What's new

mypinballs

Student
Joined
Aug 18, 2019
Messages
64
Reaction score
127
Location
Leeds UK
Hi,

Have an original taito bubble bobble pcb that i got with a pile of random pcbs awhile back. I've started to look at. it's non working with just garbled graphics on screen.

First thing is the reset line is just resetting over and over (watchdog barking). Been looking at the schematics trying to fathom whats going on. I've tested the watchdog circuit itself and i can get it to stop resetting if i manually add a pulse at jp6 so that bit works.

I found a stuck high line at pin 5 ic39 which feeds the clk line to the ls273 at IC55. Changed that and no difference though i do see a pulse on that line now

Also have changed the z80 at ic50 and socketed (for high score save stuff later to), burned new roms (ic52, ic51, ic37, ic46) and new pal ics at ic49 and ic43. No different

Changed the work ram at IC44 (was a fujitsu ram and corroded), no different

Also changed ic35 as outputs looked alittle iffy, no different

I've checked all the usual things on the main z80. clock pulsing, rd and wr pulse (when not in reset), rfsh and mreq also look ok. Only thing i notice is the IORQ line on the z80 is high all the time.

Doe anyone know what is needed to get the main z80 out of reset and running, as it seems to be a loop on itself so i assume something is just making this cpu crash immediately.

Any help or pointers would be greatly received as its starting to do my head in!!!

non boot screen1.jpg
 
Last edited:

mypinballs

Student
Joined
Aug 18, 2019
Messages
64
Reaction score
127
Location
Leeds UK
I have done some more testing and am following the enable and bank select signals for the main game rom. From what i understand it looks like rom at ic52 (27c512) ROM 1 has 4 banks (max) and the 74ls273 latch ic at ic55 handles this, the enable for this rom and also the resets for the sub z80b processor and the PS4 ic.

Having tested it as best i can in the current scenario i believe this ic is knackered as most output lines are stuck low and one is stuck high. According to the data sheet when the mr input is active (low) any output should be made low regardless of any other inputs. This is in direct conflict with this stuck high output.

Waiting on some replacements. Typical that i'm out of these right now!!

One other thing i've just noticed is that once i changed the 74LS139 which restored the pulse to output pin5 and the clk line for this 74ls273 latch ic , i started to get the random garbage on screen inverting repeatedly. This makes sense as output pin 12 is one of the few outputs here still functioning which is VHINV going off to the video board. So that makes me think the invert circuit works.

SO my overall thought is if ic55 isnt working then the rom2 cannot be accessed properly or bank selected by the z80 and results in the processor crashing and then causing the reset loop. I'm assuming that rom 2 is accessed first which tells the processor to setup the ls273 latch so it can access rom1
 

xodaraP

Enlightened
Joined
Sep 21, 2016
Messages
4,397
Reaction score
3,038
Location
Gold Coast, Australia
Step 1: Do you have a programmer? First things first: verify all main program ROMs

Step 2: do you have CLK at the main CPU?

No point replacing further parts until you have the clock signal in order at the very least.
 

mypinballs

Student
Joined
Aug 18, 2019
Messages
64
Reaction score
127
Location
Leeds UK
Clock line is fine and yes i have a good set of roms. I programmed a new set which have been verified multiple times. Clock line as you say was first thing i checked as below.

"I've checked all the usual things on the main z80. clock pulsing, rd and wr pulse (when not in reset), rfsh and mreq also look ok. Only thing i notice is the IORQ line on the z80 is high all the time."
 
Top