BAD SCOPE??

[jaunty]

Hi!

SHEESH! Can't catch a break. Okay - well I guess that's what you get when buying used equipment from ebay...! At any rate - the scope is a Tek 2445A 4ch. 150mhz model. The probe is a brand new P6106 probe from a sealed package. After playing with the scope for quite some time to familiarize myself with it, I was utterly unable to capture a useable signal from a music keyboard's KNOWN GOOD headphone output (drives a pair of headphones just FINE). I also was unable to see a good 3-5v signal taken from the analog circuitry inside (yes - ground clip was on etc and both my voltage measuring settings and triggering mode seemed FINE). Granted - I haven't regularly use a 'scope for a LONG time but i'm not a total dummy (I don't think!) when it comes to scopes. The only thing i got on the screen was a slightly irregularly shaped trace (humps over on the left and a bit squiggly) but there were NO changes to it when I changed either my voltage scaling nor my time base (!!) so perhaps there are some slight problems in the CRT drive electronics but it seems to me i'm not getting a signal through. Sorry  - too many words already (and probably the wrong kind too!). Apart from the obvious - is there anything I may not have checked? I checked probe continuity and everything was good. Though weirdly it seemed the probe was way better at picking up RF than it was picking up a healthy voltage directly (!). No - i didn't have my LF limiting filter on...  so i'm kind of confused. Seems like there's SOMETHING in the sensing circuitry blown (?) - All self-checks passed. Both channels 1 and 2 seemed to behave pretty much the same... (scratching my head)  - does this behaviour sound familiar at all?

[KJDS]

What do you get when you connect it to the scope calibrator output?

[jaunty]

pretty much zilch - unless i go down to 20mv/div and x10 mag then just  a tiny little spike in the middle of the display that doesn't move much when you change triggering mode.

[jaunty]

hang on - finally got SOMETHING but only in channel 2... very odd. i'll try to upload a shot.

[jaunty]

hope you can see that

[jaunty]

not exactly the cal signal i was expecting... (scratches head more - but encouraged by getting SOMETHING on the screen) -bizarre that it doesn't work on ch.1 however (despite same settings)

But WEIRDLY i lose track on it (becomes a blur of HF noise) when i depress HF reject. I guess that makes sense since the time scaling is 10ns/div

[jaunty]

okay dropped the time base down to the ms range (100 or so) and AM getting a square wave    though there the rise time seems too fast to register ANY sign of a vertical.. there seems to be significant overshoot too... but no matter... I think those are really tiny flaws..

so does that mean i just need to RTFM more..?

[KJDS]

https://www.eevblog.com/forum/beginners/oscilloscope-training-class-(long)/

[jaunty]

thanks - though I'm afraid that wouldn't teach me anything i don't already know.. i just find it absurdly frustrating that I can pick up a signal from the calibrator but not an audio signal (out of a jack no less - never mind a circuit board). Is a 10M probe going to block or impede small audio signals? Maybe I have the wrong probe (?)

[jaunty]

hmmmm... okay - that would explain a thing or two.... the synthesizer (keyboard) i was working on suddenly has NO output at all... lol .. okay - i'll pick this up again after i get back from vacation next week! thanks again.

[tautech]

okay dropped the time base down to the ms range (100 or so) and AM getting a square wave    though there the rise time seems too fast to register ANY sign of a vertical.. there seems to be significant overshoot too... but no matter... I think those are really tiny flaws..

so does that mean i just need to RTFM more..?

Yep.
Cal signal will be 3 or 5 V @ ~1KHz
SO for a 3V waveform your V/div setting @ 100mV/div will show a 3 Div high waveform. (10x probe)
But thats not all  @ timebase setting of 100 us/div 1 period will be 10 divisions long.

NEVER connect a scope WITHOUT an understanding of the waveform you expect to measure.
Always remember the probe attenuation used and use simple maths to set the scope for the signal amplitude expected.
Overdriving the vertical amps of a CRO could damage your scope.

Got all that?
Now go play with the Cal signal some more until you get a feel for the attenuation and timebase adjustments and report back.

[jaunty]

haha well thanks for the basics. yes that's obvious .. i recall all that stuff from the 1970s with my old dumont scope my dad gave me on my 16th bday... but- i guess you didn't see my last post... there was no signal to measure.

[KJDS]

Yep.
Cal signal will be 3 or 5 V @ ~1KHz

Except that the cal signal on a 2445 changes its frequency dependent on the timebase setting so you always see the squarewave.

[w2aew]

Yep.
Cal signal will be 3 or 5 V @ ~1KHz

Except that the cal signal on a 2445 changes its frequency dependent on the timebase setting so you always see the squarewave.

Exactly right.  Adjust the timebase down to 1ms/div or so in order to do the probe compensation.  And yes, it is NORMAL that the rising/falling edges are so fast relative to the sweep speed that you won't see the vertical portion of the trace.