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stormdogI need an old desk or something to use as a workbench in the spare room. Soon. My electronics gear has expanded to the point where it's taking up a majority of the living room table. Soldering iron and stand. Big old HP oscilliscope. Multimeter. Manuals and reference material. My little toolbox and a place to put my pliers and things. And of course a big space in the middle for whatever I'm working on. I was doing some soldering yesterday morning and I could still smell a faint odor of melted solder when we got home from my parents' place in the evening. Danae is tolerant, but somewhat unamused...
At this point, I'm primarily
learning by making wrong assumptions and then correcting them. This is fine with me, and I feel pleased with myself every time I figure out something I was wrong about and correct it.
I mentioned that I was reversing the pin numbers on the tube sockets. That helped things make more sense, but I then found that, according to my DMM, the pins connected to each side of the heating filament in the tubes were shorted together. I assumed that the transformer secondary powering the heaters must be shorted, and that's why the fuse blew. I was going to have to find a new transformer.
Then it occurred to me that a transformer winding is just a bunch of turns of wire around a core. Maybe it *should* have really low resistance. I desoldered both secondaries from the circuit, plugged the VTVM in, and switched it on, half-expecting to blow a breaker. But all was well; no short after all! I checked the voltage on the secondaries with the DMM. about 15VAC on the one powering the heaters, and 150VAC on the other. That was slightly too high; it would be driving the filter caps on the 150VAC circuit right at their rated limit. And the tube heaters should be running on 12.6; they'd probably burn out at 15. They were both too high by around the same ratio; there must be something wrong with the primary winding; maybe it's fried after all!
More reading led me to the fact that transformer output voltage actually drops a bit under load, so with no load they will read high. Maybe it does work!
So I figured I'd look at the output on my scope and see if I was getting a nice sine wave. (I got to use my scope for a real, live diagnostic procedure for the first time; yay!) The sine wave looked fine, but the voltage was crazy! It was something like 50VAC on the heater secondary and around 400VAC on the other one. Wow! Was my scope's calibration that messed up?
I checked the scope's 1VDC calibration square wave and that looked fine. Maybe the calibration signal generator was affected by the same problem affecting the reading from an external source and was giving me bad data? More reading online.
Oh, right! I'd forgotten that AC voltage of a sine wave is calculated by root mean square: about 0.707 times the height of peaks relative to center 0. I was simply measuring voltage from positive peak to negative peak, which gave a result that was well over double the RMS voltage, which is what my DMM was showing me. So my scope was working *and* the transformer was working!
So then I resoldered the transformer secondaries *back* into the circuit, and by then it was time to go visit my family for the day. So the VTVM is in the same state it was in before, but I've learned some stuff!
At this point, I'm primarily
learning by making wrong assumptions and then correcting them. This is fine with me, and I feel pleased with myself every time I figure out something I was wrong about and correct it.
I mentioned that I was reversing the pin numbers on the tube sockets. That helped things make more sense, but I then found that, according to my DMM, the pins connected to each side of the heating filament in the tubes were shorted together. I assumed that the transformer secondary powering the heaters must be shorted, and that's why the fuse blew. I was going to have to find a new transformer.
Then it occurred to me that a transformer winding is just a bunch of turns of wire around a core. Maybe it *should* have really low resistance. I desoldered both secondaries from the circuit, plugged the VTVM in, and switched it on, half-expecting to blow a breaker. But all was well; no short after all! I checked the voltage on the secondaries with the DMM. about 15VAC on the one powering the heaters, and 150VAC on the other. That was slightly too high; it would be driving the filter caps on the 150VAC circuit right at their rated limit. And the tube heaters should be running on 12.6; they'd probably burn out at 15. They were both too high by around the same ratio; there must be something wrong with the primary winding; maybe it's fried after all!
More reading led me to the fact that transformer output voltage actually drops a bit under load, so with no load they will read high. Maybe it does work!
So I figured I'd look at the output on my scope and see if I was getting a nice sine wave. (I got to use my scope for a real, live diagnostic procedure for the first time; yay!) The sine wave looked fine, but the voltage was crazy! It was something like 50VAC on the heater secondary and around 400VAC on the other one. Wow! Was my scope's calibration that messed up?
I checked the scope's 1VDC calibration square wave and that looked fine. Maybe the calibration signal generator was affected by the same problem affecting the reading from an external source and was giving me bad data? More reading online.
Oh, right! I'd forgotten that AC voltage of a sine wave is calculated by root mean square: about 0.707 times the height of peaks relative to center 0. I was simply measuring voltage from positive peak to negative peak, which gave a result that was well over double the RMS voltage, which is what my DMM was showing me. So my scope was working *and* the transformer was working!
So then I resoldered the transformer secondaries *back* into the circuit, and by then it was time to go visit my family for the day. So the VTVM is in the same state it was in before, but I've learned some stuff!
