Dec 042018

Wednesday 21st saw the LSWC hold their annual Construction Contest for the Joe Rose Memorial Trophy. Last year saw us two different versions of ATV transmitter. One being for home use including a partly assembled water cooled power amplifier (For an update see below) built by Adrian M0NWK. The second, and also the winner for last year, being a fully portable transceiver in a case built by Jason G7KWP.

Adrian M0NWK

This year Started off with a similar theme. Adrian M0NWK, started off by showing us his Arduino powered controller for a comtech 23cm ATV transmitter.








This was achieved by removing the dip switches from the transmitter and replacing them with a SSR (Solid State Relay) board. These relays are then controlled by the Arduino which also shows the selected frequency on the front LCD.

Adrian also showed us the completed Water Cooled Power Amplifier which was shown the previous year. Also shown was an Arduino controlled Temperature monitor.









Andy G0FVI

Next up was Andy G0FVI. Who has constructed an 80m CW receiver which will eventually become a transceiver in time. By choosing an IF of around 10.695 this allowed him to use bits from numerous scrap 11m radios he had laying around.












  • The bandpass filter was fabricated from two Toko type ferrite core coils.
  • The first mixer is based around an SA612 chip and also provides amplification. In retrospect this may have been a bad idea as the chip does produce a few ‘birdies’ although they are well below main signal level.
  • The VFO is an Alan Lake design for 80m with the L/C of the tuned circuit changed to give an output of 7.2-7.1 MHz.  Output is buffered in the VFO enclosure and this is fed to the 1st mixer.
  • Output of the 1st mixer is fed to a 10.7 MHz XTAL filter with a bandwidth of about 15-20KHz.
  • The IF amp is based around an MC1350 IC.  Negative DC feedback from following stages provide AGC action (fast/slow).
  • Demodualtion to provide audio out is achieved using a diode ring mixer. Such a mixer provides a nice clean output with little spurious signals. A 10.695 MHz signal is fed into the ring mixer (together with IF).
  • Demodulated audio is fed to a 2 transistor pre-amp which also provides DC feedback for AGC. Audio is sampled at this stage and fed to another transistor amp to drive the signal meter (taken from an 11m walkie talkie!).
  • The audio stage is based on a rather ancient LM380N!

All stages were built and tested using a sig gen/scope etc. before being connected. Great care was taken with the construction of the VFO, whose spectral purity I was finally able to look at using an SDRplay RSP1A and a beta release spectrum analyser app.

Controls are IF (RF) gain, volume, RIT, AGC fast/slow and VFO.

When I complete this I will add a prefabricated board for PA and some TX/RX switching (should just about fit). I will sample VFO and mix with TX XTAL oscillator, add some filtering and off to PA. Might be an idea to have a go at matching XTAL filter in the receiver too.










David G6MAF

Next up was David G6MAF. Who had constructed a RF tap, or signal monitor which covered 2-30MHz. A device like this is very useful for monitoring your own transmissions by connecting straight into the feed line. With 50 ohms through to the antenna from the transmitter, a second output is taken and attenuated for feeding into a device such as a spectrum analyser etc…











For the final item we made full circle as Bob, G4PDF, showed off his portable 5GHz ATV Transceiver. The equipment was all installed in a box which was mounted to the back of the dish which was then fixed to a tri-pod. The transceiver included a screen overlay showing his Callsign and also converted a received GPS signal into a locator which is also displayed.











The Winner

It was at this point after all had builds had been shown that contestants left the room for the remaining to vote on who’s they thought was the best. After discussions about complexity, imagination, modules or scratch built & uniqueness, the vote was in.


The contestants were called back in and the Winner was…… Bob G4PDF, with his 5GHz ATV.


The trophy will be awarded at the clubs AGM.

Apr 292018

This part of the project is to help you understand how we are going to control the motors of the rotator and we are deviating from the 2rpm motor to a higher rpm to be able to quickly move from one location to another at a higher speed then slow down to be able to follow the path of the satellite.

In the previous post I gave links to the parts needed and for this part of the project, we will be using the H Bridge, 2 motors, and the Arduino Nano.

I have found a good video which explains the principle of driving the motors, but before watching it I thought it better to explain a bit about writing code for the Arduino. The code is known as a “Sketch” and contains all the information for the Arduino IDE to compile it and load it into the Nano so that it can run the program.

