Electronics > Repair
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 ;D
Alex_Baker:
I am picking this up after a few months.
Up to this point I was assuming that there was a problem with U1 or its associated components, However following the troubleshooting section I lifted R50, which should be able to bring back the trace if U1 was a problem. Lifting this resistor did not have any effect, which makes me think that there is another problem somewhere else.
Both the "Dlyd Gate" and "Comp Gate" signals are ok.
I have attached the Gate amplifier section from the manual.
Alex_Baker:
I went around the board with a multimeter measuring voltage potentials on the pins of all the transistors, making the assumption that Si transistors tend to have about 0.6v base to emitter. With this method I found that Q14 was bad, it had become a resistor.
For the time being I replaced it with a generic npn I had in stock. This fixed the gate amplifier(I think) but we still have problems. I began by checking CRT voltages but I cant get very far into the adjustment procedure before I run into problems.
I did find that when holding the beam find button under certain circumstances I can get a very faint cloud to appear and move around on the screen to some degree, however I cannot move it very much or bring it into focus at all. Perhaps somebody that knows far more about CRTs would know why this may be. :-//
m k:
So deflection is operational, but focus is far away.
Is the scanned manual available somewhere?
U1 power should be between 53V and 30.1V and its +input no more than 18.2V below 53V.
18.2V from R22 would be far inside R23.
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