VHF NFD

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VHF NFD under lockdown is very different this year. Portable stations are banned and everyone has to operate from home. Each band is assigned a time slot to operate. I decided to operate on 50Mhz, 70Mhz and 144Mhz. Using my new Yaesu FT-991A this year!

 

VE Day 75th Anniversary

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To celebrate VE Day I took my R109 receiver out into the garden with all the celebrations going on. I listened to GB4VE quite strong on 80m.
I must admit that these celebrations were a breath of fresh air for everyone in the lockdown which is now in it’s 8th week. Beautiful weather all the way through, which made things easier and gave me chance to lots of jobs in the garden.

 

New roof and antennas

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At last after waiting 10 years we have finally managed to get the roof replaced along with my neighbour. This has given me the opportunity to take down the antennas and replace the mini 8 coax cable with f-Zero cable from Moonraker. This coax has very good specs and although the coax run is only about 8 meters it will make a big difference on 70cm, 23cm and now 13cm.  I decided to take down the 4m HB9CV on the stack and replace it with the 19 element 13cm yagi that Ross G6GVI has kindly let me have.

Before the roof replacement with the 4m HB9CV in the centre of the stack.

The new roof with new coax on the antennas and 13cm yagi in the centre of the stack.

 

Topband Regen Receiver

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Just completed a nice little project, a regen receiver for Topband. This started when I bought a pc board and circuit from G-QRP at a rally last year. The regen receiver is based on Rev. George Dobbs, which was in PW April 2013. The circuit was then improved by G4CFY of Spectrum Communications. A full kit is available including an additional board for switching bands up to 18Mhz. I may add one of these later.


At a later rally I came across an old Codar RQ10 Q multiplier. This was a valve circuit made to use with some of the other Codar receivers. I bought it to match my existing Codar equipment, but was not sure about actually using it as a Q-Multiplier for my CR70A. The regen receiver is really a Q-Multiplier working on a different frequency. Having given some thought, I decided to strip out the valve circuit and build up the regen receiver board and mount it into the case using the existing tuning capacitor and controls. The case was cleaned up and repainted.

The receiver works really well and the Codar slow motion drive is just the job for tuning.

Eddystone EC10

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I have never owned an Eddystone EC10 receiver, but I remember using one back in the 1970’s during VHF NFD Contests. I still have a photo of me operating the EC10.

The EC10 uses transistors that I am very familiar with, the OC71, OC81, AF114 Germanium Transistors. I built my very first projects in the late 60’s with these transistors. I would love to get hold of one of these receivers, but they are usually quite expensive when they come up for sale. After patiently waiting for one to come up on e-Bay, I stumbled on a very nice MKII EC10 and managed to get it at a good price. The receiver is in good condition cosmetically, but not sure whether it works or not. Seems to have the Power Supply Unit missing.

This looks like a very nice project, it is so nice to be able to work on the old transistors again, quite nostalgic!
When I powered up the receiver it worked, but seemed very deaf. My first reaction was that the RF Transistors, which are AF116, needed replacing with something more up to date, which is apparently a standard mod for this receiver. However, further investigation showed that there was in fact another fault. The RF Gain peaks in the middle of control for some reason and the AGC switch is not working correctly, very strange…
After taking measurements around the AGC circuit I diagnosed that one of the electrolytic capacitors needed replacing, after replacing the EC10 worked perfectly! I did consider changing all the electrolytics, but decided to leave it alone for the time being.
Now for the cosmetic restoration, lots of scratches on the case and front panel. I did not want to touch the calibrated tuning window, the paint on this is very delicate and even touching it will make the paint come away. A clean on the outside with soap and water was sufficient, the same with the front panel and knobs, they came up a treat! I covered the scratches on the black paint with a permanent marker, this always works well. The scratches on the silver case was covered up with Humbrol model paint. I finished off the front with a light rub with duraglit. Looks really nice.

I now need to fill that huge hole in the back where the Power Supply Unit was fitted. I decided not to make a mains power supply, but instead to make a battery unit with a socket so that a wall mounted PSU could be plugged in.

This has been a very enjoyable project, I use it with the batteries at the moment, a nice receiver.

 

ISS flyby

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Caught some nice photos last night of the International Space Station over my antennas.

The ISS fades as it enters shadow and disappears.

 

BC348 Renovation

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My latest ‘Repair Shop’ project has been an old BC348 receiver.  The BC348 was used during the war in the American B17 bombers, after the war thousands of the receivers were released and bought by Radio Amateurs. Several versions were made, mine is a BC348L and it had obviously been modified and hacked in the past. Although the receiver looks to be in good condition, closer inspection showed that wires had been cut and other components added including an additional audio pre-amp using an ECC83 valve.  I must admit that I was a bit reluctant to attempt renovating this receiver, it was going to be a challenge!

