Help me understand the 630 horizontal circuit
 

Specifically, the oscillator and control section.

The chassis I'm wanting to use for my color wheel set has a problem I haven't encountered before, and since the only experience I have is with the synchroguide circuit found in color roundies I figured I would bow to the wisdom of the B&W forum this time.

Issue is I can't get rid of the horizontal blanking bar in the raster, the instructions tell me to adjust the back of the transformer till it goes off the right side of the screen but when I do that the image gets all wavy and I lose sync.

There's a wrinkle: the flyback has a tuning cap on it that shorted out, so I removed it. Is the lack of that capacitor enough to cause a blanking bar in the screen? It's only a 68pf cap.

Can you point to the correct schematic on earlytelevision.org? I don't know anything about the circuit, but would like to follow the discussion.

If I am looking at the right schematic, the horizontal discriminator does not get any feedback back from the flyback, but only from the oscillator. If that's right, eliminating a tuning cap on the flyback or yoke probably changes the retrace speed and high voltage, but not the phasing. Phasing effects would have to come from the discriminator circuit or whatever feeds it.

IIRC, there are also two chassis versions. The Red Book at ARH has two 630 chassis.

http://www.americanradiohistory.com/...ographs-TV.pdf

Last 50 pages of the redbook.

I've never worked on one, dad had one, but swapped it and a VT71 for some test equipment. I do have a 730, courtesy of dad.

Looking at the red book schematic, I see there is a pulse shaping/coupling network between the oscillator and the H discharge (390 pf, 6800 ohms, .01 uF). This could conceivably affect phasing also.

I'm done wild-guessing now.

Have you already modified the set for CBS frame and line rates?

No, I wanted to get it fixed and running at NTSC rates first so I could iron out problems like this. Otherwise if I just skip right to CBS, I'd be chasing my tail on any issues that crop up not knowing whether it's a chassis fault or self induced. I hate that. This black bar is really the only other issue it has, the mods for focus rectifier and shunt regulator work perfectly already. Once I can clear up this issue, I can move on to CBS conversion.

That's why I asked. When modified for CBS rates, you shouldn't have any issues if running properly at NTSC prior to conversion.

Wayne, same basic circuit any 630 has.


http://earlytelevision.org/pdf/rca-630ts-sams-54.pdf


Now that I think of it though, there might not be anything wrong with the oscillator at all. Could it be that the difference in tube size is enough to physically make something like this happen? I mean, the original CRT was a 23CP4 and I have the yoke wrapped around a 10SP4. Could the difference in sweep angles between them make this happen? I'm not sure, I would think the center of a tube is the same no matter what size it is.

The change in tube is going to be an issue for you. The 10SP4 is a ~50 degree tube, the 23CP4 was probably between 70 and 90. (I don't have a datasheet in front of me.)

The degree of deflection experienced by the stream of electrons is governed both the strength of the magnetic field created by the deflection coils and the velocity of the electrons (which is itself governed by final anode voltage accelerating the electrons; we assume the electrons are essentially stationary when at the cathode.)

Whereas the final anode voltages required by 10SP4 and 23CP4 are likely similar, the magnetic field strength required to fully deflect the beam is going to be markedly different for both tubes.

If the 1975 RCA tube manual is correct the 23CP4 crt is a 8 pin keyed button base 110 degree tube. One h--- of a change. 10SP4 again if before said manual is correct is a 50 degree tube. The different h.v. needs could cause diffacuilty. I would go back to the tube that was in set and see what happens,and go from there. All the best, Tom.J

If you turn up the brightness, is there light (raster) in your dark bar, or is the scan width just too small? If it is scanning the blank region, then it's a phasing issue.

Wayne,

I took a shot with the scope triggered on horizontal rate, channel A is the h-drive wave form, channel B is video from the CRT. As you can see the horizontal blanking bar is right is the middle of the drive wave form, which is exactly what I'm seeing on the face of the tube. What I would have expected to see is the blanking bar triggering the end of sweep like it's supposed to, so the blanking bar should roughly line up with the negative going (retrace) portion of the drive wave form.

http://videokarma.org/attachment.php...9&d=1504420916


Since that's not happening, I can only conclude that for some reason the horizontal time base isn't being properly triggered by the horizontal timing pulse. The way things sit at the moment, retrace is being triggered right in the middle of active video and it shouldn't be. No matter what I do with the discriminator transformer it refuses to behave, I can adjust the phase slug to where the bar is nearly off screen but it's very unstable there and the picture gets wavy.

I really don't know what is triggering horizontal prematurely, I can't see anything else that would cause it to happen. Sync pulses coming from the sync section are well defined and have more than enough amplitude, so it's not a sync problem. I don't see any noise with no video present, so that's not it. Next thing I might try is injecting pure horizontal sync pulses to see what will happen, if it behaves like that it might give me a few ideas.

Sweep angle between the 2 CRT's aside, the only issue I was expecting was an overscanned raster horizontally. Vertical is fine, the height pot has enough range to make things work out.

p.243 of the red book (p.9 of the 630TS circuit description) shows the proper waveforms on the plates of the sync discriminator (on the left). Since the horiz is being locked, it must be developing the proper control voltage to the reactance tube, but somehow this is at the wrong phase.

Possibilities I could surmise:

1) The reactance circuit is screwed up and requires an offset input voltage in order to center the frequency (I think not likely, but...).

2) (I think more likely) The transformer that drives the discriminator (which you are tuning) is bad, or tuned way off frequency, or the .015 cap that tunes it is bad, and the discriminator is not being driven by a pure sine wave. I would check the waveforms on the sync discriminator plates to see how they compare to the redbook illustration.

EDIT: redbook p.268 bottom and p.269 top show actual scope waveforms - I see the pulse is much bigger compared to the sine wave than shown in the drawing on p.243, but the phasing is the same.

The circuit in Sams-54 is not a Synchroguide, If the set has the Synchroguide circuit, it is important to align it per the instructions. Typically, you short out the "sinewave" coil, adjust for synch with the hold control centered, then take the short off the sinewave coil and adjust it. The Synchroguide output has a distinct waveform, a sort of hump then dip and spike. The hump should be adjusted (via sinewave coil) to be slightly lower than the spike. The idea was to have a sharper rise time at the end of the waveform for noise immunity (to prevent false triggering). I'll see if I can find an example and post that.