Is My Scope Working Properly

[jhaas]

Thanks again for the added suggestions. It's been a busy week, so I haven't spent much time with it, but I did take a few minutes to do the 9V battery test, as well as the 6.3V AC filament supply test. I wasn't able to get a trace with either. I'm suspecting the scope needs some servicing.

Martin, there was no probe involved with the cal signal test, it's just a BNC to BNC cable connection. I guess it's possible the issue may be with my probe. It was NOS when I got it - the package had never been opened.

[martin manning]

Sounds like something is wrong with the probe to me.

[jhaas]

Well... last night on eBay I bid on and "won" a Tektronix 2213 for about $78 shipped, which comes with a probe. So, between two scopes and two probes, hopefully I'll either be up and running or learn that I truly don't know what I'm doing.

John

[martin manning]

Measure the resistance from the probe's tip to the center contact of the BNC. You should see 9M. If that checks out then the connector is not making contact for some reason.

[jjman]

I have a probe that is intermittently low reading. Moving the wire around resolves it but only until it moves again to the wrong position.

[LOUDthud]

Well... last night on eBay I bid on and "won" a Tektronix 2213 for about $78 shipped, which comes with a probe. So, between two scopes and two probes, hopefully I'll either be up and running or learn that I truly don't know what I'm doing.

John

An X10 scope probe is sensitive to the input capacitance of the scope it is connected to. There is a small variable cap inside the little box at the end of the cable where the probe connects to the scope. This needs to be adjusted or checked any time the probe is moved from one scope to another. Touch the tip of the probe to the 1KHz square wave and adjust the trimmer cap to get nice flat tops on the square wave. Check the amplitude of the signal on the screen. The scope should be set to 20mV/div (200mV/div with the X10 probe) and the square wave should be 5 divisions on the screen. Set the timebase to 1mS/div and check that you get one cycle per division horizontally on the screen.

[jhaas]

Thanks for the tips. I did have an issue with the way my probe was assembled. Under the spring-loaded clip piece there was a small sleeve that I think was preventing good contact. I removed that and I'm getting a much better trace.

I connected the probe to a new 9V battery that measures 9.8V with my DMM. Switching from GND to DC causes the trace to rise an equivalent of 1V. Does that translate to 10V since I'm using a 10x probe?

Next I connected the probe to the 6.9V (measured) filament supply in an amp. I've attached a picture of the resulting trace. Amplitude of 4 divs at .5 V/Div results in 2V. (or is this 20V given the 10x probe?)

Either way I can't make 6.9V from what I'm seeing on the trace.

As you can see I removed the plastic cover on the probe to get at the adjustment LoudThud mentioned. There are two tiny adjustment screws in there. Adjusting them made no noticeable difference.

Any other ideas?

[JazzGuitarGimp]

Your numbers look good. You are correct that since your probe is 10:1, 2v peak to peak on the scope is equal to 20v peak to peak at the probe tip. Now, as far as the filament supply measurement goes, your meter is showing the AC RMS voltage, but your scope is showing you peak to peak voltage. To convert peak to peak to RMS, divide by 2.828. So 20V / 2.828 = 7.07 VRMS, which is close enough to your DMM-measured 6.9 VRMS.

[jhaas]

Your numbers look good. You are correct that since your probe is 10:1, 2v peak to peak on the scope is equal to 20v peak to peak at the probe tip. Now, as far as the filament supply measurement goes, your meter is showing the AC RMS voltage, but your scope is showing you peak to peak voltage. To convert peak to peak to RMS, divide by 2.828. So 20V / 2.828 = 7.07 VRMS, which is close enough to your DMM-measured 6.9 VRMS.

ahh, I forgot the whole RMS vs. Peak-to-Peak thing. Thanks Lou! I still have a lot to learn, but now I can finally start using this thing.

-John