Dumont RA-103 Project Redux

[old_coot88]

...the bottom open circle and the arrow indicates that is the articulating contact in the relay switch correct? The top open circle is everything else and is the stationary contact?

No, in schematic arcana, the arrow is always the stationary contact and the moving contact is always what the coil "pulls" on.

[Chris K]

Originally Posted by old_coot88

No, in schematic arcana, the arrow is always the stationary contact and the moving contact is always what the coil "pulls" on.

GREAT!!! Thank you for that piece of knowledge! I think I have the relay hooked up correctly. The stationary contact has the tied together pin #8s from both 5U4s and the yellow lead from the small secondary coil soldered to it as well. I'm going to double check the wiring in the rectifier circuit then move on to checking the moving contact leads and how they trace out.

[Electronic M]

A couple of things:
The resistor that's smoking, if it tests fine then it's smoking because the short is on (or not far down stream of) the side of the resistor not connected to the high B+ rail....That resistors is the somewhat literal metaphorical smoking gun that points you to at least one killer short.

There were a few purposes to the relay. The main one is to delay the B+ until the tubes have warmed up to stable operation so when the set comes on you get a perfect picture and sound instantly without any concerning warmup instability. (Realistically you can delete the relay by shorting the contacts and the set will still work fine... At least for sets that work with the relay there) The other purpose is to protect most of the lytics from warmup surge.... Basically since the 5U4s are filament tubes they warmup almost instantly, but all the other tubes take ~11 sec.... During that time the B+ has no load from the signal/sweep tubes and skyrockets briefly this can be hard on the lytics and or require higher voltage lytics to cope.

[old_coot88]

Since the high B+ rail goes directly to the area of the focus control and H centering pot, I'd bet money that the short will be in that area (circuit-wise), most likely involving the focus control. The schematic is too fuzzy to read print, so if you could post a blow-up of the lower-right corner of the schematic, it should be pretty easy to nail the culprit.

[Chris K]

Here it is. Unfortunately, I'm at work so no troubleshooting until I get home!!!

[Chris K]

Originally Posted by Electronic M

A couple of things:
The resistor that's smoking, if it tests fine then it's smoking because the short is on (or not far down stream of) the side of the resistor not connected to the high B+ rail....That resistors is the somewhat literal metaphorical smoking gun that points you to at least one killer short.

There were a few purposes to the relay. The main one is to delay the B+ until the tubes have warmed up to stable operation so when the set comes on you get a perfect picture and sound instantly without any concerning warmup instability. (Realistically you can delete the relay by shorting the contacts and the set will still work fine... At least for sets that work with the relay there) The other purpose is to protect most of the lytics from warmup surge.... Basically since the 5U4s are filament tubes they warmup almost instantly, but all the other tubes take ~11 sec.... During that time the B+ has no load from the signal/sweep tubes and skyrockets briefly this can be hard on the lytics and or require higher voltage lytics to cope.

So I had it backwards in terms of what happens. The relay delay is for the rest of the TV to warm up before they get slammed with B+ from the 5U4s.

[old_coot88]

Originally Posted by Chris K

So I had it backwards in terms of what happens. The relay delay is for the rest of the TV to warm up before they get slammed with B+ from the 5U4s.

Exactly. That's its primary function. Less high-end designs didn't use it by designing-in enough 'headroom' that the surge doesn't overvolt the filter caps' rating.

[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!

[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.

[Chris K]

Horizontal position pot is shorted to ground. Based on the circuit, could it be a bad horizontal yoke coil in addition to a bad pot?

[old_coot88]

What point are you actually measuring from? Is it a dead short (zero ohms)? If not, how many ohms?