I did something dumb. I replaced the damper tube with what I thought was the correct tube, but in fact was the wrong tube. The TV was working fine with a 6AX4 damper. I had done a full re-cap of electrolytics, and had already replaced all of the paper caps, except for 1 or 2. I followed the schematic, and put a 6DE4 in the damper tube socket, and also replaced a few caps that I had missed. I have double checked those caps I just replaced, and it seems I made no mistakes. When I powered up the TV, with the 6DE4 tube installed, I got a howling squeal through the speaker, and no raster or HV. The little neon lamp under the chassis that normally lights up did not light. That same squeal also seemed to be eminating from the HV cage. This squeal was no where as high frequency as the squeal one hears from a tv when it's working properly. I have since replaced the 6AX4, and the set acts the same way, What did I do wrong?

here is the schematic

http://antiqueradios.com/gallery/v/temp ... ewsIndex=1

Here are data sheets for those tubes, if you want to compare:

http://frank.pocnet.net/sheets/049/6/6AX4GT.pdf

http://frank.pocnet.net/sheets/093/6/6DE4.pdf

They look pretty similar to me, unless I'm missing something obvious.

Could it be something else, like a blob of dropped solder, wire pushed over to make a short circuit, etc.? Everyone has made boo-boos like that.

It's not uncommon for a marginal part to croak when you power up an old TV for the first time in decades. I would carefully check everything in the HV section.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

The 6DE4 should work in place of a 6AX4. The 6DE4 can handle higher plate current and draws slightly more filament current. That would not cause the problem you are seeing.

Edit: If the damper had it's own filament winding on the power transformer then the slightly higher filament current could be a little hard on the power transformer in the long term. But in this case the damper is on the same filament winding as all the other tubes. In that case the slightly higher filament current is too small of a change to notice.

It seems that I messed something else up, but I cannot visually find anything wrong. I'm replacing out of spec resistors here and there, but they are not way off, and I seem to get the same results.
http://www.youtube.com/watch?v=xCs5GCCk ... e=youtu.be

You are way out of horiz frequency. Check and make sure the caps you changed are the correct values. For example maybe you put in a .005 for a .05

I cannot find any solder blobs, and I have triple checked all of the caps that I replaced right before the problem started. I even dug the old caps out of the trash to double check. I cannot find any crossed wires, but I'm not saying there aren't any. I'll continue to search for visible problems. I have replaced a few more resisters that were a little high, but not terribly out of spec. I feel like I should track down the problem electronically. Here are the voltages on the 6DQ6 Horizontal output tube

2
3
4 188v - should be 155
5 -40v - should be -60

Maybe these voltages don't tell you much, but that squeal the tv makes is loud!

What should I look for next?

Those readings aren't terribly far off. How about voltages on the other sweep tubes (V8, V9) and supply voltages (280V and 720V) in that neighborhood?

Viewing waveforms at the indicated test points with an oscilloscope would be useful, but if you don't have a scope, that's empty advice.

It's not impossible for mica caps to fail. Some of those in sweep circuits take a beating.

Phil Nelson

Here are the voltages for tubes V-8 6EM7

Should be actual
1- 25 0
2- DO NOT MEASURE
3- 20 30
4- -35 -21
5- 90 34


Voltages for V9 - 6GH8
Should Be actual
1- 85 272
2- -90 0
3- 170 276
4-
5-
6- 340 403
8- 0
9- -6.5 0

I will get a 20 Mhz scope pretty soon.

What do these voltages mean, what do the tell us?

The dampers are different in some area (perveance?). Did you change R77 ? The damper has
a high voltage cap across it and a coil in series with the cathode which shows that they had
some ringing issues. They also had to use a width sleeve on the yoke. The 6AX4 had 3 versions
during its design run. If the damper loads down with some sqedge mode, the 720 volt boost might be too low to allow the horizontal oscillator to work properly. (R77 is its plate resistor).
And C55 is the boost smoothing cap. Are there any service notes out there on this chassis?

I have replaced R77.

Where are other places to check voltage?

When I get my first oscilloscope, what should I look for?

The latest voltage readings suggest that something is amiss in your sweep sections (V8, V9). In addition to checking components in that area, you can test the basic power supply voltages to see whether the p-s is providing the right levels in the first place. In the schematic near the LV rectifier V13 is a section showing those supply voltages (280V, 130V, etc.) Are those voltages correct? In other places in schematic where an arrow is shown with those voltages, are those values also correct in those places? If you find a place where the voltages are seriously wrong, you can try to track down the cause.

Re the oscilloscope, the schematic shows model waveforms at several test points, for instance at pins 2 and 7 of V9. The idea is to connect your scope's probe to a test point and set it up as indicated in the model, and then compare the waveform displayed on your scope to the model waveform in the little picture.

If you have never used an oscilloscope before, expect to spend some time learning how it works. Your scope's user manual may offer some guidance. You may also find books and/or online tutorials on oscilloscope techniques. While learning oscilloscope skills, you may find it easier to practice on some simpler device that already works correctly -- a tube radio or amplifier, for example.

Phil Nelson

For your first oscilloscope, it would depend on whether it was a modern solid state
type or an old TV service tube model.

Some waveforms in this chassis assumes a knowledge of handling a scope. Without the
background you could you could do damage to the instrument.

I went back and replaced V9, 6GH8. The set now plays beautifully, I even backed off the AGC a bit.

It seems like Zenith was trying to make the best 19" B&W TV they possibly could.

Now with the TCM channel (Turner's Classic Movies) going strong both in broadcasting and
sourcing re-releases of the B&W classics on DVD there could be a resurgence of meticulously
restored sets. (dare I dream of a viable CRT re-builder)

Watching and listening to an old flick on an old TV with active DC restoration on the kine,
and the hoaky old speaker-sound could be regarded as a special event.

Could be an escape from the hi def media blitz of the internet.

There are no color TVs in our home. Our daily driver, main set in the living room is a 1961 zenith 23" B&W console. The bedroom set is the one I'm describing in this thread, a 19" B&W metal cabinet Tabletop Zenith set from about 1962. The living room set is not perfect, I did a re-cap a few years ago, but the horizontal and vertical synch are pretty critical, the set has some line paring, and the tuner is still a little scratchy. But we watch that TV about 5 hours a week, for a few years... knock on wood...

We have a DVD player, digital converter box, and a LaCie LaCinema digital media player. We are currently watching episodes of 30 Rock. My girlfriend has 4 seasons of Ironside with Raymond Burr on DVD.

I didn't think I would ever feel this way, but I kind of like B&W better than color, because if I watch a lot of TV on those large screen TVs, I kind of get overwhelmed and burnt out by all the overstimulation.

I am now officially old.