Bumping this one up a bit.

I recently acquired another -515 and the matching NSD-515 transmitter. My original appeared deaf next to the new arrival, so I began an exhaustive process of troubleshooting. Involving replacement of both mixer JFETs (the U310s I'd mentioned).

We'll cut to the chase: Although the Transzorb (right arrow above) isn't supposed to be conductive at low levels of RF, it apparently is. Enough to knock 30dB or more off received signal levels. So...a modification of the original overload protection circuit is in order: Two pairs of four diodes in series (1N4001, 4002, etc) with the strings arranged back to back and wired in parallel with the suppressor tube. A 1 megohm, 1W metal-film resistor may be added in parallel to bleed off any static charges.

The second receiver has been modified by Rob Sherwood. He hooked me up with all the info and parts required to do this to the first one, so when I have the supplies in-hand I'll mod the first set and place it back in service.

I will say that a -515 is one of the quietest receivers I've ever listened to, and that doesn't imply a properly working one is deaf. No, not by a long shot.