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

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...

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.

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...

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.

I'll try that either later tonight, or another night when I've got more time...

Thanks..

I managed to squeeze some time in.. I removed the chassis, sat it up on the table, I snipped the wire going to the electrolytic and connected a 100uf 50v cap to the wire, and the ground to one of the terminals that go to ground, put the chassis back in the cabinet, hooked everything back up, and there is no change.. Contrast control still does nothing..

did you try it with the cap disconnected? do some resistance check for pin 1 of the 12GN7 see what the sams say is should be. Make sure the control effects the resistance readings.

I did try it with the cap disconnected and the color looked weird.. I'll do some readings when I have time to pull the chassis out again and set on the table to where I can access it much better.. Am I able to check resistance with the tube out?

the color should have been correct, just very washed out. if it goes from very washed out to too much (cap out/cap in) then is sounds like the control is not working, check the pot, look also for any shorts, sometimes the output terminals from a pot can get moved around during chassis removal, resulting in shorted term to ground. its always best to check tube pin resistance readings with the tube installed. Depending on how its wired, an open pot ground may have the same effect. check to make sure its functioning AND the ground lead is not open.

The color did look washed out, but look like it also wasn't in sync correctly.. I'll check all this soon, when I have more time.. Thanks..

sync should not be effected. could have be a co incidental. My bet is on a bad pot (crack in element) or bad ground.

I measured resistance, Easier than I thought, I didn't have to pull the chassis out.. Not sure which one is pin 1, but I measured the two outer pins, one measures at 196ohms, and the other outer 000.6ohms, and they don't change adjusting the contrast pot..

The sams doesn't show any measurements on the schematic out from pin 1, just shows voltages

The pot according to the sams is a 600ohm, I measured acrossed it, and it doesn't go near it while adjusting..

disconnect the pot enough to be sure and isolate it, then check the two outside terms and confirm 600ohms (no breaks in element) and then the wiper to each outside, checking for correct action (0-600 ohms).
an analog meter is the best tool for this.

the pot is setup as a variable resistor, so IF its broken you MAY be able to swap the wiring around to get it to work.

I measured the two outer terms with everything still connected and it only stays at 196ohms when adjusting.. I assume the pot is bad.. Still with everything connected, I would think there would be change as I adjust the pot..

best to take off at least 2 wires if 3 are on the pot to get an accurate reading. to find out if the pot works you have to use one end term and the middle term then rotate and see if ohms change.

Oops I was doing this all wrong.. I disconnected the wires, I get 736 ohms out of each end, and it goes down to about 00.2 ohms on the lower end from wiper to each end.. So I assume the pot is good..

hmmm very odd, the pot checks out, you need to review the wiring and make sure nothing is shorting and all is hooked up correct. The washed out look is correct (degenerative feedback) the cap bypasses the cathode eliminating the degenerative feed back. this is a very simple circuit, should not be that hard to make work. IF you get the washed out pic AND you get the over contrast then the pot/cap have to be an issue.

I disconnected the green wiper wire that goes to the filter cap that's suppose to give a washed out look when disconnected, and really no change in the picture, I touch it on the terminal while the set is on, to see if there is sudden change in picture and it seems to make it slightly brighter or add a slight more contrast when connected.. Odd how it didn't give that washed out look this time, but I also disconnected the wire from the contrast pot this time instead of from the filter cap leaving the 100uf cap connected to wire and removed from ground the last time..

the way it works is the pot outside fixed resistance is in the cathode to ground circuit of the video amp, its in p with a fixed 220 ohm cathode resistor. This allows for a voltage drop across the resistor and pot when the video out tube conducts. The result is the cathode of the tube has a positive voltage with respect to the ground. This sets the bias of the tube (control grid to cathode). Proper bias is needed for the tube to work as designed.

the Cap is connected in such a way (neg to ground/pos to wiper) so when the pot is fully CCW (least contrast, least gain) the wiper is at ground so the cap does nothing. As you turn CW the cap is introduced into the cathode circuit, acting as a bypass to the ac signal generated by the tube conducting a normal video signal. With out the cap there is neg feedback (strong signal, more voltage drop, more voltage drop higher cathode voltage, higher cathode voltage greater bias off of tube. With the cap in place the bias is held constant for max gain (most contrast) of the tube.

Looking at the schematic.. The circuit only consists of 2 ceramic disc caps and two resistors, plus the 100mfd cap off the filter cap which isn't the problem.. I don't know what else it would be other than possibly one of the resistors, or one of the caps which is probably unlikely..

I checked the two resistors in the circuit which are rated at (R67) 220 and (R68) 270 ohms.. I get 229 out of the 220 rated resistor, and 195ohms out of the 270 ohms rated resistor.. Testing them in circuit, the one is off by alot, but could be due to being in circuit, and even if it is way off like that, I would still think there would still be contrast control..

doubt the ceramic are an issue, and the resistor values are not critical.

