Dumont RA-103 Project Redux
 

Hi Folks,
I'll be restarting this one again. I got a NOS delay relay installed and working Last power up, I had smoke from the area of the vertical output transformer. After much analysis revealing the value of a VTVM in testing coil winding integrity, I found no damage to the VOT. This doesn't mean I'm certain this was the source of the smoke...just that it was coming from this area. I sourced a new 1Kohm focus rheostat from Surplus Sales. I'm going to install it on the back of the chassis as the shaft is not long enough for a direct replacement. Tom C. gave me some advice on extending this but I can't remember what he said and I'm too lazy to search for it!!!! So I'll be on a journey through this in the next few months and I hope you will be along there for the ride. Just wanted to get things started!

I believe ACE hardware sells 1/4" shaft couplers...if the new shaft sticks out close to 3/8" or more one should work...just saw the shaft off the old pot after measuring how far it sticks out and stick it in the other end of the shaft coupler (and make sure it sticks out the same length as the old one did). If ace doesn't have it Amazon or some online vendors will. You can get rigid and flexible shaft couplers...This application is probably better with the rigid kind. (Last spring ETF I turned down a nice doghouse because someone had deleted the focus knob and put on wrong knobs elsewhere and I thought it looked real bad like that).

I forget where the vertical output transformer is but if it's near the candohm chassis mount power resistors those are more likely to be the source of the smoke....I haven't had any fail for me, but of the 5 RA-103s I've had 3 of them had 40s-50s power resistors cobbled in to replace those resistors or sections of them. The one near the vert osc transformer (and the osc transformer itself) are common to find failed.

Last time I was at Moyers, I started rattling off the values of power reaustors I needed for my 30s GE table radio.

Before I was done, he interrupted me and said “Candohm?”

Those things are famous for shorting and going open, and in general, just foung annoying stuff.

OK here we go. Let me preface this with two things because in the past, when I have posted pictures of work that I have done, I've received a few nasty comments about lousy soldering, poor organization etc. I started this hobby in late 2020 and there's no question some of my restorations have looked amateurish. I'm 64 years old now with shaky hands. Despite this, most of my electrical work is solid and reliable. I by no means try to pass myself off as an experienced electronics restorer. Constructive advice is always welcome, in fact, it's desired but I'm sure some here will not participate or lose interest when the journey reveals stupid mistakes or poor technique. So be it. I desperately want to get this TV working but it was altered significantly by previous work over decades and I've done my best to attempt to problem solve. With all this in mind, I'll make my second point.

I started work on this TV a couple of years ago when I had a significantly different approach to restoration. Today I use the standard method of trying to baseline the TV through a slow power up on original parts. Once that's done, I do methodical replacement of components where I think the failure points are and then check function. But a few years ago, I shotgun recapped my restorations and I had a reliable bookkeeping method but I did create errors that were then difficult to run down. That's the method I used and describes the current state of this TV. I have some rudimentary function in the relay/rectifier section but then I start to hit walls.

I had the great fortune to run across this on eBay and snatched it up.

https://imagizer.imageshack.com/v2/8...924/Wws80c.jpg

This was a great acquisition because of the quality of the available resources. I can't read the Sams schematic. The copy quality is poor and the print is microscopic. The Riders service data is much more useful and I've been using it as an adjunct to the publication above. Below is the Dumont Service Manual schematic for the RA-103

https://imagizer.imageshack.com/v2/1...923/Jkn2qC.jpg

Below is the detail of the section I have had success with. The relay works and the circuit closes powering the entire TV

https://imagizer.imageshack.com/v2/1...924/moMelw.jpg

OK, here are some pictures of the chassis. I did relocate the focus pot and I am in the middle of wiring it up. It's mounted on the side of the rear of the chassis under the HV cage. The wiring in the rectifier section is a mess and I will clean it up. I tested function on 4 relays so there was a lot of extension wiring to test it unmounted. Oh hell...I'll just post them all! Go ahead, let me have it! :bash::bash::bash:

https://imagizer.imageshack.com/v2/1...923/2bMqGY.jpg

https://imagizer.imageshack.com/v2/1...923/Zb4Jw7.jpg

https://imagizer.imageshack.com/v2/1...923/qhZYpT.jpg

Relocation of the focus pot, not yet wired up. Should I use Tom's suggestion of a shaft extender :scratch2::scratch2: I'll move it back to the original position.

https://imagizer.imageshack.com/v2/1...922/3JZYUY.jpg

The original focus pot still in place but out of circuit. The 750ohm resistor hanging off the chassis is something I put in circuit to test TV function in the absence of a working 1K focus rheostat. It's no longer in circuit.

https://imagizer.imageshack.com/v2/1...922/OrLpgc.jpg

More of the chassis

https://imagizer.imageshack.com/v2/1...922/di3UT3.jpg

Area under the HV cage

https://imagizer.imageshack.com/v2/1...922/X2a5vM.jpg

OK...so, when the relay kicks in, I have a dead short...somewhere. I'm certainly not asking anyone to review these photos and tell me where it is. I'd just like some guidance on where to start methodically running it down. My first thought was to go through my replacements one by one checking for accuracy but IDK if that's the best way. If desired, I can post photos and details of any area on the chassis and describe what's going on. Any and all advice is greatly appreciated!!!!

Sorry if my ranting about criticisms offended anyone. I'm embarrassed by some of the dumb things I do! The Top Dogs here have always been kind, encouraging, helpful and reassuring.

This post is not as comprehensive as I would have liked it to be but I wanted to get something out there to get started.

Cheers!

