Finally got some time with the TV.
Drive/width don't do anything for the vertical bars, aside from move them around a bit. It's a shame, the picture is otherwise nearly perfect.

You might try a different Damper or Horiz Output tube, sometimes a set can be fussy about which tube works better even if they are good.

Nope, not a difference. Have tried 3 different of each, and a couple 6CG7s too. I think I'm going to have to start checking resistors, etc.

Ah yes, I looked back through this thread and see you did already do that.

This is one of the more aggravating problems I run into on old sets, probably something out of spec, a resistor seems likely at this point.

There are a couple of caps and resistors inside the yoke but the caps are likely Mica and unlikely (but not impossible) to be bad, the resistors might be however.

I was pretty careful to do neat work, only replaced about 8 or 10 caps in a sitting, tried to put the new caps in exactly the same place as the old ones, but I may have moved something, it's not totally out of the question.

Is it reasonable to think the problem of vertical stripes is more likely in the horizontal than the vertical section? Right now is a bit late to get the TV on the bench, but over the next few days I plan to give it some time. Problem is fairly subtle, but for sure want to get it worked out before I put the TV upstairs and into regular service.

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.

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.

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

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.

So perhaps a pulse from the Horiz retrace is causing some "ringing" in the power supply to the video amp? :scratch2:

jr

Speaking of video.
 

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.

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

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

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.

HOT waveform
 

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)

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!)

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

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.

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.

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.

Here ya go.

http://antiqueradio.org/art/temp/RCA...2HVPartial.jpg

http://antiqueradio.org/art/temp/RCA8PT7012Yoke.jpg

If you send me an email via http://antiqueradio.org/contact.htm , I can email you the whole thing.

Phil Nelson

They all leak a lot of wax out of the Fly, it would have made more sense to put it on the bottom and the tubes on top!

A resistance check may not do much good, a shorted turn or two wouldn't show up that way.

Phil:

Thank you, will send email soon! Mine is the KCS100K, so the flyback is different than that one - I started comparing resistances, and then realized that diagram has C78 = 0.047uF, which corresponds to the earlier KCS100B chassis.

Eric:

Yep, I thought it looked upside down too - when I took apart the chassis to clean it, the first thing I thought when I encountered the wax covered damper and HOT was hmmm heat rises, what's up with this idea?

I spent about an hour or two with the TV again tonight, without any real progress. I did however get a much cleaner photo of the horizontal waveform, as below.

Schematic sent (336-14, chassis KCS100B, the only match I found in the index).

Which waveform is that last photo showing?

If you want to go beyond resistance testing, this thread from the "other forum" has an article describing how to test for ringing with an oscilloscope (haven't tried it myself).

http://antiqueradios.com/forums/view...er=asc&start=0

Phil Nelson

Hi Phil,

Thank you for the sams! Been reading through it, and it has some perspective which the RCA manual does not. It also has a much better drawn schematic, from an understanding how the circuit operates perspective, even if the sweep circuit has some subtle differences.

Last waveform is the a few turns of wire around the horizontal output tube's plate lead, into my scope's probe.

Thanks for the article - Looks like I've got something to do tomorrow night now!

radar
 

heres one that i might need some guidance on i,am looking to get and old b/w tv and convert it to radar the tx and rx of it will be around 300mhz now i know that the tubes won,t be able to handle the high freq so should i stay with ic,s or go with tube?

Your radar question seems completely unrelated to this topic. You can click the New Thread button to start a separate discussion in this forum.

Phil Nelson

Ok, had some time this morning to look at the TV. I think I want to try a new fly, and maybe width coil. The only waveform which looks wrong, according to the RCA service manual, is the horizontal.

The testing method for ringing flybacks in the above article counts on having a "known good" to compare with.

One thing about this problem, is I am sure getting well acquainted with the inside of this wonderful little tv - it's actually more fun to work on than my early 70s Zenith whose performance was my previous interest, with its massive chassis, hot chassis and overwhelming number of parts.

On another "note" I think I figured out the problem with audio buzz too.... this is a design defect caused by the 6AW8 being nonlinear when the contrast is set too high. Once I get the TV working perfectly, I am going to think about ways to have the audio not go through the 6AW8

Noticed a new 'defect', or possibly characteristic. When contrast is cranked up, text can get fuzzy. Plate voltage on video amp is off spec, quite a bit too high. Since contrast control works by varying bias on 6aw8 pentode video amplifier, and current is too low, i'm guessing either tube is bad or resistors have drifted. This design doesn't appear to have a lot of headroom built in.... Effect not terrible, it just means one has to balance contrast with sharpness. Will work on this one once new flyback installed. In the mean time have been using this as my only tv... Works great... Wonder how long until i start to dream in black and white? :)

When I turn the contrast all the way up on mine it almost looks like overloaded agc. I think what you are seeing is just a characteristic of the design and not a defect. The picture and fine tuning controls need to be adjusted for the best overall performance which on a set like this is invariably a compromise. I expect this was typical of 1950s sets. The one thing these little RCA sets would have benefited from is an aluminized tube.

I agree completely, just for the sake of improved contrast. If attached advertising (radio & tv news, July 1954) is credible, it would have been a pretty big improvement. I wonder if any aluminized replacement CRTs were made for this set.