NRD-525: A Technical Review
Dallas Lankford, 15 XII 92
There have been a number of reviews of the NRD-525 in the past, but none of them answered
certain technical questions I had regarding 525 performance, especially in the MW band. The 525
was discontinued last summer, and the remaining stock was offered at a discount by several
retailers. Not being able to resist a bargain, I bought one of these last 525's with a serial number in
the high 51,000's. Line drawings of the 525 front panel and vacuum fluorescent display are given
below.
Previous reviews of the NRD-525 include Magne's RDI white paper, Edition 2.0, 11 June 1987
(presumably there was an earlier review), the 1987 WRTH, pages 555-556, Rainer Lichte's review
in his 1987 book More Radio Receiver Chance Or Choice, pages 41-48, and John Bryant's
Proceedings 1989 review, "Wastegunner on a 525," pages R12.1-R12.9. Because these reviews
cover most basic aspects of a 525, my discussion of basic features will be brief in places. For
additional details you may consult those sources.
The front panel and vacuum fluorescent display line drawings above tell most of the story about the
basic 525 features. Some controls are conventional analog controls, while some are push button
switches. Other reviewers have fussed about certain aspects of the controls, but I haven't been
seriously annoyed by any of these so-called defects. Yes, it would be nice to have an analog Smeter. But the main reason the S-meter jumps around so much in AM mode is because the AM
attack times are not appropriate. My mod described in "NRD-525 AM AGC mod 2" takes care of
this annoying defect. And yes, it would be nice if the front of the 525 could be tilted upward to get a
better view of the front panel and keep your hand from bumping the table top while twirling the
main tuning knob. Nevertheless, the flywheel-weighted, dimpled main tuning knob turns smoothly
and is easy to use. Mode changing is easy with two switches r-. which move the mode indication left
or right through the following: RTTY CW USB LSB AM FM FAX. By contr~st, with the R8 you
have to cycle completely through all other modes to get back to the one you want. Similarly,
bandwidth changing is easy with two switches which move the bandwidth indication left or right
through NARR INTER WIDE AUX. Changing the AGC setting is not as convenient; a single
switch cycles through OFF FAST SLOW. In addition to tuning with the main tuning knob, a
frequency can be entered either in KHz or" MHz using the keypad, and the frequency can be
changed with the UP and DOWN switches.
Some operating aspects of the 525 can be changed by the user. The tuning rate of the main tuning
knob and the step increment of the UP and DOWN switches can be changed between "coarse" and
"fine" by pressing the RUN switch. In "fine" tuning mode, the main tuning knob rate is 2 KHz per
revolution and the UP/DOWN increment is I KHz, while in "coarse" tuning mode the rate/increment
is 20 KHz/10 KHz. The 10 Hz digit of the frequency display can be turned "off" and "on" by
pressing MEMO and I switches simultaneously. And the frequency indication in USB and LSB
modes can be changed between "automatic" and "manual " modes by pressing MEMO and 0
switches simultaneously. In "automatic" mode the frequency display does not change when
changing between USB and LSB (and you do not have to retune).
The NRD-525 Instruction Manual gives the tuning range as 90 KHz to 34 MHz. The 525 can be
tuned below 90 KHz, but internally generated noise begins to register on the S-meter at about 70
KHz, so the 525 is not useful much below 90 KHz. VLF DXers will need to use a VLF converter.
In the past, the merit of a receiver was often determined by the three S's - sensitivity, selectivity, and
stability. With synthesized solid state receivers like the 525, stability is seldom an issue.
Consequently, a more appropriate modern version of the three S's is sensitivity, selectivity, and
spurious responses. Nevertheless, for completeness, here is what has been published about 525
stability. The instruction manual specifies "Frequency stability +/- 3 PPM." Lichte measured "+/- 5
Hz/hr" while Magne reported "less than +/- 10 Hz at 10 MHz, excellent." I don't have the equipment
to make such precise drift measurements. Suffice it to say that I haven't observed any drift with the
525's I have used, and I don't expect to observe any.
Sensitivity should not be an issue for modern solid state communications receivers, but surprisingly
it sometimes is. Some receivers, like the R-5000, are desensitized in the MW band by design.
Others, like the R8, are not quite sensitive enough throughout the entire tuning range. Both Magne
and Lichte stated that the 525 sensitivity is derated in the MW band. Lichte even provided values of
16 and 19 microvolts at 1000 and 500 KHz respectively for a 10 dB S+N/N using the WIDE
bandwidth and AM mode, compared to 2 microvolts for the SW bands. In addition, the 525
instruction manual specifies 15 microvolts or better sensitivity for the 0.90-1.6 MHz frequency
range, and 2 microvolts or better for the 1.6-34 MHz frequency range using AM mode. However, I
do not know of any 525 with worse sensitivity below 1.6 MHz than above. As a matter of fact, the
sensitivity of my 525 is about 0.35 microvolts throughout the MW band for a 10 dB S+N/N using
WIDE bandwidth and AM mode, with a 400 Hz modulated source at 50% modulation. The WIDE
bandwidth AM sensitivity of my 525 on the SW bands is not quite as good, about 0.45 microvolts,
but still excellent. Perhaps some early production 525's were desensitized by design in the MW
band. But that appears not to be the case for most 525's. The 525 is one of the few solid state
receivers with adequate MW band sensitivity at locations like mine with low levels of man-made
noise.
Selectivity should also not be an issue for modern solid state communications receivers, but again it
sometimes is. Insufficient number of bandwidths, and inappropriate bandwidths are the common
defects. The 525 design is excellent with regard to available bandwidths because it has four
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