Tektronix 465B square wave not square

[necessaryevil]

Hello,

I'm quite a newbie when it comes about oscilloscopes. I've bought a second hand Tektronix 465B, but I can't get a good square wave from the integrated calibrator.

Since i'm new to oscilloscopes, I'm not sure if it's the oscilloscope or me.

My observations:
*AC or DC coupling does not make much difference (only the vertical position of the trace changes)
*With the 20 MHz bandlimit on the screen is less fuzzy


Some pictures:



With the 20 MHz bandwith limit on:

[Whales]

Only seeing the 'top' and 'bottom' of the waves is normal for analog scopes.  The brightness of the screen phosphor is dependant on how long the scanning electron beam stays over any one area. The 'vertical' parts of the square wave (which last almost zero time) are  drawn by the beam moving vertically very fast, so they are practically invisible.

Distortion of the top and bottom: try using shorter leads.  It may be the inductance of your current ones.

[andrewfernie]

Have you adjusted the compensation on your probes? There is normally a small screw head on each probe that needs to be tweaked to get a nice square wave.   Google "oscilloscope probe compensation" for more.

[necessaryevil]

Andrewfernie,

There is no such thing on my scope probe. Can it be that I have to do the compensation on the oscilloscope? Or do I have the wrong probes?

By the way, the square is 1 kHz.

[cs.dk]

It is possible, that you have fixed 1x probes, they lack the compensation-screw.

Any name/numbers on the probes?

[Shock]

If the probes have a slider switch on them (or are 10:1 probes) there could be a compensation screw adjustment on the bnc connector or on the probe itself somewhere.

[EV]

There is no such thing on my scope.

It is not on the scope. It is on the probe!

[PA0PBZ]

There is no such thing on my scope.

It is not on the scope. It is on the probe!

[necessaryevil]

It says Tektronix, but that's just on the removable top part. I 've found no switches or screws on the probes for so far.

//I typed scope where I meant probe in my previous post. I fixed it.

[PA0PBZ]

There's something weird going on there. If I read it correctly the square wave should have an amplitude of 300mV, yet your screen shows slightly more than 1 div with the attenuator set to 50mV. Looks like 5:1 probes but they are not very common. Evenso, if they are not 1:1 there should be a compensation possibility.

[Andy Watson]

There's something weird going on there.

Those probes are similar to the ones that Tektronix supply matched to one particular model of scope where probe is (as good as) captive. E.g. Tek 222 series.

[T3sl4co1l]

Don't recognize those probes, but they can't be very good if they don't even have simple compensation.  Dump 'em.

Tim

[necessaryevil]

Will those probes work with my oscilloscope?
http://www.ebay.nl/itm/2x-Digital-Oscilloscope-Probe-100MHz-P6100-with-clip-new-/130808400734?pt=LH_DefaultDomain_0&hash=item1e74c9cb5e&_uhb=1

[tautech]

2 Comments:
Tek hook grabbers will fit many other probes, so they may not be Tek probes.
The eBay probes look like the P6*** series and are OK quality and fine for general purpose work.

Edit

I'm quite a newbie when it comes about oscilloscopes. I've bought a second hand Tektronix 465B, but I can't get a good square wave from the integrated calibrator.

One normally would only adjust probe compensation at 10:1 probe settings to correct waveform overshoot/undershoot.

[T3sl4co1l]

Will those probes work with my oscilloscope?
http://www.ebay.nl/itm/2x-Digital-Oscilloscope-Probe-100MHz-P6100-with-clip-new-/130808400734?pt=LH_DefaultDomain_0&hash=item1e74c9cb5e&_uhb=1

I have some of these; they compensate correctly into most scopes.  Don't expect great flatness past 100MHz.  They do extend into the 100s of MHz, so as long as you aren't trying to measure precision settling or anything.

Tim

[miguelvp]

Morton controls has some UNI-T probes under discount, but the 100 MHz are long gone (well there is one but that's a HV 1:100 probe single)

https://www.mortoncontrols.com/index.php/clearance.html?dir=asc&limit=all&order=name

I got the 300MHz ones for just $32 for a pair and I'm happy with them. I believe the 100MHz ones went for around $12 a pair but I don't recall.

