Just got my Tek 2225 - defective or not? :) - Page 1
Forum section: electronics / beginners

author: Sigmoid
author_rank: Frequent Contributor
timestamp: August 20, 2013, 12:46:58 am
content: I just got my Tek 2225 scope from eBay, and it's acting really weird.When I press the find beam button, a single dot comes up on the screen. After playing somewhat with the vertical control and the trigger, I got a squarewave from the probe calibration output, but only while holding the find beam button pressed, and rather erratically at that.I couldn't get a proper trace on the screen.Maybe it's something that's really easy to fix, but if not, I'll raise an issue with eBay.

author: Sigmoid
author_rank: Frequent Contributor
timestamp: August 20, 2013, 01:53:56 am
content: Okay I got a trace by following a "how to center all controls on the scope" tutorial. Apparently intensity was turned down so much I couldn't see anything. I feel really dumb. However...Currently I'm using the probe adjust output for testing, with a 10x probe. It looks okay, until I get to .1V/div (10x). There, the trace gets quite washed out and defocused, even at the best setting with the focus knob. Also, it seems I'm having triggering problems, as I have a ghostly flashing opposite image of the square wave, in an irregular pattern maybe a few times a second, as if it was also triggering on the falling edges. Note that this ONLY happens in .1V/div, it does NOT happen on 1V/DIV x10 (Vernier button pulled out). It stops when I switch the trigger to HF reject, so I guess it's some kind of noise. This mistriggering happens no matter where I place the trigger level, and on both rising and falling, and both normal and p-p auto mode.Since I wrote the seller that I'm having difficulties and maybe he has an idea, I got offered a refund, but I'm unsure if I should keep it or send it back. Is this mistriggering a natural thing, or a problem with the device? What kind of testing should I put it through?

author: PaulAm
author_rank: Frequent Contributor
timestamp: August 20, 2013, 02:04:44 am
content: Do you have a sine wave generator? Triggering on square waves can get a little iffy. If you had a sine input, you could tell if the trigger level is working correctly.What's the amplitude of the waveform on the screen? I haven't looked at the schematic, but trigger pickoff is normally after the attenuator and an amp or two. You may have a dirty attenuator.Do you get the same behavior on the other channel?

author: Sigmoid
author_rank: Frequent Contributor
timestamp: August 20, 2013, 02:08:03 am
content: It happens on both channels the same. The wave amplitude is .54 Volts according to the scale.I'll get a sinewave and try that.Okay I hooked up my function generator and got a sinewave... At first, I didn't get the messed up triggering this time, it stayed clean down to the highest resolution. However, this time I got a very strange noise, completely messing up the waveform, that came in spells, almost as if from changing settings - and it went away after reducing the vertical resolution... Then it remained perfect when I increased it again.Also, in certain timebase/vertical resolution combinations, I still got the double triggering, as if I was getting a trigger on both the rising and falling edge at the same trigger level.Weird thing though, now that I have had the scope on for about ten minutes, the problems seem to have gone away. Almost as if it was a temperature thing. Is this normal?EDIT: Okay the bidirectional trigger glitch thing started again on ONE SPECIFIC timebase / resolution combo, after I tried switching the triggering direction. It happens there on both channels: 5mV/div, .2ms/div. Note that it does NOT happen on .1ms/div or .5ms/div, nor on .2ms/div with 10mV/div, etc.I could not reproduce the crazy noise glitch thing. I tried various things that might affect it while I had it, like touching the inputs, the coax cable and the bnc tees, etc... Now I'm wondering if it could have had anything to do with radiation from the CRT, if the cable was right in front of the trace.

author: Sigmoid
author_rank: Frequent Contributor
timestamp: August 20, 2013, 02:53:14 am
content: I'd really appreciate the help and opinion of some Tek maestro here.If it's a matter of cleaning up, and maybe replacing a dead cap, I'm definitely up for it, and most of the 2225s in such cosmetic shape go for well beyond the $130 I paid for it. However if something important and hard to fix has died, I might be tempted to take up the seller on the offer and return it. After all the low noise floor is one of the main merits of these scopes....one thing, the mains plug looks abysmally dirty and corroded. Could that cause issues?

author: edavid
author_rank: Super Contributor
timestamp: August 20, 2013, 03:53:22 am
content: It's hard to tell from your description what is happening.Dirty line plug doesn't matter.You might have a problem with dirty switches or controls. Try turning all the switches and pots through their whole range a few times each, and see if anything settles down.You might also have some electrical noise in your environment that you are picking up - line voltage, CCFL lamps,

author: Sigmoid
author_rank: Frequent Contributor
timestamp: August 20, 2013, 05:01:24 am
content: Yep I'll try cleaning the controls. Should I try to use a contact cleaner? Can I find a guide somewhere to getting to the switches themselves behind the front panel for some cleaning?As for background noise, I live in a highrise building, so I guess it can be anything. But the erratic trigger direction was quite persistent in certain settings, unlike the noise superimposed onto the sine wave.

author: G7PSK
author_rank: Super Contributor
timestamp: August 20, 2013, 07:24:22 am
content: Is the problem there if you have the trigger set to auto and is the trigger holdoff turned of and have you tried playing with the holdoff while you are having the problem.

author: Sigmoid
author_rank: Frequent Contributor
timestamp: August 20, 2013, 12:38:33 pm
content: Nope, holdoff is on min, and the problem appears in both auto AND normal mode. I tried changing the holdoff hoping it might have an effect, but it didn't.Here's the problem in detail: In certain timebase/resolution combos, behind the normally triggered waveform, I get a flashing ghost image of the waveform triggered in reverse (perfectly triggered, perfectly drawn, except the wrong trigger - rising when I'm set to falling, and vice versa.)The frequency it appears at is low and erratic. Sometime it's once every two-three seconds, then three-five times a second, then nothing for a few seconds again, etc. At low frequencies, HF reject on the trigger makes it disappear.

author: Sigmoid
author_rank: Frequent Contributor
timestamp: August 21, 2013, 03:41:37 pm
content: In the end I sent it back for a refund. Somewhat ashamed to have done so, but I'm neither equipped nor experienced enough to troubleshoot an unreliable trigger.So, once again on the lookout for a scope... I've started considering some low-end DSOs - measurement confidence above all -, but I'd still love to score one of these epic analogs if I can.