[Boatanchors] RME 69 Crystal Filter and Cc/C adjust

[Richard Knoppow]

----- Original Message ----- 
From: "Bill Carns" <wcarns at austin.rr.com>
To: <Boatanchors at mailman.qth.net>
Sent: Sunday, April 14, 2013 7:58 AM
Subject: [Boatanchors] RME 69 Crystal Filter and Cc/C adjust


>I am doing my first alignment of a RME 69 receiver.
>
> I am having trouble reading the schematic in the area of 
> the crystal filter
> but there is apparently some confusing info.
>
> There is not much in the manual about the switch positions 
> for the filter
> selector switch. In the alignment section it refers to the 
> middle position
> of the switch as the "Series" mode position.....fine.
>
> That implies that there is a "Parallel" mode position... 
> ???  Maybe.
>
> There appear to be three positions of the switch.. 
> 10:00, 12:00 (Series)
> and 2:00.   But, in the schematic there appear to be only 
> two positions and
> it is not clear what the wiring is in each position, or 
> which position it is
> shown in..
>
> I want to understand this guy before I leap in.
>
> Also, anyone out there have any words of wisdom for 
> setting the C and C(c)
> capacitors that appear to be a "factory Setting on Band 
> one, but "Should be
> kept to an absolute minimum, so as not to degrade the 
> upper bands.
>
>
> 75 years later, I am not sure I trust the "factory" 
> settings for C and Cc,
> which could have shifted or been messed with along the 
> way.
>
>  Any   ....  HELP !  is appreciated.
>
> Bill
> N7OTQ

     I read over the procedure in the handbook from BAMA. 
It appears that two crystal filter circuits were used at 
different times. The earlier one is shown in the schematic 
that Al Klase posted, the later one in the BAMA handbook. 
Curiously the BAMA handbook seem to have only the adjustment 
procedure for the earlier form which is NOT what is shown in 
the schematic!
     Both are variations of James Lamb's original crystal 
filter. While it allows the use of the phasing adjustment to 
generate a tunable null and also has some degree of 
variation of its bandwidth both adjustments will cause a 
shift in its center frequency and in its gain.
     The handbook method of finding the resonant frequency 
of the crystal is OK, even if you can't hear WLW where you 
are.  The IF of _most_ receivers with crystal filters should 
be set up at the exact crystal frequency. One exception is 
the Collins 51J series because the whole band change and 
calibration scheme assumes an IF of exactly 500 khz. The 
spec on the Collins filter crystal was very tight so this is 
not usually a problem.
     What RME wants you to do is the set up the phasing 
control to exactly balance the capacitance of the crystal 
and holder so that the crystal response is symmetrical. If 
its off somewhat it will cause a null on one side or the 
other and also cause the response on the other side to 
broaden out.  You can often hear the correct position 
because there will be a minimum of high frequency noise.
     There is no coupling cap in this filter as there is in 
National receivers.  In those the coupling cap changes the 
bandwidth of the crystal and also the gain through it and it 
must be compromised according to what you are mostly going 
to listen to, i.e. phone or CW.  The phasing control does 
the same thing.
     There are two variations of the original filter, one 
using a taped IF coil and one using a capacitive divider to 
obtain the outputs for the two sides of the crystal bridge, 
both work the same.
     I think the series/parallel switch probably changes the 
crystal from peaking to nulling, the handbook does not 
address this.
     In 1938 Hammarlund obtained a patent (can't find it) 
for another variation of the crystal filter. In the 
Hammarlund circuit a split-stator balanced variable 
capacitor is used to maintain balance while shifting from 
series to parallel operation of the crystal.  This allows a 
rather wide range of nulling without detuning the crystal. 
They also used a resistive divider on the loading coil for 
the crystal to obtain variable bandwidth.  The range of 
bandwidth with this arrangement is very wide and does not 
change gain significantly.  The same circuit, with minor 
variations, was adopted by Collins and by TMC.

   Its useful to find James Lamb's original articles in QST 
in which he describes the development of his crystal filter. 
I am not sure if the QST web site has articles sorted by 
author, I will have to look.  Its also useful to find his 
original patents.  Google patents should find them or I will 
search though my patent file for them.
   The whole idea of "single signal" receivers for CW was 
quite something in the early 1930's when bands were 
evidently fairly crowded and getting rid of audio images 
made a lot of difference. Lamb was a prolific author and a 
pretty good writer.  AFAIK, National was the first to offer 
a crystal filter in its receivers.  Previous to the crystal 
the best selectivity was gotten by regenerative detectors. 
They can be pretty selective but have a set of vices all 
their own as anyone who has built a regenerator knows.
    Probably RME would have welcomed a Dymo machine.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com