Prototype set is here

[Dave A]

John, given the riches I have with my little collection of rare sets, the spare parts I have and all the help I get around here, a small crack is nothing. My minor contributions are more important than a crack. Knowledge is everything.

[Steve McVoy]

John, your supposition that the 3 bolt version is the earliest one conflicts with the information that Dave gave me some time ago. Here is what he wrote:

This tube [the 6 bolt version] was use in the tri-color models 1 and 2 and possibly the 2A. Model 1 was the first RCA demo to the FCC. These tubes were circa April, 1950 to February, 1952. Models 3 and 3A used a 3-bolt version. Starting with the 3, they improved the trajectory of the guide bolts for the shadow mask alignment. The model 4 appears to be the first of the glass tubes. Most of what I know can be found in the RCA Color TV submission to the FCC...the "red book"...and in the second "Proceedings of the IRE" on color television.

[Dave A]

I'm under the microscope now. I can stand to be corrected from the dimunition of time. The 6 to 3 does seem to be logical as they improved as does the 14 to 21 pin base.

Perhaps we should start another thread just for these efforts. I would like to know how the mechanics of these seemingly eccentric bolts worked. These are not just locking bolts but perform a function...but what?

I am not surprised that RCA had to develop a shadow mask that was moveable for the developmental stage until mfg could catch up with precision.

It would take an artist to move these bolts to allignment under the light box used to align the shadow mask as seen in the documentation. You would think that any one bolt could distort the shadow mask not to mention 5 more.

[Steve McVoy]

John, you are almost always right.

[John Folsom]

Uh oops. Poor choice of wording on my part. What I intended to say was the 3 bolt mounting version of the CRT post dates the earlier 6 bolt versions.

[Steve McVoy]

Glad to hear you found the convergence circuitry. Now it makes sense. The next mystery is to figure out what color scheme it uses.

[Penthode]

This prototype I find incredibly interesting in light of the early development of the NTSC standard. Certainly, it should be possible to generate a 1952 CPA signal to run the set. I would certainly hope that the set not be modified from the CPA design if it indeed is still a CPA receiver.

Is the subcarrier crystal 3.89MHz?

I came across another interesting paper published in the March '53 Proceedings of the IRE. It is entitled "Generation of NTSC Color Signals" by Joseph Fisher and pertains to the earlier CPA specification.

It appears that for the NTSC '52 tests, field rate R-Y switching was employed. Vestigial sideband equal bandwidth R-Y and B-Y chroma channels were used. Color subcarrier was 3.89MHz and it appears only standard 1/2 line offset interleaving was used.

CPA at field rate as opposed to line rate probably was a better choice in 1952. The reason for CPA was to cancel phase errors in transmission and to facilitate vestigial sideband chroma channels without quadrature crosstalk. Because there was no way to electronically delay and combine alternate field or line information economically in 1952, the errors would be cancelled visually. This is very much akin to simple PAL in Europe. The field rate equivalent to "Hannover Bars" would result from transmission phase errors.

The real failure of the 1952 CPA tests therefore must have been due to the flickering akin to Hannover Bars and the 610 kHz chroma subcarrier to audio carrier beat (this was before the line and field rate was shifted to 15,734.26Hz and 59.94Hz respectively to ensure that the sound carrier was a multiple of the scan rates). In addition, the alternating R-Y axis would prevent proper 1/2 interleaving of the chroma within the luma.

Note that PAL later adopted the 1/4 line offset to help mitigate the interleaving problem. PAL also introduced a picture rate subcarrier frequency offset (25Hz) to further conceal the interference. (In hindsight, a 29.97Hz offset could have been added to help conceal the NTSC dot crawl. But the standard was already established and the simpler luma to modulated chroma arrangement in NTSC probably makes this unnecessary).

[miniman82]

Originally Posted by Penthode

This prototype I find incredibly interesting in light of the early development of the NTSC standard. Certainly, it should be possible to generate a 1952 CPA signal to run the set. I would certainly hope that the set not be modified from the CPA design if it indeed is still a CPA receiver.

Is the subcarrier crystal 3.89MHz?

*sigh*

If only there had been a crystal installed in the chassis, it would have made the mystery of what standard this chassis uses a lot easier to figure out that's for sure! Since it didn't have one though, the best I can do is trace the circuit and try to determin if it's still CPA or if it was converted to use the later 3.58MHz subcarrier. I'm still leaning towards CPA though, considering the 'CPA transformer' is the one that's still connected.

CPA at field rate as opposed to line rate probably was a better choice in 1952. The reason for CPA was to cancel phase errors in transmission and to facilitate vestigial sideband chroma channels without quadrature crosstalk. Because there was no way to electronically delay and combine alternate field or line information economically in 1952, the errors would be cancelled visually. This is very much akin to simple PAL in Europe. The field rate equivalent to "Hannover Bars" would result from transmission phase errors.

One of the things I'm itching to figure out with this thing is if the edge effects and bars with CPA really are as bad as people make them out to be, it's one of the motivating factors to get it working as it last did. I'm hoping my brother will stop by this weekend, so we can do some digging in the color section and determine for sure what it's supposed to do. He should be able to sketch the circuit in only a couple hours, and usually takes payment in beers.

[ChrisW6ATV]

Originally Posted by miniman82

*sigh*

If only there had been a crystal installed in the chassis, it would have made the mystery of what standard this chassis uses a lot easier to figure out that's for sure!

Since the crystal is missing, I wonder if it would be feasible to connect either a sweep generator plus scope, or a spectrum analyzer with tracking generator, to various circuits in the chassis to figure out their resonant frequencies and bandwidths as currently tuned. Or, get a 3.89 MHz crystal along with a 3.57 MHz one, isolate and apply power to the oscillator circuit with each crystal, and see whether either or both work in the circuit and measure relative RF outputs to determine which frequency the circuit might have used when it last operated.

Here's another idea: Try using a dip meter near the various circuits, if it is possible.

[JBL_1]

Originally Posted by ChrisW6ATV

I wonder if it would be feasible to connect either a sweep generator plus scope, or a spectrum analyzer with tracking generator, to various circuits in the chassis to figure out their resonant frequencies and bandwidths as currently tuned.

Sweeping the bandpass amplifier is a great idea! The response may not be peaked at the subcarrier. The peak could be staggered to make up for any roll off in the IF or bandpass/lowpass filtering.

But the information still could be usefull if you looked at the overall video response of the chassis.

[old_tv_nut]

An aside regarding frequency interleaving: during the analog to digital transition period in the US, digital channels that were lower adjacent to analog channels were required to have a precise frequency offset to reduce the possibility of visible beats between the digital pilot carrier and the chroma of the upper adjacent analog signal - a re-application of the principle developed by NTSC long ago.

[JBL_1]

Was thinking you will need to get some sort of power supply to power up sections of the chassis. I think the power supply from one of the many 1948-1950 B&W RCA projection sets would be a good fit. Has three 5U4G, it is an RCA chassis, just about the right period. And there has to be a couple of guys with spare chassis for these.

[miniman82]

I have many doner chassis at my disposal, I would never deprive another collector of the opportunity to restore something like a projection set in the process of getting this thing to work. I'd much rather use a common chassis, like a Maggie 6V6 amp or something that no one will miss.

[miniman82]

I guess if one fell into my lap I'd use it, but it would have to be pretty much free. I can't justify much more expense with this chassis, the last thing that will be bought for it is a nice cabinet to house it when it's finished.

[miniman82]

I doubt there are any markings under the panel, for all intents and purposes it's just another subchassis.