Videokarma.org

Go Back   Videokarma.org TV - Video - Vintage Television & Radio Forums > Early Color Television

We appreciate your help

in keeping this site going.
Reply
 
Thread Tools Display Modes
  #1  
Old 03-25-2011, 10:38 AM
DaveWM DaveWM is offline
VideoKarma Member
 
Join Date: Feb 2009
Location: Orlando FL
Posts: 5,607
noisey pic in Roundie philco

figured I would share my attempt to resolve this with the group.

the pic has excellent color, but there is always some background noise. Not snow, more like light streaking through out the pic.

I put the B&K analyst on it, the streaking is AFTER the tuner. I tried injecting at various IF stages, but with the transistor IF I had a hard time getting to the small leads to clip in.

I now plan to try signal tracing, again this time it will be the B&K analyst, with and with out a slide (what ever works best for finding the noise). I already put in a tube spacer at the video out tube. figured I would start there at the grid and plate and see if I can start to narrow it down.

it has been sugested that I check the cathode bypass cap on the video out tube (its a can cap, orig to the set) which is a good idea, but I really want to see if I can use my equipment to help me understand and trouble shoot.

I assume its a noisy plate load resistor, or a decoupler if its no in the bypass cap. Of course it could be up the chain so I am starting near the end. there is a video driver transistor and a cathode follower tube before that.

I just hope its not in the IF somewhere, if so I will have to see how well my demodulator prob works (something I picked up but have never used).
Reply With Quote
  #2  
Old 03-25-2011, 03:36 PM
DaveWM DaveWM is offline
VideoKarma Member
 
Join Date: Feb 2009
Location: Orlando FL
Posts: 5,607
well I tried the scope from the video out tube all the way back to the grid of the cathode follower tube (the 1st amp after the detector diode) I think I am still seeing the noise. I am going to try disconnecting the feed line from the detector that goes to that pin of the tube (its on a different PCB so I hope I can just find the wire that connects it) if its noise from the detector, then I assume disconnecting it will clear the screen. then it on the the video board, yuk.
Reply With Quote
  #3  
Old 03-25-2011, 03:42 PM
oldtvman's Avatar
oldtvman oldtvman is offline
Larry Melton (oldtvman)
 
Join Date: Jan 2005
Location: Mishawaka, Indiana
Posts: 772
lots of times noise can be generated from loose of missing ground or shielding. Also a noisy horizontal output tube can cause barkhousen (noisy tube). Most times these problems look more serious than they actually are to fix.
__________________
[IMG]
Reply With Quote
  #4  
Old 03-25-2011, 03:45 PM
miniman82's Avatar
miniman82 miniman82 is offline
First Light: 1952-2011
 
Join Date: Jun 2009
Location: Great Mills, MD
Posts: 4,159
Maybe the video detector diode is failing, my CTC-7 did funky things when it's detector was going out.
__________________
Evolution...
Reply With Quote
  #5  
Old 03-25-2011, 03:52 PM
DaveWM DaveWM is offline
VideoKarma Member
 
Join Date: Feb 2009
Location: Orlando FL
Posts: 5,607
maybe. if grounding the grid of the 1st video amp tube confirms the prob is before it, I will focus on video board, that will be the 1st thing I check.
Reply With Quote
Audiokarma
  #6  
Old 03-26-2011, 04:52 AM
DaveWM DaveWM is offline
VideoKarma Member
 
Join Date: Feb 2009
Location: Orlando FL
Posts: 5,607
well I decided to fool with the IF AGC (a pot on the video IF board). It seems to have cleared the problem.

I need to go thru the hole AGC setup (assuming the sams is correct) I dont think its right. the AGC control on the on the back of the chassis will not cause the pic to tear when fully CW. Maybe I should just leave it be, the noise in the pic seems to be gone, but its late at night, so I better look again after some sleep.
Reply With Quote
  #7  
Old 03-26-2011, 09:18 AM
old_coot88 old_coot88 is offline
VideoKarma Member
 
Join Date: Sep 2009
Posts: 2,541
A tip worth bookmarking.. with any kind of "noisy pic" or a 'overload' symptom, try the AGC control first, and also look for an RF AGC adjustment if a set is SS or hybrid.
Reply With Quote
  #8  
Old 03-26-2011, 11:59 AM
DaveWM DaveWM is offline
VideoKarma Member
 
Join Date: Feb 2009
Location: Orlando FL
Posts: 5,607
roger old coot. I should have just scoped the AGC line, I have chased things before that had to do with AGC one of which was hash on it from open caps, etc...
Reply With Quote
  #9  
Old 03-27-2011, 04:20 PM
miniman82's Avatar
miniman82 miniman82 is offline
First Light: 1952-2011
 