The clue is if you disconnect the can cap and the contrast gets super weak then it pretty much has to be something screwy with the pot/100uf/wiring.

you can check other things but that seems to be the case to me.

I don't know how to proceed other than to check pin one on the tube socket and check for continuity each end of the wires or just replace the pot if I can find a equivalent part.. The sharp control is also another issue, the wiper and one of the end terminals are bent into each other on that pot and soldered to each other and goes to ground, pot measures very low ohms and meter doesn't go up/down at all the way it's set up, but according to the sams, this is the way that pot is..

http://i1220.photobucket.com/albums/...V/DSCF0382.jpg

no reason to replace the pot if it check out.

Well it seems to be a Maggie problem with contrast because the contrast on my 65 is not the greatest but I decided to leave it alone.

lets recap the contrast problem as stated.

Too much contrast, contrast control does nothing.

Disconnect the 100uf cap in the contrast control circuit, contrast goes very low
colors get "weird" (not sure of what that means or if related).

My conclusion is since there are only two things in the contrast control a pot and a large cap, and deactivating the contrast by removing the cap effectively puts the contrast at the other extreme, then the problem HAS to be the cap/pot/wiring one of those are a combo.

I'll try the 100mfd cap again, this time I'll tack it on a short wire from the pot control instead of going under the chassis..

It's the 100mfd cap... For some reason now it's working.. Last time I tried it didn't work.. Possibly due to the fact I rigged it under the chassis last time, and perhaps maybe when I put the chassis back in the set something may have came loose..

Should I go ahead and just restuff the can? infact I might as well restuff them all..

if you have a cap tester you could test the others (old school eye tube type to check for leakage).

IF it checked fine I would leave it and just not use the 100uf in the can. IF any others tested bad (leaky or weak) then I would go ahead and restuf.

Am I able to test the cans while they are still hooked up, or does the wires need to be removed to test?

I was looking into the sharp control circuit, I changed out the 6GHA which had a 6LM8.. Now video went away, it's just a grey scale screen.. I inserted the original tubes, and same thing..

need to be removed for testing leakage

Well I give up on the damn thing... All I did was remove and exchange the third video IF tube and the 6G8H video amp and the other 6G8H on the sweep board... I wiggled wires around, and checked the tube sockets, wiggled tubes to see if any change and nothing changes.. I can't get video back.. I get nothing but a grey scale screen with moving retrace lines..

Here's an example: The flickering is the camera..

https://www.youtube.com/watch?v=o7847FdbnBY

its best to only do one thing at a time on a working set. That way if something goes wrong its easier to find the problem.

I looks like no signal getting thru the video since the audio is working.

1st is the tube lighting up? sometimes pcb grounds can crack, Need to be gentle when removing tubes. I use a spray lube/contact cleaner on tube before removing just aim around the base and let it set. this way the tube comes out easy and less likely to flex an old pcb.

If it was mine I would start with signal injection at the grid of the video out and work back. OR scope the signal begining at the output of the video diode.

if you lack the equipment for the above you need to do voltage test and see if the video tubes (video amp and video out) have correct voltage.

as to why this would happen, hard to say, most likely a flakey pcb or if you inserted a shorted tube then maybe smoked a plate or screen resistor.

It's something not making a connection, because it has happened one or twice before when messing with those tubes, I remember powering off, wiggling tubes and wires, and powering back on and the problem went away.. This time it's not.. I moved on to the sharpness control seeing how I already know what the problem is with the contrast... All tubes are lighting, I took the 6GH8 out from the sweep board and powered on and seems like no change.. I have to go to work so I have to look into this probably on Monday, is when I have free time..

Circuit board traces under the tube sockets probably the video output tube or more likely others. All my sets had broken connections on several sockets but you have to look closely for them . Best thing to do is resolder every tube socket and mark it off your list of problems.

what timmy is describing is shown on one of my videos working on a predicta. a tube socket extender with the test points is ideal for finding such issues. Agree using your eyes and a magnafiy glass is a good start.

I've seen Shangos recent predicta video where he actually zooms in real close to and it shows a break in the tube socket connection.. I've checked the 3 sockets that I pulled the tubed from before this problem occurred... I check with a mag lite, magnifying glass and poked in each hole with a small wire.. All of them are solid.. I'll go under the board where I have to remove the bottom metal plate, which I already did once, and this time I'll just reflow the tube sockets as timmy says.. The wire that goes to the sharp control was broken lose and the inner wires were touching each other.. But I don't think that's the problem.. I don't think anything like a resistor got burned out or else I would have smelled something or saw something cooking, unless a resistor burnt out without noticing any symptoms..

Should I spray WD40 and contact cleaner in the sockets pin holes?

I'm getting 125v at the 6EJ7 3rd video IF, sams says its suppose to be 180v, I pull the tube out of socket and the voltages jump up to about 180v.. I'm getting 150v at the 6GH8 second video amp which is about right... I get 51v at 6GH8 sync sep, suppose to be 85v.. I'm doing the test with the chassis out of the cabinet and the HV tube removed..