A variable is a number that can be changed by the sketch and is usually given a “label” that identifies it in the sketch, say for example we define “Int variable1 = 5”, “Int variable2 = 2” (Int tells the compiler the type of variable it is and in this case an integer) and later as the sketch is running we want to perform a mathematical calculation and change “variable1” to another value “variable1 = variable1 + variable2;” (the “;” defines the end of the line) the microcontroller will process this and “variable1” will now = 7 (5+2). Another way to increase the value of the variable is “variable1 ++1” which will increase the variable by +1 and tends to be used in loops. See here for more information about the syntax for arithmetic calculations and more information of the programming syntax can be found here

(For those who want more information,  when the sketch is compiled, the compiler changes the label “variable1” to a fixed memory address which contains the value of the variable so calculations result in the microcontroller changing the value of the fixed memory address.)

The next post will be on the magnetometer and how we use it to control the motors of the rotator.

Apr 172013

After serious thought on the layout of the components on this Linear Amp especially the placement of the filament transformer and the position of the RCA 813 valves for cooling, I decided that the best thing to do was to remove everything and reposition it better within the chassis. Initially my thoughts were to sort the amp out and get it working keeping the same layout, but as I progressed I was not happy with the layout, so this has now actually turned into building a 1kw Linear Amp from some recycled parts as I am now modifying a lot of the old amp.

linear amp chassisThere is going to be major changes to the top of the chassis as the aluminum screen separating the HT transformer from the tank circuit is going to be moved down slightly and the meters in that part are being taken out and repositioned in a suitable place. The HT transformer is going to be moved forward to the front of the linear to make room behind it for the filament transformer, as I didn’t like it being near to the PA tank circuit.

The plate choke is being replaced with a better one that I have made which has considerable more inductance and a very low self resonance frequency achieved with the insertion of a ferrite rod in the middle of the choke.

The valves are going to be moved to where the filament transformer is presently located and a fan fitted into the back of the chassis behind them sorting out my concerns about proper ventilation and while doing this, the vent in the front is going to be closed off and more ventilation put on the top of the amp. (You don’t want warm air blowing in your face when you are operating.)

The PA tank coil is being replaced as I have found a better design that enhances the 10M band which is the Achilles heel of the RCA 813 Valves due to the high inter capacitance of the electrodes in the valve. Using the new coil design should make it possible to get better power on the 10M band.

The last change to the amp is to replace the ceramic band switch in the PA as it is HUGE and takes a lot of room up, this will be replaced with a suitable smaller ceramic switch which I already have.

Although this may seem a lot of work, in the long run it will make the amp better and easier to work on and I only wish I had started the project with this concept rather than follow the original build. I shall put it down to experience and now go forward with the project in a positive manner.

For anyone thinking about building a linear amplifier a good starting point is the Matt KK5DR Website.

Apr 122013

Because of finding dangerous faults during this project, this Linear Amp was not a refurbishment but a complete rebuild. I would advise others not to do this work unless they are experienced in what they are doing, as inexperience could result in DEATH as LETHAL VOLTAGES are involved. For reference, this linear is not pretty and is being brought back to life as a working project and will only be used safely in my shack, it will not be entered into any beauty contest.

Old Valve Base FixingValve Bases RewiredI changed the way the valve bases had been fitted to provided better ventilation by removing the plate that the bases were attached to and using 4mm 2 inch screws refitted them below the chassis using the screws as spacers (it is recommended that you fit the base of the 813 is below the level of the chassis with just the glass envelope above it).Valve Bases From Side I had to rewire the bases as the grids had been directly wired to earth instead of via a .01uF 1000v capacitor and a 4.7R 1 watt fuse resistor (still to arrive and be fitted) which is a safety precaution as you can’t be too safe with linear amplifiers. Right of the valve bases is a new homebrew filament choke.

Things are plodding along nicely as the bleeder resistors for the capacitor bank arrived and were fitted and a mock up of the power supply using another HT transformer I have was assembled and fired up via the Variac transformer so I can start to re-form the capacitors.

Forming the Capacitor BankThere was no big bang and I am now slowly increasing the voltage across the capacitors till I eventually reach the top voltage with this HT transformer which is 2.5KV and it should then be OK for the HT transformer on the linear amp where the voltage across the capacitor bank will be 3KV. (I just hope there is no big bang or the cat gets electrocuted.)

Homebrew Filament TransformerThe fitting of the hombrew filament transformer shown on the left with the RCA 813 Valve is being delayed as I rewire the linear, as it is heavy enough turning it over with just the HT transformer fitted, I have replaced the filament choke and decoupling capacitors and wired it up and all it needs is wired to the transformer when it is fitted. The principle jobs at the minute are rewiring the mains input circuit and fitting a soft start to the HT transformer, removing unwanted components and making a hole in the front panel for the HT voltage meter so things are just plodding along.

