My friend ‘robotype’ sent me some boards for a repair.I hope this log will the first of a long row! 🙂

Let’s start with this Jigoku Meguri (the Japanese version of Bonze Adventure) :

 all

Board played fine but was silent, no audio at all.Digital part of circuit was fine, Z80 CPU was active and healthy so the fault had to be in the analog section.Main amplifer was good producing some noise when I put my fingers on its pins.Other component involved in analog section were a YM3016 DAC , a couple of TL074 and a uPC4556 OP-AMPs:

As usual I went to probe the analog output (pin 13) of the YM3016 DAC and sound was present although scratchy and distorted:

So I decided to replaced this DAC :

In this way the sound from it was fine:

but board was obviously still silent.Following the path of the signal I came to the near uPC4556 OP-AMP.Sound was still present on its inputs not nothing came out from outputs.I replaced it with a compatible LM358 and this restored sound.End of job.

Another great game on the bench, it’s the turn of Splatterhouse.

It was told me that PCB didn’t boot at all giving only a black screen.First thing I do before powering up a faulty board is to check for shorts with a multimeter.Indeed, this PCB showed a dead short between VCC and GND.Since boardset is made of a CPU and ROM board, I separate them and could locate the short on the ROM board while CPU one was succesfully tested.

I removed all the EPROMs and MASK ROM but short was still here.Usually components prone to go shorted are ceramic capacitors and seminconductors like diodes, transistors.Inspecting the board I noticed a diode on the right of the JAMMA connector @D1.According the schematics it’s a 6.2 zener diode (type 1Z6.2 from Toshiba).Its datasheet says that it’s used as transient suppressor so to protect devices from voltage spikes:

https://www.jammarcade.net/files/Datasheets/1Z6.2.pdf

Obviously, since cathode of diode was tied to VCC and anode to GND (a zener diode must be polarized inversely in order to do its job) I read it in circuit  as shorted.But, when I lifted one lead, the short on PCB was cleared :

So this diode was the culprit :

Once removed the short, the PCB booted stuck on this screen:

SInce I experienced similar issue on other Namco System 1 boards, I could quickly trace this to a bad custom ’64A1′ @M4:

Replacing it with a properly programmed HD63701 MCU (see article on our guide section) fixed the board: