N detector preamp -- check the simple things!!

Tales of woe that teach. We learn best through failure sometimes
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This is for tales of woe that teach. We often learn quickest through failure that shows us how our thinking was off, and of course, sometimes it's funny at least in hindsight. Share your "Doh!" moments here.

N detector preamp -- check the simple things!!

Postby Joe Jarski » Tue Aug 30, 2011 6:21 pm

Well, I've been beating my head against the wall for the past week trying to get these N detector preamps to work with no luck whatsoever. I thought there may have been some issues with the PCB that I had made. Some of the holes were smallish because I designed it to one fab house's specs and then sent it to another, so those holes had to be drilled out. Doug had also mentioned the possibility of parasitic capacitances being a problem and Chris brought up the point of the battery symbol on the schematic being reversed. So eventually, I went back and bread boarded a new circuit to try it that way... still nothing. The next step was to strip everything off of the bread board and try it again, because maybe I made a mistake wiring it...

With all of the parts sitting in a pile on the table I started over, picking up one of the transistors and what do you know?!!! It says 2N4124, which was odd because the circuit only uses a 2N5087 and 2N3904. I picked up the other and it says 2N3904. So I grabbed the little baggie of transistors that I got and the tag says 2N5087, but all of the parts inside say 2N4124 -- not only the wrong part, but it's a NPN instead of a PNP!!! :roll: :roll: :roll:

I guess I got too comfy just reading the labels on the baggie and not actually checking the parts!
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Joe Jarski
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Re: N detector preamp -- check the simple things!!

Postby Doug Coulter » Tue Aug 30, 2011 6:37 pm

Ah yes, troubleshooting 101 -- always check the simple stuff first, even before engaging the brain -- because it's simple and fast, and might solve it - you learn this quick when you fix stereos for commission pay -- speed matters when you can only get paid a couple bucks for a cheap unit -- you have to learn to crank them out, or starve. Too bad most modern tech support stops there with their cheezy drop down menu-list items. Some are actually hard, after all.

I had a similar problem with Lesker, trying to use those new type aluminum gaskets for QF flanges (I wanted one to work with glass blowing going on, it was going to get hot). There's an extra part, a steel ring with bent punch-ins that lets you crush the gasket just so far, and no farther. Bag had right number, wrong size part. I called them and explained the problem and asked that they check, they sent another - same problem. Third time -- same problem (no cost to me, but considerable hassle). I gave up. Wonder if they realize why they don't get repeat sales on that.

I've never known which way the battery symbol is really supposed to be -- I've seen it both ways in schematics back to the '30s of the previous century, which is why I clearly marked the polarity separately (or, well, actually Jon did, but I think he copied the short-long pattern from my scrawl so that's my fault :oops: ).
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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