I have had a HP 1740A Dual trace oscilloscope for over 20 years, with a full set of instructions and circuit drawings, and now has a common vertical deflection problem on A, B or chop. Horizontal deflection is OK. The Vertical trace is way south and the vertical adjustment pots have little or no effect. I have digital electronics experience, but no idea what voltages to expect on an oscilloscope.
A old thread by Jim Yanik recommended shorting the vertical deflection plates to each other, which did result in a correct centered trace. All power supply voltages are OK. There was a burnt connector pin and dry joints on the screen illumination side which I repaired.
With the A probe on the Cal 1v, only channel A selected and the vertical position pot on MAX, I can see the square wave pulse at the botton of the screen. Volt on 1st deflection plate 11.36v, 2nd 11.06v. When vertical position pot to MIN the 11.36 REDUCES to 10.49, and the 11.06v INCREASES to 11.24v. Adjusting (lowering) the vertical posion pot only lowers the top part of the square pulse signal trace, the botton trace of the signal is fixed right at the botton of the screen, and does not move.
I do not know what the correct deflection plate voltages should be, or what the difference in the 2 voltage should be for the signal trace to be correct or visible on the screen.
On the input to the vertical deflection card, at the transistors, the circuit indicates +12.6v for both sides. I have measured 12.6v on the one side and only 10.92v on the other side. Base reference voltages on both transistors is correct at 12v.
I do not know what is lowering this 12.6v to 10.92v or if this is the problem at all (beginning of the Vertical output card, or the output of the vertical preamp. )
I am now stuck and at the end of my voltage !!. Can anyone help or advise ?
Regards
Barry
A old thread by Jim Yanik recommended shorting the vertical deflection plates to each other, which did result in a correct centered trace. All power supply voltages are OK. There was a burnt connector pin and dry joints on the screen illumination side which I repaired.
With the A probe on the Cal 1v, only channel A selected and the vertical position pot on MAX, I can see the square wave pulse at the botton of the screen. Volt on 1st deflection plate 11.36v, 2nd 11.06v. When vertical position pot to MIN the 11.36 REDUCES to 10.49, and the 11.06v INCREASES to 11.24v. Adjusting (lowering) the vertical posion pot only lowers the top part of the square pulse signal trace, the botton trace of the signal is fixed right at the botton of the screen, and does not move.
I do not know what the correct deflection plate voltages should be, or what the difference in the 2 voltage should be for the signal trace to be correct or visible on the screen.
On the input to the vertical deflection card, at the transistors, the circuit indicates +12.6v for both sides. I have measured 12.6v on the one side and only 10.92v on the other side. Base reference voltages on both transistors is correct at 12v.
I do not know what is lowering this 12.6v to 10.92v or if this is the problem at all (beginning of the Vertical output card, or the output of the vertical preamp. )
I am now stuck and at the end of my voltage !!. Can anyone help or advise ?
Regards
Barry