AN INVERTER FOR 400 Hz POWER
                       
                              Paul Wade N1BWT
                              

        I recently acquired a 10 GHz TWT amplifier from Ken Burtchael,
        W6GHV, who has been advertising them in FEEDPOINT.  The only
        problem was that the power supply requires 120 volts AC at 400 Hz
        input.  I was able to modify a commercial inverter to supply this
        voltage, making the TWT usable from a 12 volt DC supply.  Of
        course, this might have use for other surplus goodies that need 
        a 400 Hz input.
        
        I started with the thought that a cheap inverter with a 60 Hz
        output might be simple enough to modify for 400 Hz easily, so I
        ordered the cheapest one I could find.  Well, it wasn't that
        simple to figure out - the unit had a pretty sophisticated circuit, including
        six ICs and power FET switches, to provide high efficiency and
        regulated output voltage - but in the end, the modifications were easy.
                
        The unit I modified is rated at 140 watts:  the PC-140 by
        Power-To-Go (various corporate names at the same address in
        Boonton, NJ).  I got mine from the Northern Hydraulics catalog
        (1-800-533-5545, Item# 45557) for about $65, and I've seen units
        with similar pictures in other catalogs.  It's in a small metal
        housing, and works pretty well as a 60 Hz inverter; the output is
        a "modified sine wave":

        
                ----                -
               |    |              |
               |    |              |
            ---      ----      ----      ...
                         |    |
                         |    |
                          ----
                          
        and the pulse width varies with load to maintain a constant 
        RMS output voltage.  It runs cool at 100 watts output, and is 
        very efficient - on my meter it takes about 5 amps at 13 volts
        DC to run a 60 watt light bulb.  No-load input current is
        specified at 60 ma., and measured 38 ma.
        
        I traced enough of the circuit to guess that the frequency and
        waveform were controlled by a 556 IC, and the periods were set
        by two 0.1 uF capacitors.  I changed both capacitors to 0.015 uF
        ( actually 0.01 in parallel with 0.0056) and fired it up into
        the light bulb.  Success - the frequency was now about 380 Hz,
        and everything else appeared the same.
        
        Since the TWT power supply specified 400 Hz +/- 5%, I hooked
        it up.  After the usual hair-pulling, I got the TWT running and
        putting out about 5 watts (saturated) at 10.368 GHz.  Input power
        is 5.8 amps at 12.6 volts, not much more than a typical
        solid-state amplifier might draw (if you could find one surplus).





        
        MODIFICATION DETAILS:  I've looked at three of these inverters.
        Each was mechanically different and had a different label, but
        all appear to use the same circuit schematic.  Locate the 556
        IC; near it are two 0.1 uF capacitors, labelled C13 and C16 on
        the circuit board.  If you wish to doublecheck, they are
        connected to pins 2 & 6 (C13) and pins 12 & 13 (C16) of the 556.
        Carefully unsolder the capacitors and replace them with 0.015 uF.
        Changing mine to 0.01 uF in parallel with 0.0047 uF put the
        frequency very close to 400 Hz, but other units might need a
        slightly different combination.  Reassemble and fire it up.
        
        CAUTION:  The circuit board looks just like a low voltage,
        solid-state circuit that you can safely put your fingers in.  
        Don't forget that this unit has high voltages and an output
        that is just as lethal as an AC wall receptacle.  BE CAREFUL!