Have you heard that the 515's were especially sensitive to static discharge through the antenna input? Mine was originally purchased from Gilfer Shortwave, in 1980. I think Gilfer used to do a mod that corrected this, but no idea if it was performed on the unit I have.
I really don't know much about the JRC receiver line so I did a Google search, and AI says this:
Internal protection circuit
The NRD-515's antenna input protection is located on the CMA105 board.
The original design uses a set of arrester diodes (CD83-CD86) in the front-end signal path.
These diodes are intended to shunt excess voltage to ground. However, experienced users and modifiers have found that under high-signal-level conditions, these diodes can cause significant intermodulation and cross-modulation distortion.
Modifying the internal protection
A common modification among NRD-515 enthusiasts involves replacing the stock arrester diodes to prevent distortion issues.
The recommended fix is to remove diodes CD83-CD86.
Replace them with a circuit consisting of a 1 mH choke in parallel with a neon bulb, which is then connected to ground.
The choke helps pass the desired signal, while the neon bulb acts as a high-voltage protector by ionizing and conducting charge to ground when a high voltage (like a static discharge) is present.
Idk if this is what the Gilfer modification was, they may have had their own mod that wasn't published. I believe Craig Siegenthaler of KIWA had at least one that I never saw on the Internet.