DXA-1
(EAS-4)
A Dedicated AM Broadcast Band DX Receiver
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On a professional level, I wanted to build an instrument-quality AM commercial broadcast receiver that could be used for Off-Air station monitoring,
program air checks and for STL (studio transmitter link) utility.
On a hobbyist level, I wanted something that would be a fun receiver that had professional instrument performance specifications ... a standard for others to
be compared to.
I selected the TFT/EAS AM receiver circuit. I have built four other radios, in various EAS (Emergency Alert System)  configurations, with this circuit
because of its good utility and adaptability for these purposes. It's basically a digitally synthesized, PLL architecture with good bandpass selectivity,
headend sensitivity and low noise characteristics. This receiver will also have auto-detect feature for the EAS (NOAA/NWS) tone alert switching.
The receiver board
under test on the
service bench.

Its basic layout lends
nicely for modular
systems.
The PLL tuner was designed as a "set and forget" system.

As with the previous EAS receivers I've built, the screwdriver adjust BCD tuning
switches will be replaced with rotary types.

The yellow LED is driven from a transistor switch at the detector stage to indicate an
unmodulated DSB carrier or the same carrier that contains audio modulation.

TP1 is an output from the AGC section that supplies a proportional signal level
voltage. That's the starting point for an accurate Signal Strength meter.

You can see the first three versions here:

EAS-1

EAS-2

EAS-3
Although an analog S-Meter will be used, there will also be a calibrated uV meter to
indicate the relative micro-volt signal level entering the radio at the tuned frequency.
This meter will be calibrated for 50 uV in a 50 Ohm line for an S9 reading.

The meter driver is a simple DC amplifier with a 25-turn calibration gain control. It
(typically, but not necessary here) uses a very high input impedance so that we can
bridge almost any part of the sensitive front-end circuits without causing any undue
loading or, the sometimes overlooked, capacitive effect. I've noticed this effect
caused by certain solid state junctions. A good example would be some varactor
diodes, like in VCO circuits, and protection components: transorbs, TVAs, MOVs.
The recovered audio will be
fed to two small power
amps which produce about
1 or 2 watts at 4 Ohms. The
important thing is that the
amp's output impedance is
very low to effectively drive
the remote feed lines (STL,
recording, rebroadcast,
remote switching, etc.) as
well as a local loudspeaker
for monitoring.

Here is one under test. It
drives a studio monitor
speaker to well over 90 dB
SPL ... more than enough
audio for almost any
listening situation.
Of course, it all begins with good clean power. This is the
regulated power supply which supplies the 12 and 5 volt circuits.
Measured noise is well below 1mV.
Counter
Screen grab of the actual front panel decal design. This process is the same for all the EAS type receivers, so I won't duplicate all the construction details.
This layout is very similar to the
EAS-3 radio, but with some refinements. Here I've added a tone control circuit with Bass and Treble controls. I've
eliminated the switch that changes the Signal Level Display between S-Units and microvolts, and just kept the uVolts reading. Since there will be a real
S-Meter (analog type), I felt that having a digital version was too redundant. I've added a sub-detector circuit that indicates if suitable signal level is coming
through the antenna port - the two LEDs under the Signal Strength meter marked "RF Signal" and "RF Fault." The very High-Q Preselector remains. This is
a great feature for chasing weak or DX stations, or when the propagation is not cooperating. Depending on the conditions, you can realize 2 to 4 S-Units of
effective selectivity for the standard 10 KHz commercial broadcast channel spacing - that's 12 to 24 dB ... sometimes more !
This is the full-size screen grab so that you may see the controls and functions. Each white box is cut out and laminated before being applied to the machined front panel.
An upgrade version of the EAS-3 Studio Off-Air Monitor
Parts-r-parts. And we need what we need ! Before I begin building any
project, I like to have a box full of the necessary component parts. I usually
begin this phase of the project while I'm designing the front panel and right
after designing all the circuits. In that way, all pre-assembly bench testing
can be accomplished. Here is the little LED voltmeter, which will be the
signal strength indicator. Also shown is one of the push button BCD
switches. The rotary BCD switch is another story - see below.
Innovator's Note:
In a few previous radios that
required rotary BCD switches, I
constructed (wired) my own using
10 position - 4 pole rotary
switches. See
here.
I searched high and low for a
reasonable rotary BCD switch with
a knob shaft, but couldn't find
anything suitable for less that
$23.00US.
I did finally find these Allen-Bradley (V-111) at a surplus house
for $2.00 each...amazing value! The shaft is some metric size,
about, .150 inch dia (x) .4 inch long. Nice, but not usable for
panel mounting, nor for attaching a control knob. Answer: drill
out a piece of 1/4" Delrin rod of suitable length. Now we are in
business. The switches will be PC Bd mounted behind the panel
with a 1/4" bushing in the panel to provide support and relieve
and lateral stress during use.
The front panel machined
according to the computer
graphic layout.

