Hello Roy,

 RM>> I recently built a simple r-c low pass filter to allow my
 RM>> multimeter to measure the mains freq of the choo choo inverters
 RM>> I've been playing with.

 CB>> WTF is a "choo choo" inverter - something to do with model
 CB>> trains? I thought they ran on 12V DC?

 RM> a dual inverter for running aircons on the new 2800 series QR is
 RM> buying. 72vdc input. 68HC11 brain. One inverter draws 7A and runs
 RM> the control gear and the cool air fan. The other inverter draws 60A
 RM> and runs the compressor and the hot air fan. Two people can lift

Yikes!  And to think I consider a 100 watt radio "high power" :)

 CB>> If it's something like the "modified sine wave"
inverters used to
 CB>> provide 240VAC on 4WD's, trucks, etc, then you will certainly
 CB>> need a lowpass to get a steady reading on a freq. measurement...

 RM> The last "modified sinewave" inverter I saw (a 600W DSE unit) was
 RM> actually a straight variable ratio squarewave with a small output
 RM> choke, although I didn't look at the actual output waveform at the
 RM> time.

Being that brand, I wouldn't expect any better.
Squarewaves are cheaper...

 RM> I'm having a go at building a small one with a PIC and some fets at
 RM> the moment. The highest switching freq I've tried so far is 1.2
 RM> kHz, with fair results.

I haven't seen an under 20Khz switcher or DC converter for ages.
Just curious - what's the reasons for using a PIC instead of a
purpose built switchmode controller IC like a 494?

 RM>  Got a few voltage drop problems to iron out
 RM> before going higher (veroboard and 6 amps don't mix...).

Yair, for sure.

 RM>> I suspect the "real fault" was poor layout.

 CB>> I've has a few fixes over the years where crap layout/design was
 CB>> the problem, and a modification rather than a normal repair was
 CB>> needed to get it going.  The earlier version of the Tait T555 2
 CB>> channel UHF mobile was a common one in that line for a while...

 RM> We drifted away from Tait around then. We sold a few 496s, and got
 RM> to know the "pull the vco shields off and clean all the ground
 RM> contacts" cure. A couple of 500s are out there somewhere, and
 RM> they're both horrible.

The basic design wasn't too bad (much better than their earlier
xtal controlled sets - the ones with the hand drawn PCB writing),
but was let down by a few crap components in critical areas.. like
the trimcap that presets the VCO freq.   Just replacing that with
a better quality one does wonders for "it picks up a lot of static
and rumbles" complaints... the rest of the poor quality probs I can
recall have been mechanical rather than electronic.

 RM> The new 2000s look promising, except for one in one of our vans
 RM> that sometimes draws 2A even when it's turned off. The PA
 RM> transistor takes off at switchon, and stays that way until the
 RM> radio is keyed.

And easy to find, too - just feel the heatsink :)

 RM>  I've slowed it down by reducing the value of a
 RM> damping resistor across one of the coils in the collector circuit,
 RM> but it still does it sometimes. Fun.

I've not seen the 2000's here, but the T5xx's have that problem
too from time to time.  Less than ideal coax, and certain types
of antennas (half wave gutter mounts!) seem to make it worse...

Regards,
Chris.

--- GoldED