Tektronix 454A Oscilloscope No Trace but Beam Finder works fine? - Page 1
Forum section: electronics / repair

author: neosunrise
author_rank: Contributor
timestamp: December 20, 2024, 11:09:43 pm
content: Got a Tek 454A the other day and did a quick test. It turns out that the only way to see a trace on the screen is to hold down the “Beam Finder” switch. As soon as it was released, the trace was gone. I shot a video to demo the problem:https://youtu.be/7HERqOtBGOo?si=wpvnHFCTCqLt0IzTThoughts?

author: norbert.kiszka
author_rank: Frequent Contributor
timestamp: December 20, 2024, 11:15:45 pm
content: Grab the schematic diagram and analyze probably causes - and check it with multimeter or another scope. I also suggest to check CRT voltages (be very careful, there can be deadly voltages in there).

author: srb1954
author_rank: Super Contributor
timestamp: December 21, 2024, 12:35:34 pm
content: Quote from: neosunrise on December 20, 2024, 11:09:43 pmMost of the waveform is off the screen as you have the vertical sensitivity turned up too high. With a 1Vpp calibrator signal fed into a 20mV/div vertical setting you need 50 divisions to display the tops and bottoms of the waveform.Turn the vertical range to .2V/div or higher so you can see the whole waveform on the screen. Or use a x10 probe to connect to the calibrator signal - this will bring down the displayed amplitude to 5 divisions.Also check the vertical offset is OK. Switch the inputs to GND and adjust the vertical position control to get the trace on the centre of the screen. Repeat for both CH1 and CH2 and for ALT and CHOP modes to verify no extraneous offsets are being added to the beam position.

author: neosunrise
author_rank: Contributor
timestamp: December 22, 2024, 03:20:59 am
content: Quote from: srb1954 on December 21, 2024, 12:35:34 pmTried all vertical sensitivities and it didn’t make any changes. Also tried a x10 probe, still nothing. Switched trigger to GND, nothing. When you mention vertical offset, did you mean the “position” knob? Tried that as well, nothing - in beam finder mode, it moved the line up and down so the knob works fine.I’m curious, why in beam finder mode, the waveform and the trace were clearly visible - it’s just that the screen was vertically compresses, which I suppose it normal. That said, everything disappeared as soon as I let go off the momentary switch. What exactly did the beam finder switch do? I’m assuming it does not only increase the cathode voltage, but also something else, right?

author: srb1954
author_rank: Super Contributor
timestamp: December 22, 2024, 09:32:06 am
content: Quote from: neosunrise on December 22, 2024, 03:20:59 amYou still need to reduce your vertical sensitivity to 0.2V/div to ensure that the calibration waveform fits on the screen.QuoteThe beam finder compresses the output voltage range of the vertical and horizontal amplifiers to ensure that the waveform is always within the appropriate range to be visible on the screen.Try operating the sweep mode switch and the display mode switches back and forth several times; the time-base may be stuck in an intensified sweep mode waiting for a trigger for the B sweep to run. The left edge of the trace is quite a bit brighter than the rest of the trace which might indicate that the time-base is not running most of the time. Also, try adjusting the B trigger controls to see if that enables a sweep. Switch the sweep magnify control to X1 rather than the X10 position you have it at in the video. This will allow you to see several cycles of the waveform on the screen rather than just the one you will see in magnify mode.Another possibility is that the CRT is really badly out of focus and in normal modes the trace is to diffuse to see. Scopes of this vintage quite often have problems with their focus circuits. With both inputs switched to GND switch the mode to X-Y and try and position the spot somewhere on the screen. Adjust the FOCUS and INTENSITY knobs to see if the spot can be varied.