Big Bang - Power Shooting repair log

Received for repair a Big Bang PCB (also known as Thunder Dragon ), a vertically scrolling shoot'em up released from NMK in 1993.The board was actually a factory conversion from Macross II :

It played absolutely fine but sound was missing at all:

A background buzzing noise suggested that analog circuit was doing its job hence the fault was located in the digital section.Most of its components (Z80/EPROM, YM2203 and samples MASK ROMs) were already socketed :

They were all good except the Z80 CPU which I tested as bad in another board.I swapped a good one but this didn't restore sound.So I fired up my Fluke 9010A troubleshooter and hooked up the Z80 POD to the board :

According to MAME sound memory map the RAM lies from 0xc000 to 0xdfff of Z80 address space :

map(0x8000, 0xbfff).bankr("audiobank"); /* banked ROM */

map(0xa000, 0xa000).nopr(); /* IRQ ack? watchdog? */

map(0xc000, 0xdfff).ram();

map(0xe001, 0xe001).w(FUNC(nmk16_state::macross2_sound_bank_w));

map(0xf000, 0xf000).r(m_soundlatch, FUNC(generic_latch_8_device::read)).w("soundlatch2", FUNC(generic_latch_8_device::write)); /* from 68000 */

map(0x0000, 0x7fff).rom();

I launched a RAM test at this specific offset, it failed meaning the chip was likely bad :

 

 

 I removed the RAM chip and one pin remained attached to the board :

I  installed a socket and new RAM :

Sound was restored but playing some games I noticed an entire music track was not correct because samples were played wrongly or randomly :

The audio samples (stored in two 16Mbit MASK ROMs) are played by two OKI MSM6295 ADPCM IC :

"Listening" with an audio probe to the analog output of them I was able to figure out which one was playing wrong samples.I promtly removed it with hot air : 

Then soldered in a spare :

 

It did the trick,  music was fully restored and board 100% working again.Another repair successfully accomplished.