Hello Roy,

 CB>> One gizmo I built years ago gets used whenever I suspect that
 CB>> a DC supply is taking off - it's a little box with four wires
 CB>> that goes between the supply and a multimeter. Contains basically
 CB>> a small series cap and a "voltage doubler" in series with the
 CB>> red wires, the two blacks are ground.  Shows any oscillation
 CB>> on a DC line very well...

 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.

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

 RM> The unfiltered waveform is complex, the meter thinks it's around
 RM> 10 kHz, so a lo pass helps the meter look for the lower freq.

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

 RM>  The r-c is fitted in series with a pair of
 RM> multimeter leads. The nifty bit is the box - it's a simple 35mm
 RM> film canister. Poke holes in the top and bottom, stick the wires
 RM> through and attach to the components inside, and the wires are held
 RM> in place reasonably well.

I used a small "tupperware" container and a bit of Araldite for
mine.. It can be safely mentioned - Sue dosen't know how to use
the mail reader, and she's still wondering where it went :-)

 RM>> Final cure was a teensy little .01uF across the collector-base
 RM>> of the TIP driver.

 CB>> Was there one fitted in that place originally that had maybe
 CB>> gone open?

 RM> No.

 CB>> If not, it sounds like a "modify the circuit to
 CB>> conceal the real fault" type scenario, something I try and avoid
 CB>> here if practically possible.

 RM> Normally, yes. But this circuit was too simple, I wasn't going to
 RM> let mere scruples prevent me from defeating it.

Yep.  Economics always rules the decision in the end.

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

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

Regards,
Chris.


--- GoldED