I see you have an oscilloscope - that is a good piece of test equipment. My only suggestion would be to adjust the oscilloscope so that we can see the entire top and bottom of the waveform you are measuring. Also, the timebase is a little too slow. Perhaps speed it up so that you see (say) 2 or 3 cycles on the screen. It is much easier to see more details then.
I would also set my Y shift control so that the 0V reference is exactly on the centreline. To do this, set the Y control to 2 Volts/division and connect the oscilloscope's probe tip to the probe's ground clip and adjust the Y shift until the horizontal trace is exactly on the centreline of the screen and leave it there. Every time you adjust the Y volts/division knob - you really need to perform this adjustment.
However, when measuring 5V signals, the oscilloscope trace will now only fill half of the screen. To expand the trace even further (see more detail) I would set 1 Volt/division and adjust the 0V reference to 2 major divisions above the bottom of the oscilloscope screen. This allows us to see as much detail as we can within the 0V to +5V extremes of the signal - and also allows us to see any ringing etc. that may be occurring above and below these limits.
I see a lot of ringing on the HIGH to LOW transition of the clock cycle - but this is a crude design anyhow and is to be expected from a 'budget' design machine!
Right, now to look at the 6502 CPU. Check the following pins of the 6502 CPU (UB14):
Pin 38 (SO) should be permanently HIGH.
Pin 2 (RDY) should be permanently HIGH.
Pin 6 (/NMI) should be permanently HIGH.
Pin 4 (/IRQ) should either be permanently HIGH or pulsing (i.e. not permanently LOW).
Pin 40 (/RESET) should be LOW when the power is applied (for approximately 0.5 to 1.0 seconds) and then go permanently HIGH.
To take the measurement on pin 40, you really need to clip the oscilloscope probe onto pin 40 to leave your hands free.
If you want (to make your life easier) you can solder a normally open pushbutton across capacitor C50 (0.1 uF). This will allow you to keep the power on and to perform a 'soft reset' by pushing and releasing the button. Push (and hold) the button - pin 40 should be LOW. Release the button - pin 40 should go high after approximately 0.5 to 1.0 seconds.
You can leave this pushbutton attached. It may be handy for diagnostic purposes later. Some people even drill a hole in the case and mount the button on the exterior of the case. Usually, switching OFF and then ON an old machine to reset it is asking for trouble - things have a tendency to fail!
Pin 37 (Phi 0) should have a 1 MHz clock.
Pin 39 (Phi 2) should also have a 1 MHz clock.
Pin 7 (SYNC) should be pulsing.
Pin 7 indicates whether the 6502 CPU is actually fetching and executing instructions.
If no activity is observed on pin 7, operate the soft reset pushbutton and observe (on the oscilloscope) whether there is a short amount of activity and then it stops or not.
Where we are looking for a permanent HIGH or LOW with the oscilloscope, you may need to adjust the oscilloscope timebase to see whether there is any high frequency or low frequency signals - or whether it is truly a static HIGH or LOW. There is a way to configure the oscilloscope to look for pulses and to trigger when a pulse is detected. I am not sure how knowledgeable you are regarding the use of your oscilloscope. This is very useful for monitoring pin 7 if it is not already oscillating after initiating a soft reset!
That will do for starters to keep you busy...
PS: You can also use your oscilloscope to look at the voltage rails. We are looking for the correct DC value - but you can also switch your oscilloscope into AC coupling mode and look at the mains ripple and noise on the voltage rails as well. Both of these can cause trouble if they exceed the specifications. I can explain this in a separate post.
By the way, what model is your oscilloscope (so I can look up the manual) - or please point me to an online link if you can find one.
Dave
