATA lines (PIN14-PIN17).I piggybacked it and magically all sprites were restored:

sprites_fixed

PCB 100% fixed and another one preserved!

Just a note for future reference : there are five TMM2018 (6116 equivalent) sprites RAMs and they are @IC73, @IC81, @IC84, @IC120 and @IC7 on VIDEO board.

 Posted by at 10:22 pm

  5 Responses to “Double Dragon repair log #7”

  1. As always a pleasure to read 🙂
    I noticed original board have a sort of long gray bar from near jamma connector to near black connector.
    Do you you know what is this for ?

  2. Thank you again 🙂
    That bar is only to visually separate the main CPU circuitry from audio one.

  3. Thanks 😉

  4. Hi

    I am having the same issue with my DD i got off ebay awhile back

    Originally flagged the old 63701 error which i have now got past and repaired that but completely no sprites and that messed up background issue on boot screen on the first 2 photos on this page

    Was probing around the areas you pointed out but i think i got issues before all of this only problem is this schematics i have from your site are a pain to work with at the best of times so could you tell me what pages the sprite areas are located

    Cheers

    • Hi,
      Double Dragon sprite generation circuitry is wide, most of the video board is dedicated to it.Pages are from 12 to 16 ( all the ones with the note “OBJ” on the righ site).