Re: Tektronix 475


Simon
 

Some progress, I tested the PDA voltage with an ignition lead and spark plug. It produced a good spark that would certainly start my lawnmower, and I put the CRT back. The time base is working and I have an extremely bright trace. If I connect the Y leads to the tube, I get a fuzzy vertical stripe which responds to the time base and X position and magnifier. The voltage on one Y lead responds to a test signal on both channels, while the other rises to +10V and stays there.
Incidentally I do not recommend piercing the PDA/HT lead with a hypodermic needle as it leaves a path for the electrons to escape. It is much safer to push an ignition lead into the socket and connect it to a spark plug.
I don’t know why the trace is so bright (even on minimum intensity, also responds to focus) and I don’t understand why attaching the Y leads give a fuzzy vertical stripe that goes from top to bottom with only a few volts on the Y plates.
Thanks to everyone for pointing me in the right direction. This pastime is rewarding as it forces one to revise one’s hypotheses in the light of evidence and other people’s input.
Simon

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