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

[David Wise]

Do you have the early production type of deck where the filter input and output are tuned by fixed molded mica caps?  I had one go intermittent.

HTH,
Dave Wise
________________________________________
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> on behalf of jgedde at optonline.net <jgedde at optonline.net>
Sent: Friday, July 12, 2019 6:42 AM
To: 'dog'; r-390 at mailman.qth.net
Subject: Re: [R-390] Symptoms of a mechanical filter failing?

I don't seem to have any AGC problems and whatever the root cause is it
comes and goes.  Mostly goes.  I pulled off the cover and connected a scope
to see what could be going on and everything is working fine now.  It's so
hard to fix a problem that is so intermittent....  Maybe for now, I'll pull
the IF deck and make sure the switch contacts are nice and clean.

John

-----Original Message-----
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> On
Behalf Of dog
Sent: Friday, July 12, 2019 9:22 AM
To: r-390 at mailman.qth.net
Subject: Re: [R-390] Symptoms of a mechanical filter failing?

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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