[sdiy] Diode Matching

[David G Dixon]

This is REALLY off-topic, but Ingo's plight reminds me of something that happened to me a year or so ago.  I had a circuit that just wouldn't work at all.  I went over it and over it, and couldn't figure it out.  Finally, out of sheer desperation, I started measuring resistor values on the PCB.  One of the 1M resistors was actually 140 ohms.

In 1%, the color schemes of the two are mirror images: Brown-Black-Black-Yellow-Brown (1M) vs Brown-Yellow-Black-Black-Brown (140 ohm).

I can't remember what I was actually doing, but I figure that it was probably a hysteresis resistor on a comparator.

The thing is, I had never bought 140 ohm resistors, so this mistake was made at the store where I shop (Lee's Electronics in Vancouver).  It's amazing that I found this problem at all.  I've run out of 1M resistors, so I don't have to worry about this same problem recurring.



-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Ingo Debus
Sent: Saturday, May 09, 2020 9:36 AM
To: Synth DIY
Subject: Re: [sdiy] Diode Matching


> Am 09.05.2020 um 13:09 schrieb Ingo Debus <igg.debus at gmail.com>:
> 
> Sorry, this is getting OT, but I really got curious. So I hooked my scope across the resistor, in parallel with the DMM. With the 560 k resistor, the scope showed some heavy 50 Hz hum. With the 470 k resistor there’s absolutely no hum. Then I grabbed some other 470 k resistors of a different type: voila, the hum as well as the instable reading of the DMM is back again! When I use 470 k resistors of the same type than the first one, from the same belt (is belt the correct term here?), there’s no hum, and the display is stable within one digit.

Boy, I do feel stupid now. I often do goofy things, but this is a highlight.

These aren’t 470 k resistors, these are 470 ohms resistors. I had labelled the belt with 470 k after I had bought them and put them into the wrong bin. This is the small variant (0204 I think), so I can hardly see the color rings without a magnifier. Hopefully I haven’t used them in too many projects.
This DMM has huge digits, but the k and the M letter is rather small. When switching from 560 k to 470 ohms, I didn’t notice the small letter switched from M to k.

Ingo (embarrased)😰


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