Changing Waveform Size on Analog Scope - Page 1
Forum section: electronics / testgear

author: van-c
author_rank: Regular Contributor
timestamp: August 06, 2013, 07:05:55 pm
content: I recently obtained a very clean, working Tektronix 2445A analog oscilloscope off eBay and really enjoy using it in my hobby electronics lab.But recently I noticed the upper and lower limits of a waveform (sine, square or triangle) shrink somewhat as I move the vertical position up or down on the display. After tinkering with it more, I found that the vertical size, of any waveform is somewhat less than it should be (based on the known voltage of the signal) and that the error increases as the VOLTS/DIV setting is decreased. Put another way, as the VOLTS/DIV is adjusted to make the waveform taller, the difference between the actual voltage of the signal applied and the height of the displayed waveform becomes greater.I understand that later model analog scopes store calibration data in non-volatile RAM and some of this calibration data controls the gain applied to the vertical preamp. So, based on what I am seeing, is it likely being caused by a calibration issue, or does it sound like something more insidious, like a bad IC or power supply?--Van

author: JimmyMz
author_rank: Regular Contributor
timestamp: August 06, 2013, 11:26:17 pm
content: Youtube video?

author: van-c
author_rank: Regular Contributor
timestamp: August 07, 2013, 12:16:15 am
content: Quote from: JimmyMz on August 06, 2013, 11:26:17 pmHere is one way to visualize it: Imagine a sine wave measuring 2 vertical divisions peak-to-peak centered vertically on the CRT. Now, using the vertical position control, move the trace up about 3 divisions. As it moves up, the sine wave amplitude decreases, shrinking from 2 divisions to about 1.7 or 1.6 divisions peak-to-peak. The trace is perfectly stable, no jittering or anything; it just shrinks in amplitude.I'll put a brief yt video together to illustrate the problem.--Van

author: macboy
author_rank: Super Contributor
timestamp: August 07, 2013, 01:47:23 pm
content: I might suspect something wrong with the vertical deflection amplifier for the CRT which is making it go non-linear, so that the closer to the edges that the trace gets, the more compressed the waveform gets. Do you have the service manual? You can usually find free them online for older Tek gear.

author: van-c
author_rank: Regular Contributor
timestamp: August 07, 2013, 02:55:42 pm
content: Well, the news is not good. I checked with the TekScopes group and there is a known issue with one of the hybrid ICs, a vertical channel switch, U400 on the main board. Replacements are hard to find and are expensive.Fortunately for me, the issue on my scope is fairly minor-- it's not being used professionally and I can just make approximate corrections to any visual voltage measurements I make, or try to keep the height of the waveform fairly short to avoid the problem region. I hope it has stabilized, though, and doesn't get progressively worse.--Van

author: c4757p
author_rank: Super Contributor
timestamp: August 07, 2013, 03:18:17 pm
content: Oof... yeah, that's one of the hardest to find and most expensive. Doesn't look like there's much else around it that could go wrong either, it probably is the chip itself.Make sure that's really the problem, though, I can see U600 (155-0237-0x, vertical output amplifier) causing the problem as well, and they're more common.Do you have another scope? Stick it across (differential mode, ch1 - ch2) one end of the delay line and see if you get the same behavior. If not the problem's actually somewhere after U400.It's definitely not a cal issue, though.

author: van-c
author_rank: Regular Contributor
timestamp: August 09, 2013, 05:31:52 am
content: Quote from: c4757p on August 07, 2013, 03:18:17 pmI don't have another scope to test with. But I've read about several cases describing symptoms that sound very similar to mine and it's turned out to be U400. Apparently word is out that this is a problem part and prices have skyrocketed. One ad on eBay has the part from a China supplier for $129.00 USD. Too rich for me, I'm afraid.I actually find the problem tolerable since I'm only using the scope for hobby purposes. If I keep the trace to within about the middle 30% of the screen, amplitude errors are not visually apparent, being only about 5% or less. But toward the upper fourth of the screen, top parts of the trace are attenuated by about 20% which is visible, but not blatantly obvious. Curiously, channels 3 and 4 are not affected, so if I need accurate voltage measurements, channels 3 and 4 can be used, although with somewhat less display scale variability control.I'll try to post a photo tomorrow to illustrate in case anybody is interested.--Van