What To Do With A Broken BK Precision 2120B Oscilloscope?  - Page 1
Forum section: electronics / beginners

author: Renegade
author_rank: Regular Contributor
timestamp: February 23, 2016, 03:34:19 am
content: I bought a used BK Precision 2120B, but it arrived broken. The seller gave me a refund and I don't have to return it (shipping would probably make it not worth it anyways). So, I've got this on my hands and not sure what to do with it.From reading a bit, I gather that repairing an oscilloscope is pretty much out of the question for a beginner like me. I'm ok with that, but I've got it, and I've got this "need" to do something with it.I don't need the money that I might get from selling it, and not particularly interested there. I am interested in learning more. And I'm patient. So I can take my time with this.I don't watch TV, but I do watch a lot of informational videos on Youtube, so I'm open to recommendations on material there. My "TV time" is pretty much "educational time". Be that Youtube, reading, or experimenting.Any recommendations on what a relative beginner should do with a broken oscilloscope?

author: vk6zgo
author_rank: Super Contributor
timestamp: February 23, 2016, 03:59:29 am
content: Is it literally "broken" or is it just inoperable?If the former,what bits are broken?If the latter,what happens when you try to use it?

author: Renegade
author_rank: Regular Contributor
timestamp: February 23, 2016, 04:07:41 am
content: Quote from: vk6zgo on February 23, 2016, 03:59:29 amI'm not sure what you mean there. I can get a dot or a line that extends from the middle of the screen almost to the far right side, but that's all.Quote from: vk6zgo on February 23, 2016, 03:59:29 amI don't really know. Like I mentioned above, I can get a dot or a short line, but that's all. I can't seem to get it to plot a signal on the screen.Quote from: vk6zgo on February 23, 2016, 03:59:29 amJust a dot or short line.I really have no idea what the problem is. I'm not sure whether to characterise it as "broken" or "inoperable".

author: vk6zgo
author_rank: Super Contributor
timestamp: February 23, 2016, 04:19:47 am
content: I would characterise "broken" as "has been dropped,parts of the 'scope are mechanically displaced,cracked,and/or bent""Inoperable" could cover from "it doesn't do anything" to "I can't use it to test anything".in your case ,"it is trying to work".Can you move the "dot or short line" up & down the screen using the vertical shift control?Does anything at all happen if you input a signal to one of the vertical (Y) inputs & change the volts/div through its range?

author: Renegade
author_rank: Regular Contributor
timestamp: February 23, 2016, 04:33:53 am
content: Quote from: vk6zgo on February 23, 2016, 04:19:47 amOh, I can move the dot/line, but I can't get it to plot any input. I tried everything I could think of, but it was all just dot/line.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 23, 2016, 04:43:23 am
content: Quote from: Renegade on February 23, 2016, 04:33:53 amMajor parts of your Oscilloscope still work--------you have HT for the CRT,you can move the line up & down,so a large part of your vertical deflection circuitry is OK.You now need to check & double check that you don't have any incorrect settings which would make you think you have many faults,rather than one or two.I think it may be time to try to find a workshop manual.

author: Renegade
author_rank: Regular Contributor
timestamp: February 23, 2016, 04:50:30 am
content: Quote from: vk6zgo on February 23, 2016, 04:43:23 amI'm not sure what a "workshop manual" is. How or where would I find that? The PDF manual I downloaded for it has no information other than simple operations.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 23, 2016, 05:14:33 am
content: A workshop manual,(sometimes called a "technical manual") usually contains a schematic diagram,some description of the device's circuit operation,parts lists,troubleshooting information,etc.I think the B&K 2120B is known by other names as well,so somebody on this forum may just have the information on where you can get a manual,& an insight into the possible fault cause.Check through your existing "User's Manual" to verify that you have all control settings correct.This,as I said before,is to ensure that you are only chasing the real faults.

