Tektronix 2225 noise - Page 1
Forum section: electronics / testgear

author: crash2140
author_rank: Contributor
timestamp: November 29, 2014, 12:20:24 am
content: I recently bought a second-hand Tektronix 2225 oscilloscope, and when I use the x10 magnification on any channel, I get a considerable amount of noise. Does this also happened to anyone else or does someone knows how to solve it? (My test probes are not the best, can that be the problem?)

author: JoeO
author_rank: Frequent Contributor
timestamp: November 29, 2014, 04:02:33 am
content: Some questions for you.How much noise exactly and what signal are you looking at?Is there noise when you ground the input.Is the noise the same on both channels?What model number / type kind of probe/s do you have?Do you have more than 1 probe? If yes, are both probes showing the same amount of noise on both channels?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 29, 2014, 04:30:08 am
content: And what bandwidth?

author: crash2140
author_rank: Contributor
timestamp: November 29, 2014, 08:34:17 am
content: It doesn't really have an exact value, it changes more or less randomly according to the vertical scale I'm using. At 1v/div(using the x10 mag.), I get around 0.75v peak to peak. And yes, it also happens using ground input, so the problem is not the test probe, I didn't think about that. The noise is more or less the same on both channels.

author: MartyD
author_rank: Regular Contributor
timestamp: November 29, 2014, 09:43:57 am
content: It shouldnt have that much noise at all...I just fired up my trusty old 2230 and on the photo you can see the noise I get with both channels set to 2mV DC, 5nS timebase and no probes connected.Marty

author: JoeO
author_rank: Frequent Contributor
timestamp: November 29, 2014, 02:13:49 pm
content: I am by no means a Tek expert. But I would think your next 2 steps are:1) Acquire a service manual/schematic for the scope.2) The one thing in common to both channels would be the power supply of the scope. I would start looking in that area.Do you have the skills to troubleshoot the scope, starting with the power supply areas?Also, please go to you profile page and turn on your country flag so we know where you are.Unfortunately, the third best tool to fix a scope is another scope. The first and second are your brain and a DMM.Good luck.PS Whoops, I reread your post. Do you get the noise when you are not in x10 magnification?

author: crash2140
author_rank: Contributor
timestamp: November 29, 2014, 02:36:51 pm
content: No, I don't get any significant noise when I'm not in x10 mag. By the way, the noise only starts to be visible at around 10us/div. Here's a picture I took using ground input, 1v/div and 0.2ms/div (and x10 magnification, of course)

author: c4757p
author_rank: Super Contributor
timestamp: November 29, 2014, 02:47:19 pm
content: Very curious that it only happens in x10 mode. That usually has zero effect on the vertical system, so I'm not sure why you're getting vertical noise. The two systems are usually quite separate. Are yousureit's not there in x1?And I second checking the power supply. It's the single most common failure point by far, and it affectseverything.

author: JoeO
author_rank: Frequent Contributor
timestamp: November 29, 2014, 02:53:43 pm
content: Here is the service manual.http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Tek/2225%20SM.pdfHere is a quote I found: "The low-noise vertical system operates..."

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 29, 2014, 05:30:08 pm
content: Oh so it's not just noise, but periodic...Could be the gate clock, does CHOP work correctly?Tim

author: crash2140
author_rank: Contributor
timestamp: November 29, 2014, 05:31:22 pm
content: Yes, the chop works just fine.

author: grumpydoc
author_rank: Super Contributor
timestamp: November 29, 2014, 06:04:50 pm
content: The x10 setting on the 2225 does tend to be a little noisy - however if you have the input set to GND that looks a bit high so the PSU ripple could be a bit high.There are a set of 840uF or 1000uF capacitors decoupling the +/- 8.4V rails and the +5V rail - replacing those would be a good place to start.

author: rf-loop
author_rank: Super Contributor
timestamp: November 29, 2014, 07:36:31 pm
content: Quote from: c4757p on November 29, 2014, 02:47:19 pmPerhaps he mean Vertical magnification X10 (this model have)Normally it have 5mV - 5V/div and with X10 down to 500uV/div.These (all) Tek analog models (2000 series, specially these low end models, are also very critical that all screws are in they places (example bottom screw of case near front and if there is one sscrew on side near back end, many times if they are loose or missing noise level rise. Also inside all GND connections need be ok. Also PSU faraday box if there is. Its every screw need be ok. This PSU radiate very easy some EMI.500uV/div is some amount critical and it also naturally have some noise but this natural noise is more like random noise and not this kind of periodic waveform.For OP, if you make question for help. Please show some amount better documents and explanation about problem. Good picture(s) what show settings and waveform and inputs etc.First think is check all power supply power lines ripple is inside specified levels (but there is some capacitors, as also "grumpydoc" told) what can also change without any check, due to scope age these capacitors are typically going end of life due to high stress inside PSU, so blind change is also good if scope is other way in good condition). Service manual chapter 6 page 8. If any is out of specs, repair it first before any other things and do not touch any adjustment before you know whole paragrapf "Adjustments" in service manual. Also note, if you adjust/change-8.6Vpower voltage, whole scope may need nearly full cal. (Some other psu voltages follow this voltage as reference)If scope all grounding / shielding things are perfectly ok, all boards every screw is ok, it is first thing to check carefully power supply ripples and if there is everything as specified then look other things.

