Home › Evil Mad Scientist Forums › Other kit and product support › Troubleshooting Three Fives, soldering and testing transistors › Re: Troubleshooting Three Fives, soldering and testing transistors
July 15, 2014 at 3:44 am #21937
The first thing that you should do is take a look at the circuit diagram, which is available in the datasheet.
( See our documentation page, here: http://wiki.evilmadscientist.com/555 )
You will note that some of the transistors (Q5, Q8, Q18, Q19B, Q23) have pins connected together. This likely explains your “glitchy zeroes.”
Others are connected in more complicated ways, and you need to remember that the diode tester tests *everything* connected to the two points that you are measuring. In most cases, there is not just a single diode connecting any two given points.
Most errors with the 555 are due to a misplaced or backwards component, or possibly a bad solder joint somewhere. It is also possible to damage a component while installing it, but isn’t very common. (We generally recommend heating a solder joint for about one second, with a hotter iron.)
What I’d recommend to look further is to open up our “principles of operation” guide, and see if you can work through the circuit to see what is (and is not) working. Can you see a change in the output of the threshold comparator when you change its inputs? Can you see the change in the output of the trigger comparator when you change its inputs? It sounds like you’ve got the right equipment and the inclination to work through it, and really see it works inside. We’ve gone through this once– we intentionally let the magic smoke out of one (by putting enough voltage on– way outside of its limits), and were able to trace the damage all the way through to one of the transistors in the output stage.
You can use an electrolytic cap, just watch the polarity please.