Thursday, February 20, 2014

Quick Decade Resistance Box Fix.

I picked this Decade Resistance Box (AKA Resistor Substitution Box)up at the OSU sale many years ago, before I started trying to learn electronics. I dusted it off when I began assembling my “lab” but found that it was not particularly accurate, or rather grossly inaccurate.

With all the resistors switched off it showed 34.3 kilohms  [there are three cases in which the final vowel of an SI prefix is commonly omitted: megohm (not megaohm), kilohm (not kiloohm), and hectare (not hectoare).] of resistance when it should show almost none.

Yes, I cross-checked with another multimeter (VOM, DMM). All of my multimeters disagree. This is leading me to an obsession with standards and calibration which I will write about later this year.

My favorite tool these days is DeOxit contact cleaner. There are lots of brands and types, this is what I have and use.

So I sprayed the banana jacks.

Then I disassembed the box and sprayed all the switches while actuating them to clean the contacts.

Now it measures a very low .246 Ohms.

I ran it through all the resistors and found that the 300 kilohm resistor read 30 kilohms.

So I tested the resistors….

Look at that, someone put a 30 kilohm resistor in by accident. I picked a pack of 300 kilohm resistors up at Norvac electronics after the hamfair this last weekend (interestingly enough there was a distinctive lack of pork products at the swap meet).

While I was at it I replaced the banana jacks. The old ones were wobbly and the threads stripped out.

Now it measures within the 1% of 300 kilohms. That’s my new-to-me Fluke 8840A multimeter from the hamfair.

Tuesday, February 04, 2014

Bad Toaster

I was having a toaster crisis.
Our toaster (besides being upside down, a temporary condition) was more of a bread dryer, than a device which would actually make toast.

The is the model number. It was cheap and I bought it a few years ago at Bi-Mart.

When I tried to unscrew the bottom I sheared off one of the screw heads. It was screwed into soft plastic. I don’t know how you could even do that.

I couldn’t find anything obvious wrong with it.

You can see that some of the resistive heating wire is pulled away from the insulating/reflecting surface. I tried to pull it all away from the surface with a dental pick but there wasn’t enough slack.

The only sections that glow are those proud of the surface. I’m guessing that the heat isn’t being reflected into the bread but absorbed by the insulation.

Such that, if I put it on the highest setting,

I get very dry and barely toasted toast with little evidence of the Maillard reaction.

As a bonus it was pulling 4W all the time it was plugged in. That’s 12 cents a year!

800W when one side is used.

1550W when both sides are used. It’s supposed to be 1650W, but that’s probably within tolerances.

Why not double check…

Anyway I decided to replace it. I spent some time researching and while you would think that the error was buying a cheap toaster, I replaced it with the Black & Decker TO1332SBD 4-Slice Toaster Oven, which was less expensive that the toaster it replaced, more versatile, and as a bonus? It actually will make toast. I’ve been quite happy with it for the past month.