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Lowe HF Series
John Wilson
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Lowe HF-150
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A SIGNAL METER FOR ALL:  The Lowe HF150 is a really superb little radio and I have enjoyed many hours of fun with the set.  The one major drawback that I found was the lack of any signal strength indication which was a drawback when attempting to make aerial adjustments and comparisons or trying to compare the strength of various transmissions. 

This short article describes how a very useful signal meter can be added to the Lowe HF150 (or any other communications receiver that lacks a signal meter for that matter).  The circuit design of the S meter described here is the one employed by Lowe themselves in their add-on audio-filter and S meter unit and is easy to construct and reliable in operation.

The input to the meter consists of a simply connection to the AGC line within the receiver - which does require a very minor modification described later. 

  Since the op-amp ( IC1 ) is being used as an 'inverting amplifier' in this circuit, as the AGC voltage applied to the inverting input of the op-amp is reduced (because the received signal strength rises) the result will be an increase in the voltage supplied to the meter so thereby indicating an increased signal strength.

Below are the circuit details for the meter circuit (top section of circuit) and the power regulator (lower part of circuit) which provides the +5 volt rail and a -5 volt rail required for the meter circuit, and is derived from an external 12 volt power supply.  I have in fact used the output of the 12V power supply used by the HF150 radio.

Signal Meter
The complete circuit diagram.  It is very easy to assemble this onto a piece of strip-board,
incorporating the signal meter circuit and the power regulator circuit that derives the required
positive 5 volts and negative 5 volt output that the signal meter circuit requires.

The op-amp for the signal meter circuit is a TL061 or equivalent (see below), while the voltage regulator for the power supply is a 78L09 which is rated at 0.1 amps, although I have also tried the higher rated (2 amp) 78S09CV , so this could also be used.  A suitable signal meter is available from Maplin Electronics. 

The power is provided by a simple 12 volt DC power adapter or transformer.  In my own case I used the Lowe 12 volt power adapter to supply both the radio and this signal meter.  It seems somewhat confusing at first, but the signal meter part of the circuit (built around IC1 the TL061 op-amp) needs a power supply of both +5 volts and -5 volts.  This is achieved by the power supply portion of the circuit, the main component of which are the 78L09 voltage regulator and the 5.1 v Zener Diode. 

Two connections from the signal meter circuit - Pin 7 of IC1 and the 680k resistor go to the +5 volts point on the output from the power supply section.  Another two connections from the signal meter circuit - Pin 4 of IC1 and the 460k resistor are connected to the -5volts output from the power supply circuit.  The -ve side of the meter movement itself is connected to the 0 volts point on the output of the power supply.  This all looks a bit odd since the -5v negative output is actually connected to the chassis and the -ve input from the 12 volt power supply so seems to be at zero potential.  In this case, however, -5 volts is simply the relative potential against the centre 0 volts connection point at the output of the power supply.  The + 5 volts output is merely 5 volts above the potential of the 0 volts output.  (and actually +10 volts above the potential of the chassis i.e. the  -5 volts point).  Don't worry about all this, it's just that the signal meter circuitry needs to see a +ve 5 volts and a -ve 5 volts supply in relation to the 0 volts at the -ve side of the meter movement.

[ Note from Bob Warriner: Bob wondered if it was absolutely necessary to fit the two 1000uf caps: "Hi Mike, having recently acquired an HF-150, I was keen to build this "S" Meter circuit. I planned to use the original Lowe power supply which, as you know, is already well regulated and perfectly smoothed (as tested on my oscilloscope). Omitting these components allowed me a bit more space in the enclosure that I to used. I built and fitted the circuitry into a meter case powered via the original HF-150 PSU. Without the smoothing caps, all is well as can be seen in the attached photographs (below). I have also carried out the backlight mod on the HF-150 using a cheap Chinese display unit, dismantled the backlight portion and fitted it behind the HF-150 display, connected up the incorporated LED to the 12 volts on the on/off switch via a 2.2k resistor. Best wishes, Bob."]

