Take a look at the link in the post above yours and you can read the story.

Nick, Steve Mcvoy, Ed, Terry and all, that is simply outstanding work putting together the schematic and research to match it up to the RCA patents. I am convinced that we now understand the pedigree of this unique engineering prototype, and look forward to seeing it at the ETF convention in May. :-)

Did you guys contact RCA (Yeah, I know...But still..) & try to get any help/guidance from them ?

Ah, how'd I miss that.
Great read, by the way. I'm intrigued by the presence of those ecc82's in the possible CPA positions... still need to wrap my head around the whole CPA thing..but one thing's for sure-this is a superb, rare item.

Some info has come to light that I think needs to be shared:

There has been at length discussion behind closed doors among the 'early color' crowd as to the true nature of the chassis: who designed/built it, what transitions it has gone through, and what lies in its future.


Who designed/built it-

Steve McVoy, among others, has been doing some very good digging lately, and now feels confident calling this an RCA developmental chassis- I concur. The facts so far are:

1. Its tuner, IF and 1st video stage are all carbon copies of schematic drawings of other RCA sets from the same time period.

2. There is now patent evidence showing that RCA did in fact use the 6BL7 tube, even if it was only in an engineering drawing. Those circuits closely match what the prototype chassis has. Still no proof that these drawings were ever used, but it is encouraging to see evidence that it was at least considered.

3. Evidence has been found to support the idea that the chassis had been used at one point to explore the possibility of CPA (field rate), but was later abandoned. This can be proven by careful study of the color circuits, and by the fact that the tubes required for the implementation of CPA (a pair of 12AU7's for a field recognizer) are still in the chassis though not connected to anything (only the heaters remain connected). It also supports documentation of what was going on at RCA at that time, with the field tests and so forth being conducted in connection with the NTSC. What standard the chassis uses now has yet to be determined, since the color subcarrier frequency has not been nailed down. I am working on this at the moment, by attempting to use a frequency generator rigged through the subcarrier oscillator. I will read it's output, and determine which one it is best suited for.

4. No conclusive evidence has been found thus far to convince us that it was made by any other manufacture but RCA. There is too much evidence pointing to RCA at this point, which is why we are in agreement they are those ones who made it.


Transitions-

We now know that at some point the chassis had CPA circuits in it, what is not clear at this point is when that changed and what standard the chassis now uses. There are some anomalies in the color circuits that still have to be hashed out, but I have my suspicions that it was modified to the '53 spec using the 3.58mhz subcarrier.


The Future-

Still remaining is the mystery of why the horizontal section of an apparently RCA chassis uses a seemingly Hazeltine regulator design, which is a pulse regulator type, versus the later standard shunt-type scheme.

As to whether the chassis will ever work again: it will depend on what type of standard the set is found to operate on, and if the electronics are still good. My aim has always been to see what this chassis is capable of, but at some point a decision will have to be made on whether to make it CPA or not. This is still a long way off in the coming months/years. Also yet to be determined is how they managed to stuff a delay line into an IF can- John Folsom has in his possession a chassis with such a line, but it is not in a can- only small enough to fit into one. So it's possible the chassis opening marked 'DELAY' really did have a luma delay in it, but we can't say for certain just yet. It has been found that the video cathode follower has an open circuit where the delay line 'should' be, but it has apparently been removed. Why? Who can say, there is only speculation at this point.

There will be a demonstration of the prototype CRT at this years convention, with the tube mounted in a CT-100 cabinet and being driven by a Merrill chassis. I expect it will be an enlightening experience for all who see it, true history being revealed before our very eyes. The chassis will also be on display for all to ponder, the more eyes the better. I will give a presentation on what we currently know about the chassis, and expect there will be much discussion afterwards about what has been said.

