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:
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:
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:
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 :
Comparing it in-circuit revealed it was faulty:
Once replaced it I got no more WORK RAM error and strange behaviours, game was playable but all sprites were missing:
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:
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:
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:
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:
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.
5 Responses to “Double Dragon repair log #7”
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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 ?
Thank you again 🙂
That bar is only to visually separate the main CPU circuitry from audio one.
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
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).