Caught some nice photos last night of the International Space Station over my antennas.
The ISS fades as it enters shadow and disappears.
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.
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.
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.
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.
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
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.