[Collins] Two More 51J4 Questions

[antqradio at sbcglobal.net]

________________________________
 
Grant

If you measure the resistance of the mechanical filter coils and measure approximately 50 ohms then the transducer coils are OK.  If you measure an open then either the wire connecting to the terminal is broken (see #2 below) or the winding has opened.

There are two remaining issues internal to the filter.  
1) A weld between the coupling wires and the resonator disks could be broken thus changing the filter characteristics.  
2) The foam material that isolates the transducer coils from the filter housing has turned to goo and has increased the filter loss.

#1 is catastrophic and there is nothing to be done but replace the filter.  If it is #2, the filter can be rebuild.  All it takes is patients and time to disassemble the filter, clean out the goo with alcohol and replace the foam.  There is at least one site on the web which shows the process: http://www.delphelectronics.co.uk/filter390a/

It is my opinion that the #2 failure mode will happen to all mechanical filters eventually.  They weren't made to survive as long as they have, just like the radio receiver that uses them.
Jim


________________________________
 From: Grant Youngman <nq5t at tx.rr.com>
To: "Collins at listserve.com" <collins at listserve.com>; "<collins at mailman.qth.net>" <collins at mailman.qth.net> 
Sent: Friday, August 23, 2013 10:33 AM
Subject: [Collins] Two More 51J4 Questions
 

1.  I could swear that this issue did not exist when I started work on the radio 2 weeks or so ago.  

I now find a 17dB difference in gain (well,
 ok, loss) between the 6 KHz filter and the 3.1. After aligning the radio this week, it seemed "deef" with the 3.1 filter selected.  Measured the overall gain with the 3.1 filter to be -17dB  relative to the 6 (and the F500F94 also).  The difference follows the filter, so it isn't a fault in the filter module.

Would I be correct in assuming that there should not be this much difference?  I have another 3.1 filter on the way -- perhaps it will have better characteristics.

2.  In comparing early and late schematics I found a difference in the way the filters are terminated in the filter module.  The "early" radio (which I have) has a single 240 pf capacitor that switches in parallel with all of the filter inputs.  There is a corresponding 240 pf terminating capacitor on the outputs.  I presume these two caps roughly resonate the input and output windings.  

The "late" schematic shows
 individual small value caps across each filter input and output.  These switch in parallel with a fixed 200 pf cap to yield input vales of 205, 208, and 212 pf on the 1, 3, and 6 filter positions respectively, and 212, 215, and 218 pf on the corresponding outputs

I'm at least contemplating pulling the filter unit again and making these changes.  Anyone with 51J4 experience have a thought about whether it would be worth the effort?  Are these values critical?  Collins must have decided it mattered somewhere along the line I suppose ...

Grant NQ5T

Sent from my iPad
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