A detailed description of how the meter unit is connected to the radio is offered below, but suffice to say:  Two 2.5mm jack sockets were used; one was mounted in the rear panel of the radio casing and the other in the back panel of the signal meter housing.  The tip connection of the 2.5 mm jack socket within the radio is taken to the AGC (Automatic Gain Control) circuit of the HF150 at Pin 16 of Q32 via a 470k Ohm resistor. This is shown in detail below.  The tip connection of the 2.5 mm jack socket within the Signal Meter is taken to the 47 k resistor, and thereby on to Pin 2 of IC1, as shown in the circuit diagram above.

The radio and meter are then 'hooked up' via the two 2.5mm sockets using a suitable length of thin screened cable with a 2.5 mm plug on each end.

Note that  the 0 Volts connection between the negative side of the meter and the 0 Volts output from the power regulator circuit should not be connected to the metal case or ground (since this is at a relative -5 volts potential which would make the meter malfunction).
A 2.5mm jack socket was fitted to the radio and the signal meter case to make an easily removable connection between the signal meter unit and the radio.


TL061 or LF351 or equivalent Op Amp
8 pin socket to mount Op Amp IC
Signal Meter - Rating 250uA 675 Ohms - (Maplin)
1N4148 Diode
100k Ohm Preset Potentiometer
100 Ohm 0.6 watt Resistor
47k Ohm 0.6 watt Resistor
3 470k Ohm 0.6 watt Resistor
680k Ohm 0.6 watt Resistor
1 Socket suitable for mounting on signal meter case - eg 2.5mm jack
Socket suitable for mounting on radio case - e.g. 2.5mm jack
Aluminium Case to mount  signal meter and power supply
Piece of Stripboard for Signal Meter and Power Supply
Miniature 12volt Bulb to illuminate signal meter (optional)


78L09 or 78S09CV  9 volt Voltage Regulator or equivalent
5.1 volt Zener Diode
1N4148 Diodes
220 Ohm 0.6 watt Resistor
1000uF 35volt Electrolytic Capacitors
0.1 uF Ceramic Capacitors
Miniature on/off Switch
Fuse and Fuse Holder (optional)

A Page Containing Some Links To Suppliers Of Electronic Components >

THE OP-AMP:  The Op-Amp used for the signal meter circuit is a TL061.  An LF351 works just as well, however and has the same pin layout.  If you have a TL062 or similar the differing pin-outs are shown below so that you can design your circuit board successfully.

Op Amps
OP AMP Pin Layouts

The diagram above shows the pin layout of the TL061 and some alternative op-amps that can also be used in the signal meter circuit described on this page.  The LF351 op-amp also works in this design and is pin compatible with the TL061. Another pin compatible op-amp that is usually easy to obtain, or that may be knocking about in the 'junk box' is the very common LM741. I have not tried an LM741 in this circuit, but I can see no reason why it would not work.

The TL06* series are low power versions of the TL8* series and are pin compatible with the TL7* series.

Use D.I.L Sockets:

Rather than solder any op-amp directly to the circuit board, I would strongly recommend that you use 8 Pin D.I.L. sockets so that the I.C. can be easily inserted and removed.

The completed circuit housed in an aluminium box. 

The 2 power sockets mounted to the right of the back panel are the 12v power input sockets (2 are fitted to allow 'daisy-chaining' of the power to another unit)  NB - the 0 Volts connection between the negative side of the meter movement and the 0 Volts output from the power regulator circuit should not be connected to the metal case or ground. 

The small socket on the left is the 2.5mm Jack that allows connection of the signal meter to the radio.  You can just see the single grey wire that connects the 2.5mm socket to the circuit board.  The top half of the board accommodates the signal meter components (ignore the second preset), while the bottom half of the board accommodates the regulator components.

Front Panel**

I replaced the original scale provided with the Maplin signal meter that was marked from 1 to 5, with a very simple one that I printed on ordinary paper using an ink jet printer. I produced a simple scale marked in 'S' units: S1 S3 S5 S7 S9 +10 +30 +50dB. I copied these gradations from a photograph of the original Lowe signal meter to ensure reasonable accuracy and conformity to the original Lowe design shown below.

