One of the main differences between the original R390 and the R390A was the use of Mechanical Filters in the 455KC IF chain.  The new filters provided superior filtration characteristics in a small package.  Many receivers of the time such as the R390 and SP-600 achieved multiple bandwidth selectivity through the uses of series tuned L-C filters.  The audio through the older design L-C filters as compared to mechanical filters is superior.  This is primarily due to their lack of phase delay as compared to a mechanical filter.

The new mechanicals filters employed of a series of resonant discs suspended within the filter body by fine wires.  The R390A came fitted with 4 mechanical filter bandwidths of  2, 4, 8* and 16 kc.  The narrow, 0.1 and 1.0 kc bandwidths are achieved through use of a crystal filter in conjunction with the 2 kc mechanical filter.
*(In reality, the filter specification bandwidth is 11 kc as indicated by all versions of the Technical Manual)

 
To the right are examples of mechanical filter commonly found in R390A receivers.  Both versions are made by the Collins Radio Company of Cedar Rapids, Iowa.  The 455 kc IF signal is input to the top of the filters and output through the bottom.  Input/output Impedance matching was achieved in early model IF decks through test selected, fixed value mica capacitors.  Later IF decks used both fixed and variable input/output matching capacitors. Of special note is the mounting flange seen about 1/4 the way up the filter.  When the filter was mounted in the IF deck, the inputs and outputs of the filter are above and below metal chassis of the IF deck.
 
 

To further augment isolation, additional aluminum separators under the chassis combined with the mounting panel of the mother chassis provided RF tight, interior compartmentalization for the filter suite.  As a final step, the mechanical filters have an aluminum cover to complete their shielding.  This superior design is one of the reasons there is almost zero filter "blow by" in the R390A design.  There can be no input/output coupling due to the aluminum IF deck chassis offering absolute input/output filter isolation.

The picture at the left shows an R390A IF chassis with a portion of the side removed.  It is oriented so that the front of the chassis is to the right.   The 455 kc input signal is selected by the filter input ceramic rotary switch seen just to the left of the interior chassis separator and that signal is applied to the top of the appropriate mechanical filter.  Its output is selected by the filter output ceramic rotary switch just out of view behind the flange in aluminum chassis separator.  Both ceramic rotary switches are driven by a common shaft by the front panel, "BANDWIDTH KC" control.  This arrangement not only selects the desired IF filter to be used but also provides inter-filter isolation.
 
Consider the picture to the right showing the input and output terminals of a conventional ceramic filter.  In this design,  no provision was made for input/output, physical isolation beyond simple distance. Thus, it is easy to see how the inputs and outputs of this filter could couple.  This coupling, combined with poor filter specification results in the "blow by" experienced when an off frequency signal is located adjacent to the desired signal.  The strong adjacent channel signal is contained within the 455 kc filter composite and can actually "jump" the distance between the input and output terminal of this filter thus making the interference audible in recovered audio.
 
 
Contrary to popular belief, there were several different manufacturers of mechanical as well as other types of filters used in the R390A.

This is an R390A mechanical filter manufactured by Whitewater Electronics.  While little is known about its specific use in a particular family of IF decks, I intermittently encounter this type of filter when restoring R390A's.  Typically, they are one of the 4 installed filters which leads me to believe the Whitewaters were used primarily as a replacement part versus a regular production item.   (Photo by Permission of Don Heywood, WC4G, Charleston, SC)
 
 
 

 
Dittmore-Freimuth filters were the latest design mechanical filter found in the R390A.  A full suite of these mechanical filters installed in an EAC IF deck is the true identifier of a Dttmore-Freimuth (order number DAAB05-68-C-0040) R390A in cases where the actual authenticity of the manufacturer as indicated by the front panel tag be in question.
 

This is a ceramic filter manufactured by Clevite Industries.  While they are extremely rare, Clevite ceramic filters can be found as original in certain R390A IF decks.  Because of the different electrical characteristics of a ceramic filter, these are not found in combination with mechanical design filters.  This but eliminates speculation that that Clevite filters were used only as replacement parts.
 
 

 
Clevite filters did employ flange mounting in their design as did their mechanical cousins.  Also, the 2, 4, 8 and 16 kc bandwidths were retained. (The Above Clevite Photos credit Jim Walker, Stowe, OH)
 
 

This final image is of an R390A IF deck I encountered during a restoration project.  It was in an R390A which had been custom built by Teledyne for an employee.  In fact, the owners initials have been factory stamped into the serial number plate in place of the normal serial number.  That is a feature one does not see every day.  This R390A was uniquely fitted with modules which were built by the same contractor and had the same serial number.  This IF deck is extremely unique in that it uses a full suite of Clevite Ladder Filters as its IF filters.  Again, the bandwidths are the same as found in a mechanical suite.  However, this configuration is ultra-rare and has never been encountered before by myself or R390A historians Les Locklear and Tom Marcotte.  That this configuration exists certainly lends credibility to the stories that small, contracts for mission specific R390A variants are indeed factual. (Photo by Permission of Bob Luce, W9EFK)

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Revision: 24 December, 2001
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