[Boatanchors] Odd behavior by NRD-515

[Dave Brown]

 Can you quantify the reduction in sensitivity? It's not unusual for general 
coverage receivers to have deliberately reduced sensitivity across the MW 
band and lower. The NRD-515 spec quotes AM sensitivity as (better than) 2uV 
from 1.6 to 30 MHz and (better than) 6 uV sensitivity  below 1.6 MHz. (down 
to 100 KHz) You may find that your receiver has nothing wrong with it but 
you will need to have someone check it with a sig gen to be sure. If the 
sensitivity below 600kHz increases it may have been modified to give 
increased sensitivity below the MW band.
73
 Dave, ZL3FJ


----- Original Message ----- 
From: "Howard Ritter" <hlritter at bex.net>
To: <Premium-Rx at mailman.qth.net>; <boatanchors at mailman.qth.net>
Sent: Friday, July 17, 2015 10:14 AM
Subject: [Boatanchors] Odd behavior by NRD-515


> 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
>
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