Hi Chris



 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...



I recently built a simple r-c low pass filter to allow my

multimeter to measure the mains freq of the choo choo inverters

I've been playing with. The unfiltered waveform is complex, the

meter thinks it's around 10 kHz, so a lo pass helps the meter look

for the lower freq. The r-c is fitted in series with a pair of

multimeter leads. The nifty bit is the box - it's a simple 35mm

film canister. Poke holes in the top and bottom, stick the wires

through and attach to the components inside, and the wires are held

in place reasonably well.



 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?



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.



Normally, yes. But this circuit was too simple, I wasn't going to

let mere scruples prevent me from defeating it.



I suspect the "real fault" was poor layout.



Cheers



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