Bass Pedalboard

Playing the guitar a little bit, I have often looked enviously at the pedal-boards fitted to pipe and electric organs which allow the musician to add bass notes to the music, while keeping the hands free for twiddly bits. Why not do the same for guitar?

Years ago Maplin used to sell electric organ parts, including a pedal-board, which is sadly no longer available. I suppose one could buy a junk instrument and repurpose the pedals, but rather than base the design on 13 note octave, it might be best to work around the musical intervals. A simple three-chord song would use chords rising from the tonic, the fourth and the fifth. Add the relative minor on the 6th, a minor third and so on, and you could play the root notes of all the common chords with about eight pedals.

A simple mechanical arrangement is what is needed for home construction, I’m using the springiness of the plywood pedal as a natural spring, which makes the mechanics much simpler. The switches are released when the pedal is pressed down which makes it more robust, because the pressure of the foot doesn’t bear on the switch mechanism.

The electronics is really simple. A PIC microcontroller looks at the pedal switches and sends appropriate MIDI signals out on the serial port to play a synth, or digital piano. There are a pair of switches on each pedal so that I can make it velocity sensitive – that way I can play louder notes if I stamp harder.

 So here is a picture of the two pedal "prototype" I constructed to try out the mechanics of the switch. The circuit board is a prototyping board I bought from CPC, but more of that in a later post.

I have tried three types of switches. The ones shown are too “clicky” and make a lot of mechanical noise when attached to a wooden board. I have settled on some sub-miniature ones which are much smaller, and more demanding in terms of mounting, but are much quieter.








Hugh M0WYE

Snubbing the Nordfrost Freezer

We have a small chest freezer in the house. A Nordfrost one. We have had it many years, and it has been ok, but it has required a new thermostat as the contacts on the old one burned out. And every time the compressor shuts off, it interferes with the TV, causing a momentary jerk in the motion, or pixelation, or a slight glitch in the sound.

When an inductive load is switched off, the energy stored in the magnetic field (in this case in the windings of the motor) tries to keep the current flowing, developing a big voltage across the switch contacts and causing an arc. The arc is like a spark-gap transmitter and generates radio interference. The solution, it seemed to me, was to fit a "snubber circuit" which is a capacitor in series with a resistor wired across the thermostat contacts. The energy which would have caused the spark now flows into the capacitor and is dissipated in the resistor.


I measured the inductance of the compressor - 43mH. I also measured the current drawn by the motor - 0.66 Amps. Then I calculated the energy stored in the inductance, which is "a half L I squared", or 0.5*L *I^2. There is a similar equation, "a half C V squared", (0.5C*V^2) for the energy stored in a capacitor, and I reasoned that I needed a capacitor to store about the same energy as was in the inductor. It worked out at 315nF. I had a couple of 220nF capacitors in the junk box. Importantly they were X2 rated capacitors (see the marking in the photo below) which are suitable for connecting directly across the mains.

These X2 metalised film capacitors are "self healing" - if the dielectric starts to break down the internal spark evaporates the metal on the plastic film and the area of broken-down dielectric is isolated. Some types of capacitor can fail with a big bang and a puff of smoke when the dielectric fails.

When I looked on the internet there was a lot of stuff about designing snubbers for MOSFET circuits, and Switchmode powersupply circuits. They all said that the time constant of the C and the R needed to be about 10% of the "on time". Not sure how that relates to a thermostat, but the mains frequency is 50Hz, and so each cycle is 0.02 Sec. The time constant of my 220nF cap with a 10k resistor is about 0.002 Sec so I plumped for a 10k resistor. The time constant is calculated by multiplying the C by the R. And it is the time taken for the capacitor to discharge to about 30% of it's starting Voltage when discharging through the resistor. It probably doesn't matter much as long as there is some resistance to dissipate the energy.

