Magnavox Color Roundie - Page 12 - Videokarma.org TV - Video

tvcollector · #**166** 05-21-2015, 03:34 PM

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

Thanks..

tvcollector · #**167** 05-21-2015, 04:51 PM

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..

DaveWM · #**168** 05-21-2015, 09:25 PM

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.

tvcollector · #**169** 05-22-2015, 02:57 AM

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?

DaveWM · #**170** 05-22-2015, 07:41 AM

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.

tvcollector · #**171** 05-22-2015, 01:11 PM

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..

DaveWM · #**172** 05-22-2015, 01:33 PM

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

tvcollector · #**173** 05-22-2015, 02:10 PM

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..

DaveWM · #**174** 05-22-2015, 02:26 PM

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.

tvcollector · #**175** 05-22-2015, 02:33 PM

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..

timmy · #**176** 05-22-2015, 02:45 PM

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.

tvcollector · #**177** 05-22-2015, 02:48 PM

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..

DaveWM · #**178** 05-22-2015, 02:53 PM

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.

tvcollector · #**179** 05-22-2015, 03:05 PM

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..

DaveWM · #**180** 05-22-2015, 04:39 PM

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.