[MRCA] R-390A antenna inputs

[Ray Fantini]

This R-390A had a bad twinax connector. Wanted to change the entire relay but that was not working out so decided to cut down the outside of the connector and install a transformer to convert the 125 Ohm balanced to a 50 Ohm non balanced connection.

The obvious question is why when there is a unbalanced connection right next to the connector? Answer, the unbalanced input to the receiver sucks! I have a twinax adapter that also will do this and you can hear a considerable improvement using the balanced input with its input matching network compared to the unbalanced input on the R-390A.

At 3 uV was seeing around a carrier meter reading of 25 using the new input network and 15 using the unbalanced input. Before starting by just shorting out one side of the balanced to ground resulted in about the same level of performance using that method and the unbalanced connector so figure I am picking up several Db with the proper impedance matching network.

This is the same receiver I took in for trade on some work I did on another radio, so far may have wasted several hours cleaning and lubricating this mess and now did the antenna input conversion. At this rate if I can’t sell this mess at the Hamvention may have to keep it just because I have too much invested in it!


Ray F/KA3EKH
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