Another quick repair log. I forgot to take photo’s this time.
The board tried to boot but just had a line of yellow text saying ‘XVVSIDXXVVVVV’. That usually means there’s an issue with the text layer. Using the ‘wet finger on the pins’ of the 4 SRM20256 SRAM chips at F14 to F18 showed some changes to the text when I touched the data pins on F14. Desoldering it and testing on my Wellon VP280 showed it as bad. (no continuity on the data lines). Replaced it with a Sony CXK58256P aka 62256 and got a working screen.
Now it showed ROM OK, RAM OK and SOUND CPU BAD. This most likely meant that the 6116 at F1 was bad, (although it can mean otherwise). Desoldering and testing the chip showed the A0 pin to be faulty. Replaced it with a Hitachi 6116, fired up the board and was presented with the same error message.
After looking at the Rally Bike schematics which is basically the same board, showed that the 68000 can access the sound z80’s work ram. This is done by using the BUSREQ pin to tell the z80 to get off the bus. There is quite a bit of circuitry to handle this.
Since the 6116 showed a fault with the A0 line, the z80’s A0 line could be bad too. I desoldered the z80, put in a socket and tried another z80. This new z80 was a pull and was in fact bad. The board didn’t even boot, and without the z80 in the socket it won’t boot either. I tried another working z80 and the game booted with the same fault.
Now it was time to look at the support chips that handle the interaction between the z80 and 68000.
The 68000 uses 3 74LS245’s to access the address and data bus, and a 74LS244 to set the control lines when the z80 gives up the bus.
Looking at the chips, I noticed the 74LS244 is a Fujitsu chip and if you read many other repair logs you’ll see a pattern of Fujitsu chips rapidly and randomly dying. A good candidate.
Desoldering and testing the 74LS244 showed, you guessed it, as faulty. Replacing it with a known brand pull (rather than Goldstar chips from bootlegs!) and the board fired up 100%.
Another classic saved.
PS You have to be careful desoldering chips on these board as they used a ground and power plane. This sucks out all the heat making it extremely hard even with a desoldering station to extract the chips without damaging the pcb.