70W HF Amplifier

Posted on

Last year I bought a very cheap Chinese 70W HF Amplifier kit. I bought it with the intention of adding the amplifier to my 2w PW Dart DSB transmitter. I never really got round to building the amplifier kit until now. I had already bought a suitable heatsink so I set about building up the kit. There is lots of information about building it, including the circuit diagram on the PD7MAA website http://pa-11019.blogspot.com/2016/11/diy-kits-70w-ssb-linear-hf-power.html

Building the kit was straightforward as all the SMC devices were already fitted, it was mainly mounting the inductors onto the PC board. The board seems to be quite good quality. I mounted the board onto the heatsink with plastic washers to keep the board from touching the heatsink. The Mosfets were a little tricky to mount as I had to sure that the leads were very short.
My first attempt at testing the amplifier began by applying a 12v supply that was limited to 2amp with no input and a dummy load on the output. This enabled me to set the bias voltage to the Mosfets. The recommended bias voltage is 2.7v, but reading some of the articles about the amplifier it seems that a voltage of 3.7v gave the best results. I set the bias to 3.4v for testing.  I now needed to change the 12v to another supply that delivers more current, in this case my main PSU. I fitted a 10amp fuse and connected the amplifier to my Yaesu FT857D set to 5w output.
At first the amplifier gave an output of 70w on 80m, but after a couple of minutes the 10amp fuse blew and the Mosfets went short circuit, not good! After changing the Mosfets and fuse I tried again, the same thing happened. One major problem with this amplifier is that the Mosfets seem to go into overdrive and draw more and more current until they destroy themselves. A solution to this was to add a current limiter circuit making it limit the current being drawn to 4 amps, above that shuts down the bias circuit to protect the Mosfets.

I replaced the Mosfets again and this time I backed off the bias to it’s recommended 2.7v before testing again. Instead of the Yaesu FT857D, I connected the amplifier to my PW Dart, which only gives an output of 2w. This time it worked perfectly, giving an output of around 10w on Topband. I then focused on the bias voltage again and slowly increased the voltage in steps of 0.1v so that I could get an output of 20w. This gave a bias voltage of 2.9v and works fine.

The next part was to mount the amplifier into a box and add a fan to keep the Mosfets and heatsink cool.

I added an extra relay for the PTT and an RF detector to make the blue LED flash when RF was present.

The finished amplifier with built in fan.

I tested the amplifier on air with the PW Dart DSB transmitter on Topband with Ian G0CTO one Sunday morning. Reports were very good, getting a healthy 20w output and sounds linear without any splatter or distortion.

LoRa Receiver for HAB

Posted on

Earlier I talked about tracking High Altitude Balloons by receiving the RTTY signals from the balloons and reporting back to HABHUB using FLDIGI. Ross G6GVI has been using a more up to date mode for tracking the balloons using LoRa (Long Range) on a Raspberry Pi computer. As I have a Raspberry Pi 3 going spare at the moment I decided to give it a try. First I need a LoRa board which are available very cheap on eBay (around £5 inc postage) the board is very small and needs connecting to Rpi using dupont jump leads.

Next, the software needs to be downloaded and installed, this was very straightforward and when installed it produces a LoRa Gateway to receive signals and report back to HABHUB. LoRa is a very efficient mode as the balloon fitted with LoRa only runs a few milliwatts of RF. As well as telemetry, LoRa also sends Slow Scan Digital pictures from the camera fitted on the balloon. This is decoded in the software and stored to view later. Once my LoRa system was built I arranged a sked with Ross g6GVI who is about to test his LoRa transmitter. We set up on 434.450Mhz and Ross sent me packets of data. The system worked a treat and I was able to receive and decode the signals.
Now I need to test it with a High Altitude Balloon fitted with LoRa, trouble is that this might take a while as we are going into Winter now, so not many balloons are being launched.

