[Test-Equipment] Problems debugging LCR bridge

[Barry]

> I'm not specifically familiar with this bridge but have 
> a couple of general suggestions:
> Is there a half-wave rectifier in it somewhere, for instance 
> where does the DC for resistance measurements come from?
> 60hz hum is often from heater to cathode leaks in tubes. 
> Substituting tubes is sometimes the only way to find this.
> If the AVC system has a fixed bias source see what that 
> looks like.
> Sometimes gassy tubes can cause hum, again the best way 
> to find them is substitution.
> 
> A good scope will help with this.
> 
> 
> --
> Richard Knoppow

Hi Richard,

There are two full-wave bridges.  One is a 6X4 with a CT transformer and the other is a selenium rectifier.

Yes, it has a fixed bias AGC.  I replaced the AGC cap with an electrolytic as I thought that might be part of the problem.  I plan to replace it with a 1uF film cap soon.  While the AGC line could be the source of the AC, I can't figure why it only occurs when in R mode versus C or L.

As I said, I've disconnected all external inputs to the amplifier so switching from one mode to another should not have any effect (yet it does).  If it's a filament to cathode problem, it should show up no matter what the mode.

I thought the selenium rectifier might be inducing something to the EF86 (not likely but I'm grabbing at straws...) so I enabled the selenium rectifuer while in C mode but that still doesn't cause this AC waveform to appear when in C mode.

I also tried shielding the EF86 with no change.

Still can't find it...

Thanks,
Barry - N4BUQ