[Test-Equipment] Problems debugging LCR bridge

[rbethman]

Barry,

Try - (Just for trouble shooting purposes) - use a DC filament source.

See if the 60HZ is still there.  You could place 4 D-cells in series 
just for this quick way to see if this stops it.

Bob - N0DGN

On 4/23/2012 12:00 PM, Barry wrote:
>> 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
>
>