CTC-15 line pairing - Videokarma.org TV - Video

DaveWM · #1 11-20-2011, 04:41 PM

another roundie that I am fighting line pairing with. same deal as the others but this one I cant even find a sweet spot on the vert hold that will let me dial it out. I have several sets that use the RCA multivibrator vert (6GF7) some are fine some have the line pairing issue.

I am going to start with a few tube swaps, the sync sep and the 6GF7, but exp tells me that is not going to do it.

Next up will be to scope the plate voltage (comes from boost) and see if I can see any excessive vert noise. I know there is a big capacitor on the sweep board that should be filtering that out.

scope around the grid of the multivibrator and see if I can see any hash there.

last time I fought this I ended up replacing EVERY cap on the vert circuit. and the intergrater, Kinda fixed it on another set.

The only thing I never replaced was the VDR from the plate of the multivibrator, but I just assumed that was to regulate the feedback pulse to a consistant voltage.

ctc17 · #2 11-20-2011, 05:06 PM

The VDR works as a special wave shaping rectifier.
If you need me to find the article that explains the function of that VDR I can dig it up.

old_tv_nut · #3 11-21-2011, 12:57 PM

Line pairing happens when horizontal pulses get into the vertical - check all grounds in the sync and sweep areas (both V and H). Designers sometimes had to play with grounding and lead dress on a new chassis to reduce line pairing, and some chassis designs had ground loops that just couldn't be eliminated completely, but there should be more vertical hold positions without pairing than with.

DaveWM · #4 11-21-2011, 01:37 PM

I will go thru the ground stakes again.

also noted that the vert hold only works at the extreme CCW, more that about 1/4 turn off ground results in rapid rolling to the point of locking in with two flickering pics and the blanker centered on the screen.

My guess is the resistor in series with the hold pot has increased and/or I have some leakage thru a feedback cap that maybe allowing some DC to get, either way something is upseting the bias and or time constant the vert is running at.

I will prob just replace ALL the film caps and resistors on the vert sweep. Its not that hard esp since its a PCB.

will also check the sync pulse vert signal, the sams shows pulling the multivibrator (bright down to avoid burning the CRT) and scoping the output from the integrator. the Sams show what looks like a vert pulse with 'some" horz hash in between but at a much lower PP.

DaveWM · #5 11-21-2011, 03:57 PM

boost filter cap on sweep board (between boost and B+ feed vert osc plate thru a pot) was a bit weak (would not open the eye all the way) leakage test was ok. Replaced it. A .0068 couples the feed back to the grid of the vert osc was very weak would barely open the eye, leakage test ok, replaced it.
A 82k resistor in the feed back loop had drifted to 120k, I think this one may account for the osc speed being off as it looks like its part of the RC networks that establishes the osc rate. there is one more in there that I need to check, its the .0082 right off the plate of the mutivibrator, I suspect it gets a lot of abuse in the form of some pretty high spikes. After that I will reinstall and see if the pairing has improved. All the ground stakes looks good.

old_tv_nut · #6 11-21-2011, 05:05 PM

You never know when fixing something else first may help - good luck.

DaveWM · #7 11-21-2011, 05:45 PM

well those were all in the vert and since I was having some issues with the vert hold range I figured may as well start there. No reason to chase down pairing until I get the vert range worked out.

Oh and I did try subbing some vert tubes, I know that multivibrators are by there nature are unstable and different tubes behave differently.

I checked the .0082 cap that was the one of the plate, again almost no movement on the cap tester. Replaced it, did check the replacement going in, just to be sure it was not the cap tester, worked as should. I do test the caps completely out of the circuit as sometimes the cap tester gives false readings, even if one leg is completely out of circuit. while I am in there I am replacing any resistors that would be hard to get at with the cap in place there is one that sits right under the .0082. its 100k tested at 93k so normally I would leave it be, but since I would have to remove the cap just to get at it later, I figured what not replace know.

DaveWM · #8 11-21-2011, 07:33 PM

oh well at least the vert hold is working right now, I get sync lock right in the middle, can roll it both ways by going to the extremes. I still have the line pairing issue, moving the vert hold just moves the paired lines up and down but never fills in the middle.

