Is My Scope Working Properly

[jhaas]

I'm a complete newb when it comes to oscilloscopes. I bought a Tektronix 453 quite a long time ago and am just getting around to learning how to use it.

I'm having trouble getting much "height" to the waveform, so I'd like to know if I might be making an obvious mistake.

The amp under test has a pair of cathode biased 6BM8 power tubes, which are apparently good for about 7 Watts.

I have the probe on the output jack. I'm running a 1KHz sine wave into it using my iPad as the signal generator. It seems to be doing the trick. If I turn the amp up until the wave starts to clip on the scope and back it off, my DMM on the speaker jack measures 6.8V. It is in this state that I've taken the attached picture.

So...what's got me confused is, in order to see this much height to the waveform on the scope, I have the Volts/Div all the way to 5mV. The displayed waveform is about five divs in amplitude, so that results in 25mv. I assume since I'm using a 10x probe, this is actually 250mV, which seems way too low. Given the 6.8V RMS on the DMM, shouldn't I be seeing in the neighborhood of 17V on the scope?

So, do I have a faulty scope, or am I missing something?

-John

[xtian]

Yea, that's weird. But here's how you test. Is there a way you can reliably set your signal generator at 1v p-p? or 0.1v or whatever, and then scope that directly to see if you can get the scope to report the right voltage?

Or, if you have an AC-to-AC wall wart (transformer) that puts out, like 12vac, you can scope that. Hey, you have a guitar amp that has ~6.3vac on the heaters, right? Use that to calibrate your scope.

[katopan]

Those red ends on the dials everywhere are usually for calibration adjustment. Ensure that they are wound all the way up, and they usually click or indent into the end position to keep them there.

[VacuumVoodoo]

At a quick glance I see Horizontal Display Selector set to channel "A" while your probe is plugged into channel "B". Also, kill everything that has anything to do with delayed sweep. You won't need for some time yet.
Check the probe. This scope has an internal calibration signal output, 1kHz square wave, available for checking & trimming probes etc.
Oh, and when everything else fails: RFIM

[Stevem]

Your trace is not across the whole width of the screens you likely have some electrolytic caps drying up in the scope, which is a issue I 've yet to get to in one of mine, so I shouldn't talk!
A cap ESR meter will make short work of the issue with out un soldering a thing most likely .

[mrn1ngstr]

You can find the manual here if you don't have one.

http://w140.com/tekwiki/wiki/453

[jhaas]

Thanks all,

I actually did read the manual and before doing this exercise with the amp I followed the "First Time Operation" procedure connection the 1kHz Cal output to each channel's input, and everything looked good from there.

Connected to the amp, I didn't mention in the OP, but with Channel 1 I couldn't get the trace's amplitude big enough to read regardless of the Volts/div setting. Channel 2 was a little better.

I'll try measuring the 6V3 heater supply this evening, but from the replies it doesn't sound like I'm missing any obvious setting on the scope, so I fear it may have issues.

John

[Paultergeist]

John,

Without pulling your thread off-track, can you elaborate a little bit about using the iPad as a signal generator? Is there a special adapter one can buy to feed the iPad output into the input of a amp (such as "iRig," for example) or did you cobble something together yourself?

Thanks for any insights.

[jhaas]

No problem. No special hardware, and I'm sure this isn't the best solution for a signal generator, but this is the app I used:

https://itunes.apple.com/us/app/signal- ... 41018?mt=8

(from a company called "CatEater" it's got to be good!)

I plugged a 3.5mm stereo to RCA cable into the iPad headphone jack. Then connected an RCA to 1/4" cable to the amp input. The unused RCA jacked needed to be grounded and it was good to go. I ran the iPad volume at 100%, but IIRC, from when I measured this before, the output is still less than 1V, but for my purposes it works fine.

I'm sure a dedicated signal generator would be better.

I'm curious how folks think this setup would work for tube amp work:
https://www.parts-express.com/velleman- ... y--370-402

-John

[martin manning]

You don't need an AC signal to check the vertical inputs. A super simple test would be to select DC coupling and an appropriate V/division, and then put the probe and ground lead across a battery. The trace should hop up accordingly, and you can compare that result to measuring the battery voltage with your multimeter.

[jjman]

The displayed wave in the pic is about 1 div (not 5.) You count the horizontal lines as 1 div, at least on my Tektronix model.

If the probe has the "tang" on it you do NOT multiply what you see on the screen by the probe's X factor. The "tang" is a spring loaded small tip that touches the metal surrounding the scope's bnc connector. Some 10x Tek probes have it ("coded") and some don't.

Actually I have a chart I compliled and I see you have the P6108A probe and my chart says you do not have the tang and would therefore need to multiply by 10.

[Mescalero]

Your trace is not across the whole width of the screens you likely have some electrolytic caps drying up in the scope, which is a issue I 've yet to get to in one of mine, so I shouldn't talk!
A cap ESR meter will make short work of the issue with out un soldering a thing most likely .

If you look at the "Sweep Length" knob, ch. A sweep length is not set to "Full." That is why it doesn't fill the screen.
As for vertical deflection, did you look at the scope's calibrator signal in AC-coupled mode? If it looks right when DC-coupled but not when AC-coupled, the vertical input section may be in need of some loving restoration.
For extreme Tektronix guidance, there is a yahoo groups Tek forum and also some very competent help at antiqueradios.com in the Test Equipment forum. Both are free of charge.

[Mescalero]

BTW: if there is any DC offset, perhaps due to some cosmic GNFB connection or whatever, then some DVMs may read incorrectly. Also, just to give you a heads-up in future work, most DVMs freak out when RF (such as from oscillation) is present.

[gingertube]

See the "1kHz CAL" output far right 1/2 way up (BNC Socket).

This is an output provided to be able to check your scope.

poke the probe tip into the centre of that BNC connector (or use a BNC to BNC cable) and you get a 1V pk to pk, 1 kHz square wave.

Cheers,
Ian

[martin manning]

From above:

I actually did read the manual and before doing this exercise with the amp I followed the "First Time Operation" procedure connection the 1kHz Cal output to each channel's input, and everything looked good from there.

Connected to the amp, I didn't mention in the OP, but with Channel 1 I couldn't get the trace's amplitude big enough to read...

When you were connected to the cal signal did you use the entire probe set up same as when connected to the amp??