*** IC8POF's Commander Linear Amplifier , 30 meters band mod ***

* * * IC8POF's Commander Linear Amplifier
30mt new band mod. * * *

30 meter band added

The story :

Since many years I was thinking if it would be possible to add the missing 30 meters band to this fine amplifier.
Studying the schematic once more at the beginning of december 2016 a LED switched (again) in my brain.
Looking at it every band portion of the RF_tank circuit has a piece of coil which closes to the LOAD_Capacitor,
leaving the other portions open.
Therefore adding a NEW coil with the right inductance for the 30 meters should be easy.

There were only some problems to be solved.
Your work will be much easier :-).

30mt band schematic

the problems :

a) what inductance, its size, howto install & switch

b) what relay and what PSU for it

c) would the PLATE_RFC1 choke work also on 30mt ?

d) howto make a measurement of the PI_tank

e) the INPUT PassBand filter problem

f) all the needed hardware

g) howto tune it

Last but not least: how much power will it last ?

So this winter I decided to reopen the amplifier and give a definitive look at it and see if my idea could take place.

Problem a) : what for an inductance and switching relay should I use ?

In my opinion the only usable free space is that on the left of the BAND_SWITCHING_UNIT(BSU) and the chassis.
Look at the yellow dots, the right side(red-dot) of the BSU has no points to easy fasten a relay and a coil.
As the free space is very limited an Air_Coil even with few turns is out of discussion, so a toroid should be the best solution.
What for a toroid ? The L2 40-80-160 tank coil is made with 2xT225-2 toroids, but a short test with this large item
revealed the big difficulty to put such thing in the amplifier, so I decided for a smaller T200-2 already in the junk box.

New coil:
The decision to connect the new coil between the 20 and 40 meters side (pic A yellow dots) of the BSU is given by :
to keep the new wires as short as possible using the 40mt position on the BSU,

the ease of moving the 40mt teflon wire versus the silver plated strap of the 20 meters,

the necessity to create a new hardware post to hold the relay (pic D below) on the chassis.

But nothing against a decision to use the 20 meters position.
Bear in mind only that the BSU contacts are not so strong as one could immagine
and are prone to broke. look here...PIN BROKEN

TURNS, how many :
Remembering of the previous accident on the L2 ...follow this link...
I recalled that the turns for the 40 meters coil were 7, so the turns for the 30 meters should stay in a number
between 3 and 5 and I decided to start the tests with 6 turns over the T200-2.

Problem b) the RELAY:

I had at hand a piece of Siemens VR311 relay, but if you want to use a normal FINDER 8 or 10 Amp should be OK too.
As in 30mt_OFF mode all the power of the 40mt runs in the NC (normal closed) contacts of the relay I preferred
a vacuum relay than an open-frame one.

HOWTO mount:
for me the easiest solution has been to create a new hardware post by drilling a new hole in the chassis (pic D)

... Cell 3-

to hold the relay I have used a piece of 2mm copper which is long enough to be wrapped around the relay body
and arrives to the new post.
The two legs of the copper wires are then bent around the post and kept in place by a nut.
The coil sits on a plastic cork to keep it at a fixed distance from the chassis.

PSU for the relay.
At first I was not jet sure if I could make an internal or external PSU for the relay (RPsu).
The vacuum relay works with a 26.5 Vdc @ 330Ohm that means a current of 80mA.
In the Commander PA there are three transformators , T1 for the plate high voltage, T2 for the heaters
and T3 for the services: +26Vdc +12Vdc etc.
The T1 is not usable of course.
I have tried to take the small current amount (80mA) from the T3, but it has not such extra capability.

Speaking with my friend Gian I7SWX about the RPsu he suggested why not taking that small power from the T2 heaters
transformator as it has a 13.5Vac secondary and possibly the extra 100mA to utilize by building a VOLTAGE_DOUBLER ?

	link to larger pic		-.-.-.
link to larger pic	.......	-.-.-.-

So the next step was to check if the T2 transformator had such capability by connecting a dummy load (pic I) and see if
the secondary would last the 3Amp fur the tubes plus 100mA for the new service.
In pic I you see the test on the heathers voltage with the 50W dummy load connetcted.
The test showed a very well built T2 secondary, no voltage drop, so no problems to take the extra 100mA.

The doubler solution to avoid an external transformator was very interesting so I mounted the few components
for the voltage doubler by tapping the heaters voltage as shown in the TAP pic above, but having only
the capacitors of 2200uF/63V, but quite enough to smooth the doubled Vac.
The LAYOUT of the few components is simple, look at pic H above.
In the doubler schematic are shown all the values, the resitor R has been calculated to last the 80mA + 10mA for
a signal Led and an extra capacitor 100kpF/200V as bypass has been added between the relay posts.
The doubler voltage solution has proven to be the ideal and final solution to power the new relay.