<<< Part One <<< Part Two >>> To Be Continued

 Posted by at 11:43 pm
Apr 092013

813 Filament CircuitRefurbishing the Linear Amp was still in progress, and because I was changing the old filament transformer for a proper center tapped transformer suitable for the amplifier, I had to change the circuit to take this into account.

Another modification to the amplifier was the cathode bias circuit as the old circuit did not have one. R5 is a 50K 10 Watt resistor that biases the valve at cut of when not transmitting keeping the valves cooler and D5 is a 5 volt 10 Watt Zener Diode which biases the valve for transmission.

For those not familiar with the grounded grid configuration D5 and R5 raise the voltage of the cathode above earth making the grids negative in respect to the cathode. on large transmitting valves, the heater filaments and the cathode are connected so share the same circuits.

D6 – D8 1N5400 3A  glitch protection diodes for the meters in case of a negative going High voltage spike. F4 a high voltage 1A fast blow fuse provides further protection in the event of a catastrophic failure. All capacitors are .01uF @ 1000 volts and M1 = 1 amp meter and M2 = 50mA meter

It was time to check the capacitor bank and I removed it from the Linear Amp and checked each capacitor individually for shorting / OC, I found two resistors were open circuit and there were no protection diodes across the individual capacitors so I decided to strip the bank down and replace all the resistors and add protection diodes. At the same time I hot glued the capacitors to the original perspex in way that the air flow could go round them everything was completed on the capacitor bank apart from the bleed resistors which had to be re-calculated.capacitor bank

This is where the health warning needs to be placed. Warning: Extreme Caution, Lethal Voltages, High Voltage WILL KILL  I checked the HT transformer and found this to be OK with an output of 1.2KV which will give about 3KV with no load on the voltage doubler. Good wire wound resistors are hard to find so I recalculated the the bleed resistors for 2 watts and needed 562.5k resistance so I ordered some 680k 2watt metal film resistors as they would do the trick though the discharge time of the capacitors would be a lot longer.

Diode ChainI clipped the diodes from the diode chain and replaced them with 3A 1000piv diodes, but before this I checked the balance resistors which proved withing tolerance so this was another job done and ready to fit.

While I waited for the parts I had ordered, It was time to refit the valve bases so that more air could pass through and fit the filament transformer and I now also had to make a suitable voltmeter for the 3kv voltage, things were beginning to take shape now on the refurbishment / rebuild.

<<< Part One >>> To be Continued

Apr 042013

G2FHM HF Linear AmpThis is a series of articles doing a step by step refurbishment on an old homebrew HF linear amplifier. It has to be taken into consideration that when the Linear Amp was built everything at that time would be valves which were more forgiving unlike the transistors of today. The amp was 2 X RCA813 valves wired in grounded grid configuration and the input was fed directly to the cathodes of the valves with no matching network. (this was common in the old valve days)

Blocking Capacitor Inside blocking capacitor The first step was a visual examination of the linear and I was suspicious about the wax on the chassis below the plate blocking capacitor and the coating of the other capacitor wired in parallel  on top of it had also melted. I removed the blocking capacitor and found that it had completely burned out, I would have to replace this with a suitable door knob capacitor.

Linear Inside topThe large 10v filament transformer shown in the picture above is not suitable for the linear as it is not center tapped and will cause a 50hz modulation to the output, so this must be replaced with a proper 10v center tapped filament transformer. The filament transformers are hard to find, but I have a suitable transformer that I have rewound so the original one will be replaced by my homebrew one.

The 15 watt bulb on the left of the picture above was used as a soft start for the power supply and was in circuit when the linear was initially switched on (the 813 valves did not need the time for heating up as other valves do before HT was applied) when the HT switch was turned on it shorted out the bulb for full power to the HT transformer. This was going to be replaced by a proper soft start relay so that the HT switch would be completely separate from the main switch.Diode Bridge

A 200 ohm potentiometer at the back of the linear was completely burned out and with some unconnected components and wires and two diodes on the rectifier board were shorted out with a piece of wire.I never gave this a second thought, the whole thing was going to be replaced with new components.

Because I was changing the filament transformer this meant that I had to also change the metering circuits, I decided that rather than a refurbishment, for safety this was going to be a major rebuild. I was also not happy with the cooling system on the Linear so I was going to change the layout of the fan’s for better ventilation.

