[R-390] AGC problems - need help interpreting IF resistance readings

[Dennis Wade]

Well, I think I've isolated the problem to the output side of the
mechanical filter group, so its  either a filter, a mica cap
(unlikely, since the filter would have to be leaking to ground too),
or the switch itself.

I want to go over my reasoning before I start lifting wires off the
filters.  I'm still a novice at troubleshooting (won't be able to use
that one for much longer).

I lifted the filter end of the 22K, R507 from the rotor of the switch.
 This isolates the grid circuit from the AGC line.  I still see about
120K from the rotor, and pin 1 of V502 to ground.  The pin 1's of V501
and V503 now show infinite resistance to ground.

There is no DC path from input to output of a mechanical filter, correct?

Does what I see and conclude pas the laugh test so far?

           Thanks all,

             Dennis

On 9/5/05, roy.morgan at nist.gov <roy.morgan at nist.gov> wrote:
> Quoting Dennis Wade <sacramento.cyclist at gmail.com>:
> 
> > Thanks Matt.
> >
> > I have the IF deck out of the chassis, so I'm pretty confident the
> > leakage is in the deck.
> >
> > Would a leaky filter otherwise perform normally?
> 
> It might.
> 
>  In order to isolate
> > a filter, do I need to disconnect one or both ends?
> One end is switched by the bandwidth switch..you can test that end by simply
> putting the switch in another position.  The other end may also be switched on
> the  input by the switch and grounded when not selected.
> 
> Study the schematic to dope this out.
> 
>  Is it possible to
> > disconnect all of them at once by lifting the common ground?  (if I
> > can find it).
> 
> Probably.  If you find that the trouble is in fact in the filters, then you can
> go from there.
> 
> Roy
> 
> 
> 
> Roy Morgan
> 7130 Panorama Dr.
> Derwood MD 20855
> 


-- 
Dennis L. Wade
KG6ZI
Carmichael, CA