author: Renegade
author_rank: Regular Contributor
timestamp: February 23, 2016, 05:29:29 am
content: @vk6zgo - Thanks again for the help. I've submitted a request to BK Precision for the manuals. I don't know if they'll help me, but something is better than nothing.As for the settings, I used what should be proper settings, and then experimented with a large number of permutations. Still just dot/line and no real results.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 23, 2016, 05:51:00 am
content: I had a bit of a Google:I found this thread on our very own forum--it'sv for the original BK2120,whis is apparewntly a bit different,but the similarity of the fault is interesting.https://www.eevblog.com/forum/testgear/bk-2120-oscilloscope-need-help/Note that near the bottom they show an URL where you can get a BK2120 manual.It has schematics,but I have also seen comments that the "B" model is somewhat different.It still may help,though!

author: Renegade
author_rank: Regular Contributor
timestamp: February 23, 2016, 06:09:25 am
content: Thanks for chasing that down. I couldn't download the file there, but I'll wait a day or so for BK Precision to get back to me on my manual request as that should be more accurate.I suppose that I should just open it up and have a peek when the service manual arrives.Wish me luck?

author: Stonent
author_rank: Super Contributor
timestamp: February 23, 2016, 09:32:03 am
content: Is there a switch labeled"AC - GND - DC"That is switched in the middle?

author: Renegade
author_rank: Regular Contributor
timestamp: February 23, 2016, 02:36:00 pm
content: Quote from: Stonent on February 23, 2016, 09:32:03 amYes. I tried them all. I also tried the initial starting procedure in the manual that uses it.

author: BurningTantalum
author_rank: Frequent Contributor
timestamp: February 24, 2016, 07:09:13 am
content: My gut feeling with this problem, and the first avenue to exhaust, is that it is a settings or controls issue. Personally, I wouldn't even remove the case screws until I had familiarised myself with the operation of the scope. Do you have a probe connected? I am not familiar with this scope but it must have a 'calibration' output connection/small pin/tiny loop. Whilst fiddling it would be useful to have the probe connected to that. The output is a square wave of maybe 1 volt, but it may not look much like a square wave due to other issues but at least it will be obvious that it is 'wiggling'.Regards, BT

author: Renegade
author_rank: Regular Contributor
timestamp: February 24, 2016, 07:20:40 am
content: Quote from: BurningTantalum on February 24, 2016, 07:09:13 amI went through the startup procedure in the manual with a probe (of course). I only got the dot/line I described above.For my other scope, I did the same for each probe with zero problems.I don't know what to make of it. CRT issue? Other? Dunno.At the moment, it's just some kind of decorative art for me that takes up space. I'd like to put it to some use, but I'm just not that good to figure out how yet.

author: tautech
author_rank: Super Contributor
timestamp: February 24, 2016, 07:32:58 am
content: Quote from: Renegade on February 24, 2016, 07:20:40 amWhat's your other scope? CRO or DSO?Screenshots/pics will help us.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 24, 2016, 08:44:04 am
content: If you have another Oscilloscope,you can look for the problem using that.

author: Shadetreeprops
author_rank: Regular Contributor
timestamp: February 25, 2016, 12:11:40 am
content: Quote from: Renegade on February 24, 2016, 07:20:40 amhttp://elektrotanya.com/b_k_2120_oscilloscope_sch.pdf/download.htmla link to the service manual.but sounds to me, like a input issue with the connection..i cracked into a old old old scope once..everything was fine, but it had fuses on the input, that were blown. replaced those and it worked fine..im not sure about newer scopes..But then again, i do not have much time with scopes..but i just look at stuff and normally can find the issues and fix it..not formally trained or anything..just some oddball natural talent to fix things.

author: Stonent
author_rank: Super Contributor
timestamp: February 25, 2016, 03:34:52 am
content: Quote from: Renegade on February 23, 2016, 02:36:00 pmIf it's on GND that all that does is short out the leads to get a zero point.It should be on AC or DC for any actual testing.What is your current time base and volts/div?

author: Renegade
author_rank: Regular Contributor
timestamp: February 25, 2016, 12:57:11 pm
content: Quote from: tautech on February 24, 2016, 07:32:58 amMy other scope is a Rigol DS1054Z.I'll get on top of the pics and post some later today.Quote from: vk6zgo on February 24, 2016, 08:44:04 amProbably very true, but a bit beyond my depth at the moment -- still very much a beginner, and a tad nervous about running a scope with mains power and poking around inside.Quote from: Shadetreeprops on February 25, 2016, 12:11:40 amThanks for the link. That has some additional information that the 2120B schematic manual doesn't have.I'll see about checking for blown fuses. The manuals should help me locate them.Quote from: Stonent on February 25, 2016, 03:34:52 amI did make certain to use DC first, and I did a full set of twisting dials across a full spectrum of possibilities. I tried every volt/div available. Only dot/line.I'll post back with pics later today.