Exploring 13cm

Posted on Updated on

Having tried to receive some stations during the 13cm UKAC Contests, it is now time to try transmitting. First of all I needed to register my interest in 13cm with OFCOM as the band is shared. Mark M0UFC has kindly loaned me the Bolton Wireless Club’s ‘Phoenix Transverter’ which gives an output of around 400mw. This along with a 16db gain Patch Antenna gives me the ability to work stations on 13cms.

My first contact was with Ross G6GVI on the other side of town, we successfully exchanged reports. My antenna is not in an ideal place, but was the best I could do without running a long length of Mini 8 to the garden. I now need to get a length of RG213 or similar for the next contest. In the contest I worked two stations G6GVI and Dave G4JLG/P who is on Winter Hill. Very pleased for my first attempt at this new band!

The Bolton Wireless Club ‘Phoenix Transceiver’.

Patch Antenna on a pole at side of house.

My first ever 13cm QSO

High Altitude Balloon Tracking

Posted on

Following an interesting talk by Ross G6GVI about tracking High Altitude Balloons at The Bolton Wireless Club I decided to give it a go.

High Altitude Balloons are Helium Weather Balloons that carry a payload into the edge of space. These are usually projects associated with schools and colleges linked with STEM. Telemetry is transmitted on the 70cms band with very low power, about 10mw, which can be received and tracked using software. Today there were two balloon launches around Oswestry in the Midlands, the first one is called ‘Hi-impact’ and the second one ‘Spacecamp’. I could copy both balloons on 70cm, the Hi-Impact was using 50 Baud RTTY and Spacecamp using 300 Baud RTTY. Using the dedicated software package DL-FLDIGI I was able to decode the 50 Baud RTTY with no problem, but could not decode the 300 Baud. When the packets are received the software automatically sends the data to HABHUB which shows on the tracking map that my signals have been received. All clever stuff!

Screenshot showing the DL-FLDIGI software receiving and decoding the Hi-impact balloon. In the background is the HABHUB tracking and mapping of the two balloons.

Perseid Pings

Posted on

A bit of Radio Astronomy here…

Here are some of the meteor reflections from the GRAVES transmitter on 143.050Mhz last night during the Perseids Meteor Shower. The peaks can be heard as tuneful ‘pings’ when a meteorite enters the atmosphere. The pings are displayed using ‘SpectrumLab’ software. I am using my Yaesu FT857D along with a 5 element yagi set to 143.049Mhz and USB.

R109 Receiver Renovation

Posted on

Back in 1969 I bought an R109A receiver from a shop in Bolton that sold lots of ex-government radios. It was Arthur G3JJM that told me about it and he went to buy it for me at the knock-down price of 30 shillings.
The R109 covers 1.8Mhz – 3.9Mhz and 3.9Mhz – 8.5Mhz, I used it as my first receiver and listened on Topband and 80m every night. It was powered by two 6v motorbike batteries that I kept under my bench.

Here I am back in 1969 with my original R109A receiver.

I gave the R109A away after I got a Hallicrafters SX24 receiver. Since then I have never seen an R109A anywhere, I always wanted to acquire one as a renovation project, but they are very rare and although lots of searching over the years I have never come across one…until now!
This started with a conversation on 23cm with Dave G4JLG and Ross G6GVI. I was talking about the renovation of the Eddystone S750 when Dave mentioned that he had an old receiver in his shed that had been there untouched for over 40 years. He had no idea what it was except that it was square in shape. I suggested that it could be a CR100 or maybe an R109 as these were the only ones I could think of. Dave then went down to his shed to have a look, he came back and announced that it was in fact an R109. I nearly fell off my chair!
The next day Dave sent me some photos.

The front panel, almost unrecognisable. Note the yellow ‘T’ this identifies the receiver as a Tropical Version.

The inside of the R109…immaculate!

