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: 5,598
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: 5,598
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: 720
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: 4,118
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: 5,598
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: 5,598
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: 1,911
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: 5,598
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 03-26-2011, 11:23 PM
DaveWM DaveWM is offline
VideoKarma Member
 
Join Date: Feb 2009
Location: Orlando FL
Posts: 5,598
ok noise is worked out but the pic was dim. I could make it ok by adj the CRT bias, but should not need that as I was able to get all the guns to show in the setup mode at the lowest setting. I checked the CRT pins and sure enough the cathodes were at least 50v too high (was 240v shb 190v).

So looks like a video out tube. I checked and yes the plate voltage was too high, checking the Cathode and again too high (about 5v not the 2.5 it should have been). So if the video out tube plate is high, and since its directly coupled to the CRT cathodes, that explains the bias problem and the dim pic. I did some resistance checks and it looks like a 100ohm resistor on the bottom of the brightness control may have drifted. Its mount up on the pot on the control panel so I will need to pull it out to check. Hopefully this will resolve the dim pic.

I also have a intermittant blue screen issue, Its like the G2 drops down about 10% all by it self. I will be monitoring the G2 voltage and see if that is whats going on. there are 100k resistors in series with the G2 pots so maybe the blue one is getting intermittant. I will prob just pull the chassis and replace all 3 of them while I am at it. Lastly there was a .1 cap in the grid circuit of the video out coupling the delay line to the grid. I will prob replace it as well since I seem to be having bias issues in that tube.

I will also check the PCB track for any break around that G2 lead but I am just guessing that is what is causing the intermittant drop down of blue in the crt (it like someone just turns it down a few degrees, but its quick not gradual, and it will come back and then do it again). I dont feel like breaking out the cold juice just yet.
Reply With Quote
  #10  
Old 03-27-2011, 10:32 AM
DaveWM DaveWM is offline
VideoKarma Member
 
Join Date: Feb 2009
Location: Orlando FL
Posts: 5,598
pulled the tuner out, the 100ohm was 300ohm off that pot. I need to go ahead an pull the entire chassis, will replace those other parts (the screen resistors and that coupling cap. there are some .001 disc caps across the G2 pots that I am tempted to replace while I have it out, just in case one of those is acting up causing that intermittant problem. The only thing is I never really captured it on the VTVM to confirm it was the G2 voltage that was doing it. could have been the G1 or the cathode I suppose. Just getting tired of yanking the chassis as you have to pull the tuner with it.
Reply With Quote
Audiokarma
  #11  
Old 03-27-2011, 02:48 PM
DaveWM DaveWM is offline
VideoKarma Member
 
Join Date: Feb 2009
Location: Orlando FL
Posts: 5,598
ah now that is much better. the grey scale setup worked as it should, where as before the incorrect bias of the video out tube resulted in too high a plate voltage, that when connected thru the serice switch in the normal mode would tend to cut off the CRT. Now with the CRT bias set at the min, I can put it in service mode, just bring up each gun, and then go to normal mode and get a nice bright pic with correct grey scale. while at it I put the old focus coil back in and now have a best focus point, before in never quite got there.

I still need to double check all my HV setup for current and voltage, but it now has a very good bright pic, great color, and NO noise in the pic.
Reply With Quote
  #12  
Old 03-27-2011, 02:52 PM
DaveWM DaveWM is offline
VideoKarma Member
 
Join Date: Feb 2009
Location: Orlando FL
Posts: 5,598
I will play with it some more later and see if the intermittant blue in the grey scale was fixed by replacing the G2 100k resistors. I only replace the one .001 disc cap on the blue since I only had Y5U caps and not the Z5P that it had originally. Since the other guns were ok I left them be (still replaced all the 100ks just so they were balanced, the were between 108k and 120k, but I was thinking they could have been noisy).
Reply With Quote
  #13  
Old 03-27-2011, 03:11 PM
DaveWM DaveWM is offline
VideoKarma Member
 
Join Date: Feb 2009
Location: Orlando FL
Posts: 5,598
HV is down to around 23.5 holding pretty steady. HOT current is a bit high on some bright scenes, but If I switch to serv to cut the beam curent its right at about 200ma, up to about 240ma for normal viewing. I may try some different HOT's just to see if I can get that down some. I forgot to check the resistors in the HOT feed back circuit, darn it....
Reply With Quote
  #14  
Old 03-27-2011, 03:13 PM
DaveWM DaveWM is offline
VideoKarma Member
 
Join Date: Feb 2009
Location: Orlando FL
Posts: 5,598
opps forgot to check the line voltage, will set that to 117v and check again. I always forget that...
Reply With Quote
  #15  
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: 4,118
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
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 06:39 PM.



Powered by vBulletin® Version 3.8.4
Copyright ©2000 - 2018, Jelsoft Enterprises Ltd.
ęCopyright 2012 VideoKarma.org, All rights reserved.