View Full Version : Any Zenith experts around Los Angeles?


TinCanAlley
10-16-2016, 02:52 PM
I'm tired of the jailbars on my Avante CCIII. I'm sure most here have read the threads I started when I got the set and many helped me with the restoration (for that I am very grateful), but the one issue I haven't been able to solve is the darn jailbars. There's nothing more annoying than watching a great image and then being hit with those darn light and dark bars.

I have replaced all the electrolytics, replaced suspect resistors, cleaned all pots and scoped all important points. The only true ringing I found was on the ABL, but was told by some here the problem was somewhere else in the set, but I've not been able to find it.

It seems most experts are in the mid west to east coast. I'm hoping there is someone on this side of the country that has experience with this issue. I could really use the help and am willing to pay.

Thanks!

DaveWM
10-16-2016, 03:18 PM
aren't there some diodes that are suspect with Jail bars? Did you scope around the horz blanking amp transistor? (25DC56) not sure if your model is the same. check the feed wire from the fly if it has one just for horz blanking.

TinCanAlley
10-16-2016, 07:50 PM
aren't there some diodes that are suspect with Jail bars? Did you scope around the horz blanking amp transistor? (25DC56) not sure if your model is the same. check the feed wire from the fly if it has one just for horz blanking.

Going to go back over original tread(s). I know I scoped all transistors. and several wires from the yoke and flyback.

I replaced several diodes (with NOS). One was on the horizontal output, that I'm sure.

The other issue I can't seem to get is the lack of the setup line. I flip the switch, but can't see it. I had to display a grid from my generator to adjust the screen controls.

It's the 25EC58 chassis.

DaveWM
10-16-2016, 09:43 PM
So the horz blanking pulses were in spec (amplitude)?
How good is the CRT? Maybe worn out and cut off no good.
does not make sence setup line not working, is the screen blanked out completely (cut off) or does the vert not collapse?

Electronic M
10-16-2016, 10:28 PM
If there is not line with the set up switch flipped, then crank up the screens till you DO see one...If you still can't get a line crank up the brightness, brightness limiter, drives, and perhaps contrast...If you still can't get a line check CRT base voltages and if good then your CRT must be really weak.

RCAZenith
10-17-2016, 08:08 AM
Look up shango066 on youtube. See if you can contact him.


Sent from my iPhone using Tapatalk

TinCanAlley
10-18-2016, 12:20 AM
Okay, guess I should give a little more detail. I can't get the setup line as outlined in the service manual. I have to turn up the brightness and contrast to get it to show, but then the accuracy of the adjustment is off. That's why I used the grid of the generator as allowed fine tuning.

There is nothing wrong with the CRT, it has been fully tested. I am not having any issues with this set other than not getting the setup line as described in the SM and the jailbars. The picture is darn good until the bars become noticeable. I do notice that on scenes that (and I don't know if this is the right term) have blacker than black, the bars are not noticeable and the entire screen is black. When the black is at normal, the bars are quite noticeable. Another reason I feel the ABL might be the culprit.

Thanks!

TinCanAlley
10-18-2016, 12:27 AM
If there is not line with the set up switch flipped, then crank up the screens till you DO see one...If you still can't get a line crank up the brightness, brightness limiter, drives, and perhaps contrast...If you still can't get a line check CRT base voltages and if good then your CRT must be really weak.

CRT tests strong. I think there might be something in the circuit that is not up to the task anymore. I have been toying with the idea of replacing the resistors in the circuit.

TinCanAlley
10-18-2016, 12:29 AM
So the horz blanking pulses were in spec (amplitude)?
How good is the CRT? Maybe worn out and cut off no good.
does not make sence setup line not working, is the screen blanked out completely (cut off) or does the vert not collapse?

The CRT tested strong. The vert does collapse, but the screen drivers aren't enough to bring the line up. Now if I also increase the brightness and contrast, the line is visible, but then the screen controls are set too high when I switch out of setup. I've used the grid pattern of the generator with great success.

DaveWM
10-18-2016, 08:26 AM
I would not bother with the jail bars until you get the setup sorted. With a strong CRT you should not have to max out the screen drives to get the setup line to work. I would look at CRT pin voltages.

zeno
10-18-2016, 10:14 AM
On delta tubes typically one end of all G-2 controls at abt 250VDC.
The other end is also tied together at apx 900V "BOOST".
Check the 900V with a 1KV meter !!. Probably the G-2 are at
abt 400V. Normaly a strong jug will quickly give you the line.
Also look at all the CRT voltages to be sure they are OK.
It may turn out to be all the same cause.

73 Zeno:smoke:
LFOD !

Electronic M
10-18-2016, 12:01 PM
On delta tubes typically one end of all G-2 controls at abt 250VDC.
The other end is also tied together at apx 900V "BOOST".
Check the 900V with a 1KV meter !!. Probably the G-2 are at
abt 400V. Normaly a strong jug will quickly give you the line.
Also look at all the CRT voltages to be sure they are OK.
It may turn out to be all the same cause.

73 Zeno:smoke:
LFOD !

Good advice. I'd like to add that there are sneaky ways to do it with sub-1KV meters. On some tube era sets the lower supply rail is 300-350V. I've (when I've lacked a 1KV meter) measured that 350V with respect to ground, recorded it, then measured the 900V rail with respect to the 350V rail (only a 550V difference), then added the two numbers...It's crude but it works in a pinch.:thmbsp:

Findm-Keepm
10-18-2016, 01:54 PM
Good advice. I'd like to add that there are sneaky ways to do it with sub-1KV meters. On some tube era sets the lower supply rail is 300-350V. I've (when I've lacked a 1KV meter) measured that 350V with respect to ground, recorded it, then measured the 900V rail with respect to the 350V rail (only a 550V difference), then added the two numbers...It's crude but it works in a pinch.:thmbsp:

Brilliant! Never thought of that.

I just knew that we kept a VTVM for measuring boost. Blow a DMM once, and you'll never do it again. It was a selling point for Beckman meters back in the early 80s - you can measure Boost voltage directly, thanks to a 1500V DCV range.

Ground reference to B+ - another tool in the toolbox...:thmbsp: Thanks.

TinCanAlley
10-18-2016, 03:56 PM
All those things were tested a couple of years ago when I first got this set. It's all, pretty much, documented in previous threads. Tested the voltages at the socket, etc. All were in spec when tested back then.

There's ringing in a circuit and that's what I'm looking for with the jailbars. I'm pretty sure the problem with the setup circuit is a resistor. I have the replacements and will be working on replacing them.

Thanks