Edit: It was $11 for the pair, not that it matters since those ones are gone:
https://www.eevblog.com/forum/reviews/cheap-china-p6xxx-probes-good-bad/msg485886/#msg485886

[David Hess]

I agree with PA0PBZ.  There is something weird going on with the probes.  The levels shown are wrong.

Will those probes work with my oscilloscope?
http://www.ebay.nl/itm/2x-Digital-Oscilloscope-Probe-100MHz-P6100-with-clip-new-/130808400734?pt=LH_DefaultDomain_0&hash=item1e74c9cb5e&_uhb=1

It is difficult to tell from the photo but if those are AideTek P6100 probes, then they will work fine.  They are not quite as good as the Tektronix 100 MHz probes that I have but it takes me a lot of effort to measure the difference.

[Whales]

 My analog scope's vertical amplification board starting playing up once before, but I simply had to clean the contacts on its jumpers.  It is worth switching which of your 2 probes goes into which input and comparing the results

[PaulAm]

I had a 468 that behaved as if all of the attenuation factors were off by a factor of 3 (1 V/div was .3V/div).  That was only on 1 channel though.  Tracked that down to a dirty AC/Gnd/DC coupling switch.

This could be the probes, the calibrator output or dirty contacts in the attenuators.  Not enough info to say which though.

Although I would not recommend it at this point, before a complete diagnosis has been made, cleaning the attenuator contacts is possible.  Use only IPA, lookup  and follow the Tek cleaning procedure exactly.  Be extremely careful.  It's easy to mess up the tiny gold contacts and render them useless.  Do not under any circumstances use any other contact cleaner.

[David Hess]

This could be the probes, the calibrator output or dirty contacts in the attenuators.  Not enough info to say which though.

If the problem is not the probes, then it is likely to be something in common with both channels which would exclude the attenuators and input circuits.

Although I would not recommend it at this point, before a complete diagnosis has been made, cleaning the attenuator contacts is possible.  Use only IPA, lookup  and follow the Tek cleaning procedure exactly.  Be extremely careful.  It's easy to mess up the tiny gold contacts and render them useless.  Do not under any circumstances use any other contact cleaner.

Listen and be warned.

[necessaryevil]

Thank you for your replies.

A few things:
*The volt/division error is definitely the probes. I have used a 'normal wire' with the use of a bnc to banana plug converter. The shape of the wave remained the same, but the amplitude becomes 6 divisions, which seems right for a 300 mV square wave with the knob at 50mV/div.


*There is still something else wrong. Sometimes I see this:


*I'm gonna buy the P6100 probes right now!

[necessaryevil]

I'm still waiting for the P6100 probes, but while waiting I scored a Tektronix 2205 for free. I used the probes which came with my 465B on the 2205, and yes, you were right, It was the probes.

I have to mention that I'm really proud on my new acquistion. Of course I didn't forget to give the guy which gave it away some beer. 

[israel]

try the cleaning the switches "AC-GND-DC" usually cause problems of this type. You will have to remove the cover of the attenuators to reach them.

Regards,

 

[necessaryevil]

I 've got the probes!

With the 20 MHz bandwidth limit on I can get the square wave as it should be. However, with the bandwidth limit off the trace is very fuzzy. First thing I'm going to do is to clean the rotary switches and the ac-gnd-dc (as suggested). Thank you for your replies for so far!

UPDATE:
I've got another question. My squarewave is square now, but it doesn't seem to matter much if I turn the compensation screw on  my new probes. Shouldn't be like this while turning: undershoot-GOOD-overshoot? I have this problem on both of my oscilloscopes.

I feel I have a long way to go.....

[T3sl4co1l]

They're on 10x right?

The trace naturally becomes fuzzier at high bandwidth.  If the noise level of the amplifiers is constant with respect to frequency, then increasing the bandwidth by 4x doubles the noise amplitude.  100/20MHz is a factor of 5, so you can expect the trace width to effectively double.

Tim