The outside was a mess and almost unrecognisable with the caked on dirt, however, this receiver is the R109T, which is the tropical version used during the war. When he opened it up the inside was immaculate as it was hermetically sealed, amazing! All the valves and wiring were as new, but there was no vibrator in the PSU. Dave managed to find a couple of old vibrators for me to try. He also sent me a manual and some notes on the R109.

Dave kindly let me have the receiver to be renovated, I couldn’t wait to try it out and bring back all those old memories!

Here I am 50 years on with the R109T ready for renovation.

Now where do I start with this receiver?

Well first of all I need a 6v battery!  A trip to a local motorbike spares shop got me 4AH battery very cheap. That will do fine for testing the receiver.

I removed the front panel from the main receiver ready for cleaning. This took a whole afternoon of scrubbing with warm water, detergent and toothbrush. The muck came off fairly easy and the more stubborn dirt was removed with some Sodium Bicarbonate. I didn’t use any harsh cleaner at all. When it was finished the front panel looked like new, even the decals were intact. The dial, fixtures and identification plate were removed and cleaned separately.

The finished front panel, written in pen on the yellow T is a callsign and name – G3RVR Dennis. Dave G4JLG thinks that this is where the R109 originated many years ago and was given to him.

The PSU is somewhat novel as it uses a vibrator unit in order to produce the HT voltage, also the valve heaters are directly fed from the 6v DC battery that powers the receiver. The PSU is a separate unit that can be powered on its own for testing. Close inspection of the capacitors showed that most of the electrolytics were either bubbled or leaking, so replacing these was my first job. The R109 is very well built, but unfortunately a lot of the decoupling capacitors are not easy to get to. Some are inside the IF cans that are welded to the chassis and others are hidden. I decided that I would replace all the decoupling capacitors that were accessible. It is the same with the resistors, I thought that I would leave them for the time being.

The R109 chassis with the SPU on the left and RF unit on the right.

The next job was the vibrator. I tested the PSU with both of the old vibrators that Dave gave me, but neither of them worked. Solid state vibrators are available from the 19 set group, but they are quite expensive. I did some research and reading about how mechanical vibrators work as well as finding out about the pin-out connections of the vibrators that I have. Taking the vibrators to pieces proved quite tricky, but I managed to remove the cans with a bit of brute force and ignorance! Removing the inside was quite easy and both in good condition. Using my multimeter it was obvious that the contacts of the vibrating bit needed a good cleaning. I spent around an hour carefully cleaning the contacts on one of the vibrators and then tested the unit….it works!
I put it into the PSU and switched on and monitored the HT voltage, then to my astonishment the receiver burst into life! Connecting an antenna brought in stations on both bands. All the controls are working and the BFO is also working, but very weak. The HT voltage is about 150v which is correct according to the manual.
This brings back memories of my original R109, I spent quite a long time trying to improve the receiver by aligning the RF and IF stages. I also added an S meter as well.

The receiver worked fine, but I noticed that the battery was only lasting for about 15 minutes before the HT voltage and LT started to drop. The receiver then began to shut down although the vibrator kept on going. Measuring the current drain on the battery told that it was drawing too much current. After a while the receiver stopped working altogether. After some fault finding in the PSU I realised that a 0.1uf capacitor across the secondary winding of the HT transformer had gone short circuit. After replacing, the receiver worked perfectly, now only drawing less than 2A from the battery.

A bit of touching up with the paint on the front panel, repainting the handles and painting the case green finished off the R109T.

The completed R109T receiver in all its glory.

13cm Crossband Contact

Posted on

Had an interesting X-Band contact on Tuesday night with Ross G6GVI. I set up the 13cm converter and antenna in my garden and we arranged a X-Band QSO between 13cm and Topband. I transmitted on Topband and Ross transmitted on 13cm. Copied Ross at about S7 on 13cm. Later, I listened to the UKAC contest and heard some stations. This is my first encounter with 13cm, maybe I will have a go at building a transmitter.

Here is a photo of the 13cm set up in my garden.