To make the measurements on the T2 voltages,
pre_checks for the voltage doubler and
the functions of the new relay the amplifier
must be left OPEN and running
WITHOUT the plate voltage active
That means that the T1 trafo is out of the box
its connector is closed and HV pins have to be left open
and the SAFETY switch has to be put in working position,
look at pic F.

c) d) TESTs & measurements:

Having solved almost all the hardware problems it was time to make some tests.

DRY Test 1: the amplifier is OFF and opened.
- leave the 40mt connection to L2 open
- connect the 30mt coil between the BSU/40mt position and the 20mt RF_tank post, look pic D above
- connect the MINI_VNA or other vector analyzer via a 4:1 balun adapter
- Look at the main schematic pic (TAP point+Ground)
  at the beginning of this page.
  link to larger view
- MINI_VNA measurements.
  The idea to use an RF_analyzer to let me literally "see" where the PI_tank resonated has turned around my mind
  since long time and also a discussion with Gian I7SWX led to a conclusion that it should be again one of those
  DO&TRY operations to see what happens.
  The measurements with the analyzer have been done connecting it to the TAP point marked in the schematic above and pic E.
  
  *** of course the PA is not connected to the Vac ! ***
  
  Having seen that I can easily measure and "SEE" all the resonances of the PI_tank I have made a list for every band.
  The following list shows why the amplifier did not resonate on 20 and 40 meters.
Frequency resonances   40 mt = 4.5 to 7.5 Mhz 20 mt = 12.7 to 16.8 Mhz

. The following picture shows how strong was the resonance on 30mt with the new coil.
Of course the band_width is more than adeguate to cover the whole band in the SWR 1:2 edges.

I had to try a couple of times by trimming the 30mt teflon wire and shorten the turns (finally 5) to get the
resonance in a place of the verniers where the PLATE and LOAD indicators should have a logical position somewhere
between 20 and 40 meters places.
Therefore the indicators had their final position at +-15 for the PLATE and at +-40 for the LOAD.
.....
After this first DRY TEST I decided to make an ON_AIR test but having in use ONLY the new 30 meter coil.
So a sort of 30mt MONO_band amplifier has been prepared to go ON_AIR.

The HOT_Test   has shown that the idea of the new 30mt coil was a winning idea.

Problem e) the input PASSBAND:

which PASS_BAND input filter should be used for the new 30mt band: the 20 , the 40 or none of them ?
Therefore a flying DPDT switch has been prepared with the 20 and 40 meter pass-band switching cables to let manually
change the RF input (during PA St.By) and to look at the SWR between the TRX and the amplifier input.
The 40mt input filter had shown a high SWR more than 1:5 (very high one) and the 20mt had a fine 1:2 or less.

That mean that to activate the new band I needed a TPDT(triple-pole\dual-throw) with 1 pole to switch ON the relay and a signal led
and 2 poles for inverting the PASS_BAND wires during the 30 meters sessions.

Problem f) the HARDWARE:

After having solved most of all the above problems
- HV trafo must be taken out
- New coil : ok
  
  the toroid keeps firm itself by the mean
  of the teflon wires,
  but for a sure "not_moving" part
  it is seated on a rubber button, look at pic D2
- New relay : ok
- New switcher : ok
- New relay psu : ok
- New holes : 4 ok

Remained to mount:

the relay PSU board,pic H, plus 2 new holes H1 H2 into the chassis (pic H and J).
The voltage doubler board fits exactly between the HV trafo and the front panel below the IG meter.

link to larger pic

New Wires and holes

pic J shows where the psu wire comes up from H2 hole at the bottom of the chassis
and the new wires on the passband switcher.
I have used the same colors as the original for instant identification.
To better make the new holes the chassis is put upside down (sri no pics of it)
and after having marked the holes position, drill them.

the TPDT switcher lever on the PA panel, nothing difficult .
tpdt

To better study how to put inside the new hardware I have unmounted the front panel and the HV trafo .
The HV trafo must be taken out , but the front panel is not stricltly ncessary to be unmounted to do add this mod.

baseband2

Problem g) HOWTO tune it:

Nothing special at this point, just follow the known procedures and keep the power to a reliable level
that is supported by the new coil.
The 1Kw level is easily reached. but the 6-700 Watt level is more than adeguate for a T200-2 toroid in CW.
For RTTY I think the power level should not exceed the 300W level.
Here the problem is the ventilation of the toroid in an already warm\hot environment,
but this is another story and another excperiment for the future mods.

Here I want to thank my friend Gian I7SWX for his precious suggestions and for the voltage doubler idea.

Any improvement or suggestions will be appreciated,
write to my email on ic8pofATyahooDoOTcom (copy,paste and correct)
.

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v1.0 , dic.2016 - ott.17 - gram 21ago22