Magnavox Color Roundie

[tvcollector]

I know it needs to be degauss.. I'm not worried about that right now, as space right now is an issue.. The worminess is what I can't get to go away.. I've adjusted the fine tuning, and it doesn't fix it.. HV is set all the way up, if that's what your talking about..

[tvcollector]

Purity was way off, had to adjust the yoke and the purity ring alot to get red in the middle, it was way off to the side.. I also did convergence and got it much better in the middle, all this seemed to minimize the worminess.. But the contrast is way too much.. I'll have to take the chassis out and follow the contrast pot leads and make sure none of them broke off..

[DaveWM]

does the contrast control not reduce the contrast?

[tvcollector]

It doesn't.. Not sure if you read the previous post on it.. But the pot seems to be good, but there is no effect when adjusting the control pot.. There is issues with the sharpness pot too..

[DaveWM]

check for a shorted cap, its in one of the cans, you will see the pot wire go to it, prob 100uf of something big like that, low volt, prob 50v

sub in a new 12GN7.

back of the AGC just a bit (adj clockwise till you lose sync, then back off till sync is normal is the typical adjustment. Make sure the AGC is work like this. You should be able to adjust from no pic at all (washed out) to a dark pic and then loss of sync.

[tvcollector]

I adjust the AGC control and all it does is make the pic bright or dim...

[tvcollector]

I'm probably going to replace all the filter caps, So after I restuff all the cans and caps on the sweep board, I'll see what improvements there are between the controls and the vertical width.. This thing has like 4 cans and one cardboard can and a non polarized electrolytic in it somewhere and also an electrolytic in the audio circuit.. I guess I might as well replace them all..

[DaveWM]

you would be better off diagnosing the problem and correcting the issue.
This era set rarely needs shot gun replacement of parts.

The contrast issue sounds like a video out tube since the other areas (sync/audio/color) are fine. Lack of hum bars in the pic indicates the B+ is prob ok. I typically look at the B+ voltage and scope the ripple to determine the condition of the B+ supply.

if a new video out tube does not clear it the next logical area to look into is the cathode bypass cap on the video out. disconnecting the cap would confirm this, bridging would not help since is seems to be shorted not open (open would be too little contrast).

The issue I have with shot gun replacement is two fold, often it simply is not needed the caps are fine, but more importantly is when used as a way to correct a problem you end up not learning much (what happens if the contrast problem returns? shotgun recap again?).

So best approach is to bone up on what the clues are from the set and learn the various circuits and how they function. Then with some decent test equipment you can trouble shoot and pin point the offending part or parts.

So back to the orig suggestion, try a new video out tube, if no change disconnect the video out cathode bypass cap, test the contrast control, test the bypass cap, replace if bad.

This take much less time to do than a restuffing of caps. One last problem is mass replacement of parts can create tough dogs out of pushovers IF an incorrect part value is used in the replacement. Not impossible to do. It one reason why unmolested sets tend to be more desirable than "restored" sets, unless of course the work was done correctly and the set is currently working.

Work one issue at a time, get the contrast fixed move to the vert, vert problems are tube/bypass cap/feedback cap/plate load resistor on multivibrator.

[tvcollector]

I'll pull out the chassis sometime early this week and go by your directions, and see if I can find anything...

[DaveWM]

before you pull the chassis do the easy thing 1st try a new video out tube. I always start with the easy possible 1st even if its not the likely problem. Also use the sams to check the pin resistance.

[tvcollector]

I've ordered a few of the 12GN7 tubes, so I will be waiting for those to arrive before I will be moving on..

[tvcollector]

I just tested the 12GN7 and it fails grid and shorts.. I thought I tested it before and it tested fine... I guess I'll see what happens when the other tubes arrive...

[DavGoodlin]

Originally Posted by tvcollector

I know it needs to be degauss.. I'm not worried about that right now, as space right now is an issue.. The worminess is what I can't get to go away.. I've adjusted the fine tuning, and it doesn't fix it.. HV is set all the way up, if that's what your talking about..

I'm working on a similar set, a Sylvania 580 (CTC15 clone). The alignment instructions for the 4.5 mc trap (a single slug-coil) right after the video detector diode say to put contrast at max, fine tune into the worms/beat then adjust the slug for minimum interference.

[tvcollector]

I received my 12GN7 tubes today.. There are 3 of them, and my B&K 747B tester tests them all in the weak range.. I'm not sure why.. If it's really my tester, or all these tubes are weak.. The eBay auction stated them all being strong.. one goes up around the "?" range, and I used that tube.. I get better color and the picture in all looks better, but I still can't adjust the contrast.. The worminess seems to be at the normal minimal..

with one of the tubes, After being on for a few minutes, I noticed the color becomes out of sync...

[DaveWM]

not able to adjust contrast, look for that wire coming off the contrast pot that goes to one of the sections of a can, 100uf 50v or something like that. Disconnect that, you should lose most of the luma content (looks like very low contrast). then try hooking a good 100uf cap to that wire (pos, neg to ground), see if that fixes the contrast control.

Tube testers are not much good but for testing short/grid emissions/and totally dead tubes. Don't put much faith in them, better to do tube subs with known working tubes.