DIY Analog Oscilloscope Calibration - Page 1
Forum section: electronics / testgear

author: van-c
author_rank: Regular Contributor
timestamp: September 23, 2013, 12:45:07 am
content: I have a Tektronix 2445A used as a hobby oscilloscope that needs a bit of calibration, especially the vertical. I have the Tektronix service manual that describes in detail all the steps to perform. The problem is, the calibration calls for equipment I do not have, and purchasing the recommended equipment would cost me more than what I paid for the scope itself.So, I'm wondering if there are some "poor man's" substitutes I could use, given that this is not a traceable calibration I'm wanting to do. I've searched the web, including this forum and have found some useful information. (For instance,https://www.eevblog.com/forum/suggestions/how-to-calibrate-an-oscilloscope-and-multimeter/). It would seem like, for just the vertical calibration, I should be able to use my adjustable DC power supply and very accurate Fluke 8520A bench multimeter to produce fairly precise DC voltage references to calibrate with. But for the vertical calibration, the service manual keeps referring to a "standard-amplitude" signal from the calibration generator, which I am having trouble interpreting.Here is an excerpt:Connect a 0.5 V standard-amplitude signal from the Calibration Generator to the Ch 1 input connector. Adjust Ch 1 POSITION and VOLTS/DIV VAR controls to obtain a 10-division horizontal signal.From this, I'm having trouble visualizing just what the "standard-amplitude" output from the calibration generator looks like. These directions do not sound like the "0.5 V standard-amplitude signal" is just a DC voltage. So, what is it? Square wave? Sine wave?. (I do have a frequency generator that outputs up to 3 MHz sine wave, if that would help.)I know my ignorance on this subject is showing, but hopefully, somebody else will benefit from some clarification on calibrating these older analog scopes. And if we make progress on this topic, I have a couple of questions regarding horizontal calibration too.TIA.--Van

author: c4757p
author_rank: Super Contributor
timestamp: September 23, 2013, 01:02:13 am
content: I've done my 2445A. You're going to want a digitally-controlled function generator (doesn't have to be DDS, but it does need to have precision frequency and amplitude settings, like my analog-with-digital-control HP 3325A) for most of the procedures. You might be able to get by without, but I wouldn't recommend it at all. Ideally it should go to 50 or 60 MHz - the 3325A goes to 20 MHz on its main output, and has an auxiliary output with fixed waveform and amplitude on the rear panel that does 60 MHz - that was enough for me. You'll want a square wave for the "standard-amplitude" signals.You also need another oscilloscope, and it really ought to be an analog one. You're using it to synchronize two alternating signals on one output, and the trigger on many DSOs seems to be utterly defeated by this compound signal. (I had to use a DSO, and I had to set it to a four-second persistence, which was extremely obnoxious to work with.)

author: van-c
author_rank: Regular Contributor
timestamp: September 23, 2013, 10:55:45 pm
content: I found a manual for the calibrator generator (PG 506) mentioned in the service manual for my scope. The output in the "standard amplitude" mode is a 1 KHz square wave of amplitude that depends on the calibration step, ranging from 20 mV to 10 V. The square wave from the PG 506 has a short rise time, of the order of 1 ns; my signal generator is nowhere near that precise.I'm beginning to think I can live with my slightly uncalibrated scope. Horizontal seems to be pretty good. Vertical, on the other hand, is about 2.5% off. A 400 mV internal calibration square wave signal from the scope shows about 410 mV on the screen; A 20.0 V peak-to-peak external square wave measures about 20.4 V on the screen; 9 V DC measures about 9.2 V. With this pretty consistent 2.5% over estimate of voltages, for hobby purposes, I can make a manual 2.5% correction in measurements, when I really need that precision.

author: Unknown
author_rank: Unknown
timestamp: September 23, 2013, 11:20:55 pm
content: 2.5% is pretty good in my opinion, since scopes generally have a vertical accuracy of about 2%.

author: c4757p
author_rank: Super Contributor
timestamp: September 23, 2013, 11:30:52 pm
content: Yeah, I wouldn't worry about 2.5%. Hold on, I'll check mine...4V deflection on a 1V/div scale was -1% out (displayed 3.96V).7V deflection on the same scale was -0.9% out (displayed 6.94V).So you're only a bit further out than a scope that was calibrated in the last two or three months. You're fine.Is it the physical vertical display that is out, or the electronic cursor? They're two separate systems and the former is a bit easier to adjust, if you really wanted to.

author: van-c
author_rank: Regular Contributor
timestamp: September 24, 2013, 02:08:18 am
content: Yeah, I'll take the 2.5%. I would like to learn how to do the calibration, though, because it's likely I'll need to if I keep the scope.It's mainly the actual trace (not the cursors) that are off. It's easy to observe the errors by aligning a square wave of known amplitude with the center graticule line. Using the cursors to measure the voltage, though, also agrees very well.I replaced a vertical switch hybrid IC recently and was told the cursors should be re-calibrated; but they seem spot on to me. Before I replaced that IC, voltage was non-linear past about 1.5 vertical divisions from the center and I had errors around 15% - 20%; so I suppose I should be happy with the improvement.--Van

author: c4757p
author_rank: Super Contributor
timestamp: September 24, 2013, 02:15:27 am
content: IIRC you can adjust the vertical gain of the CRT with one trim pot.R638, near U600 (vertical amp).

author: van-c
author_rank: Regular Contributor
timestamp: September 24, 2013, 03:01:24 am
content: Quote from: c4757p on September 24, 2013, 02:15:27 amI believe R638 adjusts the cursor gains. Vertical (preamp) gain is adjusted using the multiple-step automated calibration procedure by connecting various calibration signals and pressing a switch to indicate when each signal is applied. The automated procedure saves the calibration data in NVRAM. Horizontal calibration is done in a similar manner, although it is more timing critical. Unfortunately, with a few exceptions, this scope doesn't use trim pots for calibration as much as the older models did.--Van

author: c4757p
author_rank: Super Contributor
timestamp: September 24, 2013, 03:05:18 am
content: R638 hangs off the vertical output amp, I'm pretty sure it directly adjusts the vertical gain of the CRT driver.You're right, most of the calibration is done in software. Butthispart is not.

author: van-c
author_rank: Regular Contributor
timestamp: September 24, 2013, 03:31:16 am
content: Next time I have it cracked open, I'll give R638 a tweak.

author: BravoV
author_rank: Super Contributor
timestamp: September 24, 2013, 03:35:04 am
content: Make sure you have a powerful fan blowing that board when you are doing the adjustment, without the enclosure, those hybrid ics will be overheated.

author: c4757p
author_rank: Super Contributor
timestamp: September 24, 2013, 03:40:48 am
content: Be careful not to twiddle any other pots - many of them interact with the software calibration, especially thehorizontalgains. You'll be one pissed dude if you stick it in the wrong trimmer.

author: van-c
author_rank: Regular Contributor
timestamp: September 24, 2013, 03:46:39 am
content: Very good advice. I try to always have air blowing on the open chassis. And as for tweaking pots, I'm a bit AR. I use a check list. Literally. Used to be a pilot.--Van

author: c4757p
author_rank: Super Contributor
timestamp: September 24, 2013, 03:48:09 am
content: And I used to be a student pilot, I know how that is!Good.

author: van-c
author_rank: Regular Contributor
timestamp: September 24, 2013, 03:52:23 am
content: All pilots are student pilots. Some of them just don't realize it.

author: Dark Prognosis
author_rank: Regular Contributor
timestamp: September 24, 2013, 04:19:35 am
content: I have flown in a plane once so does that count?