A good way to run down a hard short is ohming out the main B+ line. If you can detect the short with a DMM unhook all branches from the main line and recheck the main. If the main isn't shorted check the unhooked branches. If one branch has a short leave it unhooked (you can reconnect the others) and disconnect what branches from it and check those. Rinse and repeat till you find it or narrow it down to a few parts and wires and check those.

In one of your photos a red and green wire meet in the foreground and don't appear to be soldered... might want to do that.

Yes that’s the focus pot hook up not yet soldered

The first place I would start is the candohm resistors. They have a nasty habit of shorting to chassis. If you replace the candohm resistors be sure to unhook all the wire connections to the terminals on the candohms. You cannot use the terminals as tie points for your new resistors. If they are not shorted now they may become so in the future.

Measure all the terminals of the candohms to chassis and see if that is where your short is then disconnect the wire and measure again if it is the candohm or the connections that are shorted. If it is not the candohm then follow the wires and the short to where it goes, disconnecting things as Tom has mentioned until you find it.

Use your eyes and check for errant solder blobs and wires that may be touching.

Great. I will try that this evening. Not to dismiss Tom’s suggestion but disconnecting B+ leads in bulk sounds too daunting for me right now. Both original resistor cans are in circuit and in use. I’ll take a comprehensive look and document it then post.

On most other sets I'd say have more faith in yourself. Dumonts with those harnesses usually aren't fun sets to trace leads on....Says a guy who made one good harness out of 2 half mouse eaten ones (it took a day and was a huge PITA, but I did it).

Yeah it's a spider web, a mouse trap and hedge maze held together with hundreds of 1948 style zip ties...these annoying wax covered string knots. I had a thought. The main distribution switch for the B+ appears to be the selector switch on the front for phono, FM Tel and Tel without pilot light (why?). Do you think it would be helpful to power it up with phono selected and see if it shorts when the relay closes? Do the same perhaps with the FM position selected? Could this approach eliminate some sections as the source of the problem?

I relocated the new focus rheostat in the original position and hooked it up. I don't need new problems that I might make from running new wires etc...

The original canned resistors seem to be OK. The only terminal that has continuity with the chassis is the one that is connected to ground.

Well it was worth a try. I have had 3 sets with one or the other candohm shorted to ground. Yes, that seems a good idea. Easy way to separate B+.

Ditto on a Candohm resistor shorting to the chassis.

OT: I restored a 16" Admiral set, 26R12 and all was well. Reading many threads on the Antique Radio Site, there was much mention about Candohm resistors shorting to the metal casing. Many radios used them.

The set was unused for a while and I wanted to watch a movie. When I turned on the set, I was greeted with quite a light show inside of the 5U4G (Admiral Labeled!) It burned open the filament of the tube rendering it useless of course. Fast forward, the Candohm was in fact shorted internally and shorted the B+ to Ground. I replaced it with a terminal strip and a ceramic resistor.

(Note: this is a picture before I restored the set)

Well, maybe not so fast on my dismissal of the Candohm resistors. I tried it on the phono setting and it didn't dead short however, R285, the 1.28K resistor began to smoke and got blazing hot. That's the Candohm right off the choke with 435V DC on it. It didn't test as shorted to the chassis when I tested it for continuity but something major is going on there.

I isolated R285 and tested it to ground and for resistance. It tests perfect. I am going to replace all of these CO resistors at some point but while I'm troubleshooting, I'm not going to complicate things. I have my suspicions the issue is in how I hooked up the relay. I'm going to start with the lead that feeds the articulating top contact. As it sits, the relay closes when the 6AL5 tells it to do so. The issue is after that happens.

So this is the area of interest...again!

https://imagizer.imageshack.com/v2/1...923/zhBxjm.jpg

Here are the relay details.

https://imagizer.imageshack.com/v2/1...922/prJvKA.jpg

I thought the purpose of this was to give the 2 5U4 rectifiers time before they are connected to the rest of the TV but this schematic confuses me a bit. The rectifier circuit feeds into the relay at the point indicated by 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.

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.

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.

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.

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

https://imagizer.imageshack.com/v2/1...923/NMjFwq.jpg

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.

…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.

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.

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?

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

No not a dead short...48 ohms. From the two tied terminals on the pot. Am I measuring the horizontal yoke at 48 ohms?

Try lifting the lead that goes to the yoke coil.

I just did and forgive me now for the fact that I'm getting a little confused so I might give you information that doesn't make any sense or that makes no relevant sense to this troubleshooting. I lifted the lead off the positioning pot that goes to the horizontal yoke. There is no continuity to ground on the pot now..no tone on my DMM and the tab on the VP pot measures 98 ohms. The lead I removed reads a tone to ground at 48 ohms, the same reading I got when the lead was attached to the pot.

Is that the lead that goes to R286A 200 ohm candohm?

Yep

I think you may have found your problem.

The terminal on R286A reads 48 ohms to chassis ground. And I lifted the leads from it and it measured 45 ohms to ground with a constant tone

yep, you found it. Replace the 200 and 250 ohm resistors and disconnect all terminals of the candohm. Use 15 watt for the 200 and 10 watt for the 250.

Thanks Tim, Tom and "88"!! I have a parts chassis where all of the CO resistors were replaced with Ohmite ceramic wire wounds so I'm going to clip those out and just replace everything. I'll get new Ohmites but for further testing and resurrection, I'll go with the old resistors. I'm absolutely certain that's NOT the last issue of unknown origin I'll need help with running down but for now, I'll take a few hours tomorrow and replace the components. Thanks again all!

:thmbsp::banana::banana:

Glad you are making progress.
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!

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!

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).

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!!!:banana::banana::banana::banana::banana:

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!

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

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