Dumont RA-103 Project Redux

[Chris K]

…and maybe when u get time Tom, for my education, can you explain how you knew the short was after the CO resistor when it got screaming hot? Some things about DC voltage I just don’t get.

[Electronic M]

Glad you are making progress.

Originally Posted by Chris K

…and maybe when u get time Tom, for my education, can you explain how you knew the short was after the CO resistor when it got screaming hot? Some things about DC voltage I just don’t get.

The short answer is doing a fair bit of ohms law and voltage divider math and seeing the principles in action under similar circumstances.

The short answer is that every resistor has a power rating (watts = amps X volts), and will smoke if that power rating is exceeded. The short will increase the load the resistor is feeding and thus the power it has to feed it's load. There are a few ways of looking at it mathematically...The resistor and the things loading it (the loads can be modeled by a resistance who's value can be obtained by ohms law and by measuring the load current and voltage drop) are basically a voltage divider. If you short the load resistance dead the voltage across the candohm resistor increases (as does current thanks to ohms law) so power through the resistor will increase even faster.
Another way to look at it mathematically if the short isn't a dead short, is to say that the load resistance has dropped which in turn lowers the total resistance of the divider...If B+ hasn't significantly changed then ohms law arranged amps= volts/ohms indicates that current will increase which will increase the power through the candohm.

Kirchoff's current law and voltage law are also helpful in understanding voltage dividers and other series parallel resistor networks.

If you want a fun/useful exercise in designing voltage dividers and have some tube battery radios designing LM317/LM337 voltage regulator power supplies for unregulated rectified AC is a interesting way to practice.

I remember some of my college EE labs where some transistor or voltage divider circuit was to be tested and a LOT of my peers not bothering to check power requirements in the math and smoking half watt resistors. In some cases you could adjust the math to get what you need within available wattage, in other cases the professor made the error and to do the assigned problem you had to know that the parts desk had some wire wound power resistors and be first to ask for them, or build your circuit with several series/parallel 1/4W resistors to make a single 1-5W part...often if you needed that many resistors you'd have to justify it to the parts room troll, or build the circuit with undersized resistors and collect the data before it burned open...Good times!

[Chris K]

Thank you Tom. I was able to follow your reasoning. Best thing for me is to start reading the basic texts I have on electricity, magnetism, resistance, power and inductance. Life is really busy right now so the attention I want to give to it is probably a couple of years off. I'm a science guy but its all biochem and medical research. I do love physics and I'm fascinated by the theoretical musings from the big names but the details regarding electron movement, capacitance, vacuum tube function et. al. just don't jibe, at this point with the rhythms of my brain. I had a discussion once with an EE about my inability to understand alternating current and how do you get any work done with electrons having a net movement of zero. He said "you're not turning a paddle wheel with it. You're doing work. Think of sawing a board with a hand saw. The net movement in the saw's reciprocation is zero but the work and its movement is down through the board. Now I don't know if that's a ubiquitous explanation of AC in the EE world but it was a revelation for me!

[old_tv_nut]

Originally Posted by Chris K

I had a discussion once with an EE about my inability to understand alternating current and how do you get any work done with electrons having a net movement of zero. He said "you're not turning a paddle wheel with it. You're doing work. Think of sawing a board with a hand saw. The net movement in the saw's reciprocation is zero but the work and its movement is down through the board. Now I don't know if that's a ubiquitous explanation of AC in the EE world but it was a revelation for me!

Yes, part of the analogy is friction (in the case of the saw) or resistance (in a circuit).

If instead of a saw going back and forth, you had a pendulum in a vacuum with a frictionless pivot, it would go back and forth forever without losing any energy.

The action of the frictionless pendulum is transfer of energy between potential and kinetic forms - the pendulum at the top of its swing has potential energy, and at the bottom of the swing that has been exchanged for kinetic energy. The electrical analog is an LC circuit with an oscillating current. At one point of the cycle the electrons have maximum potential energy (voltage on the capacitor) and at the other part of the cycle they have maximum motion (current in the inductor).

[Chris K]

Thank you so much for your insights and for further clarifying the picture!

Speaking of which, I did a horrific bodge job with the harvested power resistors from the parts chassis completely substituting for all of the Candohm resistors in my set. I prayed, chanted and lit some candles before I turned it on. When the relay kicked in...I HAD AN INSTANT PICTURE WITH RASTER!!!

That is first light on this Dumont! I have the resistors on order from DK and Mouser. Thank you guys for helping me get further with this TV than I ever have before and thank you Tim and Tom for pointing to the can resistors almost immediately in the process! Whoopeeee!!! Further updates and confusion/questions to come!

[Chris K]

Lots and lots and lots of work to do but...Holy Crap guys! Thanks again!

[old_tv_nut]

Originally Posted by Chris K

Lots and lots and lots of work to do but...Holy Crap guys! Thanks again!

Super!

[old_coot88]

Originally Posted by Chris K

Think of sawing a board with a hand saw. The net movement in the saw's reciprocation is zero but the work and its movement is down through the board. Now I don't know if that's a ubiquitous explanation of AC in the EE world but it was a revelation for me!

Another good AC analogy is a steam locomotive. The force-driven reciprocation of the driving rods translates directly to forward propulsion of the whole machine down the track.