Is this oscilloscope worth buying?

[Frankentronics]

Greetings,

I am considering the purchase of an analog oscilloscope. But there is a problem. Here is a photo.



The image shows the same triangular wave on CH 2 as is displayed on CH 1. But obviously it is not displayed correctly.

I am guessing it is very likely that the only thing wrong is a capacitor that is charging and discharging incorrectly. If that is the case, is that a faulty capacitor that I am likely to find with an ESR meter (which I still have to build, BTW)?

Thanks...

[hexreader]

I notice that the volts/division settings are different, so maybe some attenuation as well as distortion.

When I had a similar problem, it was a bad switch wafer that was the cause.

Switch replacement was not practical, so I ended up throwing the scope away.

Not saying that this is the problem with your scope, but switches tend to be less reliable than capacitors.

If the scope is cheap enough, then it might be worth the risk of buying it as a single-channel scope. Who knows, you might get lucky and fix channel 2. Perhaps a bad solder joint...  a bit of switch cleaner might fix it short term EDIT - NO DO NOT - SEE BELOW ....   or it might be beyond repair altogether.

I am no expert on scope repairs though. The risk is entirely yours.

I recommend waiting for other replies, as there are far cleverer people than me on the forum.

[ataradov]

Whether is it worth buying depends on the price. For most modern applications (MCUs and digital circuits), analog scopes are mostly useless.

In this case the channels also have different VOLTS/DIV setting, so there is also an amplification difference between the channels.

Old analog scopes have very high chances of fixing the issues, but you will need to remember that you may have to work with a high-voltage device in its on state. And if you don't have experience with that, you may be zapped once of twice, hopefully not in a fatal way

And having another scope for fixing this one will help a lot in some cases.

I would not pay more than $20-30 for this, even if I was desperately needing a scope.

[capt bullshot]

To me, this doesn't look faulty. It appears someone set up a triangular and a square wave to show the oscilloscope is working.

Otherwise, the square wave could be a differentiated version of the triangular wave. If so, its amplitude should vary with the frequency setting. Might be a defective probe or whatever. How is the function gen connected to the scope?

[Frankentronics]

Thank you all for your replies.

To clarify, CH 2 is displaying the exact same triangular wave, but it is completely distorted.

Yeah, I think I'll just pass on this scope. Fixing scopes is not exactly a project for newbies such as myself. And as ataradov pointed out, I would be working with high voltage (locked up alone in my basement).

It is a "good price" but perhaps it falls under the category of, "I can't afford to buy cheap".

I did find another scope for $100 cash (local pickup). This one. And working correctly, comes with probes and power chord.



Would this one be a good deal?

Thanks...

[ataradov]

$100 may be a fair price, but it all depends on what you actually want to do with the scope. You may still need to go over all the switches with a contact cleaner. BNC connectors also look slightly oxidized, which is typical for the old equipment, they will probably work fine anyway.

I would replace the probes with modern ones. 100 MHz probes are very cheap (less than $15 delivered), and they they will work and feel better than the old Tek probes.

[Frankentronics]

Thanks, ataradov.

I mostly want to work on audio equipment. Learning to build amplifiers, especially battery powered guitar amps.

I actually do have a 20MHz, 2 CH scope, but the image is skewed (for example, a square wave is tilted backwards at around 85 degrees, instead of being 90 degrees). So, I thought I might as well just get a better scope. Either a decent analog one, or spit out the price for a digital scope. But I'm not sure I can justify the price of a new scope, being that I'm just starting to learn. OR perhaps I should just try to fix the skewed image on my existing scope. But I wouldn't even know where to start.

What do you think...?

[ataradov]

For pure analog audio stuff, analog scopes are fine. It is probably the only application where they can still be useful.

Do you have a picture of the distortion? It sounds very strange, almost like there is some inter-dependency between the vertical and horizontal amplifiers.

What happens if you apply a negative voltage (just reverse the ground and the tip of the probe)? Does it still go back? Or leans even more forward?

[rhb]

DO NOT SPRAY TUNER CLEANER ON THE ATTENUATOR SECTION SWITCHES!!!!

The Tek manual for the 465 has an explicit warning about this. I learned this the hard way by doing it before I was able to get a service manual.  I washed everything *many* times with 99% isopropyl.  But read the service manual before doing any servicing, even if you think it's trivial.

As for the "analog scopes are useless" I doubt that anyone saying that has fixed much equipment.  Analog scopes have limitations, but so do digital scopes. I have both.

