Tektronix 460A oscilloscope problem.

[stratogazer]

Hi, all. Hope you can help me. I have a Tektronix 460A scope I bought used. All self tests pass. all waveforms generated internally look as they should.

The problem occurs when I try to measure a waveform made by something else - a battery operated voltage controlled oscillator for example. The VCO generates triangle and square waves. I see the wave form but SUPERIMPOSED ON TO A 60 HZ SINE WAVE.  In fact, everything I try to measure is on top of the 60 HZ sine wave.

I replaced the input filter. This had no effect. I inspected the power supply PCBs. There is no obvious signs of damage. The house AC is well grounded as is the scope. What else should I check??

Thanks in advance.

[SeanB]

Probe ground lead open circuit? Check continuity of the probe to the scope case, and from the tip to the centre set to X1.

[stratogazer]

I checked the probe continuity as you suggested. The ground clip has good continuity as does the probe tip to the input connector inside. The ground clip has continuity all the way back to the AC socket.

Any other ideas? Appreciate all suggestions. Thanks in advance.

[TerminalJack505]

You are connecting the ground clip to your circuit's ground, right?  And have the scope switched to DC coupling?

[stratogazer]

Yes, I've tried connecting the ground clip to earth ground, circuit ground, case ground, any ground I can reach - same result.

 About the DC coupling, I'm not sure. I'll have to check to make sure.

Thanks very much for the suggestion.

[stratogazer]

Thanks TerminalJack! That was exactly the problem! When the 460A powers up the status page indicates that it is DC coupled. HOWEVER! Upon selecting the "triggering" page I found the scope was, in fact,  using AC coupling. When I switched it to DC coupling the scope worked as it should. Pays to double check. Thanks again for the solution.