There is plenty of information on the internet about the BC348 including all the manuals. So where do I start with this beast?
First of all I needed to digest the circuit for my model. Originally, it had a dynamotor to supply the HT voltage, also the valve heaters were wired in series. There are seven huge oil filled capacitors on chassis which I assume are all decoupling capacitors. On the circuit they are 0.5uf capacitors and they all need replacing along with the other capacitors that are called ‘micamould’, they look like mica capacitors, but are in fact paper. Once I had a better understanding of the circuit and wiring layout I started to simplify things by removing the modifications and try to bring the receiver back to it’s original state. I removed all the string lashing that holds the wiring looms so that I could trace wires easier. I also removed the large Jones Plug as this is no longer necessary.
Now it is time to replace the ancient resistors and capacitors. Most of the components are mounted on tagboards, four in all. The RF and Mixer stages are inside the tin boxes.  I started with the oil filled capacitors and replaced them with modern ones and mounted them on an additional tagboard. This gave a lot more room on the chassis to work on the other boards.

New tagboard of decoupling capacitors to replace oil filled ones. Note the ‘micamould’ capacitors on the rear board that still need replacing.

Chassis after replacing all the resistors and capacitors. Looms have been undone so that wiring can be traced.

Tracing the wires from the circuit was very time consuming. Some wires had been cut or disconnected and others had been soldered into the wrong places. I just had to take my time and work my way through it logically so as not to make mistakes. The big switch at the back was another challenge, again, wires disconnected and some soldered in the wrong part of the switch. I had to draw diagrams of the switch and it’s wiring to be able to work it all out.

The heater wiring had already been changed for 6.3v parallel wiring, so I left this as it was.
At this stage I was beginning to see the light at the end of the tunnel, I now need a power supply in order to finally test this receiver.  There is a space on the top chassis for a power supply (where the dynamotor was) so I set about building one onto a piece of aluminium that was the right size.

The new power supply fitted into the receiver. I later added another piece of aluminium to mount the audio output transformer on the right.

All ready for testing now….I nervously switched on the power supply and to my amazement the BC348 sprang to life! I could hear broadcast stations, but not very loud. Further investigation told me that something was wrong with dual volume / RF Gain control, they were wired wrong. I managed to solve the problem and rewired the controls, switched on and it worked perfectly!  I was so pleased to hear this receiver working again after all the hard work. Still a few things to iron out, the bandswitch is very dirty and needs a good clean, the BFO is not working and needs looking at and the receiver needs aligning. No real problems solving these, the BFO was working, but has a very low output injection, I solved this by running a wire from the BFO output and wrapped it around the grid of the IF valve top cap. Now there is ample injection. Had to open up all the RF, Detector and Mixer cans to get to the bandswitch and give it a good clean. Alignment was quite easy with the BC348, all the trimmers are clearly marked.  The receiver works really well on the lower bands, but is a bit deaf on the higher bands, even after re-alignment. I will come back to that at a later date.

To finish off it just needs a bit of cosmetic attention. I cleaned the chassis with wire wool and metal polish. Cleaned and lubricated the drive mechanism. The front panel and case is in really good condition and just needed a good clean with water and detergent followed by a wipe over with WD40 which really brings up the black wrinkle finish. I used a black marker to cover up scratches.  A couple of holes in the front panel made by a previous owner needed filling, so I put an on / off switch in one hole and found a suitable lamp to fill the other.

The finished BC348 receiver. Very challenging, but this is one of the best renovation projects I have done.

 

Codar CR-70A

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The CR-70A when I received it, case a bit tatty.

For a long time I have wanted to get hold of an old Codar CR-70A to renovate and use along with my Codart AT5 Transmitter. The CR-70A was produced in the late 60’s as an all band receiver. It has a very nice slide rule type tuning drive. There were two versions of this receiver, the Mk1 and MK2, sometimes a MK2 would appear on eBay, but always sold above £100, the Mk1 is quite rare. The CR-70A is an all valve receiver, single conversion and very much like a normal domestic radio. The MK2 was built on a rather tacky printed circuit board, I didn’t want one of these, the original MK1 had all the valves mounted on the chassis like the AT5 Transmitter, much better built.

The CR-70A after a good clean and repair to the silver knob.
After years of trying to get hold of one I was lucky when an original MK1 appeared on e-Bay with it’s main transformer missing. I managed to get it for £30!  The receiver is in quite good condition, just needs a good clean and case repaint. I got a transformer from someone at my local Radio Club which fit perfectly.

CR-70A after fitting the mains transformer.

I fitted the transformer along with some fuses for protection (in those days they didn’t bother with fuses!) The receiver worked perfectly on all bands, just needed a quick tweek to align the RF stages.

Completed Codar Cr-70A with repainted case.

This is a nice little receiver, S Meter working perfectly and a great addition to my Codar collection.

70W HF Amplifier

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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

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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.