Signal Meter

Lowe HF-225 signal meter
Above, The Lowe HF-225 signal meter

I found an interesting page by Frank OK2FJ who produced an excellent meter scale for the Yaesu FT-857 and FT-897 amateur radio transceivers. The meter for the Yaesu rigs shows much more information than just the signal level, so I decided to simplify it so that it could be used in an S Meter project such as this. I saved the image that Frank produced made some changes using an image editor. The finished S Meter scale is shown below. It is a large image being 1795 pixels wide so that the print quality will be high.

When printing, you will need to scale the image very accurately to fit the particular meter that you use. This can be done by trial and error or by mathematics! Click on the image below to save it.

Signal Meter - S Meter - DIY Homebrew Scale - by M0MTJ and OK2FJ
You can download and save the above image to print out at a size that suits your particular meter movement

Printing The Scale: The scale can be printed on paper or thin card and possibly laminated, which is what I did. White paper or card might be the obvious choice, but cream, light green, yellow or light blue card would also make a good background colour.

When printed, the image will need to be scaled quite accurately to suit the size of the particular meter movement being used, otherwise the needle will not line up properly with the scale and the indication will be inaccurate. This can be done by trial and error until the correct size is found - a bit of a fiddly and a rather wasteful method. Alternatively a bit of simple math's can be used.

My image editing program allows scaling of the print-out using a sliding scale that shows the total width of the image when it's printed and the dpi (dots per inch) output to the printer. Knowing the total image width isn't especially helpful since what is needed in this case is the dimension that is the distance between the left and right end markers of the S scale - the top curve. My simple image editor does not allow an accurate measurement of a portion of the image, so I did a test print, estimating that the resultant image would need to be 50 mm wide, the output in this case was 920 dpi. I then measured the width of the top curve on the test print, from end marker to end marker - it was 40mm. The scale of the original microammeter is 34mm wide, so the print had to be scaled down in size.

The magnitude of the size reduction can be found by dividing that measurement, 40mm, by the required measurement - in this case 34mm.

40mm  ÷  34mm  =  1.176 (the scaling factor)

The original test print produced an scale that, at 40mm, was too wide. It needed to be 34mm wide. The original image width of the test print was 50mm and therefore this needed to be divided by the scaling factor of 1.176

50mm ÷ 1.176  =  42.5mm

The calculation suggests that 42.5 mm is the width required for the whole image. The image was printed again at that width and the reulting print measured. It was found that the width across the top curve from end marker to end marker was, indeed, the required 34mm.

The other way of doing the scaling is to note the dpi output of the original test print, in this case 920 dpi, and multiply (not divide) that by the scaling factor. The original dpi figure is multiplied, rather than divided, becuase the dots per inch will increase as the original image size is shrunk. In this case the new, and correctly sized print, is 1082 dpi. Whichever method is used, the second print should produce a scale of the correct size.

Reference: This is the original meter scale image that was produced by Frank OK2FJ, I altered this to produce the meter image that is shown above.

To make sense of 'S' units, the table below shows the relationship between 'S' units and terminated voltage - in microvolts:

12.5 25

Note how relatively small changes in voltage at the lower end of the scale ( in the S1 to S8 range) produce quite noticable swings in the readings, while really quite large changes in signal voltage at the higher end of the scale ( S9 to S+50 ) produce quite small variations in read-out.  This non-linear effect is quite intentional:  Increases in signal strength from S1 to S9 will produce dramatic improvements in the received signal to noise (S/N) radio while above around the S9+10dB signal level the receiver is approacing the best acheivable S/N ratio and further large increases in signal strength will make less if any improvement to the audible S/N ratio, so the S Meter does not really need to reflect these changes in such minute detail.

The signal meter is invaluable when making aerial adjustments and comaprisons as well as being useful for comparing the strength of various transmissions.

A Note About S-Meter Calibration

Although there is no set standard for S-Meter calibration, it seems widely accepted that the S9 reading is calibrated for a signal input of 50µV. (Although some manufacturers may calibrate S9 at 100µV). Additionally the gradation between each S unit is generally accepted as 6dB. i.e. S2 is 6dB greater than S1 and S9 is 12dB greater than S7 etc. As can be seen in the table above 6dB represents a doubling of the voltage received by the radio.