100_0486Turning my attention to the electrolytic capacitors on the power supply, visually they seemed OK and the bleeder resistors across the capacitors were all wire-wound and looked OK. The next stage was to make a new diode chain as I did not like the old one and try to re-form the capacitors and check them to see if they were all alright, I would use a variac transformer so I could slowly increase the voltage across them.

813 valve basesI did not like the wiring on the valve bases so decided to cut all this out and rewire the valve bases and make sure that they had a good earth as I was not happy with this part of the build. I removed all those components and the valve bases and while everything was disconnected it would be a good time to alter the chassis to improve the air flow round the valves.

My final check before starting anything was to check the aerial switchover relay and this proved to have been arcing so a new relay would have to be fitted.

I had set my stall out on what needed doing, all I had to do now was find the time to do it. Time to buy the XYL some flowers and a big box of chocolates so I could work on rebuilding the Linear Amp.

>>>Part Two

Mar 302013

Electrolytic’ Capacitors may smooth out our ripples but they do need some TLC or they will blow your equipments main fuse. They do benefit from a regular top up of electricity but some are neglected and need a little bit of extra TLC to bring them back to life.

electrolytic capacitorThis short article is covering the poor neglected electrolytic capacitors that have not had an electron pass through them for a very long time and is to give you advice on giving them that extra TLC. More information on electrolytic capacitors can be found on the Wikipedia. While this topic is on unused equipment it is also relevant to New Old Stock capacitors recently bought.

Important!! There is controversy about using the methods in this article  on switch mode power supplies. If your equipment has a  switch mode power supply, then use at your own risk!

If a piece of equipment has not been used for some time that has a linear power supply, the electrolytic capacitors electrolyte deteriorates and can in fact blow the main fuse of the equipment straight away. Caution is required before switching on the equipment and a reduced voltage on power up usually helps.

A technique called re-forming is used to bring the capacitors back from the grave and all that is involved, is to apply a small voltage to start with and gradually over time increase the voltage till you reach the maximum voltage. On equipment this can be achieved by powering it up with a Variac transformer and slowly increasing the voltage over time. A quick method that can be used is to put the equipment in series with a 40 Watt light bulb and the same effect can be achieved though not as good as using the Variac Transformer. Basically what you are doing is using electricity to re-form the electrolyte dielectric see Wikipedia.

Warning! It’ is OK to start with reduced input but does not always guarantee ‘no bang or smoke’ !!

Mar 302013

Having recently started being interested in HF Linear Amplifiers I needed a voltmeter to measure the High voltage on the anodes of the valves, I have a Bradley meter but it will only go up to 1.2 KV and I needed a 2.5 KV voltmeter to monitor the voltage on the Linear Amp that I was building.

100uamp MeterI had a 100uA meter in my junk box with a scale of 0 to 25 so I though that I would use that with a series resistor, (forget what it says on the scale as this can be easily changed) so the next thing to do was the calculation for the voltmeter. To find out the value of the series resistor using the formula V/I = 2500/.0001 = 25 Meg-ohms minus the internal resistance of the meter which was 540 ohms so the series resistor would need to be 24,999,460 ohms for FSD and to find the power to be dissipated in the resistors using the formula W=V X I = 25000X.0001=.25W. (I would recommend a 100uA meter because of the power rating of the dropper resistor, a 1mA meter would need a power dissipation of 2.5Watts)

For the dropper resistor I decided on a chain of .5 watt metal film resistors, these have a voltage rating of 500volts so you would need a least 6 resistors in series to be on the safe side. I decided to be extra safe and have a chain of 19 X 1.5 meg-ohm resistors ending with a variable resistor for calibrating the meter. Although not necessary it is best to put the variable resistor and the meter on the earth side of the circuit where the voltage in the circuit is low.Dropper Resistor Chain

To calibrate the meter, I used a Variac transformer to set the voltage on the power supply to 1KV with the Bradley meter then using the variable resistor set the voltage on the 100uA meter to 1kv and the job was done and the meter was ready for wiring into the linear amp.

Although this meter has been designed for 2.5 KV using the formulas above you can measure any voltage. This article comes with a health warning HIGH VOLTAGE KILLS, make sure your dropper resistor chain is properly insulated as high voltage can arc across an air gap. If you want to build a stand alone voltmeter for high voltages make sure you use a good insulated box to put the meter in and hard wire the high voltage leads into the insulated box, High Voltage Probes are essential.