Need help with my analog Tek scope (Video) - Page 1
Forum section: electronics / general

author: FenderBender
author_rank: Super Contributor
timestamp: January 10, 2012, 01:25:09 am
content: Hey guys,Today I was going to play with my new Tek 465M, but when I put the probe in the Cal, I started getting some really weird waveforms that I have never seen before. Before I was getting almost perfect square waves out of the cal. Looked great, now I'm getting this and I'm really uncertain as to why or how.Is something wrong internally or are my settings messed up? Probes?Any ideas?Video is here:

author: PeterG
author_rank: Frequent Contributor
timestamp: January 10, 2012, 01:31:17 am
content: Try setting the scope to 0.5ms/div and adjusting the trim on the probe until you see a square wave on the screen.If that fails, try another probe.Regards

author: FenderBender
author_rank: Super Contributor
timestamp: January 10, 2012, 01:35:45 am
content: I was trying but the trimmer didn't seem to affect much at all...I would turn it but nothing would happen. I have barely used the scope or probes and they were working great before. Hmmph

author: The_Penguin
author_rank: Regular Contributor
timestamp: January 10, 2012, 01:42:03 am
content: I don't think it's quite triggering properly, once the waveform is stable and isn't moving, then I'd trim the probe to square up the waveform. You are triggering on ch-1 in normal mode right? Did you adjust the trigger level at all?

author: FenderBender
author_rank: Super Contributor
timestamp: January 10, 2012, 01:56:31 am
content: Well I looked at the manual and set everything to the specs they said to calibrate the probes on, and when I tried adjusting the probe compensation trimmer, nothing happens, the rolloff doesn't move or anything. Does this seem like a bad probe? It WAS just working...

author: lowimpedance
author_rank: Super Contributor
timestamp: January 10, 2012, 02:32:21 am
content: Quote from: The_Penguin on January 10, 2012, 01:42:03 amAs The penguin said, you should try to get a stable trigger, ie ch 1 source , internal (not ext or line!) trig on AUTO or NORMAL and adjust the level (note set coupling to ac not lf or hf reject if on your CRO, not familiar with that model).Also increase your vertical sensitivity (next do the same tests above on channel 2 with the trig source from ch 2).Do you have another probe to try. As well another signal source? function gen. , or you could rig up a good old 555 astable at 1khz. powered by a 9v battery if you don't have a bench supply.Does look like a probe issue or the cal signal and not the CRO, unless you cannot get a stable trigger on any input (with properly set trigger controls!).John

author: The_Penguin
author_rank: Regular Contributor
timestamp: January 10, 2012, 02:34:17 am
content: Here's a good video tutorial on triggering:

author: FenderBender
author_rank: Super Contributor
timestamp: January 10, 2012, 02:40:11 am
content: yeah I got it to trigger right when I used the Tek specs for calibrating. It was stable. Maybe I had my probes bundled up a little too tightly? They were used probes, but I wasn't like tossing them around really. But I doubt the cal signal all of the sudden went kaput...I don't have any other probes and I'm still waiting on my function generator to come in.Omg can't even stand it.Well the fact that the trimmer doesn't do anything makes me suspicious of the probes.

author: FenderBender
author_rank: Super Contributor
timestamp: January 10, 2012, 04:06:39 am
content: When it's triggering properly, this is the waveform I get:Seems like that trimmer maybe went kaput or something. It looks quite a bit like what you would get if you kept on turning the trimmer, except when I turn the trimmer, nothing happens. It's not a pure wave here. You can see some parts of the signal are missing which would lead you to think that it's just a messed up square wave.Sorry it's a little big.

author: BravoV
author_rank: Super Contributor
timestamp: January 10, 2012, 04:48:06 am
content: Assuming you've turned & tested the trimmer at the probe, looks like your probe's compensation range is not within your scope's input.Just make sure when you buy probe, it's compensation range is within the scope input capacitance.

author: Gridstop
author_rank: Contributor
timestamp: January 10, 2012, 04:53:53 am
content: The crawling waveform made me think you had the trig set to HF, when I have mine on HF the calibrator input crawls in exactly the same way.Anyway, I'd bet money it's a probe problem, although I guess it could be a bad calibrator. That is a large amount of capacitance creating a low-pass filter. If I had to test it, I guess I'd just poke a little hookup wire from the calibrator output to the middle of the BNC (be careful not to touch the ground). It won't square up like a real probe on the 10x setting could, but it should behave pretty close to what a 1x probe would do, the rising edges won't be exactly vertical, but it shouldn't be anywhere near that rounded off. If it still is, I'd say bad calibrator (or something horribly wrong with the input itself?)

author: FenderBender
author_rank: Super Contributor
timestamp: January 10, 2012, 09:37:50 pm
content: Well is there anyway I could measure the capacitance of the probe directly? Also, the probe was working acceptably fine before, and the cal made a more than decent square wave, so I'm not sure what's going on.I'll try some hook up wire and set the volts/div down lower and see what happens.Thanks.

author: benemorius
author_rank: Regular Contributor
timestamp: January 10, 2012, 09:51:49 pm
content: Quote from: FenderBender on January 10, 2012, 04:06:39 amIt isn't that parts of it are missing. It's that the signal is changing very quickly in those parts. That's just what happens when the beam moves quickly - the trace becomes very faint. Turn up the intensity a bit and the missing parts will reappear.

author: FenderBender
author_rank: Super Contributor
timestamp: January 10, 2012, 09:59:06 pm
content: Ok you're right. I wrote that at like 2 in the morning. I was meaning to make the point that it looks like a really augmented square wave, with a lot of roll off caused by excess capacitance on the probe.

author: FenderBender
author_rank: Super Contributor
timestamp: January 10, 2012, 10:30:43 pm
content: Thanks for the advice. I just tried hooking a piece of hookup wire from the cal to the ch1/2 bncs. It produces almost a perfect square wave, so I guess it would be the probe.Shucks. I payed $34 for that probe...