This is one of my favourite arcade games ever!
I’ve got this GOLDEN AXE PCB a SEGA System16b board on the bench from a friend of mine, Daniele, for a repair. The board boot fine, the game starts in the right way, but there is no audio at all.
This should be a quick fix, i think. This is the picture of the board :
The PCB is in very good conditions and no scratches were found on the solder side. So it’s the time to get hands dirty and take a deeper look at the audio section of the board.
First of all i noticed the C18 cap is missing, but according to the System16b schematics i found on the net, the missing cap shouldn’t be involved in the issue, the board il totally silent.
We can look at the missing cap on the red circle, this picture is taken from the sega System16b schematics:
… and this one is taken from the IC datasheet:
For a first quick and dirty test of the audio power amp, a Fujitsu-MB3735, i used an old school method and an absolute high-tech probe … a finger! : )
Adding some interference and extra capacitance the amp should oscillate abnormally and should reply with the classic “hum” noise. This will tell us that the amp may do it’s job, and it seems working.
So we can take a deeper look at the audio chain of the board:
So we can go backward through the audio circuit in the preamp section a quad op-amp marked NEC uPC804C, it’s an equivalent of a better known TL084. Probing it with the scope i found that it was totally silent both inputs and outputs. The inputs are feeded by a Yamaha YM3012 DAC, a digital analog converter, obviusly all the output stages were silent but there is an apparently activity on its inputs. The serial data input pattern looked at the scope seem to be too regular, it should change in some way when differents sounds were played. The input pins of the DAC are connected to the output pins of the FM sound generator chip, a Yamaha YM2151. They are a classic audio ICs tandem, found on several arcade boards.
Also the NEC uPD7751 ADPCM Decoder was silent too …
My suspect is that the CPU in some way should misdriving the sound generators ICs. Before probing around address and data lines on the PCB, we can notice that on the SEGA System16b boards the audio CPU, a classic Zilog Z80, is mounted on a socket so we can easily test if the culprit relies on it by simply swapping it. Using a good Z80 didn’t solve the audio issue, and testing the mounted Z80 reveals it was good, so we need further investigations.
It’s time to take a look at the CPU signals, the clock signal was good, CPU control signals (RESET, HALT, WAIT, …) seemed to be healty, system control signals reveals the CPU is stucked in a READ state no WRITE activity on the RAM, this should be strange in a good working mode. So i started poking with the scope over the address and data lines, it reveals a lot of activity and the displayed patterns seemed to be the usual ones.
But when pinned the probe on the D3 line …
… The yellow pattern is a good D0 line, the green pattern is (inverted for a better reading) the D3 line and it is stuck high. Checking and beeping around the D3 signal for continuity traces testing between CPU, 6116 SRAM, an the FM chip reveals they were all ok. The test points are the ones with the red dot on the upper image of the audio section.
So only few IC remain to test on the shared data bus, the EPROMs. They are placed on a daughter board attached to the mainboard with four connectors. I started from the Z80 sound EPROM a standard 27c256 marked EPR12390, according to the datasheet the D3 signal should be on PIN 15.
The digital multimeter reported no continuity between CPU and EPROM on D3, maybe they can be wired in a different way so i check D3 on the CPU against all the pins of the EPROM data bus, but no audible beep can be heard. I checked the whole data bus and it was ok and only D3 was missing. So this should be the culprit, a broken trace, maybe on the connector, between EPROM and CPU data bus on D3 signal.
I pulled out the EPROM for further investingation and i spotted this:
… so i lifted the pin, reseated the EPROM on the socket and ready for the “smoke test” … and it worked! I was greeted with the usual Golden Axe audio effects and voices, and the well known music theme.
The damaged pin prevent the CPU bootstrapping and executing the right program code, probably leaving it in a endless loop of trash bits with wrong data.
It’s time to replace the missing C18 cap to completely fix the board. Despite of the lenght of this post, it was a few minutes easy fix.
Now, Daniele should be happy! : )