[Test-Equipment] Problems debugging LCR bridge

[Barry]

I'm still working on my Marconi TF1313 LCR bridge.  The problem it's exhibiting is in R mode, I can't get a deep null reading like I can in C mode.

I have traced this to a 60 cycle signal that surfaces in R mode even when there is no signal coming from the output of the bridge.

This model uses an EF86 for the first amplifier followed by two more stages of amplification that use both halves of a 12AX7.

I have disconnected everything except the AGC line from the grid of the EF86 and have disconnected all other connections from the bridge (one coupled to the plate of the EF86 and one to the input of the 2nd amplifier stage) to the amplifiers.

When in C (or L) mode, the output of the 3rd amplifier shows a very small 1kc (or 10kc depending on the bridge frequency setting switch) at the output of the final amplifier.  This small signal, once detected, is too small to cause any reading on the meter.

When in R mode, though, there is a significant 60 cycle signal that, once detected, is sufficient (about 4v peak-to-peak before detection) to move the meter well off the NULL point.  When I'm measuring resistances, this makes for weak null indications and that's what I'm trying to fix.

With the only thing connected to the grid of the first amp (the EF86) is the AGC line (that is sourced from the detector via a 10M resistor), I can't figure out why the 60 cycle signal is introduced in R mode and not in C or L mode.

I assume that with nothing else connected to the input, then (in a perfect world) the output should be zero.  With nothing but the AGC line connected, even in an imperfect world, I don't understand why I see the 60 cycle signal in one mode and not the others.  If it were heater noise, then I assume I would see this in either mode.

Any advice as to what to look for?  The heater of the EF86 has a hum balancing pot and I can somewhat minimize the 60 cycle noise with that, but I can't get it to disappear.

Sorry this ramble a bit but I've done several things to try to isolate the source of this stray signal and wanted to try to paint a clear picture of the current state of the amplifier/detector.

Thanks!

Barry - N4BUQ