When the motor is not running the snubber will have quite a lot of Voltage across it - most of the mains Voltage. A 220nF capacitor will pass quite a few milliamps at 240V. It is possible to work it out with phasor diagrams and power-factors and things, but I simply connected the circuit across the mains and measured the Voltage across the resistor - which was about 135V. Since power = V squared over R, that is about 1.8 Watts being dissipated in the resistor. In fact it will be less, because the motor is in series. At first I paralleled up 10 x 100k metal film resistors, but that is quite a fat bundle. I found it was better to series 10 x 1k resistors and cover them with heatshrinkable tubing. Each resistor is good for about 0.6W, so in theory my chain should be ok dissipating 6W, but it is good to have some margin of safety. The long thin arrangement dissipates the heat better. I put five on each leg of the capacitor.
So I had an arrangement like this:

The Nordfrost thermostat is mounted in a plastic box in the front of the cabinet, so it is quite easy to get at - there is also a fair amount of room inside the box to accommodate extra components. I used some of the "piggy-back" type 1/4" push-on connectors to connect the snubber circuit. Since these just crimp on to the wires there was no soldering to do. Here's a picture with the cover removed - there is rather a tangle of wires, but nothing is danger of shorting to anything else so it is ok!

So I installed the snubber this evening, and we have been watching TV. Here's the interesting thing. The interference when the freezer turns off has completely gone ... but ... there is sometimes a bit of picture disturbance when it turns ON !  Not sure what is going on there. I wonder if it is something to do with the PTC starter circuit on the compressor - which is an extra winding on the motor that is connected at switch on, but is disconnected by a "Positive Temperature Coefficient" resistor once the motor is running ... I need to think about this.

The other slight downside to the snubber circuit is an increase in power consumption - the result of having a small current flowing in the resistor when the motor is off - I guess it is about a Watt of electricity wasted during the off period of the freezer.

Perhaps an in-line RF filter might help with the TV interference.
At least the snubber should extend the life of the thermostat contacts.

73
Hugh

Simulating the FIlter in the Microphone

Having measured the values of the filter components in the Retevis Speaker-Mic, I thought it would be interesting to simulate the circuit and find out roughly what the cut-off frequency was.
I use the free simulation software from Linear Technology, called LTSpice. http://www.linear.com/solutions/ltspice
Here is the circuit ...

At this stage I used 50 Ohms for the input and output impedances, and got this nice curve:

The cut-off is surprisingly low at around 5Mhz but the attenuation at 433MHz is (in theory) 120dB.

However ... the input impedance is the input impedance of the audio input of the radio, modified by the long curly lead and jack plugs. The output impedance should match the input impedance of the electret condensor capsule. Both of these are unknown, and only really measurable with complicated test gear. But I know the capsule measures about 2000 ohms with a Digital Volt Meter, so I did another simulation using that impedance.

I also extended the lowest frequency down to 100 Hz to see what was happening in the audio range. It is very different. However the flat response up to 100kHz will still pass all the audio frequencies ok, and the filter still appears to offer significant attenuation in the UHF region.

The dashed line, by the way, is the phase response, using the scale on the right hand side. The solid line is the amplitude response using the decibel scale at left.

It would appear that the designers of the filter may have simply designed a 50 ohm impedance filter and hoped it would do the job.

Another thing I noticed when measuring the inductors on the board was how low the Q factor was. The meter said 0.031, with a series resistance of 0.186 ohms. It made me wonder whether the inductors were of the conventional kind, or whether they were the special EMI absorbing sort, like these from Wurth Electronics WE-CBF SMD EMI Suppression Ferrite Bead
I think these tend to be deliberately lossy to absorb the RF energy, while at the same time having a low d.c. resistance. There are no markings on the parts fitted on the board, so it is difficult to tell what they are. One would normally expect the Q factor of a surface mount inductor to be, say 30 - 40.

Still not sure what to put in my hands-free set-up!
73
Hugh

Microphone and Speaker Connections for TYT MD380

Just over a week ago I bought a new radio, a TYT MD-380 DMR handheld. A new repeater, GB7AS has just opened, about 6 miles from here, and I am keen to access it because it has very good coverage in this area. Indeed, with the links to other repeaters in Kent it should provide opportunities to work many of the amateurs in the the Folkestone area that have been in the shadow of the North Downs for me here.