(It may also be worth pointing out that for a receiver to receive double the voltage from a transmitting station, the transmitting station would have to increase its effective radiated power level by four times.)

In real life the signal meter on many radios may not be accurately calibrated to 50µV at S9 and each individual S unit may not, indeed, accuratelt represent 6dB intervals. Some meters may represent the interval as 5dB or 4dB. However an S Meter's visual representation of relative signal strengths will still be very useful in judging differences between signal strengths of different stations and particularly useful with antenna experiments.

However, without an accurate signal generator, it will be down to individual judgement as to how this "home-brew" signal meter is calibrated!


A minor modification to the HF-150 is required to enable an external signal meter to be connected.  The AGC (automatic gain control) line on the receiver's printed circuit board needs to be identified and an explanation was proveded by Lowe for the HF-150, but a similar connection could feasably be made to the AGC line of any other receiver.  This may be marked as a pin on an IC or next to a transistor or other component.  You may be able to find the circuit diagram for your radio in the handbook or be able to abtain one from the manufacturer or from a search of the internet.

A very thin wire can be soldered to the board at this AGC point and taken, via the 470k resistor, to a socket on the back or side panel of the radio.

I used a 2.5mm jack on the back of the HF-150 to facilitate easy connection and avoid the method described by Lowe below which involves cutting the PCB track to the earth terminal and exiting the signal meter feed by that terminal - thus the radio would no longer have an earth terminal.  Instead I carefully drilled a neat hole in the back plate and fitted the new dedicated socket for the signal meter.   The radio and the meter are then easily connected together by a short length of screened cable with a 2.5mm plug on each end.

This is the modification as described by Lowe:

Modification for 'S' meter operation

Modification of the HF 150 for use with the 'S' meter if your set has a serial number of 142617 or less it will require the addition of a 470k resistor.

To modify your HF150 you will need to remove any power including batteries if fitted, lay the set upside down on a piece of cloth or similar to prevent scratching the case.

Now remove the bottom panel;  With the front panel facing you locate the wire aerial connector, the black terminal being earth, if you now look at diagram No.1  you will see a thin track connecting this pad to the earth plane, you will need to cut this tiny track*, now you can solder a 470k resistor .25W from the black terminal to pin 16 of IC Q32 at the point where C97 is soldered onto the circuit board.  Please refer to diagram No.1.

For sets after No. 142617 the 470k resistor will be fitted and all that will be needed is to cut the thin track to earth as to allow the agc signal out from the black terminal.

* see my notes in the text

The panel below shows the modification required to be undertaken to the Lowe HF150 itself.  The original modification shows that the 470k resistor is taken from Pin 16 of Q32 to the Earth terminal.  I instead decided on the much neater option of fitting a dedicated 2.5mm jack socket on the back panel for the signal meter output:

Lowe Signal Meter Modification

* The Lowe explanation of how the signal meter is connected to the AGC line of the radio via a 470k resistor, and output through the earth terminal.  I instead opted to fit a dedicated 2.5mm socket to the back panel to take the feed out the the signal meter - the 470k resistor is therefore connected to pin 16 of I.C Q32 and taken directly to the 2.5mm jack socket.  This makes a much neater way of connecting up the external signal meter to the back of the Lowe HF150 I feel - as can be seen in the photo of the rear panel below:

Rear Panel
Rear Panel - showing additional 2.5mm socket

The photograph above shows the rear panel of the HF-150.  The additional 2.5mm signal meter output socket is mounted in a neat hole drilled just below the letter 'F' of the HF-150 logo top centre, above the battery housings.

There is just one single thin wire to connect on to the circuit board within the radio at the AGC line.  This wire is taken to the 2.5mm jack socket via the 470k ohm resistor.  The body of the 2.5mm socket is grounded to the aluminium panel of the receiver.  It is then a simple matter to use a short piece of screened cable, terminated with a nice solid 2.5mm plug at each end, to connect the radio to the external signal meter - the centre conductor of the cable taking the AGC voltage to the meter while the braid screen simply connects the ground of both the radio and the signal meter casings together.