Join Date: Jun 2009
Location: Great Mills, MD
Posts: 4,159
Yep, all the readings I took the other day were at full line voltage (120). No doubt it would run cooler if I hook it up to my RCA TV Isotap transformer. I generally run old sets at 117/115.
__________________
Evolution...
Reply With Quote
  #10  
Old 03-27-2011, 04:39 PM
DaveWM DaveWM is offline
VideoKarma Member
 
Join Date: Feb 2009
Location: Orlando FL
Posts: 5,607
if you get a chance would you pls snap a pic of your CRT neck. I don' think the blue lateral and the purity rings are in the right posistion on mine.
Attached Images
File Type: jpg crt neck.jpg (52.8 KB, 19 views)
Reply With Quote
Audiokarma
  #11  
Old 03-27-2011, 05:26 PM
miniman82's Avatar
miniman82 miniman82 is offline
First Light: 1952-2011
 
Join Date: Jun 2009
Location: Great Mills, MD
Posts: 4,159
Wouldn't do you any good, what I have for blue lat is completely different than yours. The one mine has is metal...might not even be the right one. I also learned today that my set is missing the degaussing coil! Guess that explains the vacant 4-pin plug on the chassis. Anyone have one????
__________________
Evolution...
Reply With Quote
  #12  
Old 03-27-2011, 08:16 PM
DaveWM DaveWM is offline
VideoKarma Member
 
Join Date: Feb 2009
Location: Orlando FL
Posts: 5,607
wow, wonder how that came to be (no coil),
Reply With Quote
  #13  
Old 03-27-2011, 08:25 PM
DaveWM DaveWM is offline
VideoKarma Member
 
Join Date: Feb 2009
Location: Orlando FL
Posts: 5,607
ok blue intermittant still there, I poked around on one of the 6BL8's and you can make it flicker, but that just may be the shock going thru the board. I will try cleaning the sockets (again) but I need to try and isolate exactly where the problem is. Heck could be a tube with a short I guess (that would certainly account for a thermal issue). I am tired or working on it, so will button it up and give it some time off for pretty good behavior. One thng for sure, it has EXCELLANT color demodulation, no bleeding what so ever, passes the homer simpson test with ease.
Reply With Quote
  #14  
Old 03-27-2011, 10:47 PM
DaveWM DaveWM is offline
VideoKarma Member
 
Join Date: Feb 2009
Location: Orlando FL
Posts: 5,607
I fooled around with the scope, I get about 180pp on the horz drive, which is close to the 190 called out in the sams. I need to break out the VTVM to check but I am guessing based on the scope the drive is fine. I also checked the screen but did not use my VTVM and I dont trust a DMM on that reading since there is a lot of pulsing going on, need to use the VTVM like it says to, I just dont think they react the same to pulsed DC. On the DMM I had 175v (should be 210 per sams).

I set the horz bias so the HV is 25.5 per the sams setup (brighness all the way down). It has a great pic, brighness range looks right, but the HOT current goes up to near 300ma in some scenes (all white screen) most of the time its around 230ma. The HV with the brightness set to normal is about 22-23kv. My only issue is the fly gets pretty darn hot after about 30-60 min.

I tried bridging another VDR and was able to adj the bias to get it down to about 200ma with normal scenes, but then the HV was around 19 and the horz sweep was falling short about 1/4 inch on both sides.

I tried several 6KD6's all about the same. I wonder if a HV rectifier or Damper could affect the fly load?
Reply With Quote
  #15  
Old 03-28-2011, 07:49 AM
old_coot88 old_coot88 is offline
VideoKarma Member
 
Join Date: Sep 2009
Posts: 2,541
Quote:
Originally Posted by DaveWM View Post

I tried bridging another VDR and was able to adj the bias to get it down to about 200ma with normal scenes, but then the HV was around 19 and the horz sweep was falling short about 1/4 inch on both sides.
Dave, in cases of borderline width and you've exhausted all avenues, there is a little trick that often helps. If you happen to have some 6KV rated disc ceramic caps of around 68pf, 82pf and 120pf, what you do is take the smallest one first (the 68pf) and put it directly across the damper, cathode to plate. This will kick the width out a quarter inch or so on both sides. If it's not enough, sub the 82pf, and if still not enough, try the 120pf.
Each stage will also drop the HOT current by 5 milliamps or so, which you can compensate for with the bias or by upping the screen grid voltage.
You can only take this mod so far, because each stage also drops the HV a bit (as in the old saying "there is no free lunch"). But there is usually a compromise where you have adequate width with reasonable HOT current and decent HV regulation. This situation also often crops up in Admirals using the 6KD6.
Bill(oc)
Reply With Quote
Audiokarma
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 06:53 AM.



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