Everybody knows this but me.......

Wildcat- did you solve the problem?
Full pix can be a function of setting the correct aspect ratio.
 Black lines are typically- if I am thinking of the same thing- vertical interval pulse lines.
 IIRC there might have been a cure for them, but my mind is getting a bit fuzzy going back that far.
 Possibly setting the vert HT & Lin might have been the cure.
 I remember some sets you just had to live with it.

BTW- If that set is a CTC 176-187 chassis, if you start getting int vertical, don't bang on the set. 
 Cause is bad tuner grounds (!!) and they will spike the EEPROM blowing it out. Severe cases required the tuner IC replacement too.
 If the vert IC fails, it means replacing the bloody EEPROM and total realignment of the parameters. Requires a VCR and a special piece of factory only test equipment. At one point the EEPROMS did come "pre-programmed".  Also resolder ALL grounds in the tuner. Big pain, but that is what would take out the vertical. Sounds weird, but I made a very nice nut for several years doing the repairs. there was also a special tuner mod kit for grounding. (Gotta love that horrible leadless solder and the poor soldering bath the boards went thru.)

 I usta be able to do the basic job in about 1.5 hrs, but if the whole shebang is borked, it was a 4-5 hour job from step 1. I hated the total alignment jobs, and it required the OEM factory manual for the info on how to properly do it too. Sams was lacking in some areas.

Do you have a VCR hanging around?
 They often have an RF mod in them. Run cable box into VCR, hook rca conn from VCR to TV, and set for ch 3 or 4. I remember running into this 10+ years back a few times. Sometimes it works, sometimes not.

The lines can be really PITA at times.
 Sometimes an RF Mod will help.
 Some of the fancy RF mods had multiple types of outputs too.
 Typically little use for them and are G sale items, or Badwilly . Should cost no more than 3 bux for a used one.

Amazon has them new from $10.00 and up. Depends on how fancy & what brand you want.

My wife recently broke her hip and she is stuck in the living room until she mends a bit.  She is "forced" to watch our old 1992 RCA Home Theater 32" CRT TV.  We have fiber optics TV signal, which is beyond crystal clear.  The picture on this old set looks like a miniature HD TV picture.  The problem is those silly black lines.  One at the top, one at the bottom.  "COZI" TV gives close to a full screen picture, for some odd reason.  Anyways, I'd like to get rid of the black lines.  I've read an RF modulator is what I need.  I can get one at Walmart for about $20.  I have been trying to find information on RF modulators.  All the descriptions say is that they provide an "S" video input for a TV that does not have one.  I need to know if an RF modulator, connected between the cable box and the TV will give this old RCA that we paid almost $1400 for when it was new, a new lease on life and a full picture.  I assume I'd connect the three audio/video cables from the cable box to the RF modulator, then connect the modulator to the TV with the antenna coax, then tune the TV to channel 3 or channel 4, whatever is appropriate to the device.  The old TV guys do something to get a full screen, so i know it's possible.  Sorry for such a silly question, and I realize I'm making this harder than it should be.  Thanks for your help.