Introduction
This article describes changes made to the "Herring-Aid Five" receiver
presented by Jay Rusgrove, then WA1LNQ, in the July 1976 issue of QST to
allow the design to be duplicated with readily available components. The
original circuit was designed so that the constructor could obtain all the
parts from Radio Shack and many of these parts are no longer available.
In keeping with the theme of using food containers as chassis for these
designs this receiver was built onto an oval shaped Scottish Herring can
hence the name. Construction was similar in general layout to the Tuna Tin 2.
The first change necessary was to get a PCB which suited the new component
foot prints and allowed for changes to the circuit where needed, to this
end Doug Hendricks, KI6DS, laid out a new PCB using CirCad. Prototypes of
this board were generated by Gary Diana, N2JGU. The new board is
rectangular which will allow the constructor more flexibility in the choice
of chassis or enclosure.
The parts substitutions were made in such a way as to have as little
impact on the original design as possible. The main changes have involved
the substitution of toroids for the original solenoid style inductors. This
then lead to the addition of trimmer capacitors, as inductance adjustment
was no longer practical by squeezing up or stretching out turns. Audio
transformers T1 and T2 were replaced with readily available equivalents as
were the semiconductors.
The Receiver
This receiver is a "minimal" direct conversion design in which the
designer has traded a little performance for ease of construction with the
then widely available components. I think the most unusual part of this
receiver is the use of a single unbalanced BJT as the mixer in a 40 meter
receiver, I imagine this unusual choice would cause severe problems where
broadcast station interference was present. As the designer of the
original receiver noted audio bandwidth was left wide enough to allow for
the reception of SSB and AM signals. The up side of this is that it allows
for the use of the receiver for these modes, the downside is that it
compromises the receiver's CW performance.
The Circuit
The incoming 40-meter signal is coupled to the source of Q1 via a tuned
circuit consisting of L1, L2, TC1 and C1. Q1 is a grounded gate RF
amplifier and has its source tapped down L2 to preserve the Q of that tuned
circuit. The output of the RF amplifier is coupled to the Mixer by another
tuned circuit consisting of C2, TC2, L3 and L4. These tuned circuits
provide all of the 7 MHz selectivity. Q2 then mixes the VFO with the
incoming RF and the resultant Audio signal is amplified by Q2 and coupled
by T1 to the audio gain control. Q3 and Q4 are conventional common emitter
audio amplifier stages; the audio output is coupled to low impedance phones
by T2. The VFO consists of Q5, which operates as an un-buffered voltage
tuned VFO. Ordinary silicon diodes such as 1N4148 are used as varactor
diodes in this circuit. The tuning pot provides a voltage variable from 0
to approximately 800 mV, which allows the oscillator to tune approximately
100 kHz at 7 MHz.
Construction
Note: The description by Doug, KI6DS and Dave, AD6AY of their debugging of
an early prototype which appears on the NorCal page is invaluable companion
reading to this section. I commenced by building and adjusting the VFO.
The VFO was designed to cover a range of any 100 kHz of 40 Meters for 180
degrees of rotation of the tuning potentiometer, which suits the vernier
drive originally, used. I first wound L7 which consists of 45 turns
occupying about five-sixths of the T-50-2 toroid. L6 consists of 5 turns
immediately adjacent to the top of L7 and noting the phasing in the
circuit. L5 consists of 4 turns over the ground end of L7. VFO output with
the pot at the low frequency position (ground) is fairly low so as only 180
degrees of pot travel is used I set the pot to about 20 degrees up from
ground and then adjusted the frequency to be 7.000 MHz with TC3 (across
C4). If necessary a turn or two can be added to or deleted from L7 to
achieve the desired range, it may be easier to replace the relevant
capacitor, C19, with a slightly different value. This gave my receiver a
range of 7.000 to 7.100 MHz. If the oscillator does not oscillate try
reversing the terminations of L6.
The 180 degree requirement can be addressed either by using a vernier
drive similar to the original or constructing a frequency scale with 7.000
MHz at 9 o'clock, 7.050 MHz at 12 o'clock and 7.100 MHz at 3 o'clock and
simply ignoring the unused portion of the tuning pots travel.
The rest of the receiver can now be constructed. L2 consists of 45T on a
T-50-2 toroid, tapped up 5 turns from ground; L1 consists of 4 turns over
the ground end of L2. L3 consists of 45T on a T-50-2 toroid with L4
consisting of 20 turns across L3. When completed C2A and C4A can be
adjusted for best signal strength using live signals or an appropriate
signal source such as a 40-meter QRP transmitter into a dummy load. C2A and
C4A should each allow 2 peaks in incoming signal level per 360-degree
rotation of the trimmer. If only one peak is obtained it may be necessary
to add a turn or two (if the peak corresponds the plates fully meshed) or
remove a turn or two if the peak corresponds to the capacitor plates fully
unmeshed. Alternatively it may be easier to increase or reduce
(respectively) C2 and C4.
Performance
The performance of the Herring-Aid Five is better than I expected however
it has some limitations due to the earlier mentioned design tradeoffs.
Additionally the receiver is able to copy SSB signals; this is a useful
feature for the beginner but is a limitation when using CW. I have been
unable to find Scottish Herrings in VK and in any case I prefer to build
this type of equipment on a wooden base with a front panel for the controls
so that I can see and play with the circuit at any time
Conclusion
This receiver offers a number of attractions. It allows you to experience
the performance of the humble single BJT mixer. Nothing is hidden in IC's;
all voltages are available for observation. It provides a platform to
experiment with one circuit block while leaving the others constant. The
effect of a modification is more clearly seen e.g. a high gain IC audio
amplifier could be constructed outboard and audio fed to it from the audio
gain pot to observe the benefit or otherwise of higher audio gain on
overall performance.
I have made numerous decisions and assumptions in converting this design
to modern parts and have not explained these decisions in any detail.
Please feel free to contact me with any suggestions, questions or
observations. I am not an expert but rather a keen learner and I make no
claim that modifications I have made are the ultimate. I have enjoyed
"playing" with this circuit and believe there is a lot of fun to be had
"Back in the Future."
I would like to thank Jay Rusgrove for the fine original design and
article, Doug Hendricks, KI6DS, for his tireless work in laying out the PCB
and co-ordinating the project, Gary Diana, N2JGU, for prototyping the PCB,
Dave Fifield, AD6AY for his work with Doug on one of the prototypes and his
suggested improvement to L4 and finally Doug DeMaw, W1FB (SK) for giving us
so much.
The Herring Aid 5 Parts Layout
Troubleshooting the Herring Aid 5
Back to the Future with NorCal's Tuna Tin 2
The Herring Aid 5 Parts Layout
The Herring Aid 5 Schematic
The NorCal Page