Videokarma.org

Go Back   Videokarma.org TV - Video - Vintage Television & Radio Forums > VideoKarma Classifieds

We appreciate your help

in keeping this site going.
Reply
 
Thread Tools Display Modes
  #16  
Old 06-06-2012, 01:28 PM
jmetal88 jmetal88 is offline
VideoKarma Member
 
Join Date: Jan 2008
Location: Derby, KS
Posts: 250
I actually can't find the sawtooth wave anywhere. Well, I can see a sawtooth-looking wave at quite a few places if I set volts/div to a ridiculously small number, but even at its smallest amplitude, I should be seeing a 15V peak-to-peak wave, and it simply isn't there. I've been removing, testing, and replacing all sorts of resistors and capacitors, as well as the selenium rectifier, but it just isn't showing up no matter what I do. Looking at the schematic, I'm thinking it might have something to do with the 6BU8 sync separator circuit, but I'm not sure what.
Reply With Quote
  #17  
Old 06-06-2012, 02:09 PM
DaveWM DaveWM is online now
VideoKarma Member
 
Join Date: Feb 2009
Location: Orlando FL
Posts: 5,604
did you calibrate the scope. do you have a 1x 10x setting on the leads? Have you a manual on the scope? have you read it?
Reply With Quote
  #18  
Old 06-06-2012, 02:13 PM
DaveWM DaveWM is online now
VideoKarma Member
 
Join Date: Feb 2009
Location: Orlando FL
Posts: 5,604
oh not a good idea to replace "all kinds of resistors" unless they check out of spec. The more parts that get replaced, the more likely you are to cause errors that add to the problem. You need to check the schematic and check for wave forms. You can also check the voltages if listed in the schematic, if you have one you should use a VTVM for this (the schematic should tell you what kind of test equipment to use). A DMM will "prob" work but I prefer to use the test equipment that was used when the schematic was noted.

Ok I re-read your post, so you should be ok if you checking and replacing. Have many checked bad? How are you checking caps?
Reply With Quote
  #19  
Old 06-06-2012, 02:46 PM
jmetal88 jmetal88 is offline
VideoKarma Member
 
Join Date: Jan 2008
Location: Derby, KS
Posts: 250
I don't have my proper leads yet, so I'm just using a piece of wire to see if I can see anything. I can definitely see the 90V peak-to-peak wave on the horizontal oscillator's grid, but that's about the only thing I can see right now. I don't have a manual on the scope.

Nothing has actually checked 'bad' so far. A little out of tolerance, maybe, but not actually bad. I've replaced a few resistors that I thought were too far out of tolerance. I have a multimeter with a capacitor testing function on it. Unfortunately, it gives inaccurate results even on known good capacitors, so I'm basically just checking to see if the capacitors actually have any capacitance. I don't have a vacuum tube voltmeter, but I know there's one in the physics lab up at my college that they probably wouldn't mind me taking in the fall (since my professor said they're probably going to try to get rid of all their analog meters soon anyway).

I did notice something that's probably an indication of some problem, though. The 10k resistor in parallel with the contrast control is getting really, really hot! Also, when I twiddle the contrast control, I can see what looks like it might be one of the sync waves starting to form on the oscilloscope display (when checking the 6BU8). It could just be noise, but it happens only when I move the contrast control back and forth within a very specific range.
Reply With Quote
  #20  
Old 06-06-2012, 02:53 PM
DaveWM DaveWM is online now
VideoKarma Member
 
Join Date: Feb 2009
Location: Orlando FL
Posts: 5,604
I am lost now.

1) Is there any HV (use HV prob to check) at all

2) Is there a grid drive signal at the horz out tube

3) with the plate cap disconnected to you see B+ at the cap lead (not the tube)
repeat with the plate cap disconnected.

4)Have you check the pin voltages with at least a DMM around the horz out tube, esp the screen and control grid.
Reply With Quote
Audiokarma
  #21  
Old 06-06-2012, 03:10 PM
jmetal88 jmetal88 is offline
VideoKarma Member
 
Join Date: Jan 2008
Location: Derby, KS
Posts: 250
1) I don't think so. At least there's no power getting to the HV rectifier.

2) There is no drive signal at the horizontal output tube. The signal that's supposed to be there also seems not to appear anywhere else in the TV.

3) The plate cap for the 6DQ6 (horizontal output tube) seems to not be getting any supply whatsoever. In fact, when I test the lead, it shows a slightly negative voltage. The cap lead connects directly to the flyback.

