Help needed for 1986 RCA Colortrak tv.....

[DonD]

Hi,
Am in the process of rebuilding my TV Typewriter from 1976 (TV Typewriter - Wikipedia) and needed an old TV to see if the RF modulator still worked. I was given a 1986 RCA ColorTrak TV, model F20517WNA01, chassis CTC146. The TV mostly works including the OSD text and menus, but can’t get the signal from my TV Typewriter to vertically sync on the TV screen. I can see all of the characters, but the picture keeps rolling. I tried another RF source from a vintage colorbar generator on the TV and it would not vertically sync as well. Attached is a partial schematic of the circuit in question. The U1001 chip is a A2302-1101-01A. I have verified the various waveforms on my scope and voltages appear to be in spec. I’m wondering if the A2303 chip has partially failed. Have tried to find more than a block diagram for the A2303 chip, but no luck. I also tried placing a 50k pot between pins 56 and 58 of the A2303 chip and could alter the vertical roll, but could not get the picture to sync.

Thanks for the help,
Don

[DavGoodlin]

The "TV Typewriter cookbook" by Don Lancaster is in my Library, so I will do some more research but for starters, replace that 1 mf cap C2603 (?) that feeds the sync into the countdown circuit.

[DonD]

Thanks DavGoodlin for the suggestion. I replaced the cap, but no change. I thought the problem was vertical sync, but seems more like vertical fold over. Using the TV Typewriter, I cleared the screen of characters and typed HELP. It shows up five times on the screen and is stable, but should only appear once. I uploaded the picture, but my phone camera only got a few of the HELPs instead of all five. Sorry for the confusion.

[Notimetolooz]

Possibly the vertical is running at a multiple of the correct frequency. I don't think that can be described as "foldover'.
It is possible with an item built even that recently that an electrolytic capacitor, maybe in the power supply, that has gone bad.

[DonD]

Thanks Notimetolooz for your thoughts. What is interesting is the characters are about the right size and are not distorted in shape. When I fill up the screen with unique characters I cannot see all of them which to me implies folding. Of course line/frame blanking could be hiding some of the characters. In regards to the power supply, I used a variac to vary the AC voltage between 90-130 volts and the vertical display rate did not change. Still had five instances of the word HELP and their positions on the screen remained the same. Going below 90 volts and the width shortened and brightness dimmed.

[DonD]

Hi, was hoping I could get some additional help. I really hate to take the TV to the local landfill. Thanks for the suggestions - Don