Problem

Lots of things as it turned out, but initially it just sat in a constant reset within milliseconds of power-on.

The list:

• Watchdog reset.
• Graphics corruption.
• No sound.
• No P1 gun input.

Diagnosis

A little bit of Googling showed there’s a simple link to make to bypass the watchdog (regards to Caius). I gave this a try and it started. It’s a good trick – gives you breathing time to see what might be playing up, should the game be capable of running in a faulty state – this board was.

It was clear that there were graphics problems and there was no sound (just blips and a lot of heat from the amplifier IC).

The graphics issue seemed to be at the colour mixer stage – after all layers are combined. It seemed intermittent and gradual so I took the hack route of putting my hand over the pins on the back of the board. This had an effect (normally it shouldn’t as working logic drive outputs can out-do the capacitance effects of your fingers), so I honed it down to a 74LS151. You can see in the (partial) schematics that these are used for the colours. Scoping the IC, it was clear that the output pin was floating around (when enabled – always check relevant output enable pins are set when scoping to see if an output is alive). I replaced the IC and all was good, colour wise.

The audio ‘custom’ PCB showed clear signs of corrosion in addition to a missing cap. Scraping off the black stuff showed a completely dissolved track. With that fixed, sound returned. In the ideal world I’d refurb/replace the PCB, but not this time around (time, effort, cost etc.).

With the jumper unlinked, the reset still occurred. A look at MAME showed only the main CPU writes to the watchdog, so this link was the first route to check. Schematics show pin 1 of the watchdog IC is the CPU ‘clock’. Dead.

Tracing this back to the 74LS74 at location V1, the IC output (Q’) was fluttering around 5V – no pulses to reset the watchdog. The (unused) complementary Q output was pulsing as expected, so Q’ was definitely not working. A diode-bias check with the multimeter showed open circuit on the pin (put the red probe on 0V, the black on the pin and look at the reading, then compare to other output pins or other instances of the same IC).
New IC, and all seemed fixed.

But it wasn’t. As I didn’t have a gun, I didn’t really test the inputs. It turned out that P1 gun wasn’t working.

As not even the trigger was registering, I hoped that would be the issue (scanline guns don’t have a moving cursor – you only see where they’re aiming when you pull the trigger and that wasn’t working).

Looking at the schematics the trigger is through a custom (passive filter) IC and into a 4:1 multiplexer, feeding the data bus. The custom was OK, so it really had to be the 74LS253…

…it was. All fixed.

Hateful, evil PCBs though – I’m presuming there’s no thermal reliefs on any pads as you almost need a blowtorch to get the IC pins out – I pity anyone who ever wants a through-hole device off intact.

Fix

In short, various Fujitsu logic ICs and the custom sound module. Feels like the sort of board where there could be many different things to fail, so it’s good that some schematics are knocking around (even if they’re not 100%).

John Bennett

Not Phil