[tooki]

DO NOT SPRAY TUNER CLEANER ON THE ATTENUATOR SECTION SWITCHES!!!!

The Tek manual for the 465 has an explicit warning about this. I learned this the hard way by doing it before I was able to get a service manual.  I washed everything *many* times with 99% isopropyl.

Out of curiosity, what’s the effect of using the tuner spray on them? (And did the IPA rinses resolve it?)

[Frankentronics]

Thanks for trying to help, ataradov.

I damaged that old scope 2 days ago. Here's what happened.

I was probing the main's voltage on an amp power supply, to see if there was noise. I touched the probe to one of the AC in connections. Then the ground alligator clip that was hanging off the probe momentarily touched the other AC in terminal. There was a spark and the scope only displayed a line that was slightly curved and tilted at about 10 degrees. The scope would not longer display anything else. Just that same line, unchanged, on both channels, no matter how I turned any of the dials.

I turned off the scope and opened it, to see if anything had burned or exploded inside. Nothing was visibly damaged, no capacitors bulged. After some time I tuned the scope back on and it was working, but the image was skewed and remains the same 2 days later.

I took 3 pictures. The square wave is from the .5V test signal on the scope itself. If I reverse the ground/tip of the probe I can't get that test signal to show up.

I don't have a function generator, but last week I built a simple sine generator that was displaying a nice sine on this same scope. Now the sine is skewed. If I reverse the ground/tip it is skewed in the same direction, as seen on the photos.

Can you draw any helpful conclusions from this.

Thanks again...

[james_s]

The 2235 is a nice scope, they perform well and are reasonably reliable.

Nothing wrong with older Tek probes. They have one big advantage over generic Chinese probes and that is the probe coding BNC that automatically tells the scope whether the probe is 1x or 10x and sets the readout accordingly. Also you can trust the published specs, with the generic probes you have to rely on testing done by others because they'll print whatever they want on the package.

[hexreader]

Tek scope looks nice, and local pick-up is a bonus.

I would happily risk $100 if that one is as good as it looks.

...  but second hand comes with no guarantees of course....

But then again...  a modern DSO is much more practical, and does come with a guarantee if bought new.

[alsetalokin4017]

1. The 2235 for $100 with local pickup is a fair deal. Test it thoroughly (but gently!) when picking up. Consider this: 100 dollars in today's money translates to about 20 dollars when that scope was new.

2. While the Tek probes may come with the readout pins, this is irrelevant for the 2235, as it does not have the capability of auto-setting probe attenuation. Go ahead and get a pair of the modern Chinese probes anyhow; it never hurts to have spare probes.

3. Don't probe high voltage or line voltage unless you _actually_ know what you are doing! If not using the ground (reference) leads, unclip them from the probes so they can't dangle into anything!

4. Or save up another three hundred and get a modern DSO. But just about any working scope _now_ is worth more than an imaginary DSO sometime in the future.

[Frankentronics]

Well, the 2235 got sold. 

I'd still like to consider an analog scope, so if I can't fix my 20MHz unit, would you say a 50MHz should be the lowest end I should be looking for?

[ataradov]

I'd still like to consider an analog scope, so if I can't fix my 20MHz unit, would you say a 50MHz should be the lowest end I should be looking for?

For audio applications, it does not matter that much.

You may actually even get away with one of those cheap and cheesy 300kHz / 10 MSPS pocket DSOs.

Or something like this https://www.aliexpress.com/item/Jinhan-JDS2022A-Double-channel-handheld-Digital-oscilloscope-20MHz-Bandwidth-200MSa-s-Sample-Rate-automotive-oscilloscope/32804357438.html Super frustrating UI, but actually performs quite well for the price.

[rhb]

DO NOT SPRAY TUNER CLEANER ON THE ATTENUATOR SECTION SWITCHES!!!!

The Tek manual for the 465 has an explicit warning about this. I learned this the hard way by doing it before I was able to get a service manual.  I washed everything *many* times with 99% isopropyl.

Out of curiosity, what’s the effect of using the tuner spray on them? (And did the IPA rinses resolve it?)

It affects the dieletric properties of the front end. I should suppose mostly the switch wafers.  The rise time checked out OK with my Tek 106, but I never had a selective level meter, so I couldn't really test the flatness of the response  properly.  Needless to say, I was quite distressed when I manual warning.

Two questions for the OP.  What's wrong with your 20 MHz scope and where are you?