There is plenty of information about DMR on the Internet already ... although, later, I may blog a bit about using the programming software. I need to get to grips with it myself, first!  One of the things I wanted to do was to make a hands-free set up for the car so that I can take it mobile. However the manual contains no pin-out information for the microphone and speaker, and I couldn't find much about the subject on the web. The situation is complicated by the fact that the same sockets are used for a USB programming cable. So I thought I would write a bit about that here.

I decided to buy an inexpensive speaker mic for the radio, and use that to find out what the connections are. So I bought one of these: http://www.ebay.co.uk/itm/121920693163
It is a Retevis 2 Pin PTT Speaker Mic for TYT MD380 (and other radios).

Picture of it plugged into the radio at left.

The microphone is surprisingly small, but quite comfortable to use. The sound is even more thin and tinny than that from the radio, which already has a sharp and penetrating quality.

In the photo at left, just where my little finger is and where the cable enters the microphone body, is a socket to plug in an earphone. You can see this more clearly in the photograph of the circuit board below.

There is a useful clip on the back which partially obstructs the top screw, you need a well-fitting screwdriver to avoid slipping out of this.

The PTT button has a nice feel to it with a tactile click to let you know the contacts have closed.

After plugging in the speaker mic and checking that it basically seemed to work, I set about removing the screws from the back.

There are a further three screws to hold the circuit board to the moulding, and you need to be careful not to loose the little rubber pad which goes between the PTT lever and the button on the board.

Quite a nice PCB inside, with rather more components than I expected.

Tracing out the circuit, I can see that the surface mount capacitors and inductors are for filtering. Now this is interesting, because I have always taken the approach that you want to prevent RF from entering the microphone socket of the radio, but here the filter seems to be preventing RF from entering the Microphone capsule. Since all electret condesor mic capsules contain a little pre-amplifier (usually just a FET transistor), RF voltages picked up on the microphone cable could potentially demodulate in the microphone. The situation is made worse by the fact that the microphone is likely to be used in very close proximity to the aerial. I have traced out the circuit, but not yet measured all the values. The measurement of the L's and C's requires their removal from the board as the other components affect the reading on the meter.

The connections are helpfully marked on the PCB, so all that was required was a continuity test to find which part of the jack plug was connected to which wire in the microphone. The unused (tip of the 3.5mm plug and ring of the 2.5mm plug) are presumably for the USB programming connection.

I note, from the specifications in the radio handbook, that the audio output is 1W into 16 Ohms. I measured the d.c. resistance of the speaker to be 10 Ohms - suggesting probably IS a 16 Ohm part (The d.c. resistance of a speaker usually measures a little lower than the a.c. impedance, as the latter includes the inductive reactance of the voice coil).

At this stage, I am not sure whether to include the filter circuit in my hands-free set-up. In the car, the antenna is further away from the microphone, so the fields should be much less. I may just put some small capacitors across the mic and see how well it works.

The other question I need to mull over is whether to get hold of a 16 ohm loudspeaker, or to try using the 3 ohm speaker that I am currently using in the car. If a 16 ohm speaker is not available, I guess I could build myself a little audio amplifier - but that adds extra complexity and would need a 12V feed from an accessory socket in the car.

So that is the story so far. I hope my look at the audio connections might help you make useful accessories of your own.

73
Hugh M0WYE

Update 13/6/2016: I measured the component values in the filter circuit and added them to the circuit. Also corrected a slight mistake where the PTT button connects to ground.

DX TV signals from Poland

The Sporadic E season is well and truly "happening" with an opening to Scandinavia on the 25th, when I worked SM5ZCJ, OZ2KEC and 9A2RD all on 6m. The next day it was even better, and I worked SM6CXS, YL2SW, ES5QD, LA5YJ and YL2GP. On this day I heard 2M0UAL working another UK station, this Scot was only 520 miles, much less than the usual 1000 miles. A shortening skip distance often indicates that higher bands are open too and Facebook posts on the 70MHz group confirmed that that band had been busy too.

