[Boatanchors] Odd behavior by NRD-515

[Howard Ritter]

Sorry for the semi-off topic post (somewhere between a Premium RX and a boat anchor!), but where else can you get this level of expertise?

I have just had the embarrassing and expensive experience of having an eBay seller return the Japan Radio Co NRD-515 receiver he’d bought from me, after finding severely reduced sensitivity between 600 and 1600 kHz. I’d never used it for BCB listening, so this problem had escaped me. 

Briefly, this is a synthesized solid-state GC receiver that tunes a lot like the Collins 51-S1, with one knob to select one of 30 1-MHz ranges and another knob to tune through the 1-MHz range that’s selected. It’s possible, with continuous turning of the tuning knob, to move through the ends of any 1-MHz range and into the next. Freq is digitally displayed to 0.1 kHz. I simplistically envision the process as that of synthesizing a base frequency as specified by the MHz knob and mixing that with a variable freq determined by the tuning knob. That’s why I couldn’t picture why a decrease in sensitivity would affect just part of just a single 1-MHz band. I was skeptical until I got the receiver back and put it on the air.

Sure enough, I found just what he did. In tuning through the single step from 599.9 kHz to 600.0 kHz, the background hiss goes from normal to virtually absent, a very striking change. Very few stations are received, weakly, up to around 1100, above which there is still abnormally little atmospheric hiss but a few (strong?) stations are heard. Continuing to tune upward, background noise reappears but remains low up through 1599.9 kHz, after which it abruptly returns to normal at 1600.0. It’s probably significant that this change in frequency is accompanied by the sound of what I take to be a relay clicking inside the radio, although there is no such sound at 600, or anywhere else in continuously tuning except for 4999.9–5000.0 (although when I switch ranges with the MHz knob, the relay clicks at 2, 3, and 4 MHz). I have tuned the entire spectrum with both the dial and the UP—DOWN switch and have found no other place where the sensitivity drops out like it does from 600 to 1599.9. The decrease in, and return of, hiss occur whether I tune into the 0–1 MHz and 1–2 MHz bands with the MHz range knob or by twirling the tuning dial, and whether I approach from above or below.

The band of decreased sensitivity corresponds so closely to the MW broadcast band that I wonder whether this is a coincidence. It would make sense if the receiver were to automatically switch to a different antenna input when it’s tuned to the BCB, and that could explain the sound of the relay—except that there’s no separate antenna input for a BCB or ferrite antenna. It may be significant that, outside the affected frequency range, the background hiss drops markedly when the antenna lead is disconnected—in fact, to the same level as the background hiss within the affected range. And, within the affected range, the hiss does NOT drop when the antenna is taken off. This suggests to me that the RF from the antenna is not getting amplified within that range. Still, why or how a problem could affect just part of one tuning band and the adjacent part of another band, spanning exactly 1000.0 kHz, remains a mystery.

Any ideas?

Thanks, 
—howard n7exn