[Milsurplus] Progress on Auzzie AT5-AR8 Set

[David Stinson]

The WWII Australian AT5 / AR8 set is on-the-air,
transceiving and making contacts.
I have had a great deal of help from our Australian members,
for which I am very grateful.

http://home.netcom.com/~arc5/AR8AT5/AR8AT5.jpg

Above the transmitter is a temporary junction box I built
to provide microphone bias, antenna and RX/TX switching.

I've connected them for 12 VDC operation.
The AR8 receiver is powered at 90 VDC using a homebrewed,
regulated 12V-to-90V DC/DC power pack.  
The radio has excellent, loud audio and good operation. 
I see no reason to pound the old veteran with 250 V 
so I intend to keep it running at 90V, 
even if I am fortunate enough to acquire an original power supply.  
I'll just regulate it down.

The AT5 transmitter is running at full 550V B++ and 300V B+
using a rebuilt Hallicrafters 12V mobile supply.

While I have the normally-included external antenna tuner unit
which, in the original installation, handled antenna and T/R
switching, space is unavailable in the upcoming display 
to include it.  Adding the third box would give a footprint
half-again bigger than an ART-13.  I need them operational
for HamCom, so I built a small external junction box to 
handle these functions.  When I build the permanant rack for
my station, I plan to install the ATU on a shelf above the TX/RX,
similar to this installation in an RAAF rescue launch:

http://home.netcom.com/~arc5/AR8AT5/oppics/AT5_AR8Launch.jpg
 
Here is a diagram of the homebrew switching box:

http://home.netcom.com/~arc5/AR8AT5/AR8AT5JB.jpg

Notes on the AR8:
I went through this set several years ago and 
it's been in the "Round Tuit" que until now.
IIRC, every cap in it was good.  If I changed any,
it was one or two.   It needed the usual lubrications and 
De-Oxits.  The MF dial is cracked so I reenforced that.
The BFO needed to be re-centered, which was easy.  
The dial calibration on HF is a little off.
Since it needed no other alignment, I decided to leave 
the glyptol-locked adjustments alone and live with a 
little "mental correction" on the dial reading.
Replaced a couple of short "rotten insulation" wires.
One curious thing:  The filaments are strung to either
be wired for 24V or 12V, DC or AC.  When 
running them 12V, the manual calls for power pin 
1 to be A+ and to tie pins 3 and 4 together for A-.
These A- leads get "earthed" in the external power 
supply.  However- in my set, if you connect it this way,
the chassis becomes +12 volts to "Earth," and if you 
ground pins 3-4, you have a short.  I don't know why yet.
I simply reversed the wires, using 3-4 as A+ and 1 as A-.
This worked fine and there's no electrical reason not to 
connect it thus.  It's actually OK with me, because I had
to do exactly the same thing with the AT5 transmitter,
but for other reasons.

Notes on the AT5:
Unlike the receiver, the AT5 required a nearly complete re-capping.
The original antenna connector is broken, so I've installed a 
temporary "N" connector (no metal changes, of course).
I put a small screw between two turns in the tank coil,
effectively removing a turn and making the tank
suitable for loading a 50-ohm antenna:

http://home.netcom.com/~arc5/AR8AT5/AT5TANK.jpg

This delivers 25-30 watts of grid-modulated AM and
50-60 watts of CW out to my 50-ohm dummy load.
Rig is chirpy on CW, even when running original crystals.
I haven't looked into that yet.

In the original design, the microphone element is not
run against ground; it is series-connected between the
microphone supply in the external power unit, the microphone
element and the microphone transformer primary, which then
goes to "earth" via the A- and thus "earthed" filament string
connection to  power connector pin 3.  U.S. designs place the
microphone power running through the mic transformer, a 
dropping or divider resistor, then the microphone element 
and thence to ground.  Since I'm using one of the microphones 
I have on hand, I connected power pins 3 and 4 to A+ and 
pin 1 to A- (the manual calls out the opposite)
and that provided A+ through the mic transformer primary,
out to a voltage divider/audio bypass and then through the 
mic element to "earth."  This worked well with good modulation
and plenty of audio and the filaments operate either way.
The transmitter VFO is surprisingly stable after warm-up.

For Safety's sake, I will be adding one actual modification:
if the plate blocking cap, C24 leaks, B++ will show up at 
the antenna connector.  I'm going to add a 2.5 mHy choke
from the antenna connector to ground so any such leakage
will be safely shunted.  

One problem I have run-into:  a low-frequency parasitic
oscillation which is creating low-level "spurs" either side
of the carrier.  You can see it on the monitor scope and 
hear them on a monitor receiver.  While I haven't 
worked on this yet, I'd bet coffee and donuts the problem
is the missing transmitter case and covers.  All these long
cables running around and next to the PA are causing 
low-frequency feedback.  Covers for these sets are 
made of "unobtainium" so I'm going to fabricate covers
for the transmitter and watch the parasitic go away... I hope ;-).

Speaking of parasitics, I need some advice:
The PA plate leads lack VHF parasitic suppressors
and are made in such a way that it would be destructive
to attempt to insert them at the plate caps of the 807s.
Should I decide they're needed, I'm thinking small ones
in the cathode and/or screen leads would work as well.
What do you think?

Now to build the display stand and work on those covers...

73 OM DE Dave AB5S

P.S. More AT5 / AR8 operational photos, 
courtesy of our Australian members, at:
http://home.netcom.com/~arc5/AR8AT5/oppics/