Pot Luck 4/20/2013

For those who missed the pot luck last Saturday, here are some of the highlights:

Walter Clark brought in his Wimshurst Machine and did a very interesting demonstration of static electricity and electric fields. Most of the things we think of as electrical use magnetic and electromagnetic effects. Static electricity only shows up in a few modern devices, such as the electrometer dosimeter.

Here's Tayopa cranking the machine.

Also included in the demo was a simple foil electrometer that could either acquire a charge (when touched to the non-grounded terminal of the machine) or detect the charge when brought near.

The sparks from this thing are also impressive. The breakdown voltage of air is 10-30,000V per cm. So the machine is generating over 25KV in potential.

Next, we had the usual potluck stuff, including an awesome dinner and clown bike rodeo.

And we fixed Tim's early 1980s Roland Piano 70 synth keyboard. This one was kind of tricky - we checked the DC power system, used a scope and signal generator to check out the amp stages, and finally figured out that the oscillator boards weren't working.

Going over these again with a multimeter, we discovered that the 3 virtually identical boards did not have identical voltages on the 2 power rails. A close look revealed a corroded trace and a couple of bad solder joints on the +15V rail.  Fixed!

One other thing: One of our mysterious neighborhood characters left a baggie in the parking lot with a bit of crystalline residue. We put it under the microscope at about 25X before disposing. Can you ID this mystery substance? Is it sugar? Is it crack?

A couple of notes on taking pictures through a microscope: If you don't have a scope with a camera port (we don't) then your best bet is to use a longer lens or zoom setting and manually focus at infinity.

When you have that done, disable the flash, set a high ISO and put the camera right up to the objective. This one was done with a 90mm equivalent lens, ISO 1000. Was very easy.


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