author: Renegade
author_rank: Regular Contributor
timestamp: February 25, 2016, 03:34:32 pm
content: Well, I managed to get "something" after fiddling around for a while. The best I could manage is shown below with only the focus changed between the 2 pictures attached here.Time and volts are adjusted to make it as visible as possible with the maximum area. I could not adjust it lower on the screen or further to the left. It is firmly in the upper-right corner with no ability to get it centered properly using the position, volt/div, or time/div dials.The signal is a square wave from an Atari Punk Console.

author: jwelch68
author_rank: Newbie
timestamp: February 26, 2016, 04:29:31 am
content: I have the exact same scope you have except mine is 20mhz version. I contacted bk precision through this linkhttp://www.bkprecision.com/support/request.htmlAnd requested the user manual, service manual, and cal procedures and received them by email within a day or two.Sent from my Nexus 6 using Tapatalk

author: tautech
author_rank: Super Contributor
timestamp: February 26, 2016, 08:19:44 am
content: Quote from: Renegade on February 25, 2016, 03:34:32 pmSomething is surely amiss.With both Position controls set mid-way the waveform "should" be close to the middle of the display.So now the fun starts; you have to make the call as to passing this on to someone as a broken scope OR have a go at fixing it, possibly with forum guidance.As the "B" series Service manual is not freely available, I'd suggest you upload the copy you have to a free manual repository so participants can download it and then we're all on the same page.From the sticky in the Repair board:https://www.eevblog.com/forum/repair/repair-documents-and-links-sticky-me-please-mods/Here's a couple that many find popular:http://www.ko4bb.com/manuals/http://www.ebaman.com/index.php/remositoryFirst steps are to check all the low voltage PSU rails are within spec.Study of CRO principles will guide your understanding of the blocks of circuitry used and how they deliver a waveform to the display:https://www.google.co.nz/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=CRO+principles+of+operationWhat we see in your pics is an imbalance in the CRT plate voltages, ie. the Position controls are set to extremes just to get the waveform on the display. Those CRT plates (horizontal & vertical) use electrostatic deflection of the electron beam and for the waveform to be in the middle of the display the voltages to the plates must be in balance. This is a simplistic overview to hopefully point you in the right direction.Another doc that many find useful is this Tek troubleshooting guide:http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdfThere are many of us here that can step you through this repair, you have another scope, no doubt a DMM and soldering iron too and if you want to go the distance you'll have another working scope.

author: Renegade
author_rank: Regular Contributor
timestamp: February 26, 2016, 10:12:43 pm
content: Quote from: jwelch68 on February 26, 2016, 04:29:31 amMaybe I should email them for the calibration procs too.Quote from: tautech on February 26, 2016, 08:19:44 amYep.Quote from: tautech on February 26, 2016, 08:19:44 amIt struck me as odd as well.Quote from: tautech on February 26, 2016, 08:19:44 amI would like to repair it, but I know it will be slow going. The only way I'll get it done is by asking a LOT of questions and reading a truckload.Even if I never manage to get it working, I figure that merely trying to will serve as a decent education and I'll learn a lot.Quote from: tautech on February 26, 2016, 08:19:44 amThanks. I've uploaded both the manual and the schematic to the first site there. I imagine the fellow that runs the site will get around to posting it in a week or so.Quote from: tautech on February 26, 2016, 08:19:44 amI'll get on top of reading about that. Looks like I have some work cut out for me.Quote from: tautech on February 26, 2016, 08:19:44 amThanks. I'll keep my eye out for related materials when I'm reading.Quote from: tautech on February 26, 2016, 08:19:44 amThanks!Quote from: tautech on February 26, 2016, 08:19:44 amThanks again!I have pretty much everything I think I'll need, other than spare parts. I've got a couple cheap DMMs, and 2 old Flukes (8050As), and a Weller on my desk. So, I'm pretty much ready to get started once I can read enough to point my screwdriver at the scope.