Using this type of shielded connecting cable arrangement is probably not really necessary if you are running the signal meter and the radio from the same power supply transformer, as I do, and a single piece of hook-up wire between the radio's AGC and the signal meter circuit could do the job - but I used this method for neatness and reliability.  Using metal cased 2.5mm jack plugs looks more professional too!

Once the meter is connected to the radio the sensitivity is adjusted by the 100k Ohm preset potentiometer - in essence this involves disconnecting the aerial from the receiver and ensure that no radio signal is being received (even hash or interference) and then to set the 100k preset so that the needle rests at minimum deflection i.e. as far left as possible or zero .

**  I actually ended up constructing the signal meter for my listening post within the case of an existing audio filter unit that I had previously constructed and which had some space left inside.

I hope you have a go at building this little project.  A signal meter can be a very useful tuning aid and I have found it invaluable for making various aerial adjustments and comparisons, as well as for general listening.

The LOWE SP-150

Hans Kröger writes regarding the Lowe SP-150 loudspeaker, audio notch filter and signal meter unit:  "Last summer I considered myself lucky to find a matching speaker/notch-filter/S-Meter at a HAM-fest, the SP-150. The missing instruction-manual I downloaded from the internet easy enough. When I connected the set to my HF-150 with an appropriate cable I found the S-Meter not to be working at all and the sound to be distorted at times. Some damage in the SP-150’s circuitry I thought and so began searching the internet for a source of repair.

That’s how I came upon the information that for receivers below a certain serial-number (just like mine) a “modification” has to be done by connecting the ground-terminal to an IC by means of a resistor (after having isolated this ground-clamp from the circuitry-ground first of all). That was easily done as well. While it struck me as absurd to rid the radio of all means to ground it, the obvious question has been on my mind ever since: How does the AGC-voltage that’s now available at the receiver’s ground-terminal (or at the tip of a 2.5 mm connector as per your approach, which I wish I’d read earlier), how does this AGC-voltage get to the SP-150’s S-Meter???

As per the manual a cable-connection must be made between the radio and the SP-150’s input. There is only one such connector at the set’s backside, which (sure enough) connects to Pin 2 of the IC described in the circuitry on your website (only in the SP-150 it’s IC 5 or 6). So where (and how) does one feed in the AGC-voltage? I cannot possibly be the only user facing this problem which I why I hope you’ve had reason to cope with this situation in the past and help me out.

What I will do in any case is re-connect the radio’s ground-lug to circuitry-ground, drill a small hole like you did and solder the 470K-resistor to a separate 2,5 mm-connector. Some advice and support would be highly appreciated! Many thanks in advance and all the best from Hamburg".

Solution - How to connect the audio and AGC signal meter drive to the Lowe SP-150: Well, fortunately the solution is relatively straightforward. Looking at the Lowe SP-150 circuit digram shows that both the audio from the Speaker Out socket on the rear ofthe HF-150 and the AGC voltage derived from the modification to the HF-150 described above is connected via a three conductor jack socket ('stereo jack') on the rear of the SP-150. The audio input socket serves this dual function which, sadly, is not made clear in the manual.

The three conductor ' stereo' jack on the SP-150 is wired as follows: The body of the plug is ground, the tip is audio and the centre ring is for the AGC voltage for the S-meter.

Lowe SP-150 three conductor audio input jack also carries AGC for signal meter
Above: cut-out section of the Lowe SP-150 circuit diagram showing that
the three conductor audio input jack socket ('stereo jack') also carries AGC for signal meter.
Body of the plug is ground;  the tip is audio; the centre ring carries the AGC voltage for the S-meter.

Download: Lowe SP-150 Loudspeaker, Filter & Signal Meter User Manual

Hans Kröger replies to our clarification: "Good morning Mike, I just wanted to let you know that the cable-connection suggested by you did indeed make my HF-150 and SP-150 work together. Just a quick and dirty job I had time for, pretty much “on the fly”, but it worked as it should. Thanks a million for your assistance and all the best from frosty Hamburg.
Sincerely, Hans Kröger".

Bob Warriner's "S" Meter
Bob Warriner's "S" Meter

Bob Warriner's "S" Meter
Bob Warriner's "S" Meter

Lowe HF-150 backlight modification
Lowe HF-150 back-light modification by Bob Warriner

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