author: crash2140
author_rank: Contributor
timestamp: November 29, 2014, 07:52:00 pm
content: Yes, I mean the vertical magnification, sorry if I wasn't clear enough. I get the noise when I pull these knobs:

author: crash2140
author_rank: Contributor
timestamp: November 29, 2014, 11:10:26 pm
content: Haha! Apparently, the noise is normal (go to 17:09):

author: tautech
author_rank: Super Contributor
timestamp: November 29, 2014, 11:32:25 pm
content: Quote from: crash2140 on November 29, 2014, 11:10:26 pmI would second others opinions, because "your noise" IS periodic, it is not what one would consider to be noise.Environmental or as I would more likely presume internal source IMOPSU MUST be to spec first.Show us a image with a larger signal on a faster timebase.My guess is the periodic signal will be ~ 40 KHz

author: barry14
author_rank: Regular Contributor
timestamp: November 30, 2014, 12:17:04 am
content: However in the video, the scope is set for 500 uV/div so the observed noise is only about 150 uv peak to peak. In your photo, the scope is set for 100 mv/div (1 V/div and X10 mag) so the observed noise is about 50 mv peak to peak, a 300 to 1 ratio. Something still seems wrong with your scope.

author: grumpydoc
author_rank: Super Contributor
timestamp: November 30, 2014, 09:25:41 am
content: QuoteWhich is almost certainly that the PSU ripple is out of spec.I don't fully understand the circuit but the x10 gain in the input amp comes at the cost of lower supply ripple rejection so using it can cause some pick-up of the switching frequency (39kHz IIRC) - I didn't really pay attention to the attenuator settings but wouldnotadvise using x10 at 1V/div - stick with it only at the 5 or 10mV/div settings.If there is no other input signal it is usually possible to trigger on the induced ripple - as per the OP's photos but otherwise manifests itself as a broad and fuzzy trace which "won't focus". The fact that it's there with the x10 Y mag and not otherwise is a bit of a give-away that the problem is PSU ripple. It also tends to be fairly independent of the input settings which is another clue.I've had a couple of 2225's which I repaired - at least two had half a division of ripple/noise before changing the caps and about 0.1 division afterwards. It doesn't seem to be possible to eliminate it entirely when the Y mag is used in my experience but the trace should be sharp with no observed ripple in the x1 positionOtherwise, as already suggested, pay careful attention to the grounding and shielding around the input attenuator board.

author: crash2140
author_rank: Contributor
timestamp: November 30, 2014, 12:14:16 pm
content: Here are some picture I took:GND input, 0.1v/div and 20us/div (with x10verticalmag.):Measuring the scope's 500mv p-p 1KHz square wave, 1v/div, 0.2ms/div, with x10verticalmag.Same square wave, 0.1v/div, 0.2ms/div, no x10verticalmag.I will try to check the PSU, I hadn't had time to do it.

author: crash2140
author_rank: Contributor
timestamp: December 07, 2014, 12:48:13 pm
content: Quote from: grumpydoc on November 29, 2014, 06:04:50 pmI checked the power supply voltage levels, and I got:5.2V is at 5.02V-8.6V: is at -8.6V (this is the only one I could correct, in the SM they didn't mention any other potentiometer to adjust the other voltages)8.6V: is at 8.73V37V: is at 37.9V100V: is at 98.8VI couldn't measure the ripple, as this is the only scope I got, so it seemed a bit dangerous to measure its voltage levels using its own channels.By the way, exactly what capacitors should I replace? There are three of them near the jumpers I used to test, all 1000uF, but I didn't see any 840uF caps. Are those the ones I need to replace? And isn't there any way of testing them before desoldering them from the board?Also, the seller told me that he used an isolation transformer, and he didn't get any problems.

author: tautech
author_rank: Super Contributor
timestamp: December 07, 2014, 01:01:33 pm
content: Check the bottom of the PCB under those caps for any signs of rework. They may have been replaced, but not with low ESR types.Try your DMM on AC V or mV on each of the supply rails for a ripple measurement.Another scope will show you the voltage and frequency of the ripple which could help you track down its source.Not so likely a DMM will do that.

author: MartyD
author_rank: Regular Contributor
timestamp: December 07, 2014, 01:19:36 pm
content: Quote from: crash2140 on December 07, 2014, 12:48:13 pmI just wanted to remind you that adjusting the -8.6 volts supply requiresALLother calibration steps to be performed according to the Tek service manual. All measurement depends on it whereas the others are not mission-critical.Marty