Scope Probe Enhancing Performance of Circuit

I had something like this happen once. We had to ship scope probes with the product. :-)

I had a circuit that would only work when my middle finger was touching a particular point.

So I bridged my finger and added that much capacitance ;-)

...Jim Thompson

How many pF is your middle finger?

gt5513e's problem sounds even more expensive. He will have to ship a Tektronix THS720 portable scope with each unit;-)

Don't remember now... 40+ years ago, but IIRC just a few pF.

...Jim Thompson

I did consider this option, but only as a quick fix for the first million boards. Any changes will be for rev 2

If you don't need a scope attached to the probe perhaps you should just ship each unit with a probe attached :).

I do need the scope! I can fit the probe in the design budget but the whole scope, that's a different story! :)

I threatened to do this once. The circuit (an ECL logic circuit, at that) only worked with a scope probe attached.

Yep.

Tried that (adding the scope capacitance); no joy. Turned out that the wire to the pin was broken. The scope probe added enough weight that it made contact. I chased that one for a while!

No, that's unaccounted for "parasitics". ;-)

Sometimes you just gotta drain the swamp.

What more do you expect? After all, it is J.T.

On Fri, 10 Feb 2006 09:08:05 -0700 in sci.electronics.design, Jim Thompson wrote,

Alternately, a drop of super glue between the part and your finger would have done.

In

with

I once got a job at a consumer electronics repair shop because of scope probe capacitance! I walked in to put in an application, showed my resume, and he said he had just hired someone. He saw I had some Sony experience and said he had a projection TV that no one had been able to fix, he ask if I'd be interested in looking at it. The projection TV had no raster, no output from the tubes. I sat it up and started probing around and after about ten minutes he looked over and said "hey you got it going". I had hung the probe on part of the high voltage osc. circuit and it fired up. I was on the backside and hadn't noticed light was coming out of the tubes. Turns out someone previously had replaced a cap. with one only 1/10 of the correct value. The scope probe added enough capacitance to keep it running. So he started thowing repair work my way. Mike

Reminds of a true story... over in Sun City, our local retirement community, a gal suspected hubby of infidelity.

While he slept she super-glued his "privates" to his leg. When he awoke, in need of a pit stop, there was sheer panic... made the newspapers because he called 911 ;-)

...Jim Thompson

a écrit dans le message de news: snipped-for-privacy@g14g2000cwa.googlegroups.com...

I'll fix it for free, then you give me half the scope price for each of the first million boards.

It happens.

A while ago I hung an old Heathkit signal tracer onto a test point in a fancy Fluke signal generator. The test point was in a PLL lock circuit.

Turns out the PLL would lock only with the signal tracer probe on the test point. Very weird.

After a little investigation it turned out the signal tracer had a leaky input capacitor. Substituting a resistance decade box in place of the probe showed the circuit needed between 11 and 12 megohms on that point to achieve lock.

The capacitor leakage: 11.6 Megohms. How cool is that?

But bvack to your problem-- first I'd put the scope PROBE on the circuit board ground, and leave the scope ground lead dangling.

Apparently there's a fair amount of hash on that, if just hanging a piece of metal onto it makes a difference.

I've seen PLL designs that would work better with the application of a finger. From memory, one would reduce its phase noise by about 10dB when touched. It turned out that the leakage current would cause a steady state phase shift that moved the phase detector away from its dead zone.

I hate the 4046.

Allan