On 07 Feb 2016 19:15:56 -0800, you wrote:
Guys, this is my first post here. I've been tinkering at Tek scopes for along time - since I first bought a 531A in the late 70's. I'm looking for some tips on a 7904.I assume you measured the output of U1635 and found it to be within a
couple volts of ground.
A high cathode voltage could be produced by leakage through CR1639
which protects the non-inverting input of U1635. U1635 could also be
On a whim, I tweaked the CRT grid bias (R1674) and can now get a faint trace. It's well focused, just dim. Pushing Beam Find does not h elp - it just gets blurry, reduces the sweep span, but it's still dim.If the cathode voltage is high then the horizontal and vertical
deflection sensitivity should be proportionally lower. Is that the
case? You can check the calibration using the probe calibrator output
I would defocus the beam before conducting further tests in case you
fix the problem so that the CRT phosphor does not get burnt.