gary wrote:Andrew, have you looked at your 'clean' 12V unconnected to the LEDs with the 'scope? If not then it is possible that this supply is not so clean after all so have a look at it.
I had done this, but having just re-checked, I can confirm that it is a nearly dead-flat 12V measured on the scope, and 11.81V via multimeter.
gary wrote:As a sanity check, if you have a couple of 9V batteries handy you could connect them in series to provide a guaranteed clean DC supply to the LEDs.
I will do this tomorrow.
gary wrote:At first glance the level of the unwanted signal seems to be too high to be just 'hum'. Its almost as though the rectifier is halfwave rather than fullwave although if this were true you would still expect it to be much smoother due to the smoothing capacitor.
It is important now to identify the signal at the emitter of the transistor, I am pretty sure that the intention of the circuit to not have any syncs at this point (truncated by the .7V drop across the transistor base-emitter). If they are there then the DC level of the input to the transistor base is not right (this could be just a matter of setting the brightness).
I had wondered, too, if I should be seeing synch signals, and now I think I understand how they're being 'dropped', thanks. I have done a whole new suite of pictures, each with their own comments, below. I have been more careful to identify the readings this time.
gary wrote:The images of your scope seem to indicate that it is out of focus or maybe that is just an artifact of the snapshot itself.
The scope is producing quite crisp images. Good enough, anyway. I am having trouble holding the probes on the correct pins while at the same time taking the pictures with the correct focus The signal doesn't synch, either -- so I am using the knob to reduce the scan width to try and get the image to remain stable oncreen. Usually when I am photographing it, it is moving horizontally, slowly.