HP 1727A no display

[Alex_Baker]

Hello

I have an HP 1727A oscilloscope that needs repair. There actually was a very long thread on this unit two years ago, but I would like to start fresh to limit confusion.

The symptom I have is that there is no trace on the display, I have spend some time troubleshooting and I seem to have a problem with A13, the gate amplifier. I checked the inputs to this board based on the provided waveforms and they were good, but I have no gate output. I tested a fair number of points but the big thing that I found is that the op-amp U1 is railed, its output is high all the time. I tried adjusting both front panel controls and internal pots with no luck. The two inputs to the amp are always 0.6V apart.

From what I understand U1 is an auto intensity controller, getting feedback from a mesh in the crt, so that if beam current gets too high it will automatically reduce the gate pulse that controls the main beam. Since U1's output is always maximum the gate signal is decreased (or gone completely) as it should be. So essentially something is making U1 think that beam current is too high, and I dont know what it is.

[Alex_Baker]

The annoying thing is that I had a display when I put this piece of crap away a couple of years ago, it still had issues then, but no display was not one of them.   

Also bumping this topic