RCA 6T75 Restoration (16GP4)

[dobulee]

I'll make the blanket statement that any resistor that shared a node with a paper or black beauty cap was taken out of ckt, measured, and replaced if grossly out of tolerance.

So that leaves mainly the IF and tuner sections unchecked at this point. I'll look there.

[Bill Cahill]

What about resistors to screens, plates, tuner, plate of audio amp, cathode of audio output, etc.?
A blanket statement like that just isn't enough.
Come on. You're a designer... Think. Look at the repair end of things. There are many resistors that can be bad. Also, it's very common for hv rect., and, or, hv cap to short.
And, tube testers won't show shorts because the voltage applied is only a very small protion of the tube's operational voltage.
Same with 5U4's.
Bill Cahill

[dobulee]

For what it's worth, I came up w/ a way to align the horizontal oscillator frequency without yet having any NTSC sync pulses from the detector stage to use:

I used my computer soundcard to output a 15734Hz sine wave (went for the color standard, not sure why), and used this to trigger my scope. Then I brought in the horizontal on the 2nd channel, and turned the slug until it synchronized. After that, I adjusted the sine wave slug for equal peaks as shown in Sams. However, I ran out of adjustment just before the peaks equalized. At least it's stable now.

Have a little bit of faith in me, here, Bill!

The blanket statement immediately lets you know what I have and have not checked. I haven't gotten to everything yet.

Plate, Cathode, screen and grid of audio amp and audio output - Yes. Checked. These all shared tie points with new film caps and were measured.

The same is not true of resistors in the tuner and IF sections, because I haven't had anything out of circuit there yet. All in due time, but thanks for pointing me to keep looking there.

[dobulee]

Well, with the horizontal running well, I hoped to have HV, but no such luck. All the biasing on the HOT looks pretty reasonable. With John's troubleshooting help, I tried the 'arc to a screwdriver' trick on the cap of the HOT and the cap of the 1B3.

Nothing going on the HOT, but I can pull a small arc from the 1B3. That gives me even more faith that the flyback is good. No change when I pull the HV cap. Here's hoping all goes well with a new 6BG6 HOT. I should know something more in a few days.

In the mean time, I put some video on Chan 3, and followed through the Tuner, IF, and amplifier stages. I had video at the Detector, but not at the 1st video amp -- I eventually found the 36uH coupling inductor to be open. I popped in the closest thing I had - 47uH. Voila, video the whole way through to the DC restorer, and stable DC referenced video at the grid of the CRT. Incidentally, there is an open 500uH inductor in the shunt leg between the 2nd video amplifier and the DC restorer. I haven't replaced it yet - I'm interested to see what difference it makes once I have a picture to look at.

I took a couple of pictures of horizontal lines and vertical fields, with the horizontal and verticals sweeps on the 2nd channel as appropriate. The horizontal hold is rock-solid, but the vertical is extremely touchy. With a little finesse, it pulled it.

Enjoy!

Alex

[dobulee]

For fun, here's the output response from a Color Bar pattern. (provided by Comcast in NTSC on Cable 119. Go Figure).

The bottom trace is the detector output, and the top is the CRT Grid.

I'm not sure exactly how much horizontal bandwidth I should expect, but there is exactly ZERO color subcarrier left on that signal by the time it hits the CRT. Those are pretty soft luminance steps, too. And, I don't think thats really color at the detector either. Maybe IF out of alignment?

Alex

[dobulee]

...sorry for the 'A Christmas Story' Reference....

With some local help from Wade, we confirmed that this CRT tests good! Also, for what it's worth, it is a replacement tube.

Anyhow, armed with a new 6BG6, we installed the chassis and fired up the set. The HV clearly came up, but the picture was off the screen. A quick tweak of the ion trap brought it into view.

After some vertical size adjustments, the picture fills the majority of the screen. Not Bad! Even after tweaking the vertical linearity control, the linearity leaves a little something to be desired. If I can't tweak it in better, I'll check the bias resistors, caps, etc. in that circuit. Also, the vertical hold is still VERY touchy.

Other than that, the sound doesn't want to tune with the picture, but comes in at the very end of the fine tuning adjustment. Wade suggests replacing all the tube shields may bring things back into alignment.

Lastly, the Width adjustment seems to have more of an effect on contrast than width -- not sure what the deal is. I'll have to trace through the circuit again and make sure I didn't flub a cap placement. But, here's to not having made any GROSSLY serious mistakes in component replacement!

