Something like that. I liken these exercises to riding an old mountain railroad for the first time: plenty of hidden curves ahead; somebody was there before us to build the thing, but it's all new now... :yes:

Pete

Nick, The earlier tubes had a 14 pin stem and base. I would speculate that this was because they had a 14 pin stem and base in the inventory. Later on, it became obvious that additional electrical isolation was needed between the convergence pin at [edit] 7-9KV and the other pins, so a 20 pin stem and base were developed, allowing 3 blank pin locations on either side of the convergence electorde.

I just checked my Sparton metal bell CRT, and it is a 20 pin 6 bolt CRT just like yours, with a type number of C73547, just like yours. My bell rim number is D255, and I believe Pete said you rim number is D205 (?). This all seems to make sense. My Sparton set is a early 1953 NTSC set, as best I can tell.

Update: took the tube out of the box, and it's definitely D225. Only 30 made between ours, if the assumtion of a production serial code is correct. :yes:

http://videokarma.org/attachment.php...4&d=1308705194

Thanks, Nick. Well it is all very interesting.

Makes perfect sense.

Hey, speaking of convergence...

This thing has a long string of megohm resistors coming off the HV rectifier, with the convergence pot right in the middle and more resistors after it. It looks electrically just like the way it was done on the CT-100, except by that time they actually had a 50meg resistor to use in the circuit. What do your proto sets have for convergence?

Your set doesn't appear to have a convergence transformer like the CT-100, so the question becomes how did they accomplish dynamic convergence, or maybe the question is "did they use dynamic convergence". I would be curious to know if there are other connections (capacitors to be specific) to the convergence grid of the CRT?

Nick, Here is a photo of the bottom side of he CBS prototype. It appears to use large high-voltage resistors, but it has been many years since I was under the chassis, so the details are vague. I have never had the HV cage cover off the Sparton, but as it appears to be a close cousin to the production Sparton set shown in Sames, I expect it will also have 50 megohm (or similar) high voltage resistors. Both of these sets are, shall we say, more mature than your set. :-)

If this prototype dates from 1950-51, how far along was it's development?

Has anyone identified whether this is a set still running on the dot sequential principle or whether the design had progressed to the constant luminance principle with interleaved chroma?

Is there any schematic information with this set? It would be very interesting to reverse engineer this set to find out what the design comprises of.

Note: seems I missed an earlier part of this thread. I see from Ed's post that progress had moved to CPA and possibly later to the early '53 NTSC development closer to the final standard.

It will be interesting to see what the reverse engineering reveals.

That's exactly what I'm wondering. The other end of the resistor string has a wire that goes under the chassis, with connections to many different parts. So perhaps it does have dynamic, but it's done in a different way?


Thanks for the picture, John. I can see the 50meg resistors on that set, so you're right- it is more 'advanced' than mine. :music:

Nick, Attached is a scan of the schematic from the June 1951 RCA symposium on tricolor picture tube development. While this features a RF HV supply and differs considerably from your sets configuration, it does show both vertical and horizontal dynamic convergence transformers. It would seem quite odd if your set did not incorporate some type of dynamic convergence.

Nick, if you follow the wiper of the convergence pot, where does it go? This is where the convergence stuff would be.

To the CRT, nowhere else.

I think that means that the set has no dynamic convergence, which is strange. But a full schematic will tell for sure.
If you look at the schematic that John posted you'll see a 10kv capacitor from the convergence pot to the circuitry that provides the waveforms for convergence.

There is a doorknob cap mounted under the chassis, but I have no idea what it's connected to.

For John's request and for comparison, my 6-bolt, 14 pin prototype tube is paper tagged;

Developmental type - C73293C
Serial - L3046R

Rim stamped - D229

And it has acquired a swooping crack right across the faceplate but not edge to edge. No idea how that happened. Have not looked at it in 5 years. Gotta be careful now.