Oscilloscope is more than adequate:
A few pinouts from the wiki that will be useful:
nesdevwiki:CPU_pin_out_and_signal_descriptionnesdevwiki:PPU_pin_out_and_signal_descriptionnesdevwiki:Mask_ROM_pinoutGiven that the PPU is drawing a solid white picture:
With a cartridge in, test the following-
* Vcc should be between 3.7V and 5.2V
(CPU)
* CLK should be 21.5MHz (26.6MHz PAL) → broken trace (because the PPU is still drawing a picture and so is still receiving the clock)
* /RST should be high → broken trace (because the PPU is still drawing a picture)
- - if it's pulsing low, the CIC is broken or the connection to the cartridge isn't good enough
* M2 should be 1.8MHz (1.7MHz PAL) → dead CPU or power supply issues
All subsequent tests could fail if the connection to the cartridge isn't good enough:
* A0 should change approximately every 2-3 M2 cycles
* some of A1..A15 and R/W should change ever
(74'139 adjacent to CPU)
* pins 11 (/INTSEL) and 9 (/ROMSEL) should pulse low quite often → if they don't, and the inputs are changing, the 74'139 could be damaged.
* pins 4 (/INTRAMSEL) and 5 (/PPUSEL) should pulse low often and occasionally, respectively
(PPU)
* /INT should pulse low ever →
maybe a damaged PPU
- - if It doesn't, trigger on /PPUSEL, look at contents of A0..A2 (b'010'), D7 (should usually be high but occasionally low), R/W (high)
- - - → more likely a damaged PPU
If all of those check out ok, then maybe the problem is the RAM. Trigger on /INTRAMSEL, and, when R/W is high, see if any of D0..D7 change mid-cycle. If it doesn't, then that RAM might be damaged.
If it plays music and makes sound effects in response to user input, the PPU RAM might be damaged.
/ROMSEL is also on the cartridge, and /INTRAMSEL and /PPUSEL are also on the RAM adjacent to the CPU and on the PPU respectively.