Hi John,Thanks very much for suggesting this explanation - certainly could be the culprit. Brilliant suggestion actually. I'm familiar with both AM stations you mentioned, being here in the Boston area. Considering that nobody, including Dr. Google, seems to know anything about a 170 kHz broadcast station, "something fishy" does indeed seem to be going on. Perhaps you've identified the fish.
Thanks also for your suggestions of inserting a tuned trap inline with my receiving system. I'll keep that in mind but will investigate further first. For about 0-160 kHz, I use an LF Engineering L-111S - LF/VLF Converter/Antenna System with RX up-convert frequency at 4.0-4.160 MHz on my TS-590SG xcvr, but I've found that above about 160 kHz, I get better results (lower noise, better received sigs) listening directly on frequency using my ham station end-fed Zepp (67' element + 67' 450 ohm window line) but bypassing the tuner. The L-111S system has low pass filtering below the AM broadcast band, supposedly limiting or eliminating intermod. If the 170 kHz signal is indeed an IMD signal, I suspect one vs. the other RX system will show significant differences in signal strength. Will play around with that hopefully this coming weekend. I did modify the L-111S a bit to optionally allow bypassing the converter but keeping the active probe with remove preamp (with filtering) intact, so I'll try that too. BTW, the run from active probe/preamp to converter is 50 ohm coax. I'll report back here once these experiments are run.
Thanks again for the brilliant hypothesis. I work in the sciences so love a good hypothesis and the chance to design experiments to test them. Thanks again and all the best.
73, Bruce WA1HGJ