Apr 272013
Dambusters QSL Card

On the16th/17th May 1943 19 Lancaster bombers of 617 Squadron RAF took off from RAF Scampton in Lincolnshire armed with the now famous Bouncing Bomb invented by Barnes Wallis. The raid was lead by Wing Commander Guy Gibson VC The aircraft took off in three groups, Group One lead by Gibson took off in three waves of three aircraft at ten minute intervals starting at 21.39,, their mission was to attack the Mohne and Elder dams. Group Two consisting of 5 aircraft took off at 21.28 taking the longer northern route to attack the Sorpe dam. Group Three also consisting of five aircraft took off later at 00.09 they were a reserve group if the Mohne, Elder and Sorpe were breached they were to attack the Lister, Ennepe and Diemel dams. The code words sent back in Morse code to the operations room in Grantham were “Goner” meaning bomb dropped “Nigger Gibson’s Black Labrador who was run over by a car on the evening of the mission” meaning the Mohne was breached and “Dinghy” meaning the Sorpe was breached. Five bombs were dropped before the Mohne was breached. The Elder dam took three bombs to breach it and Sorpe despite several attacks was only slightly damaged. Only eleven aircraft returned to Scampton out of the nineteen aircraft which took off.. Of the survivors, 34 were decorated at Buckingham Palace on 22 June, with Gibson awarded the Victoria Cross. There were five Distinguished Service Orders, 10 Distinguished Flying Crosses and four bars, two Conspicuous Gallantry Medals, and eleven Distinguished Flying Medals and one bar. The Lincoln Short Wave Club shack is less than 1 mile from RAF Scampton 70 years ago you could have watched the aircraft take off and count them back in again.

On the 16/17th May 2013 the Lincoln Short Wave Club are operating a Special Event Station in memory of the Air Crew and German civilians killed during the now famous 617 Squadron’s raid on the great Ruhr dams of Germany. The station will be on RAF Scampton’s bomb dump which is where Barnes Wallis’s bouncing bomb would have been delivered on 16th May 1943. The station will start at 15.00 local time on 16th May 2013 and transmit through the night on 40 and 80 metres 3.617 MHz band conditions permitting. Transmissions will cease at 15.00 local time on the 17th May 2013. A sked has been arranged with an amateur who will be visiting the Mohne Dam on the 17th probably the first time a qso has taken place between the dam and RAF Scampton since the raid. A very special Limited Edition QSL card will be available of the Dambusters raid details on QRZ.com

Latest Update: Operations will now start on the CW DX section of the 40mtr band at 1300 UTC.  Ian G4XFC will operate for at least 1 hour +- pileup , when 80 opens about 20.20 UTC when the first wave would have been taking off we will use 3.617 will also transmit at some point on AM. We will also be sending a signal via 23cm to GB3VL to allow video to be broadcast via the BATC stream .

Apr 252013

RAF AWACS AeroplaneTonight club member Alan M0XVR  A.K.A. George the Porsche gave a very enjoyable and instructional talk on the role of the Airborne Warning and Control System which he was involved with up to recently.

Alan talking about his role in the AWACS

 Posted by at 12:38 am
Apr 212013

Club of  the Year 2013Club of the Year Award

We arrived at the Institution of Engineering and Technology just after 11 a.m. after climbing 193 steps to get out of Covent Garden underground, apparently that is equivalent to 15 storeys, for the 86 Annual General Meeting of the RSGB.

The AGM was over quite quickly and was followed by the presentation of the various awards the RSGB Club of the Year being the last to be presented.   In third place was Warrington ARC Region 3, in second place Newbury and District ARS Region 9 and in first place Lincoln Short Wave Club Region 13.

In the top photo from left to right are Steve Burke (LSWC), Mark Holmes (LSWC), Mark Frances (W&S) Pam Rose (LSWC) and Jeff Stanton (W&S).  Waters and Stanton are the principal sponsors of the competition.

 Posted by at 3:02 am
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 112013

It was a successful well attended evening at the Village Hall in Aisthorpe for the Lincoln Short Wave Club Used/Surplus equipment sale (some junk), with our resident auctioneer Bob G7AVU trying to extract the last penny from the bidders. As usual there was a lot of humor with the usual suspects trying to wind Bob up, and people frightened to scratch their head as Bob would take this as a bid. Steve M5ZZZ was running round the table trying to find the items for Bob to sell and at times struggled to find the items (if only they had been placed in order round the table!). Pam G4STO has set up a table inside the hall, selling old radio amateur books for a £1 a time to raise money for the club.

At the end of the sale, club members and visitors drifted down to the BSA Club for a drink and natter to end the successful evening. Many thanks to all the visitors from the other local clubs who attended and brought items for sale as they contributed to the successful evening.

 Posted by at 1:45 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