[MRCG] 800Hz AC Power for Navy Radios

[Jay Coward]

Mark,
 I'm not too familiar with s/s inverters but I would think there would be a reference oscillator/frequency divider/ PLL that gets you accurate 60  cy. How does the 60 cy of the inverter compare to the line  60 cy? I would presume the inverter manufacturer would attempt an accurate output frequency. Also, the output filter would not be a good filter if it passes harmonics of 60 cy so you may need to look into that.
 Have fun!
 Jay KE6PPF


-----Original Message-----
From: Mark J. Blair <nf6x at nf6x.net>
To: West Coast Military Radio Collectors Group <mrcg at mailman.qth.net>
Sent: Sun, May 12, 2013 5:17 pm
Subject: Re: [MRCG] 800Hz AC Power for Navy Radios



On May 12, 2013, at 16:50 , Chuck <j-mcclurg at sbcglobal.net> wrote:
> I am keeping an eye on this as you know.

Since you mentioned it, here's a status update:

I received my inverter from Amazon yesterday. It's a Power Bright model 
APS-1000-24V:

  http://www.amazon.com/Power-Bright-1000-Watt-Inverter-110-Volt/dp/B002EA006S/

I opted for a 24V input inverter rather than a more common and cheaper 12V one, 
because the GP-7 would suck too much power for my 12V radio room supply. This 
inverter will run off my PP-4763A supply. I also selected one with a "true sine 
wave" output.

I took a gamble by ordering this one, without knowing anything about what's 
inside of it. So far, things look promising.

It does not appear to contain any highly-integrated components (i.e., no 
microcontroller, custom inverter controller IC, etc.). The control circuitry 
appears to consist of standard off-the-shelf 4000-series CMOS logic, linear ICs 
such as op-amps, and discrete components. This is about the best possible case 
with respect to having access to the innards and the ability to change the 
design, but on the other hand, my job will be more complicated than simply 
googling up a datasheet for a custom IC and then changing an R-C on the 
frequency-defining pin.

Initial blind probing appears to have verified my guess that it would generate 
the sine wave by pulse-width modulation with an output filter to roll off the 
switching frequency. It appears to switch in the ball park of 25 kHz. There is 
one crystal in an oscillator + divider circuit, and one trimmer potentiometer 
which turns out to adjust the output voltage.

That's where I am right now. I've identified an area on the PCB where I suspect 
the 60 Hz might be coming from, so I need to study the circuitry some more to 
figure out what's going on. I think I'm going to order a high voltage 
differential scope probe before I poke around much more, and I found that poking 
some nodes in that area with my regular scope probe made the supply emit a 
disturbing noise... luckily, the magic smoke stayed inside! Those HV diff probes 
are EXPENSIVE. The cheapest one I've found is made by LeCroy and costs around 
$390 new. Tektronix makes a particularly nice one which would interface well 
with my Tek digital scope, but those ones appear to cost $600+ used on eBay! 
I've wanted this sort of probe for a long time for working on power supply 
circuits, and I finally have a good excuse to take the plunge and get one.

I've been taking pictures, and I'm working on a web page post with my status so 
far. I don't know yet whether I'll publish it before I've dug into the circuit 
deeper. We'll all find out later this evening, I guess. :)

I presently suspect that I have three major tasks:

1) Figure out how the 60 Hz waveform is generated, and modify the circuitry to 
make an 800 Hz sine instead.

2) Make sure that the output filter can pass the 800 Hz sine without heating up 
too much. Hopefully the corner frequency is high enough that it will work 
without modification at 800 Hz.

3) Figure out the physical details of my modification, including tearing out the 
Edison plugs, installing something else in their place, possibly adding a switch 
for 60/400/800 Hz if practical, etc.




-- 
Mark J. Blair, NF6X <nf6x at nf6x.net>
http://www.nf6x.net/

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