[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

[DaleHCook]

What make and model of isolation transformer are you using? Just because something calls itself an "isolation transformer," especially newer ones, does not mean that it actually is one.

[SRv2]

Hi Dale, Hmm, that could be the problem? Here is a link to the model purchased on Amazon. If this is not an isolation transformer and only a variable transformer, what new model is recommended? And do you see other issues with my process?

[DaleHCook]

That is not an isolation transformer - it is a variable autotransformer. It provides variable line voltage to the device under test, but it does not provide isolation from the power line and ground. It sounds like you need an isolation transformer to eliminate your problem. Note that only the device under test (your radio) should be plugged into the transformer. Your signal generator and any other test equipment should be plugged into non-isolated outlets.

Some of us use a vintage isolation transformer. I use an RCA-Viz TV Isotap Model WP-26A from the 1960s. Note that the rather crudely modified power cord added by a previous owner has since been replaced.

[Image: https://plymouthcolony.net/starcity/radi...wp-26a.jpg]

[462ron]

Hi George, so when you connect the ground lead of the signal generator to the radio chassis that doesn’t kick out your GFCI but touching the .22ufd cap to the control grid does!  That indeed is strange. If your new generator has a 3 prong plug with ground, try using one of those 3 prong to 2 adapters and as mentioned earlier, make sure your isolation transformer is indeed a true isolation! You can ohm it out to see there is no connection between the AC power input leads and the output!

Ron

[SRv2]

Hi Dale and Ron, 
Looks like I am going shopping for a proper isolation transformer. Definite rookie mistake on my part, thanks for drilling down on that. Once I have that does it sound like my process is good otherwise? The new generator just has bnc outputs so I’m trying to think of a better way to add the capacitor inline when signal injecting. Any tips there are appreciated.
George

[morzh]

does 22mF stand for "22 microfarad"?
If yes, change it for 22nF.

[SRv2]

I am using a 0.22uf cap, but I can change that to your suggestion, thanks.  Any tips for how you connect that to your signal generator?  Are you using some jumper cables, or do you literally touch one end of the cap to the grid or injection point, with the other lead on your positive probe?

[morzh]

220nF is probably fine. At RF frequencies 10-20nF is plenty.
It is connected, usually, GND to chassis and central via a cap to your point of destination. Sometimes the original documentation of a DUT asks to leave the local circuit (like a resistor from the grid cap to GND) in place; sometimes they ask to remove it.

There are many culprits: leakage in your IsoTap and such, but if your radio is AC/DC with hot chassis (or chassis chopped via a 0.1cap from the outlet's pin it is connected to), you could have, especially with a large cap, like 220nF (this is in fact huge) a significant current imbalance in your GFI and it will get tripped.
There are various ways for the current to go.
Ideally it should not, but then I do not know your equipment and the integrity of it.

A smaller cap of 10nF (this is what I often use) might (I am not saying "will") help, but you should investigate nevertheless as the leakage is never welcome.

One of the leakage paths is a leaky tube you are trying to work with, or ... some RF coild connected from your grid to chassis and then via that 0.1uF to the outlet.....not sure.

And do check your IsoTap for isolation.

[SRv2]

Thanks for the feedback and help. I have a new (old) isolation transformer on order. And I’ll use a 10 nf cap. I’m slowly getting my bench built back up, 40 years later. Blowing the cobwebs off my memory, so I do appreciate the help.  George