Some weeks ago I bought this game for my collection in really great conditions.
While playing, I messed up with the external potentiometer of my supergun and menaged to detach it for a brief moment . Result was that the power supply jumped to 7V on the 5V line and 18V on the 12V line.
With my surprise, the game was not totally fried, sprites were still good but all the background was totally black
Also the game had no sound ( but the amplifier was still good)
For the graphics problem, 2k sram @H15 and H16 had all the data lines in the grey area.
Changing them restored completely the graphics.
The sound had also another UMC 6116 sram and the data lines were completely in the grey area.
Changing it led to no improvement but after checking the connections I found out D6 line was not connected to the Z80 but only to the YM2151.
I probably broken it during the exhange. Restoring the line led to a fully working game.
In the end it appears that UMC parts were the weakest devices of the pcb , I replaced all the ones which were present on the pcb