4) Yeah, and they seemed to be nowhere near where they should be.
Reply With Quote
  #22  
Old 06-06-2012, 04:20 PM
Zenith26kc20's Avatar
Zenith26kc20 Zenith26kc20 is offline
VideoKarma Member
 
Join Date: Apr 2009
Location: New Orleans
Posts: 801
The plate cap of the horizontal should have close to B+ on it (remove it to test or the test gear may be damaged from RF if present). If no B+ I would check the damper circuit first. Is there screen grid voltage on the horizontal output tube?
Also, be careful on a horizontal circuit with no drive to the output tube. It can be damaged by overcurrent quickly. Leave the cap off the tube until you get drive.
Do you have the schematic for this set?
Reply With Quote
  #23  
Old 06-06-2012, 05:52 PM
jmetal88 jmetal88 is offline
VideoKarma Member
 
Join Date: Jan 2008
Location: Derby, KS
Posts: 250
Yeah, there is screen grid voltage, but it's supposed to be 135V, and I'm only seeing 25V. Both resistors connected from the 250V B+ to the grid measure correctly. Hmm...
Reply With Quote
  #24  
Old 06-06-2012, 06:30 PM
jmetal88 jmetal88 is offline
VideoKarma Member
 
Join Date: Jan 2008
Location: Derby, KS
Posts: 250
Turns out the damper tube was bad, according to my dad's tube tester. I replaced it with a good one, but it's having no effect on the operation of the circuit. There's still no B+ on the plate cap for the output tube.
Reply With Quote
  #25  
Old 06-06-2012, 06:32 PM
jmetal88 jmetal88 is offline
VideoKarma Member
 
Join Date: Jan 2008
Location: Derby, KS
Posts: 250
Oh, and yes, I do have the schematic. I have the whole Photofact folder, in fact. Would you like me to scan the schematic sheet so you can see what I'm trying to work with?
Reply With Quote
Audiokarma
  #26  
Old 06-06-2012, 06:35 PM
DaveWM DaveWM is online now
VideoKarma Member
 
Join Date: Feb 2009
Location: Orlando FL
Posts: 5,604
that screen resistor is prob getting very hot or its increased a lot in value, check its rssistance.

sounds like maybe the tube is conducting thru the screen, you need to trace the B+ begining at the damper and follow it thru the fly (take the horz output tube out of the socket to avoid damage to it) to the horz out plate cap.

Generally speaking HV problems should start with a subbing of the horz out/damper/HV rectifier tube (and HV regulator if a color set). Tube testers do not always do a good job on these. If Subbing brings it back you can start replacing one tube at a time with the old ones.

Another easy way to see if there is any action is to use a neon bulb (attached to a plastic straw) and hold it near the plate cap of the horz out, if its working then the bulb will glow. no wire need to be attached just get it close to the plate cap of the HOT.
Reply With Quote
  #27  
Old 06-06-2012, 07:09 PM
jmetal88 jmetal88 is offline
VideoKarma Member
 
Join Date: Jan 2008
Location: Derby, KS
Posts: 250
Yeah, it is getting really hot. It hasn't increased any in value, though, it still measures 15k.

As far as I can tell, the B+ goes into the flyback, but doesn't come back out. Which still leads me to believe that the flyback is the problem.
Reply With Quote
  #28  
Old 06-06-2012, 10:16 PM
DaveWM DaveWM is online now
VideoKarma Member
 
Join Date: Feb 2009
Location: Orlando FL
Posts: 5,604
well back to the schematic and a checking of the continuity and resistance measurements of the FLY I thought you had done that already? make sure you are careful with the cap lead, sometimes that can be corroded and broken etc...
Reply With Quote
  #29  
Old 06-06-2012, 11:50 PM
jmetal88 jmetal88 is offline
VideoKarma Member
 
Join Date: Jan 2008
Location: Derby, KS
Posts: 250
I'll check the DC resistance of the flyback again in a few days, probably. I'm kind of tired of working on this for now, so I'm gonna take a break for a while.
Reply With Quote
  #30  
Old 06-07-2012, 12:10 AM
jmetal88 jmetal88 is offline
VideoKarma Member
 
Join Date: Jan 2008
Location: Derby, KS
Posts: 250
I guess I lied, I went ahead and checked it. What did I find? A broken connection to the fourth terminal, which is where the B+ should be entering the flyback. Must have happened when I reattached the flyback after checking the resistance the first time. Maybe it'll work if I re-solder it.
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 10:04 AM.



Powered by vBulletin® Version 3.8.4
Copyright ©2000 - 2017, Jelsoft Enterprises Ltd.
ęCopyright 2012 VideoKarma.org, All rights reserved.