The rack mount holes and the
chassis (2U) mounting holes
have not yet been drilled.

Below the panel are some of the
PC Board sub-assemblies. It's
good to have as much of this
preliminary stuff completed and
tested before actual assembly
begins. It makes laying out the
chassis architecture easier and
usually more logical.
The main audio monitor boards; preamp with bass and treble
control and the little 4-watt speaker power amp.

I bought actual PC Bds for these circuits, rather than using
proto-perf boards, since the physical layout can cause
problems with ground loops, hum pickup and oscillations.
Other sub-assemblies include (clockwise from top left) VU
meter audio driver, S-Meter DC driver amp (incomplete), the
3-digit microvolt meter (yellow), a 20dB RF preamp for the
preselector, and a little audio driver board (SMT) to drive the
remote 600 Ohm balanced audio feed line output.

I am not a big fan of SMD/SMT assembly. I've never really
had a problem, but the micro-size of everything concerned
seems to take some of the fun out of building. It also makes
future mods and possible repairs somewhat difficult
As can be seen from the front panel decal artwork, this is a
dual-watch receiver. However, unlike previous designs, I
decided that a "
diplexed" dual tuner is simpler yet just as
good as the previous "
duplexed" versions. There doesn't
seem to be any functional or performance reason to have
two identical tuners and IF strips when just simply switching
the tuner sections will do the job.
Since this is a BCD-Logic tuned PLL receiver, it was
necessary to isolate the two BCD tuner sections. That was
easily accomplished with diode logic (left). There are 9
diodes per tuner section, for 18 total. The layout is: 10KHz
BCD 1,2,4,8, then 100KHz 1,2,4,8 and finally a single diode
for the one/zero for the 1MHz position. One final improvement
in this model over the previous is that the BCD switches are
full-rotational. So when I'm band scanning and reach the "9"
position, I don't have to go back counterclockwise to get to
"0", it's just a matter of one more click in the same direction -
a small, but convenient, difference.

Here's the panel prior to assembly. I'll let it sit for a few days
so that all the adhesives and paint are fully cured
Initial assembly of the front panel. Everything seems to fit without interference to chassis components and mounting brackets, etc. I'm anxious to get this thing
playing since it incorporates all of the mods and additions combined from the three previous models. Note under the digital display are the two LEDs that are driven
from the RF Amp and indicate whether there's a viable signal available (see layout at the top of this page). If the antenna link is lost or if there is no carrier on the
incoming line, the "Fault" LED will light. When a usable signal is present, the "RF Signal" LED lights. In conjunction with that, in the Detector module, the "Carrier"
and "Signal" LEDs indicate the status of the incoming signal's modulated data. As with previous models, the dual tuners are selectable as rotary switches or decade
push-button switches. The S-Meter calibration pot is glued to the back of the meter.
Front and back views of the diode switching logic board. Since this is a DC Control
circuit, the diodes are only needed to keep the two tuners from back-feeding each
other. The BCD extended switch shafts have bearings in the front panel to
eliminate any lateral stress on the little PC Board switches
The finished and working unit.