[Milsurplus] Operating the TBW Transmitter on the #800-1 800 Cycle Rotary Inverter

[Richard Brunner]

Operating the TBW is a problem because it wants 120 volts at 800 cycles, so 
I am using the #800-1 aircraft inverter, which is 24-28 volts input to 110 
volts at 800 cycles output.  The problem with the inverter is it is very 
noisy; 91 dB at three feet, which is equivalent to three feet from a subway 
train running by.  For noise reduction I constructed a 3/4 inch plywood box 
lined with 2" fiberglass panels.  The box joints must be air-tight or noise 
will leak out the minutest joint, so make all joints glued and screwed, and 
calk the joints inside.  For air intake and exhaust, construct chimneys or 
ducts about two feet long, also lined with 2" fiberglass panels. (and mind 
the joints)  A small computer fan will move lots of air.  This box reduced 
the noise from 91 dB to 50 dB, which is tolerable.

A curious thing about the #800-1 inverter is the reactance seems to vary 
with input voltage.  At low voltage, 24 volts, the output voltage droops 
with key-down.  27 to 28 volts is the sweet spot with about 2% regulation 
key-up to key-down.  At high voltage, 30-32 volts, the output voltage is low 
key-up, and comes up to normal key-down.  Changing the compensating 
capacitors in the TBW didn't help, and using external series capacitors made 
it worse.  It's possible I optimized the carbon pile and excitation settings 
at 28 volts, and I think I will leave it alone.

I have some optimizing to do and some joints to calk, and think I now have a 
good system.  The TBW sounds good.

The fiberglass batts are not a retail product and you won't find them at 
your local lumber yard or hardware store.  I called Owens-Corning, was 
referred to the district sales manager, and was sent to a distributor not 
too far away.  I believe they are semi-flexible duct insulation, and come in 
2 x 4 feet, 4 x 8 feet, and 4 x 10 foot panels.  The smallest standard 
package of 2 x 4's is nine panels, which is a bit more than twice what I 
used.  They cut nicely with a sabre saw, but wear a mask.  I attached them 
with contact cement.

Richard Brunner, AA1P