Next up will be to check the B+ for horz noise, check the HOT screen decoupler cap a .1 right off the screen.

DaveWM · #9 11-21-2011, 10:21 PM

well while messing about with the tv I noticed the shunt tube seemed to be glowing hotter than normal (and the FLY was getting quite hot as well even though the HOT current seemed right a 185ma) I need to check the V drop across the shunt cathode resistor before doing anything else. The voltage regulation seems fine, able to dial in right at 23kv, but I did not check that shunt tube current so will do that before I do any more testing on the pairing issue. I did try several 6Gf7's much to my surprise the pic vert and lin remained pretty constant, as did the line pairing issue. It seemed like one tube did a little better but its not right. if I take it to the edge I can get rid of it, so I know there is room for improvement. My scope lead took a dump so I will have to fix that and go in search of horz rate noise.

DaveWM · #10 11-22-2011, 06:59 AM

checked the shunt tube current/v drop across the 1k cathode resistor. between .5 and 1.0 so no excess there. I did see a rather ghastly green which may be normal considering the voltages, but I replaced it with a C (had a B installed) tube. Don't see the green anymore, v drop about the same.

checked the voltages at the HOT screen was about 165v grid was -55 tube current was 185ma so all looks good there. I still don't under stand the high heat of the FLY but for now will just have to live with it (and not run set very long, I never leave any vintage set unattended).

I will get back to the pairing after Thanksgiving holidays.

DaveWM · #11 11-28-2011, 03:27 PM

while waiting to get a new scope prob (to check the PP on the output from the vert integrater) I decided to look at the input from the sync sep to the integrater. there is a 220k resistor from the output to ground, could not test it in circuit, so lifted a leg and get 1.2meg, hmmm I am hoping that this maybe allowing too high a PP input into the integrator, which would then be too high and output (including horz noise). A hint MAY have been the 8ka8 in place fo the 6ka8 tube used. Perhaps a tube jockey attempting to reduce the sync output but running a tube cold. keeping my fingers crossed. If this works it will be something I check on some of the other sets that seem to have problems with line pairing. I don't recall the plate voltage (the resistor acts like a voltage divider as its on the plate), I should have checked there but since its already out I will just replace and try again.

DaveWM · #12 11-28-2011, 04:43 PM

replacing that resistor brought the resistance check amount right in line with sams. Guess now is a good time to go thru the rest.

DaveWM · #13 11-28-2011, 08:04 PM

well not sure if there was any improvement, I still seem to get an excess amount of pairing but it does seem like I can dial in and out a little bit with the vert hold.

dont think I will be able to do much more till I get my scope leads fixed. I want to poke around some and look at the vert sync pulse and the b+ for any errant horz hash.

DaveWM · #14 01-17-2012, 09:57 PM

more pics

1st is the closeup of the vert sync pulse off the plate of the sync sep tube, this is using a composite video from the converter box injected at the grid of the video amp. this produces a very nice pic, no line pairing or as I have recently read "scan lines".

2nd is using the tuner and a RF signal from the same converter box, this results in "scan lines" that can not be dialed out with the vert hold.

3rd is the grid of the 1st video amp, I prob should have looked at the plate from that to see if the video amp acting up as well, that is the bottom trace, the top is the composite video from the converter box.

Penthode · #15 01-18-2012, 09:09 PM

Picture 2 and 3 appear to show differentiated pulses. This would imply poor low frequency response. Vertical sync needs the lower frequency response to work properly.

Perhaps poor low frequency response in the video amplifier is the culprit. It could be due to bad (low capacity) coupling capacitors or open screen decoupling or cathode bypass capacitors.

The RF/IF response could also be the cause: if the video carrier is too low on the nyquist slope of the IF response or if the IF or RF response is not correct, the low frequencies would be attenuated. To check this out, simply adjust the fine tuning control where the color disappears and when the video begins to get a bit smeary. If the interlace improves, that may be it.