Prototype set is here

[miniman82]

The prototype tube is slightly shorter in overall length (about 3/16" shorter), which could mean the electron guns are at a different distance from the dot plate. If that is so, it could mean it will be a little different than a 15G. I should have the parts to experiment with by Monday, so I'll know more then.

[Penthode]

Now that it has been suggested that the HV for this set's CRT should be only about 15 to 16kV, I wonder if now the static convergence will be achieved.

The lower beam velocity resulting from the lower ultor voltage should allow now enable static convergence?

[Steve McVoy]

I determined today that the chassis puts out 7-8 kv for the convergence grid with an ultor voltage of 16 kv. Nick can now try these voltages and see if he can get it to converge.

I also have the regulator circuit working in the range of 14-16 kv. With everything considered, I'm fairly sure that the chassis was designed for an ultor voltage of 16 kv.

Today I found a connection cut and a bad resistor in the negative power supply divider network. After fixing that and doing the alignment again, I now have working AGC and sync, plus proper gain for the IF strip.

Tomorrow I'll work on the reference oscillator.

[John Folsom]

16KV is pretty much the lower limit for the ultor voltage, with 20KV being the upper limit (for the prototype tube). The convergence G4 electrode is specified at 42.5 - 51% of the ultor voltage. You can do the math. :-)

[miniman82]

The HV resistors for the convergence divider showed up in the mail today, I'm making the necessary changes to my CTC-2 chassis right now. I will try the 100 meg ones first, then 25 meg.

[miniman82]

Here are the changes required to remove the HV pot from the end of the convergence string, isolating it from any changes in convergence current draw. This should make sure that anode voltage doesn't jump around when convergence potential varies. Just lift the wire coming from the 50 meg resistor, move it to ground. Then take a wire from terminal 1 on the flyback (B+Boost), run it though a 2 meg fixed resistor, then to the open terminal on the HV pot. This arrangement gives you 420v on the high side of the pot, 270v on the low side. Pretty close to what I measured originally when it was still in the convergence string, which was 468v high side, 303v low side. During testing, the HV regulator works just like it should. I am able to adjust HV throughout its range from 20kv, to low enough to make the 6BD4 start redplating. For now, I left HV set at about 19kv. Tomorrow if I get some time, I will install the proto CRT and attempt to converge it with the factory 50 meg resistors in the string. If it doesn't work out, I'll play with the new ones. Here's what things look like right now in the cage.

[miniman82]

Steve reports the first attempt of getting video through the chassis, connected to a spare 15GP22. There's light on the screen, but some sweep issues have been encountered. Steve hopes to have these sorted shortly, and we might see some color images very soon.

Test setup.



With vertical sweep.



Without vertical sweep.

[ChrisW6ATV]

Very nice to see ongoing progress!

[miniman82]

We have color on screen!

[Phil Nelson]

Very encouraging!

Phil Nelson

[Bill Cahill]

Is this the protoyype chassis? Or, just the pix tube?
Looks encouraging....
What's causing waviness?
Is this the new color system, or the original to this set?
Looks very interesting.
Bill Cahill

[Steve McVoy]

You can read about the restoration here:

www.earlytelevision.org/rca_cpa_restoration.html

The waviness is due to ringing in the horizontal circuit because we are using a CT-100 yoke rather than the original. After I took that photo I was able to get the original yoke to work, and the ringing is gone.

This picture is NTSC. The colors are wrong, and we have a way to go to get it right. After the chassis was used for CPA it was modified to run on NTSC.

The next step is to build a CPA signal source and change the chassis back to CPA (not difficult). Then we will be able to watch CPA color.

[Bill Cahill]

Thanks. I had beren reading the thread, but, lost track some months ago...
Making cpa video amp: will it be hard to do?
I imagine you might have schematics for original one?
Didn't hat set have a bad flyback? If so, what did you end up using?
I'm fascinated with this set....
What are you going to end up using for a cabinet?
Was this with the original Pix tube, or, a 15GP22?
Bill Cahill

[Steve McVoy]

Bill, here are answers to your questions:

1. What has to be added for CPA is the circuit that alternates the phase of the color demodulation every field. There are two tube sockets on the chassis that were originally used for that purpose, and we have some hints of the schematic from a RCA patent. We will also have to install a 3.89 mHz crystal and retune the oscillator circuits.
2. No, the flyback is good.
3. The set belongs to Nick Williams. He has some ideas for a cabinet, but I'm not sure what his final decision is. My own thought is that the best thing to do would be to make a plexiglass cabinet so that people can see the guts.
4. Nick has the original tube, so I've been using a 15GP22. The specs for both are very similar.

[miniman82]

I thought the flyback might be bad at first too, but I think it came down to the grid of the output tube not being tied to the correct negative voltage source. Once the tube was biased correctly, HV came up to 20kv and stayed there. Which brings up an interesting point: This chassis gives us the opportunity to document the operation of the pulse regulator circuit, I would very much like to convert another set to operate this way since it takes a little stress off the flyback.

We already discussed that the cabinet will be the TC-168 with doors, but I'm open to making a clear display case for it. Such a case would make the guts easier to see, though if you ask me it's pretty compared to production units. It definitely has a face only an engineer could love, if you know what I mean. In that respect, it is indeed a thing of beauty.