[R-390] Symptoms of a mechanical filter failing?

[dog]

What I have found actually happens is you loose AGC action first. Notice 
the filter output is directly attached to the AGC line. Any resistance 
to ground will really screw with that AGC action and that is what I 
found when defective, there is a definite resistance to ground from the 
coils in the filter which you don't want. The insulation degrades over 
time. I rebuilt 4 of mine to some degree of success, but it's not 
perfect. The other thing that happens is the magnetostrictive wire falls 
off. That's nearly impossible to put back on, but I've done that too but 
not with that hair of a wire they use. Another large problem is you 
really need to take the filter out of the circuit to measure anything. I 
used my Fluke 189 in the nS (nanoSiemens) mode to measure that coil to 
ground resistance, it seemed to work. Good luck with it, if you need any 
hints, I'll be glad to help. I have some info on testing them out of the 
circuit also.

Dave N3DT

On 7/12/2019 08:48, jgedde at optonline.net wrote:
> G'Morning,
>
>   
>
> On my R390A, the 2 kHz bandwidth is acting weird intermittently.  With a
> strong signal it works fine, but it works strangely with weak signals.  I
> get a high frequency oscillation or the radio loses almost all sensitivity.
> It's kind of like a squelch if a detune a carrier.  It goes from receiving
> the station just fine, to a squeaky oscillation or no signal at all.
>
>   
>
> I'm thinking it could only be the filter or the BW switch, but I'm leaning
> toward the filter.  Whaddaya all think?  Is it time for a foray into the
> guts of my 2 kHz filter for repair?  That'll be a first for me, but
> thankfully a repair procedure has been well documented online.
>
>   
>
> Thanks!
>
> John
>
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