[R-390] Filter repairs

[Graham Baxter]

Hi Rich

With the radio cold (so the valves don't conduct) unplug the multipin
connector from the IF module.

The pins are numbered with tiny numerals (at least the sockets are).
Look for pin number six. Using your multimeter on its highest
resistance range, measure between pin six and ground. Ideally the
positive lead should go to ground and the negative to pin six. You
will see a low resistance reading initially. As the capacitors charge
up it should go higher than a standard multimeter can measure.
(Usually > 10 megohms)

If it passes this test, you can either be happy, or if you are like me
perform a more rigorous test. I measure mine using a high resistance
meter which has a 250 v measuring potential. With this, I am seeing
around 50 Megohms for the entire IF AGC line.

If you have a low resistance, less than ten megohms would probably
start to have a noticable effect, the next step is to remove the IF
module. Disconnect R507. Pull V502. Measure between pin 1 and ground.
Any leakage here can only be a filter or possibly C512.

If you have leakage, remove the filter can. C512 is easy to disconnect
so it is worth checking first. It is the disk ceramic close to the
2kHz filter. If you still have leakage with it disconnected, it is
down to one or more of the filters.

To decide which filter(s), you have to disconnect them one at a time
and measure them individually. Individually I am seeing about 200
megohms at 250V .

The thing about the leakage is that it is not ohmic, in the sense that
it might be 10 megohms at a volt, but 100 kilohms at 100 volts. So it
is difficult to say what is a problem and what isn't, but there
shouldn't really be much leakage at all.

Good luck

Graham


 18/08/07, rbaldwin14 <rbaldwin14 at nc.rr.com> wrote:
> Can you give me a method to determine if my filters are leaking.  Not
> working at all is easy to figure, but I am not sure how to measure or
> quantify the leaking situation.
>
> Thanks
>
> Rich Baldwin
> KD6VK
> ----- Original Message -----
> From: "Graham Baxter" <graham at delphe.co.uk>
> To: "r-390 at mailman.qth.net" <R-390 at mailman.qth.net>
> Sent: Saturday, August 18, 2007 12:40 PM
> Subject: [R-390] Filter repairs
>
>
> > Hi all,
> >
> > Thank you for your kind comments and supportive feedback. I think I am
> > now happy with the latest version of the filter repair article. So,
> > major spelling mistakes excepted, you can consider it stable.
> >
> > The latest version is at:
> >
> > http://www.delphelectronics.co.uk/filter390a/
> >
> > Please refresh your browser and see what you think.
> >
> > If anyone has a proper R390A site and would like to host this, please
> > let me know. I am a bit limited for space.
> >
> > Would you believe I had a third faulty filter? I had not noticed this
> > before because although faulty, the only symptom was blow-by
> > manifesting itself as twittering noises on SSB. I didn't replace the
> > coils in this one, just a good clean, new foam and a new earth strap
> > cured it. I have taken the opportunity to add a couple more photos
> > courtesy of this filter.
> >
> > Take care
> >
> > Graham
> > _____________________________________________________________
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>