So, we have learned a great deal here recently. Progress will proceed as fast as possible on the chassis, which will be shipped to Steve at the museum before the convention so he can study it in person. That's all I have for right now, more updates as they become available.:thmbsp:

Nick, I'm sure we can get it running as a CPA set, assuming the flyback is good. There are replacements available for all the other parts, and I'm sure we can figure out the cut wires. Even if we can't find a delay line to fit in an IF can we can put a standard one under the chassis. And I'm sure Darryl can make us a converter to generate a CPA signal.

We'll have to figure out which route to go after you have time to sit down with the chassis, just let me know when to ship it and I'll work on getting it boxed up. Do you need the power supply along with the main chassis?

Don't think we'll need the power supply. If we need to power it up I'm sure I have something at the museum that would work.

I could bring my bench supply if needed.

You might see if there is some way to restore the set such that it could easily be switched between CPA and whatever state you determine that the engineers left it in. I think that having it in the last state that it worked in is as historically valuable as having it work as a CPA set.

It would be interesting to see what a CPA set is capable of, and Darryl has said he can program a magic box to output that standard. I think we should at least try it, for S&G's. After that, it will probably end up as an NTSC set.

Here is what I'd to. To get it working as a NTSC set:

1. Finish documenting the chassis as it is.
2. Figure out what it needs for power supply voltages.
3. Figure out how to connect the cut wires.
4. Determine what parts are missing (delay line, etc.) and find replacements.
5. Fire it up as a NTSC set and see if it works. Make whatever alignment changes are necessary to get a NTSC picture.

Then, configure it for CPA

1. Design the missing CPA circuits and install the components.
2. Fire it up as a CPA set and make whatever alignment changes are necessary for CPA.

Hi All,

I have been away and I have been reviewing the documentration we have collected in recent months regarding CPA.

Upon comparing Nick's prototype and the RCA patent for the CPA switching, there are striking similarilties in the components. The CPA transformer and components around it in the prototype follows the patent even down to some components.

I am trying to derive a reliable field switcher using only four triodes (the two 12AU7's). This is proving a challenge because one pair of triodes would provide the bi-stable device for alternate fields, one triode for field recognition, one triode for alternate field bi-stable switching and an extra triode as the field pulse buffer (as shown in the figure 13 Hazeltine drawing). The field pulse buffer is the fifth triode so the question to ask is: is there a spare unused trode on the chassis besides the two 12AU7's? Or is there possible a spare low impedance winding on the vertical output transformer which could produce the pulse?

Also the transformer on the plate circuit of the 6U8 3.89MHz oscillator buffer appears redundant. The CPA transformer is all that would be required and I wonder if it originally attached to the buffer plate directly? I find the separate subchassis for the oscillator curious and wonder if it was later fitted to fulfil subsequent modifications?

In the timeline of development of CPA with symmetrical vestigial sideband chrominance channels thru to the development of the IQ asymmetrical sideband solution, I suspect there were intermediate but "unofficial" modifications to the system. If I were in their shoes in 1952 when CPA with the 3.89MHz subcarrier operation was proving difficult to tame and growing unpopular, I would have been inclined to experiment with CPA back at 3.58MHz. The benefit of 3.58MHz CPA would be to make each chrominance channel sidebands more symmetrical to help reduce the CPA flicker.

Introducing the IQ solution would mean different bandwidth I Q chrominance channels which would mean a second delay line for the I channel. At this stage they may or may not have thought about the design of a "simplified" narrowband chrominance channel decoder. Without seeing any evidence means that discussion of this is pure conjecture. Making the set a switchable 3.58MHz CPA/non-CPA receiver would not be much of a problem but full IQ demodulation I feel would be out of the question.

I plan to complete my diagram in Visio which I will hopefully tonight. Any thoughts, comments or criticisms are most welcome.

Terry

Seems reasonable and historically cogent. Particularly since there is no longer 1953NTSC IQ to drive the set anyway.

Pete

Terry,

There is a 12AX7 and 12AT7 near the vertical section. One of them is for the convergence, but we don't know what the other one does. I suspect you have a total of 5 triodes to work with. When I get the chassis the first thing I'll do is trace those circuits.