Zenith 23XC38Z Chassis Losing Horiz. Width
 

Hey everyone,
My daily watcher 1967 Zenith with the 23XC38Z chassis has been slowly losing Horizontal width at both edges of the screen over the past few weeks, however, the shrink has been much more noticeable over the past couple times I've had the set on. It gets about 2-4 hours of use a day, five days a week, and Ive been in using it in this capacity since about 2011. In that time, I've replaced a few bad electrolytic caps, the focus rectifier, a horizontal output tube, and a Damper tube. Does anyone who has any familiarity with these late 60's zenith Tube Chassis sets have any pointers on where to look for problems with Horizontal Width on the 23XC series chassis? Any help would be greatly appreciated.

I've been putting off pulling the chassis to install the NOS Channel Master Rebuilt 25GP22 I picked up a few months ago. It looks like this problem gives me a reason to get off my butt and finally do that.

Thanks in advance.

The first guess would be a weak H output tube of couse. There's a 'quick and dirty' way to check it without subbing the tube.
Turn the set off for about 7 seconds, and back on. Does the width take a while to fill back out to where it was, or not? If it does, it's likely the H out tube.

(The '7 seconds off' test can also reveal borderline weak H osc., 3.58 color osc., Vert out, RF amp tubes, etc.)

Your set probbaly uses a 6BK4 HV regulator. In later Zeniths using beam triode (6HS5, 6HV5 etc.) regulator, a gassy regulator can cause loss of width. The only test for these is to sub the tube, even though it tests 'good' on a tester.

I subbed in a spare NOS 6JS6C tonight and unfortunately it made no difference. I also swapped the damper tube.

This set uses a 6HS5. I subbed that with a NOS one too and it made no difference. I'm guessing a cap or resistor somewhere. The Horizontal oscillator is part of a 6u10 triple triode compactron. I subbed that too. Basically, I did all the easy stuff first hoping for a low effort solution.

Looks like all left is the damper. Also there is a VDR in
the HV reg that causes a minor width problem. On newer
sets HV control gets a bad spot but also only causes minor
problems most would ignore. Power supply also poss.
but you should get hum bars in that case.

BTW the OEM Zenith VDR's are going bad on the shelf.
There is a sub for it, was talked about a few months ago.
Since you will be pulling it probably best to replace it and
the 2 vert integrators, all high failure stuff.

73 Zeno:smoke:

Your set likely has a width jumper. It should have two positions 'increase' and 'normal'. Move it to increase if it is on normal. If that don't help, and the HOT is still strong, then you have a definite problem.

I would start by checking the B+ and HV

note this set uses a voltage doubler, so if B+ is low I would start there. Also while I doubt its a problem, there is a Thermistor in the B+ circuit, just in case you may want to check the voltage drop across it after it warms up.

If the cans are orig, see if they are warm (remember on doublers one of the cans while have voltage on the case with respect to the chassis, should have a cardboard cover).

Confirm line voltage ok (I go to the easy stuff 1st).

If B+ is ok then I would start checking voltage on the 6JS6 tube mainly the control grid voltage (-60v)and the screen (135v).

other things, are those large white tube caps, think at least one is a boost filter cap, maybe leaky if not replaced already. .1 1kv could be loading down the fly.

Bingo!

The caps on the doubler were going south. I'm going to go ahead and replace them and the last of the other electrolytics I haven't replaced yet. Thanks for the help guys!

Also, does anyone have a service manual scan for this chassis? I want to get all the cap values so I can buy the caps in one shot.

Also just an FYI but my local electronic surplus house just got a big load of NOS Thordardsen Yokes and flybacks for various tube color sets from an extinct contract service center that used to do repairs for major department stores in NE Ohio. I was able to grab two spare yokes and a spare fly for this set for $30, and I talked the owner into holding onto the rest for a while and not selling the lot for copper scrap. Just thought I'd let you guys know.
http://www.electronicsurplus.com/cat...?q=+THORDARSON

Alas, the usual criminals are missing: Fly277, Fly308, Fly312 -all of RCA's most vulnerable. Still some goodies to be had - cheap too!