A few screen shots from the local news on WGAL, channel 8. Enjoy. That last picture was shot at 1/60s and shows only a single field, not a full frame. Hence the seemingly large area between scan lines.
Alex

[John Folsom]

Alex, you may have to tweak up the sound IF to align it with the video IF. Good 'ol RCA and their damnable split sound sets! RCA set are also noteworthy for having rather poor video bandwidth, sub-3Mhz is not uncommon, though I do not have first-hand knowledge of this particular chassis bandwidth performance Don't ever try an put a Col-R-Tel converter on a late 1940's vintage RCA!

Great job getting the set going!

[Phil Nelson]

Originally Posted by dobulee

I haven't removed the CRT or the speaker, nor have I made any long wiring harnesses yet, so I put the chassis back in the set

This may be too obvious to mention. Anyway, I was whining in another venue that I wanted to adjust the HV on my RCA CTC-11, but the adjuster was under the chassis and I was too lazy to build extension harnesses. Someone pointed out that you can often turn a chassis sideways and connect everything with the original wiring.

Duh! Time out while I slap my forehead

I wouldn't do a lot of soldering like this, but it beats muscling the chassis in and out of the cabinet every time you want to check/adjust some little thing.

The second pic shows a daffy moment 10 years ago when I convinced my wife it would be fun to help hoist the RCA 21T227 cabinet onto my workbench. Can't tell you how many honey-do hours I spent working that one off!

Phil

[dobulee]

Thanks for the Tip Phil! Thats a great idea - some of the best ones are really simple!

John - thanks for your encouragement! Without everybody's help, i'd have fuddled around a lot more before getting things right.

Now that the set is 'working', my pace on it will probably slow down a bit, but I'll try to post occasional updates.

Any tips on tuning up an IF stage without a sweep generator? I can probably find one to borrow if needed....

Hopefully this set will eventually make it up out of the basement, and into at least occasional use somewhere in the house...
Alex

[John Folsom]

For the sound IF alignment, you can probably attempt a "by ear" alignment. Tune for best picture, then peak each sound IF and discriminator primary & secondary for best/loudest sound. Make note of the # and direction of turns on each coil you adjust so you can find your way back if need be. I would not recommend this for the video IF alignment.

[dobulee]

On John's advice, I executed a 'just tweak it' alignment of the sound IF and discriminator inductors. The result is quite acceptable - except that things only seem to line up on channel 3. So either the tuner or the entire IF chain is still out of alignment. Its good enough for now, since the TV is living it's life w/ an RF output on channel 3 anyhow.

I also picked up a Heathkit IG-57A post marker/sweep generator (minus the attenuator and RF probes - but these can be refabricated), and thanks to a fellow hobbyist, I have attained the schematics and instructions. It actually seems to be in working condition -- the markers are set around 45Mhz IF. However, the marker circuits are pretty simple tuned circuits with crystals for reference in the base of a miller oscillator. I forsee no issues in swapping out the crystals w/ the closest match for 21Mhz IF and adding some capacitance to re-center the tank circuits.

The sweep width and center frequencies are set via a variable saturable inductor. DC current is run through the inductor primary to drive it partially into saturation and lower the effective inductance, changing the center frequency. to get the sweep, some AC current is injected at line freqeuncy to modulate the inductance. I haven't measured much directly, since my scope is only good to 20Mhz, but it looks like its possible to sweep down to the needed frequencies, as-is. I think I'm in good shape, since the Heathkit also has provisions for 10Mhz FM IF.

If and when I get around to it all, I'll post some pictures of the true IF curves Vs Sams.

[dobulee]

Well, things are still running well, and I'm getting around to centering the picture.

As it was, it was rotated about 20 degrees CCW, and is off center vertically.

I had previously centered it horizontally using the adjustment on the CRT neck, and was able to bring the picture to correct rotation by loosening the deflection coils and rotating as needed.

However, the picture is still off center vertically 'low'. Does anybody have advice on how to adjust vertical centering? The the adjustment doesn't seem as obvious, and Sams doesn't seem to give any advice.

Right now I'm able to fill the screen by playing with the vertical linearity, but it just stretches things out at the top of the screen.

Any advice?
Thanks!
Alex

[Bill Cahill]

There is a centering lever on the side of the magnet assembly.
Up and, down adjustment for horizontal, side by side for vertical.
Loosen the phillips head screw first.
Be sure to tighten when finnished.
Bill Cahill

[dobulee]

Thanks Bill!

I got the up and down for horizontal part -- Didn't realize that same adjustment would also do vertical if moved in and out!

[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.