Tektronix 2235 Oscilloscope Channel Repair Help - Page 1
Forum section: electronics / repair

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 29, 2013, 08:10:35 pm
content: Hello everyone!I'm a amateur electronics hobbyist, I've watched a lot of and enjoy the EEVblog videos on YouTube, I've been learning a bit here and there about electronics for about a year now, here and there, and that's on top of what I know about electrical stuff. Thus I am fully aware of the mains potential and also HV for the CRT inside these and understand how to go about my poking around safely.Back in my early days of learning about electronics I had accidentally fed the second channel of my Tektronix 2235 oscilloscope 800v, much more than the 400v limit. Ever since then when I try to use the second channel, whether its in AC, DC, or grounded, I get no trace. It's not a real big deal for me to fix this channel because I rarely use the scope and 1 is usually good enough, but if its easy enough to fix, I figure why not.I decided to crack it open today and look for anything obvious, unfortunately nothing stood out in those mountains of complex pcb's.I am wondering if there is anything in specific that I should check out because as I said, I saw nothing obvious and the service manual I downloaded is also no help.I can provide pictures (low resolution) if needed.Thank you in advance.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 30, 2013, 02:14:32 am
content: Anybody?I'd like to get this fixed guys.

author: mtdoc
author_rank: Super Contributor
timestamp: July 30, 2013, 02:39:01 am
content: In case you haven't seen it yet,THISmight help.Good luck.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 30, 2013, 03:05:25 am
content: Quote from: mtdoc on July 30, 2013, 02:39:01 amNo I've not seen that yet.I took a look at it and began the process for troubleshooting the vertical section and the strangest thing happened.I went to change the trace thing to show channel two and that was when I noticed, it actually showed a trace for channel two. I was intrigued and I connected a battery with some wire to the input and I'm very delighted to say that it does indeed seem to be working now!I think I know why too. When I had the scope opened up earlier today I was examination the rotary switches used. They are very different to most others and definitely a world different than the rotary switches in the tektronix 2225 tear down video, these are pieces of thin metal that are parallel to the vertical PCB and they are pushed down at certain positions by the rotation of the rod connected to the switch because of bumps on the rod, the only way I can think to explain this is like a camshaft hitting the pushrods, instead of pushrods, its contacts.Since I found these contacts interesting I sat for a while and just played with the knobs as Dave does. I think that perhaps when the 800volts was applied a year ago that the ticking noise I heard when the cap (the sorce of the 800v, its a long story) discharge might've welded some of the contacts down with some very tiny force and me playing with the switches freed them up.Either way both channels are now working! The only problems remaining on this oscilloscope are my 1) lac of actual probes, 2) missing clear plastic indicator with numbers on it that should be around the knob of the second channel voltage selector (its not a huge problem, I just have to look at channel 1's knob to see where the second knob is at as a reference), and 3) I do not know what state this oscilloscope's calibration is in.Any suggestions on how I can calibrate this? The only relevant things I can think of to calibrate it is the scope itself and a 555 chip I have, I dont have a function generator or anything.

author: mtdoc
author_rank: Super Contributor
timestamp: July 30, 2013, 03:44:51 am
content: Glad it's working!Reasonable probes can be had for cheap ($10 or so) on ebay, etc.Depending on what you're going to use it for it may not need calibration.Have you checked the probe compensation output? It's probably a 1kHz 0.5V square wave. If that output looks ok you're probably good to go for most basic electronics.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 30, 2013, 04:36:51 am
content: Quote from: mtdoc on July 30, 2013, 03:44:51 amI did see some probes on eBay for about $10 but was hesitant to buy them because they are so cheap, I guess I'll get some sometime.Even if it doesn't need to be calibrated I'd like it to be for a couple of reasons, just so I know it is, just so I know how to do it, and also because I plan to do some PWM stuff and might need it to be somewhat accurate.How likely is it that the probe adjustment output is actually incorrect? I get a good squarewave but it doesn't vertically go as far as it should, however measuring DC voltages with the scope seems pretty accurate.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 30, 2013, 07:09:24 pm
content: I did some more testing and it appears that both channels seem to measure dc voltage pretty accurately so I think perhaps the probe adjust is out.How often is it that the probe adjust is wrong and is there a way I can get that probe adjust back to normal?

author: Bryan
author_rank: Frequent Contributor
timestamp: July 30, 2013, 09:43:54 pm
content: The only compensation you can perform on a probe is to adjust the "Square" of the square wave as it appears on the scope.http://www.evaluationengineering.com/articles/200801/eight-hints-for-better-scope-probing.php

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 30, 2013, 10:00:24 pm
content: Quote from: Bryan on July 30, 2013, 09:43:54 pmOh well that's not really the problem. It's as square as any other square wave i've seen, its just not 500mv p-p as far as I know.\I guess I'll leave it like that then.Also, will that affect my ability to adjust probes? (I've never adjusted a 10x probe before but I'm getting one soon)

author: mtdoc
author_rank: Super Contributor
timestamp: July 30, 2013, 10:22:22 pm
content: Just to be clear - the " probe adjus"t on the scope is a square wave output that is used as a reference signal t make compensation adjustmenton the probe itself- not on the scope. There is usually a small screw on the probe that you turn to make this adjustment. See the scope manual for more details

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 30, 2013, 11:00:26 pm
content: Quote from: mtdoc on July 30, 2013, 10:22:22 pmLooks like I was a bit too tired last night and was reading the oscilloscope incorrectly.I know what its used for, but the scope says it is supposed to output a 1khz 500mv p-p squarewave, and when I used a 1x probe I read it wrong and thought it was only 250mv.Just now I realized that's because the wave goes up and down, I must've been very tired last night lol.I guess everything is fine with it.Do you think the probe adjust is accurate enough to 1khz to use it as a source to calibrate the scopes horizontal?

author: Unknown
author_rank: Unknown
timestamp: July 30, 2013, 11:12:35 pm
content: No, and neither is it accurate enough to calibrate the scope's vertical ranges. It only services to adjust the low frequency compensation trimmer on 10x/100x probes. You might be able to use the probe cal output together with a frequency counter to calibrate the timebase, although the jitter might be too high for that. It's just going to be a lousy RC oscillator, after all. What gives you the impression that calibration is necessary?

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 30, 2013, 11:18:26 pm
content: Quote from: alm on July 30, 2013, 11:12:35 pmWell this isn't looking good.I'd like to have it calibrated so I can measure stuff accurately, I never did before and I'd like to get some practice using an oscilloscope to measure frequency.I guess it might also be helpful with some PWM stuff I plan to build.

author: Unknown
author_rank: Unknown
timestamp: July 30, 2013, 11:23:40 pm
content: With PWM it's often the duty cycle (relative pulse width) that matters. That should be fairly accurate unless the scope has severe issues. Also, don't expect accuracy better than say 2% or so from a scope even after calibration, either timebase or vertical. It will not replace a frequency counter, although it will give you an estimate of frequency. Which is all you need for many purposes (don't tell that to the RF people).

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 30, 2013, 11:39:23 pm
content: Quote from: alm on July 30, 2013, 11:23:40 pmI know its usually duty cycle with pwm. I guess I'm just looking for something to do.I'll leave it as it is because its only the horizontal that is possibly out, the vertical looks fine.Thanks for all of your help guys!