[SRv2]

Hey everyone,

I need some help please with a signal generator usage issue.  I am learning and practicing signal-injection and getting to know this new Siglent signal generator. I have made progress using it and I can inject a signal at the top of the volume control and get a tone out on the speaker.  In that case my test cables are set with the ground clipped to the metal chassis of the unit under test.  And the positive probe is clipped to a .22 mf electrolytic cap, then the other end of the cap lead I touch to the volume control.  That works fine.

But if I try to inject into the grid of the IF amp, as soon as I touch the tip of the cap to the tube grid pin, I trip the GFCI outlet that my entire bench set up is connected to.  I have also tried connecting the negative black lead to B- and as soon as I touch the lead to B- the GFCI trips again.  

I am using an isolation transformer for the unit under test.  

I get that the signal generator has a 50 ohm resistance to ground, so I am guessing as soon as the GFCI outlet senses current flow on the ground it is tripping my bench.  But I feel like I am just making a rookie mistake in how I am using the signal generator.  Do I need to isolate the ground also with a capacitor?  

And is there a more elegant way to do this than using a capacitor?  How are you all making the connections?  Is there some BNC device that can connect between the signal generator and probe set to more easily do the isolation of the DC voltage?

If this is a rookie mistake, feel free to give it to me, I have thick skin.   I am just trying to learn how to use the signal generator more effectively.  Also trying not to blow out a channel on this new piece of gear.  My goal is to use signal injection methods of troubleshooting (whereas I have only used signal tracing with a scope up to now), and to use the signal generator for calibration/adjustment once I am more familiar with it.

And just to complete the picture, the unit under test is a 46-350.  The unit works and powers on and I have can tune stations and play them on the speaker.  There is some 60hz ripple/hum because I haven't replaced all the caps yet.  But this is my test unit that I often use to try out new test equipment since I know it works and I can use it for troubleshooting.  

What am I doing wrong?

Thanks,
George