Commander Linear Amplifier PI improvements




It was not only my fault if the amplifier suffered such accident.

   

The accident happened almost at the end of an RTTY contest on 40 meter band.
The fault is partially my because I did not tune the LOAD at its optimum setting, the power output was less than 2/3 of its capability.
The most part of the fault is due to the project. Later I will describe that situation.
It happened on february so the temperatures were not high.
At one time a strong smell of burned plastic came from the PA. Of course going on stand-by and putting an extra fan to help
the air to circulate was done at once. Strange as I have done already many contests.
Well I finished the contest at reduced power and had to wait a couple of days till I can open the PA for inspection.

The inspection of the PA showed the accident as you see in the above pics.

Of course I contacted the Commander owner Pat Stein, but I discovered that the company had been sold to the Palstar company.
Nor Pat nor Palstar could\would help me in getting the new delrin support and discs, so I had to look "around".

• What happened ?
  
  • The LOAD tank toroid which carries the coils for the 40-80 and 160 meter bands is adequate for a normal SSB traffic,
    but in RTTY it is another story.
    If the 80 meter coil is almost adequate for the rtty traffic at 2/3 power level, the 40mt coil with its 7-TURNs on
    a very little toroid surface is not sufficient to well transfer the generated heat to the AIR .
  • the ventilation of the tank toroid is VERY poor in RTTY mode.
    See pics nr 1 es 2 below.
    

The repairing was not so easy as it could seem.
The delrin support was impossible to find as new piece on my side of the world .
So I found via ebay a seller in the Usa that had some ceramic pieces complying the needed dimensions and ordered a pair.
But the Luck has turned his head on the other side so the first packet went lost in the Postal Mare Magnum.
On the second try the small packet arrived quite soon, but the seller had only 1 support left of the proper dimensions....
The support has also two screws, one holding the support on the chassis and is of steel and the other holding the front disc of delrin,
both old screws were good to be screwed into the new ceramic support.
But the toroid support is of 3 pieces , one cylinder and two discs.
If for the cylinder I was lucky to find a replacement, not so was for the two discs, so I had to clean and reuse them.



Toroid ventilation.
This is the most important part in this accident.

poor ventilation   improved ventilation


The original project provided internal and external toroid ventilation, as I discovered after the accident.
The internal ventilation was provided by 5 holes, visible in the pics, but too small in respect of the delrin disc.
Holes with the red dot show their original dimension as are also used on the output balun toroid.
So my thought was to enlarge these holes, blue dot in the pic 2 as the delrin disk had already larger holes.
Aligning the holes between the chassis and the delrin disc is to be carried carefully.
The outside body of the toroid ventilation is VERY VERY POOR, it should be given by the means of two small openings in the RF box,
see the yellow circles in pic nr 2.
This is the weakest point I have found untill now after 15 years of use in this project .

More: the air coming through the five holes excaped between the first delrin disc and the toroid, so I added a sort of obstacle
with a bottle cork, pic 3.
air control
In this way the air flowing in the inside is forced to refresh the inside part of the turns.





Coil rewinding.
This was the less complicate thing, buy the new teflon wire and rewind the coils in the same way as they were the old,
doing the taps for the 40 and 80 meters is quite easy task.

Having solved the supporting parts problem the next steps were to solve the VENTILATION of that compartment.
The 40mt tank on the toroid occupy with 7 turns a very small place on the toroid and during the rewinding phase I decided
to put 2 more turns to increase the the space on the toroid and let the heat to spread on 9 instead 7 turns.
That mod should let the LOAD capacitor to reduce its value so there were no problems here.
But that mod alone did not jet solve the poor VENTILATION and I had to invent an ADD_ON to bring more air.



More Air add_on.

After some turning over in my mind on How_To bring more air in the RF deck of the Commander amplifier....
there is no space at all for any mini_fan inside the PA, a lamp switched in my brain: why not use an aquarium pump
to bring more air via an external tube ?
Only one shop on the island had an aquarium pump and I did not want to wait for a web order.
It had a capacity of 2 outputs x 190Lt hour and its dimensions were not excessive (12x6x8cm LWH) so I purchased it.
For your reference this pump is given for aquariums up to 700 liters of water, but a smaller unit would be also adeguate.
air pump

As you see in the pic above the pump is quite small and its noise is well hidden below the noise of the amplifier fan.
The red dots show the 2 tubes running into the RG213 tube, which then runs into the RF_Deck box.

Now in feb.2016 after some years and contests in rtty (specially in 40mt) I can say that using this air-pump gives really better
ventilation of the toroid tank and also the whole RF_Deck keeps a lower temperature.

For a visual indication use a digital thermometer putting its sensor on the hot air flow.

Of course LONG RUNS in 40-80mt rtty at levels of about 500W are much safer and short peaks of 2/3rd power
level are held very good.   HIGHER LEVELS are possible of course, but keep them short: how short should they be is your decision.



How to bring this new air quantity in the RF_deck was just a matter of how_to use the materials ready at hand in the shack.
more air ...    more air....

As you can see in the pics 5 es 6 the solution found was a piece of rg213 tube with a plastic pen-tube on which I made some holes
of about 2mm diameter (red dots). As already written before I was not quite sure of how much air should be necessary.
I made some test in a water can and the air flow was strong, but how would be in the RF_Deck was to experiment.

The idea\solution of bringing more air into the RF_Deck has proven to be the FINAL touch to get this fine amplifier again ON_AIR.

Any improvement or suggestions will be appreciated,
Write to my to my email on ic8pofATyahoodotcom (copy,paste and correct)
.

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