Last night was fairly quiet, although I did work EA7JUR (and his pet cat).

Well today, it has been Poland coming through loud and clear. I worked Roman SP9RM and Bogdan SP4LVC. While I was waiting for my chance to work Bogdan, I noticed that there was a buzzing sound, which is TV frame buzz. It is quite distinctive because the sound of the buzz changes each time the camera angle changes. So I quickly got out my 5" black and white portable TV set and connected the 6m antenna to it.

I was quickly rewarded with, first synchronisation bars, and then, pictures. Well they are pretty grainy, but at times they were clear and watchable.

I also tried to make a little video to show how the pictures were constantly fading in and out. I put that up on Youtube, here. https://youtu.be/gL466VSMKaU

Unfortunately, in Europe, the TV frame rate is 25 frames per second, which doesn't work at all well with the 30fps frame rate of the camera. The result is an unstable image with black bars which weren't on the original... but it kind of gives the idea.

For the record, my antenna is very simple, just a 1/4 wave ground plane - that's a 1.5m long whip, poked through a hole in the ridge of the roof. The felt underneath the tiles has an aluminium foil covering, and that is connected to the shield on the coax, so the whole roof becomes a ground plane.

73
Hugh

External Speakers on FT847

When operating a special event station it is useful if the visitors can hear the audio output of the radio, and it is also useful if the operator can wear headphones - because there is always chatter and other noise going on in these places. But, when you plug headphones into the Yeasu FT847 the loudspeaker is switched off, so no one else can hear.

I found a fairly easy way round this. There is a second audio output available on the rear of the radio which has a 200mV output level, that is independent of the Volume control on the front panel. This is on a stereo 3.5mm jack socket marked Data In/Out. It is intended for connecting a packet TNC, or other data-mode device to the radio, but the audio quality from this socket is good.

Red arrow shows the Data In/Out connector on the back of the radio.

 The output is about the right level to drive a pair of PC speakers, but there is a snag ... The audio out is on the ring of the jack and the tip is used for PTT and for audio in, so if you plug the stereo plug from the speakers into this socket the radio goes straight into TRANSMIT! This is because the input impedance of the speakers is sufficiently low to make the radio think that the "TNC" has selected transmit. So I made up a simple adaptor out of a plug and socket, wired as shown below ...

Do test the speakers with the radio on transmit before using them at the event, because PC speakers are notorious for being sensitive to RF fields and may demodulate your SSB transmissions in an unpleasant way! Fortunately the speakers lent to us by Chris, G6AFY, were of high quality and behaved very well.

So here is a picture of me operating GB8WW with my FT847 and the PC speakers plugged in the back, the stylish, wedge-shaped thing in the foreground. If you look carefully you can see the adaptor-lead plugged into the back of the radio.

The headphone volume is controlled by the AF gain control on the front of the rig and the PC speakers are controlled with their own volume control.

One of the criticisms of the FT847 is the "steppy" digital AF gain control on the rig, making it hard to get the right listening level - so using an external speaker on this audio output might be a way round this in the shack - so this adaptor might be useful for more than the occasional special event station.

Happy Operating, and 73
Hugh M0WYE

Headset adaptor for FT847

It is nice to have both hands free when using the Amateur Radio - especially when operating a Special Event station or contesting, because you can fill in the log, and fiddle with the radio controls while talking. And  ... your thumb gets tired using a fist mic!

With the popularity of gaming and Skype, some nice headphones with microphone booms are available quite cheaply. I bought one of these: "Gearhead" AU3700S Computer headsets for £10.91. This one has 3.5mm stereo jack plugs for the microphone and headphones. I noticed that quite a lot of the headsets now come with a USB plug, which presumably has a USB soundcard built in. Not much use if you want to connect to an old analogue wireless set! Make sure you get one with jacks.

