[R-390] filters

[Bob Camp]

Hi

If you dig into the subject, they designed the filters mathematically. That eliminated a lot of trial and error. The yields were quite good (>90%) on the type of filter in the R-390A. 

Conductive foam is probably not a good idea. Light weight / low density flexible foam is much closer to the original material.

Bob
KB8TQ

On Feb 7, 2013, at 1:01 PM, Norman Ryan <nnryann at yahoo.com> wrote:

> Hello, all,
> 
> I imagine the learning curve for making these mechanical filters at the factory must have been steep.  Even so, the rejection rate must have been considerable, given the delicate construction.
> 
> Sadly, we poor slobs have only one shot at getting the repair right, and we're faced with the problem of finding a more durable replacement insulation material as well.  Anyone have thoughts on an alternative way to close up the filters, say, using two-part epoxy such as JB Weld?
> 
> It may turn out that foam pipe insulation as suggested by Graham Baxter will work just fine as it normally is exposed to heat from hot water pipes and thus ought to last a long time inside a working IF deck.  Heat from resoldering the end caps may be too much, thus my suggestion of epoxy as a possible workaround.
> 
> One of these days I want to undertake Graham Baxter's repair procedure and will share results here.  It likely will be some time down the line, though.
> 
> 73,
> Norman
> 
> --- On Thu, 2/7/13, Cecil <chacuff at cableone.net> wrote:
> 
> From: Cecil <chacuff at cableone.net>
> Subject: Re: [R-390] filters
> To: "Bob Camp" <ham at kb8tq.com>
> Cc: "John Wendler" <wendlerjrv at gmail.com>, "r-390 at mailman.qth.net" <r-390 at mailman.qth.net>
> Date: Thursday, February 7, 2013, 12:21 PM
> 
> We know there is some kind of foam material in them that turns to gum that impedes propagation through the filter that probably accounts for the filter loss seen in these aging filters.  There have been those that have opened them up..cleaned them out and replaced the foam with some positive results.
> 
> I guess we had get good at this since that is where they all are heading unless we find a suitable replacement.
> 
> Cecil
> 
> Sent from my iPad
> 
> On Feb 7, 2013, at 5:45 AM, Bob Camp <ham at kb8tq.com> wrote:
> 
>> Hi
>> 
>> The mechanical filters are made up of resonators. They work by acoustic resonance. You need to be very careful about how you mount them.
>> 
>> Bob
>> KB8TQ
>> 
>> On Feb 6, 2013, at 9:31 PM, John Wendler <wendlerjrv at gmail.com> wrote:
>> 
>>> Would any of the knowledgeable folk on the list care to comment about the
>>> donut material and whether the old material was chosen as an acoustic
>>> isolator to prevent microphonics?
>>> 
>>> 73 de John, N5CQU
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