RCA 6T75 Restoration (16GP4)

[Bill Cahill]

Originally Posted by dobulee

Wow...the vertical centering adjustment was very stiff -- I'd have never known it was even there!

Even with the locking screw loosened, I had to use a small screwdriver as a lever to persuade the adjustment to loosen up. I was pretty scared about how much force it took, but eventually it moved and all is well!!

I was able to get it centered, and can now make the vertical linearity look great and still fill the screen. It really made a big difference. The set is very watchable right now.

The sound is relatively decent -- as good as I could get without going through a full IF alignment, but the ratio detector has quite a low SNR. I'm not sure how 'quiet' I should be able to get it. Its very listenable as is, but I don't really know what all I should expect out of it. I'll try to post some new pictures and maybe a test pattern if I can find an easy way to get it from PC to NTSC.

Actually, that's not a screw adjustment. That tightens the lever when finnished. The lever does all the adjusting. You didn't need to pry it. Merely loosen the screw...
Bill Cahill

[dobulee]

I got it -- but the adjustment is just extremely stiff in the 'in/out' direction for vertical. The 'up/down' horizontal adjustment is very fluid. Im not sure whats keeping it from moving freely, but it definitely isn't the locking screw.

At any rate, here are some test patterns. These are aliased no fewer than 3 times.
1)Original to Camera
2)Camera to CRT
3)CRT back to camera

The RCA doesn't get much above 250 lines resolution on the horizontal. It also still has some fairly severe 'ghost' type smear that I haven't been able to tweak out yet.

For comparison, I've shown the same RF feed on Philips that has a good digital comb filter and high horizontal bandwidth - it resolves about 400 lines although hard to tell in this picture.

Finally a couple of 'live' shots. Sorry for the boring subject matter, but I wanted something with high contrast text to show!

[Bill Cahill]

First, your width on the right side, that's your right,the tv's left, is too far out.
The horiz. centering is not quite right.
Next, your vertical is stretched. I have the same problem, only worse.
I suspect that if you try to tweak the controls, your vert. will compress, and, you'll have blanking lines on top. I know I do. This is not normal, and, shouldn't be that way.
Next, the resoution, also like mine, isn't right.
It's not properly focussing in the center, and, that's why you only see 250 lines. If anybody has any ideas here, we both need your help.
On mine, all caps, and, electrolytics are new.

Manny resistors in power supply, and, some in vert. circuit are also new.
This has continued to be an on going problem here, as well, except worse than his.
Bill Cahill

[dobulee]

Ya, some of these as you note, continue to be minor problems.

But, I think I've about reached the point of decreasing returns. I'm not sure I need this to be perfect, just good enough to be enjoyable to watch.

The vertical isn't stretched too badly after adjustment. The 'height' of the test patern squares is almost unchanged from bottom to top. The width at the extreme right is somewhat compressed, as you note. However, with real programming it is barely noticeable.

The resolution is more than focus for me. The IF section has Sub 3Mhz bandwidth right now. At least I can be 100% sure that there is a steep cutoff before 3.5Mhz, because the color carrier is completely stripped off. (check out the color bar staircase from a few posts ago)

Focus isn't terrible, as the scan lines can be made out with uniform clarity across the entire viewing area. Though, again, I don't have any other B/W sets to really compare with.

Hope we both get a few of these things nailed down!

[dobulee]

I figured lots of you guys would have experience with this, so here goes.

Im currently using a VCR to RF modulate for the set, which works great for downconverting all the non VHF channels from analog cable.

But, if I want to watch a DVD (or downconverted HD, or streaming video players) using the VCR as vehicle to RF modulate, i'm having Macrovision Copy Protection Woes, where the macrovision pulses screw with the VCR's AGC circuitry and cause annyoing fading and loss of vertical sync.

Have you guys had luck with external RF modulators for this purpose? I assume these do not usually strip the Macrovision, but leave the TV to still deal with it.
My questions are:
1)Does the AGC in your Vintage TV typically deal with Macrovision gracefully
2)Do the macrovision vertical sync pulses show objectionably in the vertical retrace period?

If results are poor, I could buy a video stabilizer to strip the Macrovision before RF modulating -- but i'm trying to keep the 'number of wires and components' count to a minimum!

Thanks,
Alex

[Eric H]

I use a DVD player with an RF Modulator and it usually works very well, some sets handle the Macrovision better than others and some RF Modulators are better than others, you have to experiment a little.
I'm currently using an RCA branded one and it works great.

If you rip and burn a DVD on your PC using DVD Shrink or something similar it leaves off the Macrovision Flag and that usually eliminates a lot of the lines and junk at the top of the screen.

I've also burned Public Domain videos from the Prelinger Archives to DVD and they work very well.

[dobulee]

Thanks Eric,

I'll try an off the shelf RF modulator - I can always add a video stabilizer if needed.

Right now, the majority of my movies come from a Roku Streaming Video player, which also inserts macrovisoin, which surprised me.

[dobulee]

I am curious to bypass the RF and IF stages and apply baseband video directly to the video amp section. Just wonder what the set would look like with better video bandwidth.

Sometime in the past (I think it was on a CT-100 restoration site) I remember somebody had a schematic for an add on circuit to do just this. Does this ring a bell with anybody? I tried a quick google search but couldn't come across it.

I could probably come up with something reasonably easily, but this would save some time, and might make a fun weekend project sometime!

Alex

[miniman82]

Not sure about your set, but on both my Philco and Sylvania Halolight I was able to inject a composite signal directly to the video amp tube's grid. Made a fantastic picture bypassing all the RF/IF stages, but you shouldn't need any add on circuits to do it. Just make sure you use a shielded cable to make the connection, to avoid interference. Same thing can be done with the audio stage, but make sure you make the connection so that the volume still functions.
I know some 'purists' don't approve , but my Philco has 2 RCA jacks on the back, and I didn't have to cut anything to do it- the round heat holes were the perfect size for mounting the jacks!
Try it, you might be surprized at the picture you see.

[ChrisW6ATV]

Folks, DON'T try that at home with a hot-chassis TV, though.

[miniman82]

That wouldn't be good, no.