Wobble on X Channel of Oscilloscope in DC XY Mode

[allenwp]

Hi! A few months back I posted about getting blanking working on an oscilloscope for a video game engine. Good news: I got a new oscilloscope with a DC coupled Z-input and blanking works great!

https://youtu.be/AkLNzdGMfsE

...But at the end of that video you can see that there seems to be a wobble on the X axis. It almost seems like the X input isn't properly DC coupled. I whipped up a test signal that shows the problem in an isolated way:

https://youtu.be/Di9QhIRAa9s

You can see that all AC or DC modes for both inputs work great except for DC mode on the X input channel, which has a bit of a wobble to it. The wobble is seen at the beginning and end of the video clip.

Here is the oscilloscope I am experiencing this problem with: https://longweielec.com/products/l-212

I have another 14 days or so to request an exchange or return it for a refund.

Because I haven't had much experience with oscilloscopes, I'd like to ask: is this type of behaviour expected and reasonable on lower end oscilloscopes like this one? Or is this a faulty unit / faulty design?

I do like some of the features on this display like the analog Volts/Div knobs, being able to increase beam intensity with a negative voltage on the Z-input, and the extremely intense brightness I can get with this brand new screen... So I'd like to get a better feel for whether this is "just the way some oscilloscopes are" or if there's something wrong with this specific unit or it's circuit design.

Thanks as always for your help and thoughts!!

[T3sl4co1l]

That's a shelved response, it would be probe compensation if the time constant were shorter and you had a probe in circuit.  Internal compensation (in the divider chain) shouldn't have that long of a time constant either.

Would be possible to trace the circuit, but may be a PITA.

They don't give specs detailed enough to say whether that's expected behavior for the X axis.  It is definitely not intended behavior on any (properly specified) scope in existence which supports X-Y mode.

Tim

[allenwp]

Thanks, Tim, for the response! I know you can't say much without more detailed specs, but just knowing that this isn't a very common or expected behaviour with an oscilloscope in XY mode is immensely helpful to me. I might return this and try my luck on eBay with a used Tektronics scope. My only fear with this route is that the phosphors might not be as as bright, which is something that is important for my use, or that the screen is out of focus like the first cheap "as-is" oscilloscope that I bought.

If anyone knows of a brand that still manufactures analog oscilloscopes like the one I bought here, but that are well designed and built please let me know! Searching on Amazon just returns a bunch of no-name generic scopes that all look the same but are made by different manufacturers, like the one I got from Longwei.

Cheers,
Allen

[james_s]

Nobody still manufactures analog oscilloscopes, or CRTs of any type to my knowledge. The last CRT rebuilder in the world shut down around 8 years ago so what's out there is it.

If you swap the X and Y inputs does the wiggle remain in the horizontal?

[allenwp]

As shown in the video, the wiggle is always on the horizontal. I haven't attempted to disassemble the scope at all: https://youtu.be/Di9QhIRAa9s

[Jwillis]

Transporting equipment can cause loosening of connections. Many times you can fix issues with analog scopes by re-seating connections inside and giving the pots and switches a good cleaning with Deoxit or equivalent . If you can't return the scope then that would be a good start . 

[allenwp]

Thanks for your thoughts on this! I just managed to nab a Tektronix 2445 4-channel 150 MHz that has 2V full blanking on the Z-input. Apparently the CRT is in good condition and it's fully tested. I'm in Canada and very happy that it's being shipped from the same province that I'm in. I managed to get it for $430 CAD, which is a tad cheaper than this new one that has the wobble on the X channel, so I'm going ahead with returning the new one and from what I've heard I should be happy with this good 'ol Tektronix scope.

Thanks again! I'm really happy to have everyone here to bounce these thoughts off of! <3 <3 <3

[james_s]

That was a *new* analog scope? Hmm I wonder if it was old stock? Is there a manufacturing date on it? I suppose it's possible somebody in China is still building scope CRTs, or maybe they found a stock of them.

That seems like a lot of money for an analog scope but I know that stuff does tend to cost more in Canada. I've picked up a handful of analog scopes for free over the years and given away a few of them. It's possible that supplies are drying up though.

[allenwp]

Hmm, there doesn't seem to be a manufacturing date, but it has RoHS is printed on the back along with all the other specs, so it can't be too old. There are a ton of the exact same scope, same model number (L-212), but with a different brand label on the front that all come from China and are sold through Amazon. The one I got was the only one "made" by a company that actually had a website and manual associated with them. From a first glance it seems like a clone of the Hitachi V-212 ¯\_(ツ)_/¯

I was about to say that it's hard to get a scope that has everything I need for less than $300 CAD after currency exchange, shipping, and customs fees, but I did just find one pop up on eBay for $215 CAD (base price of $100 USD, which is $215 CAD after everything) which is a really good deal from what I've seen. I'd like to have this ASAP, and I've got a sizable budget for the project, so I'm just happy to get something from inside Canada that can arrive quickly and not go through the hassle of customs, etc. Honestly, having a 2Vpp Z-input is worth a lot to me because this now means I don't need to amplify my DAC to get full blanking I'm a software guy, not a hardware guy

[james_s]

Yeah the analog blanking is kind of a niche thing.

That video is impressive, I've always loved vector graphics. If you ever have any interest in trying to code for one of the vintage vector arcade platforms I have some vector games running on FPGA boards and a guy I know has been busy recreating some others, last I talked to him he was looking for someone to create a new game for the Atari Star Wars hardware. The monitors use magnetic deflection so you can't move the beam around as quickly as you can with a scope and the 40 year old hardware pretty much mandates assembly language.

[allenwp]

That's really cool! My personal interest currently lies exclusively with exploring what is possible using vector displays and modern high-end signal generation (a powerful modern PC with a high sample rate, high resolution DAC, like the 192 kHz 24bit 4 channel audio DAC that I'm using right now). So I don't think I currently have an interest in working with any of the signal generators of the past...

That said, I've got an old Vectrex sitting right beside me that I would like to wire up to my DAC at some point to see what I can do with a magnetic deflection screen... This is many months away for me, but please let me know if you have any connections with people or groups who have experience hardware hacking Vectrex displays! (I know a number of people have, but I think a lot of them use the built-in signal generator rather than wiring up their own...)

My end goal is to create games for festivals and expos. For example, the Alt.Ctr.GDC showcase at the Game Developers Conference. For this purpose, small vector displays like the Vectrex or an oscilloscope are ideal because I can take them as carry-on on a plane

[james_s]

I do actually, I know a fellow named Fred Kono who has done just that. https://hackaday.io/Thedreamarcade

He eventually built a new deflection board to drive the Vectrex tube as it was found that the original audio amp IC based deflection circuit was just not fast enough to display arcade games properly.

[allenwp]

Ideal! Thanks!

[bdunham7]

Just FYI, you can still buy a new CRO from a somewhat reputable manufacturer, but they aren't cheap.

https://www.bkprecision.com/products/oscilloscopes/2160C-60-mhz-analog-oscilloscope-with-probes.html

[allenwp]

Yes, that’s the other one I saw on amazon that seemed to be new stock — from looking at their website, their lower end models have no z-input, so I can see how it’s a lot cheaper to get a few used ones for the price of one new one from BK Precision.

Thanks!

[james_s]

MSRP of $1200? It's like that page was created in 1995 and never changed.