[Test-Equipment] Problems debugging LCR bridge

[Barry]

Bob,

I had thought about that and may try it.  What's odd, though, is if it's a filament problem, why it only happens when the bridge is in R mode.  With all the inputs disconnected from the amplifier stages, I can't understand why that makes a difference.

73,
Barry - N4BUQ


 On Mon 23/04/12 11:55 AM , rbethman rbethman at comcast.net sent:
> 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
> >
> >
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