Re: 475 questions


ciclista41@...
 

Hi Graham!

Thanks for your contributions and supportive words! Yes, what I have learned has mostly been internet sourced, including the book available at allaboutcircuits.com, but I am only at Chapter 6 of volume 1. I am picking up more from eevblog, various youtubers, and both TekScopes and TekScopes2. I did take both semesters of the University Physics intro (weed-out) course and passed them both, although I tried to do them in summer school, which worked for the first semester, but not the second, which was mostly on electromagnetism. I did okay in the calculus parts, but the fact that this was more than two decades after a weak foundation in both high school electronics and physics along with a full-time load as a half-time parent meant that my grasp was barely enough to pass the class the following fall semester. I was shocked that I was given a C, as I think I only got about 40% on the final exam. I did get an A in the one-credit-hour lab, which was the hardest I ever worked for a single credit in my life. As I mentioned in an earlier post, this new hobby is partly about fixing that less-than-satisfying part of my life.

Thanks for the advice about checking out other Tek 4xx manuals. I will try that. I am still struggling to get through the 475 explanations, but I did download "Troubleshooting Your Oscilloscope--Getting Down to Basics" from Tektronics last night. I expect it will be helpful in giving me a more systematic approach.

Your explanation of the full wave rectifier confirmed my correct understanding of that circuitry! That felt good, as I had not much clue about that a couple of weeks ago.

I have found no sign of overheating on any components, except for smoke residue on components under the "Warning High Voltage" aluminum shield, though no sign of any of them being bad. One exception: the two neon bulbs (still no idea what these are doing there) next to each other, DS1382 and DS1383 do not glow when the power is on.

Another observation I made last night was that after running the board for several minutes, I shut it off. I shorted the big caps for safety, but while C1462 gave me a good "snap" and C1472 gave me an even louder one, none of the other four held any detectable residual charge when shorting them. Neither did any of the caps under the aluminum shield.

Yes, you are right that I am confidently running the scope directly plugged into the wall, because nothing bad seems to happen. With the scope running, I get the following measurements:

DC across +50 to ground ~ 39V AC across the same ~ 87V

DC across C1412 ~ 65V AC across C1412 ~ 150V

DC across C1414 ~ 45V AC across C1414 ~ 103V

DC across C1442 ~ 20.5V AC across C1442 ~ 47V

DC across C1452 ~ 9.5V AC across C1452 ~ 21V

DC across C1462 ~ 12V AC across C1462 ~ 27V

DC across C1472 ~ 22.5V AC across C1472 ~ 51V

Each AC reading seems to be about 2.2 to 2.3 times the DC one. Not sure if that's what should be happening, but it was consistent, so the readings are probably correct even if the information isn't useful. Am I doing something wrong here? This doesn't seem to distinguish any of these sub circuits as being a culprit.

Sorry, no infrared camera available.

Bruce

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