The headset is actually terminated in a "four ring" jack plug, suitable for Apple devices (apparently) but comes with a Y adaptor with a separate plug for headphones and microphone.

To use the headset with a radio we need some kind of adaptor box. This will have socket(s) for the headset and leads to plug into the microphone and headphone sockets on the radio. It will have a Transmit/Receive switch or PTT button, and with supply a bias Voltage to drive the electret condensor element in the microphone.

So this is what I built. I decided to stick with the "Y" adaptor on the headset, because it means that the box becomes more versatile having separate sockets for mic and headphones. I wanted the box to be well screened because there maybe high RF fields around, so I used a diecast aluminium box, from Hammond Manufacturing (Pt. No. 27969PSLA) - it is one I picked up last year at a Radio Rally. I found some flexible 9-way screened cable - more conductors than needed but I doubled up the ground connections to try and keep the impedances low.

But we have to be careful not to introduce ground loops. There is the cable screen, there is the microphone ground, there is the headphone ground and there is the PTT ground, and really they should all be kept separate. Trouble is a lot of jack sockets have a metal fixing nut that connects the barrel of the jack to the metal box that they are mounted on.

Fortunately I found some inexpensive, 3.5mm Jack sockets on E-bay which have a single (8mm dia) hole fixing, but are completely insulated from the metal panel.

Gold-plated too!

The FT847 manual has a useful diagram showing the microphone connections. The headset connections are fairly standard - in fact there is an industry standard which specifies the microphone and speaker connections. On the microphone the tip carries the signal and should be capacitively coupled, the ring carries the bias Voltage which must be less than 5.5Volts and sourced from an impedance greater than 2.2 kilohms. I used a 10uF solid aluminium electrolytic capacitor and a 3k9 resistor for coupling and bias resistor. Seems to work well.
Here is a circuit - drawn on CAD as you can see :-)

The mic connector is viewed looking at the front-panel of the radio. So it is viewed from the solder-terminal side of the (female) microphone plug.

The "spare" pins on the mic connector are used for "up" and "down" buttons on the microphone - but I haven't used them here.

The headphone circuit is kept completely separate - partly because the radio offers the facility to listen to your own SSB transmissions, so we don't want any audio getting back into the microphone socket. I used RG58 for the interconnecting cable, as we only need a mono signal for the 'phones.

the TX/RX switch is just a bog-standard DPDT toggle switch.




So here are a couple of pictures of the "innerds".

 I have tywrapped the incoming cables to the body of the jack socket. There may be more elegant ways of strain-relieving the cable but this seems to work ok.

The big fat grey wire, is actually the braid from the multicore cable being soldered to a solder tag on the die-cast box. Might have been better if this was a shorter connection from an RF screening point of view, but hopefully it will keep HF frequencies out of the box.








The finished box looks like this:

On air reports have, so far, been favourable. One thing to note is that the output of the electret condensor mic is much higher than the standard fist mic supplied with the radio. I used a deviation meter to check the FM modulation level, and found that I had to reduce the FM microphone gain from 32 to 12 in the menu system. Menu 25 [FM-PSET]. I also went through the procedure for setting the SSB microphone gain - setting the meter on the rig to measure ALC using Menu 24 [TX-MTR]. Strangely the SSB mic gain is set close to where it usually is. The "Moni" feature on the radio is great for listening to your own input, you can set the Monitor Volume using menu 20 [MONI-VOL] - but unfortunately you can't monitor when using the speech processor as it uses the same circuitry!

So there you have it, a Bank-holiday-Monday headset to FT847 adaptor box.
73
Hugh M0WYE

Further notes: 

The 3.5mm Jack is an EST Part: Part no. MJ073H, from Ebay, here http://www.ebay.co.uk/itm/351607508978

8 pin microphone connector also from Ebay: http://www.ebay.co.uk/itm/111965139338
But found the thread on the locking ring was poorly made, and won't screw up on the socket - so I will have to change it ... a pigging nuisance!