Street Smart (SNK A7007)

[Jacmar]

Picked up 2 of these PCB's from a member on here, both faulty, in the hope I could at least get one working game from the 2 PCB's. The 2 pcb's differ however because SNK released this game on 2 different boards and this one is the A7007 version (version 1) which I believe uses the same hardware as Ikari Warriors 3 .. there are only partial schematics available for this board ... (Version 2 uses the A7008 board)

Board booted (good start!) to a mainly black screen apart from the good text layer, with some (but not many) signs of the game running and no sound (just a steady hum from the speaker)

Looking at the PCB it's not in the best of condition, the usual dust/grime covering and areas of corrosion .....



Gave the board a good scrub and removed as much corrosion as possible, checked all the ROM's and only one didn't ID correctly - number 6 ... rather than burn a new one I borrowed the same rom from the other board/version (A7008) of the game that I got which checked out good. (I think only ROMS 1 & 2 are different in versions 1 and 2). So with a clean board and all good roms, fired up the game ...



Obvious graphics issues, still no sound ...
Found some damage to the solder side of the board, a gouge which has taken out 2 traces meaning a loss of continuity between a couple of LS153's and a couple of other IC's under the ROM daughter board...



Repaired these traces which had a big improvement on the graphics issues ...



Looks like the sprites are all back but the colours look wrong ..... so I checked out the 2 palette rams (6116) at C4 and C5, all address lines and data lines had seemingly healthy signals.
Thankfully the partial schematics for this version includes a page on the colour circuitry ...The data lines from these 2 rams go into 2x LS273 flip flops at D4 and B4 ... All signals in and out on D4 seemed good, but when checking B4 I found a few output pins with stuck signals while all inputs were toggling. Sliced B4 and it failed as expected....



Replaced the LS273 at B4 and colours fully restored ......



Just the sound to sort out now .... tbc ....

 

[Jacmar]

OK so total loss of sound. Speaker produced only a low buzz that tracked with the volume pot (so I knew the amplifier section was alive). The partial schematics available for this A7007 board does not include a page on the sound generation but there is full schematics for the A7008 version and the circuitry seems to work exactly the same just the layout of the IC's is different...



Made a simple audio probe with an old pen, wire, 3.5mm jack, capacitor, small battery speaker and probing the outputs of both sound ICs (YM3812 FM chip and D7759 sample chip) showed no audio activity, just the same buzz coming from the D7759 output pin.

Probing pins 23/24 (the resonator oscillator pins) of the D7759 showed no clock activity. At first I suspected the 640K resonator or its 220p caps, so I swapped them. No change. Assumed then the chip must be faulty so dropped in a known working D7759. Still no oscillation. I was pretty confused at this point, then I discovered something pretty important ...... the D7759 doesn’t free-run its clock – it only oscillates when the /ST (start) line is pulled low. That explained why the clock was dead.

The /ST pin comes from an LS139 (at E17), via an LS32 (at D17). Tracing backwards, the LS139 gets its inputs and enable directly from the Z80 sound CPU. Both logic chips passed slice. Moved on …

The YM3812 FM chip was also quiet. /CS was toggling, /WR was toggling, /RD was high – but its audio output was dead. Both sound chips being silent told me the problem clearly wasn’t just a failed D7759 oscillator.

The Z80 was alive: good 4Mhz clock, address/data lines active, MREQ, IORQ, RD, WR all pulsing. The LS139’s enable (Z80 /IORQ) sat high most of the time with the odd blip. Output Y0 (chip select for the YM3812) was pulsing occasionally, but output Y1 (the D7759 /ST line) never went low. So, both sound chips were effectively never being told to do anything.

Since the Z80 appeared to be running, I suspected bad instructions or bad data. The Z80 fetches its program from a 27C512 ROM and uses a 6116 SRAM as work RAM. Out came the scope … address lines all looked healthy but the data lines not so much ….



The ROM had previously been dumped and verified so I knew the ROM data was good. Popped it out the socket and the dodgy looking data signals were still present on the sockets pins. This ruled out the ROM and left the Z80 and the 6116 SRAM as prime suspects corrupting the data bus.

Obviously went with swapping out the RAM first and thankfully this turned out to be the faulty component. Replaced with a socket and good known 6116 and the game has full sound again. Hooray! So just a bad 6116 work RAM (at C15) killing the entire sound circuit.

Shame I went down a bit of a rabbit hole with the D7759 sample chip. Assumed it must be faulty as it wasn’t oscillating but just didn’t understand how that chip works, now I do I won’t make that mistake again. Took me ages to replace it as well because this board is an absolute nightmare to de-solder from. I had to cut it out in the end and ultimately didn’t even need to, it was fine 😖😖😖😖



But all’s well that ends well and that’s this one finished, even replaced the little red +5v power LED that had broken off during repair (taken from a SNK mechanized attack donor board) so everything is fully working again