[MRCG] 800Hz AC Power for Navy Radios

[Scott Johnson]

Mark-
I am wondering why you didn't just modify something like a CF electronics
700VA 400Hz inverter.
It is a sine wave synthesized MOPA design that uses a programmable divider
to set the frequency, so the move from 400 to 800 Hz would be a piece of
cake.
I believe they were used to replace the older inverter in the MRC-108 set,
and they seem to spring up everywhere, they are not TSO'd for aircraft, so
they go cheap.
I think BPBA and American Mil-Spec still have them.  Failing that, I may
have one.

Regards,
Scott W7SVJ

-----Original Message-----
From: mrcg-bounces at mailman.qth.net [mailto:mrcg-bounces at mailman.qth.net] On
Behalf Of Mark J. Blair
Sent: Sunday, May 12, 2013 5:18 PM
To: West Coast Military Radio Collectors Group
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/

______________________________________________________________
MRCG mailing list
Home: http://mailman.qth.net/mailman/listinfo/mrcg
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:MRCG at mailman.qth.net

Website: http://www.mrcgwest.org

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html