New Project - RCA 8PT

[Eric H]

Yes, it is a problem with the horizontal sweep section, it's not sweeping across evenly and that causes the bars.

I'm not sure if it's the Damper circuit or the Sweep circuit that causes this, or if it's a mismatch between the Yoke and the Flyback (possibly caused by bad resistors and caps inside the yoke)

I doubt it's related to anything you replaced, more likely to be something you didn't replace yet.

[maxhifi]

Checked resistors in yoke, and most of horizontal circuit. Nothing is way off, but not dead on either.

Read -30V on G1 of HOT, versus -36 on schematic, and 105 versus 126 on screen of HOT, with 225V B+ versus spec of 216 (probably due to silicon rectifiers),

Attached is photo of set tuned to inactive channel, with contrast and brightness tweaked to emphasize problem.

[maxhifi]

Noticed something new - maybe I am looking in the wrong place.

When contrast (pix) is turned up all the way, the lines are very visible (first attached pic)

When contrast turned all the way down, the lines nearly dissapear.

See attached

[maxhifi]

Still at this - I think I'm starting to understand why Phil didn't recommend this TV for beginners

Anothing thing I've noticed is the problem seems to get worse the longer the tv is on, stabilizing once the tv hits operating temperature.

I tried shorting the cathode and G2 of the CRT to ground through a capacitor, to no effect, so it really must be in the horizontal section. The old book "Television Simplified" said says that problems on the left side of the screen are damper related, the right side of the screen more likely horizontal output tube or oscillator related. The only components related to the damper I haven't subsitituted are L156 and L157, but not sure how to test this. I did short L156 with a jumper, and it seems to affect nothing. It reads 0.8 ohms, but I'm not sure what it's supposed to be.

Have tried a few brands and even values for C173, also to no effect.

Regarding attached - green stuff has been checked and/or subsitituted, red has not.

[jr_tech]

Originally Posted by maxhifi

Noticed something new - maybe I am looking in the wrong place.

When contrast (pix) is turned up all the way, the lines are very visible (first attached pic)

When contrast turned all the way down, the lines nearly dissapear.

See attached

So perhaps a pulse from the Horiz retrace is causing some "ringing" in the power supply to the video amp?

jr

[maxhifi]

When I put a clip lead at X, and use it as an antenna, near the horizontal output section, the following picture results.

The old C121 had very long leads, and the new one shorter leads... I have a feeling placement in the area must be critical, so will spend some effort here.

EDIT - even with the 6AW8 pulled, and the plate grounded through a capacitor, problem still there as always, just not emphasized as below. Also, the problem is still there when the front and rear chassis are separated. I'm starting to wonder if a NOS 6AX4 might be the answer... all of the ones I tried test exactly at the minimum value on my TV-7 tube tester.

[IsthmusTV]

I think swapping in another 6AX4 damper tube would be a good thing to try. Also C174, the 330pF silver mica cap could be the problem. It's got the full plate voltage across it and may suffer from silver migration.

Good luck,

Clark

[Phil Nelson]

One of my old TV books illustrates insufficient damping with a photo of vertical bars in the left half of the screen. These would roughly equate to ripples in the ascending part of the horizontal waveform. The purpose of a damper tube is to smooth out the ripples.

So, it sounds sensible to try a new damper tube and examine whatever else is in circuit with it (typically not much, IIRC).

Phil Nelson

[maxhifi]

Thanks much for the input! Sorry to drag this out so long, but hopefully the result will prove useful to someone else at one point.

Found a 6AX4 GTB which tests a bit higher than the original, and all others I have on hand. Bars still present as ever - no change. C174 is a tiny ceramic disc, not a mica, but jumpered it with a 0.01uf disc also to no result.

Looks like in this circuit, the damping is taking place through C173, since the boost voltage is in series with the B+. All the same, I have changed it out already without results.

The bars are present either with a program, or with no signal and a blank raster.

[maxhifi]

I wrapped a couple turns of wire around the 6BQ6, to try to get the horizontal waveform (I have no high voltage probe). Sure enough, it does reflect what's on the screen of the TV. (Note: Polarity was set to reverse on scope, so waveform is actually flipped upside down)

[maxhifi]

Damper plate waveform (this was jumping around on the scope a bit. Scope is an old Stark from 1962 with all original parts, so it's hard to get stable sometimes)

Horizontal output tube G1 waveform

RCA Manual waveforms for comparison (Thanks Phil!)

[miniman82]

Looks like bad ringing in the output stage, I think the damper can cause issues like this.

[maxhifi]

have tried probably 5 damper tubes and 3 or 4 outputs, including a 6DQ6 just for kicks. Even tried the 1V2 from my scope just to be sure. I am starting to suspect either the width coil, or the flyback itself as all other major parts have been checked.

[Eric H]

I have seen a bad Flyback cause this, on a Silvertone, it was obviously bad though, burnt to a crisp but still making HV. Replacing it got rid of the problem.

I'll see if I have a spare Fly somewhere.

[maxhifi]

mine leaked out a ton of wax at some point. Hardly a surprise given its location directly over the power tubes! I could (and likely am) overlooking something simple here, but the flyback looks more and more suspicious . Have you got the sams for this tv? I think it might be prudent to check the resistance of the windings at least, and of the yoke too for that matter.