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 282013

A recent conversation with Des G4UHZ prompted me into writing this post about the changing ways of today’s Radio Amateur. For those who are wondering what a Boat Anchor is, in radio terms all vintage electronic valve equipment becomes encompassed with the term because it is a heavy piece of equipment with large transformers and tied with rope is suitable for mooring a boat. This is theoretical and I am pleased to say that vintage equipment it is not normally used for this purpose.

This post is not about restoration, but more on the loss of essential skills that are dying out because of the way that amateur radio changed. Today most radio equipment is digitized and computer controlled all squeezed into a small black box that makes it impossible for the normal amateur to repair and is generally sent back to the manufacturer for repair.

Today more and more radio amateurs are becoming operators and although the terms on the radio license also state it is for experimentation and self training, this part is being ignored. We are losing the ability to repair our own equipment and the skills used for this are disappearing, the self satisfaction of bringing a radio back from the dead is no longer being felt by the majority of radio amateurs.

Information is readily available on the internet and search engines can be your best friend, if you have a radio or any other equipment you would like to find out more about and try and get the manual for then you could be lucky. You will find that someone somewhere has already done what you are looking to do and a mass of information will be available to download. A typical search for “boat anchor” turned up the Boat Anchor Manual Archive

More information on this topic will be published soon

Mar 262013

Eddystone 730/4

Built during 1956 – 1962 the Eddystone 730/4 is a professional commercial receiver based on the 680X  with 15 valves, 2 RF stages, 2 IF stages (450kc/s), BFO, IF cathode follower output, variable IF selectivity, xtal filter, xtal calibrator, adjustable scale, noise limiter, S-meter, audio filter for CW, fully tropicalised. Cost at time of manufacture £230. This receiver was ordered by the Ministry of Defense in large quantities and used by the Royal Signals

The opportunity arose for me to purchase an Eddystone 730/4 spares or repair from the family of a Silent Key at a reasonable price, so I was pleased to purchase it as a project without prior knowledge of its condition.

On examination of the receiver I found that the tuning dial was not working and underneath the chassis there was about 10 wires disconnected so I had to research the receiver to be able to get it in a condition that I was able to apply power to it without blowing myself or the set up.

My first port of call was the Eddystone User Group website which is full of information to refurbish old Eddystone radios and also to join the group Forum

I downloaded the User Manual and other useful articles from the Eddystone User Group and set about the task of taking the front panel off so that I could sort out the problem with the tuning dial. Before starting this long winded task I gave all grub screws on the knobs a squirt of WD40 and left them for a couple of hours to loosen them up as previous experience with old sets had found undoing them could be a pain.

Taking off the front panel is a major job and several different size screwdrivers a large allen key and spanner are necessary to detach it from the main chassis. Full information on how to do this task can be found at the Eddystone User GroupEddyston 730/4 opened up

The picture above shows the front panel with the tuning capacitor gearbox attached, the gearbox was seized and had to be completely dismantled and the rust that had formed round the steel drive shafts that was stopping it turning was cleaned off.  Everything else in the gearbox was also cleaned and greased so that the gearbox rotated freely then the pointer drive cord was fitted and the front panel was ready to be fitted back onto the chassis. Before fitting the front panel back, I thought it would be a good opportunity to clean all of the switches and variable resistors as I did not want to have to take the front panel off again.

Before power could be applied, I had to figure out what had been done to the receiver which was a bit of a mess with disconnected wires under the chassis.Eddyston 730/4 Chassis Comparing it to the pictures I had of untouched receivers, I found out that a tag strip was missing from the right side of the picture where all the wires from the wiring loom should be attached, so I had to rewire it up again and I also found  that a couple of valve heater wires had also been disconnected from the bottom of the valve bases. The switch under the chassis was the AF Filter switch and where that should have fitted on the front panel, a  variable resistor had been added and fitted to the front panel where it had been wired in. I removed the modifications and rewired the receiver to its original condition and switched it on. Much to my delight everything lit up and after a few seconds sound came from the loudspeaker, I tuned into a AM broadcast station and everything sounded OK so the receiver was working and now it was just a case of checking voltages and replacing faulty resistors and caps and tuning it up properly to get it back to A1 condition. PROPER JOB!

I would like to thank the Eddystone User Group Forum for all the friendly advice and help I was given with this project.