Videokarma.org

Go Back   Videokarma.org TV - Video - Vintage Television & Radio Forums > Vintage TV & Radio Tech Forum

We appreciate your help

in keeping this site going.
Reply
 
Thread Tools Display Modes
  #1  
Old 11-22-2020, 09:31 AM
DavGoodlin's Avatar
DavGoodlin DavGoodlin is offline
Motorola Minion
 
Join Date: Aug 2011
Location: near Strasburg PA
Posts: 3,400
Convergence rectifier question

It was impossible to bring in some dynamic adjustments on a Zenith 21" so I figured there was something I forgot about these. This has not affected static convergence, center screen is all OK.

One reason that I suspected the convergence rectifier pack was it looks to be selenium. Then one tested open, so I put in three 1N4005s, which are silicon rectifiers. There were also two paper caps, one a bumble bee and the other a white tube-stuffed one.

Replaced two 100 ohm 1W resistors in series with these rectifiers, and a (blue-horz top) pot connected to the third. Then also, there was an 82 ohm 1 watt I replaced.

Since I replaced these three diodes with silicons, the negative voltage to ground as shown Sams is higher at resistors. The red vert lines are impossible to pull in on both sides. The 100 ohm I put in both run hot.

I noticed that later sets like Z-models used 220 ohm resistors in place of both 100 ohm 1 watts. Zenith schematic also shows a current of 25 ma. Maybe I answered my own question.

Pretty sure RCA, Magnavox and others used similar circuits, and surprisingly I never went into the weeds on convergence boards of all things. I'm aging into being a perfectionist or just started doing what was expected of a tech 50 years ago.

Has anyone else had this issue?
__________________
"When resistors increase in value, they're worthless"
-Dave G

Last edited by DavGoodlin; 11-22-2020 at 09:36 AM.
Reply With Quote
  #2  
Old 11-24-2020, 07:19 PM
DavGoodlin's Avatar
DavGoodlin DavGoodlin is offline
Motorola Minion
 
Join Date: Aug 2011
Location: near Strasburg PA
Posts: 3,400
After looking at a schematic for a 16Z8C50, 1969 model, it was assumed the convergence rectifiers were silicon not selenium.

On this newer model, two 100 ohm resistors were replaced with 220 ohm, 1 watt. I used 250 ohm 5 watt since I bought a large quantity for other projects.

All other parts and adjustment pots were the same, leading to my conclusion.

I also found the right side R+G vertical adjustment coil was missing its slug. It was impossible to screw another one in. The coil ran hot, causing the distortion preventing the insertion of a slug.

Luckily, I found my tap set and had a fine thread 1/4" fit and reamed out melted plastic, so a slug could be inserted.

I always considered convergence circuits as black magic, affecting only parts of the raster. All together these adjustments affect each other. Lots of back and forth between static and dynamic convergence.
__________________
"When resistors increase in value, they're worthless"
-Dave G
Reply With Quote
  #3  
Old 11-25-2020, 01:10 PM
DavGoodlin's Avatar
DavGoodlin DavGoodlin is offline
Motorola Minion
 
Join Date: Aug 2011
Location: near Strasburg PA
Posts: 3,400
One more issue with the vertical sweep needs fixed before we wrap up but the convergence has been successfully repaired. I actually had a parts chassis to pull a R+G vert convergence coil from as the original was literally melted from excess current due to open rectifiers. Somebody was all over this set trying to get it converged judging by the missing slug and other two spun almost out of the coils.

Lingering issue- I cannot seem to get linear vertical sweep without stretching out the bottom or top. Centering strings are missing from the yoke. Trying both 6HE5/6JB5 and 6BA11 tubes of course made little difference. Like Bob's Motorola color showing an adaptation of SNL's coneheads- LOL

Voltages indicated on the schematic were very close, with plenty of variation via adjustment of height and linearity. Resistors are not suspect and output tube cathode bypass cap has been replaced, so I'm looking at four mylar caps and possibly the integrator (wave shaping) couplate.

Most Zeniths I have worked on of this vintage have some vertical issue. If not a higher value resistor, VDR or control. Capacitor replacement should get it close enough to forget needing a centering control. The secondary of the VOT powers the convergence board which is a current-driven morass. The good thing is, you cannot get shocked by it.
__________________
"When resistors increase in value, they're worthless"
-Dave G

Last edited by DavGoodlin; 11-25-2020 at 01:43 PM.
Reply With Quote
  #4  
Old 01-08-2021, 02:17 PM
dsndblm's Avatar
dsndblm dsndblm is offline
Keepin the blue smoke in
 
Join Date: Aug 2008
Location: Lost in the desert between Phoenix and Tucson
Posts: 23
Good information. I have some of the same issues with my CTC 11. My RCA convergence board is made basically the same as you described your Zenith.
__________________
Tube audio guy. Tube collector.

Last edited by dsndblm; 01-08-2021 at 03:27 PM.
Reply With Quote
Reply


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 01:54 PM.



Powered by vBulletin® Version 3.8.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
©Copyright 2012 VideoKarma.org, All rights reserved.