I pulled the Chassis tonight. It was much less of a chore than I had anticipated.
http://i.imgur.com/TT9NWdsl.jpg
Tomorrow I'm going to replace the rest of the electrolytic caps I haven't already replaced, install the NOS 25GP22 rebuild, and do the convergence and setup.
If anyone asks, that's a last gasp 1978 Portacolor peeking in the right side of the frame. It has the stripe Phosphor tube and click-stop UHF tuner. I've been using it temporarily because I've been too lazy to dig into the Zenith for the past few days.

the plug in tuner makes it a easy chassis pull. Its going to look great with a new CRT. watch out for the little black electros with red tops.I had one of those go one me in the AGC circuit, made it HYPER touchy in the adjustment.

I put this off for days thinking it would be a massive project, but Zenith really designed this chassis for easy servicing. It took less than 30 minutes to pull the chassis. Zenith was smart enough to emboss all the chassis connection points for the power switch, degauss coil, speaker, etc.
DaveWM: Thanks for the advice, I'll look out for those little caps.
I'm still amazed I got almost four years of regular use out of this set, with mostly original components. It was still hooked up to a digital cable box and plugged in when I removed it from it's original home in a Duplex in the Collinwood neighborhood of Cleveland back in 2011.

And if you pull the tuner there is a few spare screw on the back to mount
it to the chassis. RCA also did it.

Our first color set was one of these ( or a Y line). Got it for the '67
world series. Under warranty a rectifire shorted but after that ran
til abt '77 no repairs. Sold it used & only replaced the 6HA5 RF
amp to perk it up.
NOTE when changing a 6HA5 try to find the short envelope version.
For some reason all the "tall" ones were noisy, enuf to kill a deep
fringe signal or make a fringe station grainy.......

73 Zeno:smoke:

Well, I replaced all the caps and the bottom of the horiz has started to collapse again. I can't get the HV over 22KV. The Flyback still looks good and resistance is in spec. I looked at the VDR, and it's already been replaced at least once, some other tech tacked a pink and white Zenith replacement VDR onto the leads of the original. I ordered one of the replacement Speer MOV's that Doug recommended in another thread. It should be here today. I'll let you all know the results once I solder it in. It would be hilarious if the caps were either fine or marginal and the VDR was the real culprit. If that's the case, I guess that'll teach me not to trust a $30 Chinese ESR cap tester.

I soldered in the new VDR. HV control is back, but the picture is still shrunken horizontally on the bottom third of the screen. It gets worse as the set warms up. I know for a fact that the Yoke is not the issue, as this is a NOS Thordarsen replacement. Once the set warms up, the bottom corners of the CRT shrink to black and the picture almost looks like a trapezoid. There are three white vertical lines centered in the bottom of the screen that fade out about a third of the screen up. Where the ticker on a cable news station would go, as an example, the text passes these lines, it shrinks and expands back to a normal size as it passes the vert. lines.

I really need help with this. I'm stumped.

a pic would be helpful

fyi, part of the pincushion correction in a zenith uses a pulse from the vert to modulate the output from the horz out tube. If you look a the circuit you can see a connection to I think a 40 uf cap (C7) that ties the screen of the horz out to the vert out.

This presents as a subtle narrowing and slight keystone of the pic (much less keystone than a bad yoke).

Sounds like the yoke....... yah, I know it's a new one.... you still got the original....?

We need a picture, several... when it's first on, and after a while...

.

its the after a while bit that makes me think its the pin cushion correction cap, if its just a 1/2 inch narrowing from top to bottom or bottom to top, then I would put money on that cap being an issue. Its even mentioned in the zenith tab manual case history as something that is often thought to be a yoke issue. Its just a lot less.

found it, knew I had messed with this before

http://www.videokarma.org/showpost.p...2&postcount=39

the full description stars on post 33

That entire thread was pretty good...... Boy you talk to yerself a lot....

.

I do that a lot, where I live its the only way to have an intelligent discussion.

This is really late, but thanks, Dave. That fixed it.

Works for where I live too.........

.

Great, glad that did it. Odd kind of failure, too subtle to be the yoke.