Hi all -
I just assembled the Three Fives kit, and connected the suggested test circuit. The test circuit failed (no light), and I'm looking for troubleshooting advice. With the circuit energized, I found voltage drops across most but not all components. I also disconnected the circuit, set my meter to diode testing mode, and probed each transistor, first with Com on B and V on each outer lead, then V on B and Com on each outer lead, expecting the meter to read about 0.660 with V on B for the NPNs (2N3904) and with Com on B for the PNPs (2N3906), and 0L in all other configurations. Unfortunately, only 14 out of the 26 transistors passed this test completely.
Re: transistor testing, I saw the following (presumably) failing tests:
* Q5 (PNP), Q8 (PNP), Q18 (NPN), Q19B (PNP), Q23 (PNP) : The BC connection was a glitchy 0 when it should have been 0.660. I say glitchy because the meter would jump when I'd first touch the probes. I'm pretty confident I made proper contact with the probes, and retried each one.
* Q13 (PNP), Q14 (NPN), Q16 (NPN), Q17 (NPN), Q21 (NPN), Q22 (NPN), Q24 (NPN) all showed 0.660 the right way around, but showed high (1.2 or 1.8) with the probes inverted on one or both outer pins. (I recorded more detail if the distinctions matter.)
Are these both symptoms of transistor failure? Would the diode-meter test produce an incorrect result in-circuit under some circumstances with Three Fives? (I was sure to disconnect the test circuit before doing the test with the meter.) Are there other testing techniques you recommend for in-circuit transistors that have already been through-hole soldered and leads clipped?
Re: soldering transistors, I was wary of overheating the transistors, so I tried to be careful. My Weller iron was set to 520 F, and I methodically avoided spending more than 10 seconds on each lead, and paused 10 seconds between each lead. I did not use a heat sync clip, and I did all of each transistor's pins in a row; clips and longer pauses (such as doing one pin on a bunch then the next pin on a bunch and so on) would have been sensible additional precautions that I didn't take. How likely is it that I damaged the transistors during soldering? Do my test failures above indicate which transistors need to be replaced? Which precautions against overheating transistors during soldering do you recommend?
Any other troubleshooting steps you recommend? I'm eager to treat this as an opportunity to learn more about the 555's design based on the excellent educational supplement that accompanies the kit, assuming I've done what I can to verify the transistors individually. I have a simple scope (the Rigol DS1052E) if that's worth using in this case.
Thanks for any help you can provide!